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Famous Scientists - Biography of James Chadwick
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James Chadwick - Student Encyclopedia (Ages 11 and up)
James Chadwick (born October 20, 1891, Manchester , England—died July 24, 1974, Cambridge , Cambridgeshire) was an English physicist who received the Nobel Prize for Physics in 1935 for the discovery of the neutron .
Chadwick was educated at the University of Manchester , where he worked under Ernest Rutherford and earned a master’s degree in 1913. He then studied under Hans Geiger at the Technische Hochschule, Berlin. When World War I broke out, he was imprisoned in a camp for civilians at Ruhleben. He spent the entire war there but nevertheless was able to accomplish some scientific work.
After the war ended, Chadwick returned to England to study under Rutherford at the University of Cambridge . He received a doctorate in 1921, and in 1923 he was appointed assistant director of research at the Cavendish Laboratory , Cambridge. There he and Rutherford studied the transmutation of elements by bombarding them with alpha particles and investigated the nature of the atomic nucleus, identifying the proton , the nucleus of the hydrogen atom , as a constituent of the nuclei of other atoms.
After the discovery of the proton, physicists had surmised that there were likely additional particles in the atomic nucleus. Elements heavier than hydrogen had a greater atomic mass than their atomic number (the number of protons). Theories for the additional particles included additional protons whose charge was shielded by electrons in the nucleus or an unknown neutral particle. In 1932 French physicists Frédéric and Irène Joliot-Curie bombarded beryllium with alpha particles and observed that an unknown radiation was released that in turn ejected protons from the nuclei of various substances. The Joliot-Curies hypothesized that this radiation was gamma-rays . Chadwick was convinced that alpha particles did not have enough energy to produce such powerful gamma-rays. He performed the beryllium bombardment experiments himself and interpreted that radiation as being composed of particles of mass approximately equal to that of the proton but without electrical charge—neutrons. That discovery provided a new tool for inducing atomic disintegration , since neutrons, being electrically uncharged, could penetrate undeflected into the atomic nucleus and led to a new model of the atomic nucleus being composed of protons and neutrons.
In 1935 Chadwick was appointed to a chair in physics at the University of Liverpool. In 1940 he was part of the MAUD Committee , which was to assess the feasibility of the atomic bomb . The committee concluded in 1941 that the 1940 memorandum of Otto Frisch and Rudolf Peierls was correct and that a critical mass of only about 10 kilograms (22 pounds) of uranium -235 was needed. Chadwick later said he realized “that a nuclear bomb was not only possible, it was inevitable. I had then to take sleeping pills. It was the only remedy.” The MAUD Committee’s results were influential in giving an impetus to the American atomic bomb program. He became head of the British delegation to the Manhattan Project in Los Alamos , New Mexico , U.S., in 1943 and formed a close rapport with its head, Gen. Leslie Groves .
Chadwick was knighted in 1945. He returned to Britain in 1946 and became the British scientific adviser to the United Nations Atomic Energy Commission . He became master of Gonville and Caius College, Cambridge, in 1946, and he received the Copley Medal of the Royal Society in 1950. He retired in 1958.
May 1932: Chadwick Reports the Discovery of the Neutron
By 1920, physicists knew that most of the mass of the atom was located in a nucleus at its center, and that this central core contained protons. In May 1932 James Chadwick announced that the core also contained a new uncharged particle, which he called the neutron.
Chadwick was born in1891 in Manchester, England. He was a shy child from a working class family, but his talents caught his teachers’ attention, and he was sent to study physics at the University of Manchester, where he worked with Ernest Rutherford on various radioactivity studies.
In 1914, Chadwick decided to travel to Germany to study with Hans Geiger. Unfortunately, not long after he arrived, WWI broke out and Chadwick ended up spending the next four years in a prison camp there. This did not entirely stop his scientific studies. To keep from being bored, he and some fellow prisoners formed a science club, lectured to each other, and managed to convince the guards to let them set up a small lab. Though many chemicals were hard to get hold of, Chadwick even found a type of radioactive toothpaste that was on the market in Germany at the time, and managed to persuade the guards to supply him with it. Using some tin foil and wood he built an electroscope and did some simple experiments.
After the war, Chadwick returned to England, where he finished his PhD in Cambridge in 1921 with Rutherford, who was then Director of Cambridge University’s Cavendish laboratory. Chadwick was able to continue to work on radioactivity, now with more sophisticated apparatus than tin foil and toothpaste. In 1923, Chadwick was appointed assistant director of Cavendish Laboratory.
Photo: AIP Emilio Segre Visual Archives
James Chadwick
Rutherford had discovered the atomic nucleus in 1911, and had observed the proton in 1919. However, it seemed there must be something in the nucleus in addition to protons. For instance, helium was known to have an atomic number of 2 but a mass number of 4. Some scientists thought there were additional protons in the nucleus, along with an equal number of electrons to cancel out the additional charge. In 1920, Rutherford proposed that an electron and a proton could actually combine to form a new, neutral particle, but there was no real evidence for this, and the proposed neutral particle would be difficult to detect.
Chadwick went on to work on other projects, but kept thinking about the problem. Around 1930, several researchers, including German physicist Walter Bothe and his student Becker had begun bombarding beryllium with alpha particles from a polonium source and studying the radiation emitted by the beryllium as a result. Some scientists thought this highly penetrating radiation emitted by the beryllium consisted of high energy photons. Chadwick had noticed some odd features of this radiation, and began to think it might instead consist of neutral particles such as those Rutherford had proposed.
One experiment in particular caught his attention: Frédéric and Irène Joliot-Curie had studied the then-unidentified radiation from beryllium as it hit a paraffin wax target. They found that this radiation knocked loose protons from hydrogen atoms in that target, and those protons recoiled with very high velocity.
Joliot-Curie believed the radiation hitting the paraffin target must be high energy gamma photons, but Chadwick thought that explanation didn’t fit. Photons, having no mass, wouldn’t knock loose particles as heavy as protons from the target, he reasoned. In 1932, he tried similar experiments himself, and became convinced that the radiation ejected by the beryllium was in fact a neutral particle about the mass of a proton. He also tried other targets in addition to the paraffin wax, including helium, nitrogen, and lithium, which helped him determine that the mass of the new particle was just slightly more than the mass of the proton.
Chadwick also noted that because the neutrons had no charge, they penetrated much further into a target than protons would.
In February 1932, after experimenting for only about two weeks, Chadwick published a paper titled “The Possible Existence of a Neutron,” in which he proposed that the evidence favored the neutron rather than the gamma ray photons as the correct interpretation of the mysterious radiation. Then a few months later, in May 1932, Chadwick submitted the more definite paper titled “The Existence of a Neutron.”
By 1934 it had been established that the newly discovered neutron was in fact a new fundamental particle, not a proton and an electron bound together as Rutherford had originally suggested.
The discovery of neutron quickly changed scientists’ view of the atom, and Chadwick was awarded the Nobel Prize in 1935 for the discovery. Scientists soon realized that the newly discovered neutron, as an uncharged but fairly massive particle, could be used to probe other nuclei. It didn’t take long for scientists to find that hitting uranium with neutrons resulted in the fission of the uranium nucleus and the release of incredible amounts of energy, making possible nuclear weapons. Chadwick, whose discovery of the neutron had paved the way for the atomic bomb, worked on the Manhattan Project during WWII. He died in 1974.
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Famous Experiments: The Discovery of the Neutron
. for his discovery. Using kinematics, Chadwick was able to determine the velocity of the protons. Then through conservation of momentum techniques, he was able to determine that the mass of the neutral radiation was almost exactly the same as that of a proton. This is Chadwick's equation: , or little neutral one. for his pioneering work.
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The Existence of a Neutron
James Chadwick and the Discovery of the Neutron
James Chadwick (1891 – 1974)
On February 27 , 1932 , English physicist and Nobel Laureate Sir James Chadwick published an article in the scientific journal ‘Nature ‘ about the discovery of the neutron , a previously unknown particle in the atomic nucleus .
Youth and Education
World war i.
After the outbreak of the First World War in 1914, he was imprisoned. During his imprisonment in the “Zivilgefangenenlager” in Ruhleben, however, he was still able to carry out his own experiments, albeit with clear restrictions. After his return to England and Rutherford’s assumption of the management of the Cavendish Laboratory in 1919, he became its close collaborator and assistant director of the institute. They worked together on the investigation of gamma radiation and the structure of the atomic nucleus.
In Search of the Neutron
Returned to Cambridge , James Chadwick discovered an until then missing piece in the atomic nucleus in 1932, which was later known as the neutron . The search for the particle began around 1920, when Ernest Rutherford published his ideas on its possible existence. In his understanding, the neutron was to be a neutral double consisting of an electron that orbited a proton. About a decade later, Viktor and Dmitri Ivaneko however proved that the nucleus could never consist of protons and electrons and in the following year, German scientists found out that in case of alpha particles being emitted from polonium and falling on beryllium , boron or lithium , radiation was produced, which they took for gamma rays . Iréne Joliot-Curie (daughter of Marie Curie ) and Frédéric Joliot proved that the previously discovered radiation ejected protons of high energy, when falling on a hydrogen containing compound.
Chadwick’s Discovery
The next person known to have been experimenting on the gamma ray theory was James Chadwick himself. He performed many experiments, stating that the radiation his German colleagues talked about contained uncharged particles of about the mass of a proton. The particles were called neutrons and his theory spread quickly, earning a great reputation amongst other scientists all over the world. Chadwick published an article in the journal Nature on 27 February 1932 on his research into the existence of the neutron. For his achievement he was awarded the 1935 Nobel Prize for Physics. Chadwick subsequently devoted himself to building a cyclotron at the University of Liverpool, where he became Lyon Jones Professor of Physics in 1935. In 1940, the device was used to prove that a few kilograms of enriched uranium would be sufficient for the production of an atomic bomb, not the previously estimated quantity of at least one tonne.
The Atomic Bomb
Chadwick’s discovery was critical in the sense of general physics and especially in concerns of nuclear fission. Chadwick was a member of the MAUD Commission, which discussed whether the construction of a nuclear weapon was possible. He wrote about it later:
„I realised that a nuclear bomb was not only possible, it was inevitable…I had then to start taking sleeping pills. It was the only remedy.“
The Italian scientist Enrico Fermi was through Chadwick’s achievements motivated to investigate various nuclear reactions which led to Otto Hahn and Fritz Strassman discovering the first nuclear fission…but this is already another story.[ 6 ] Together with other British scientists, Chadwick worked in this MAUD commission on the construction of such a weapon, which reckoned with the availability of a British nuclear weapon until 1943. An appropriate facility for the production of weapons-grade material was built in Canada. After the USA entered the war in December 1941, the American government also intensified its efforts to build a nuclear weapon. In 1943, the governments of the two states decided to coordinate their nuclear programmes. Together with other British scientists, Chadwick was sent to the USA to work on the Manhattan project. He led the British mission to the Manhattan Project and remained there until 1946. The uranium produced in Canada was used for further research and thus contributed to the completion of the first atomic bomb.
Later Years
After the war, Chadwick returned to Liverpool and participated in the development of the British nuclear energy programme. He also helped to establish a synchrotron at Liverpool University and was instrumental in the UK’s decision to participate in the development of CERN, the European Nuclear Research Centre.
James Chadwick passed away on July 24, 1974 in Cambridge.
References and Further Reading:
[1] Biography at nobelprize.org
[2] Chadwick at the atomic archive
[3] Discovery of the Neutron
[4] Chadwick at Cambridge Physics
[5] Ernest Rutherford Discovers the Nucleus , SciHi Blog, December 20, 2012.
[6] The first Self-sustained Nuclear Chain Reaction , SciHi Blog, December 2, 2012
[7] George B. Kistiakowsky and the Manhattan Project , SciHi Blog, November 17, 2017
[8] Harrison Brown and the Isolation of Plutonium , SciHi Blog, September 26, 2017
[9] Marie Curie – Truly an Extraordinary Woman , SciHi Blog, November 7, 2012
[10] Pierre Curie and the Radioactivity , SciHi Blog, April 19, 2016
[11] Hans Geiger and the Geiger Counter , SciHi blog
[12] James Chadwick at Wikidata
[12] Tyler DeWitt, Atomic Structure: Discovery of the Neutron , Tyler DeWitt @ youtube
[13] James Chadwick Timeline at Wikidata
Harald Sack
Related posts, edward condon – pioneer in quantum mechanics – scihi blog, sir alan hodgkin and the giant axon of the atlantic squid, emilio segrè and the discovery of the antiproton, edward teller and stanley kubrick’s dr. strangelove.
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Chadwick's Discovery of The Neutron
Hsc physics syllabus.
investigate, assess and model the experimental evidence supporting the nuclear model of the atom, including:
Discovery of the Proton and the Neutron
This video discusses experiments that led to the discovery of the proton and neutron.
Rutherford’s atomic model did not show what was inside the positive nucleus. Besides protons, he realised that there must be other particles because the mass number of an atom was always found to exceed its atomic number (number of protons).
Experiments that led to the discovery of the Neutron
When alpha particles were fired at a thin block of beryllium, a nuclear transmutation resulted in the production of neutrons.
However, the neutrons produced this way were initially hypothesised to be high-energy gamma radiation because they were unaffected by electric and magnetic fields. All particles known at the time (e.g. electrons, protons) were charged. When further experiments were conducted to investigate the nature of this radiation, the hypothesis was rejected.
The 'radiation' was projected onto a proton-rich paraffin block, causing protons to be emitted. Analysis of these protons' momentum and kinetic energies provided an estimation of the energy of the gamma radiation. However, the energies of alpha particles that caused the emission of gamma radiation were far too small to allow for this possibility without violating the law of conservation of energy.
When this 'radiation' was used to irradiate metal surfaces, no photoelectric effect was produced. If this gamma radiation had sufficient energy to eject protons from the paraffin block, it should have been able to eject electrons as the latter would require much smaller amounts of energy (work function).
Chadwick's Experiment and Discovery of the Neutron
Chadwick conducted the same experiment using beryllium and paraffin block but provided a different interpretation. He claimed that this unknown radiation was actually neutral particles – neutrons.
By applying the law of conservation of momentum and conservation of energy , Chadwick determined the mass of a neutron. Chadwick reasoned that a neutral particle could eject a proton from the paraffin by imparting its momentum onto it (this explanation accounted for the kinetic energies of protons measured in the experiment).
Using the kinetic energy and momentum of emitted protons, Chadwick showed that the mass of a neutron is slightly greater than that of a proton.
Chadwick's discovery of the neutron added to the understanding of the structure of the atom as the atomic mass is now accounted for.
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October 12, 2021
Physicists announce the world's most precise measurement of neutron lifetime
by Whitney Clavin, California Institute of Technology
Particles called neutrons are typically very content inside atoms. They stick around for billions of years and longer inside some of the atoms that make up matter in our universe. But when neutrons are free and floating alone outside of an atom, they start to decay into protons and other particles. Their lifetime is short, lasting only about 15 minutes.
Physicists have spent decades trying to measure the precise lifetime of a neutron using two techniques, one involving bottles and the other beams. But the results from the two methods have not matched: they differ by about 9 seconds, which is significant for a particle that only lives about 15 minutes.
Now, in a new study published in the journal Physical Review Letters , a team of scientists has made the most precise measurement yet of a neutron's lifetime using the bottle technique. The experiment, known as UCNtau (for Ultra Cold Neutrons tau, where tau refers to the neutron lifetime), has revealed that the neutron lives 14.629 minutes with an uncertainty of 0.005 minutes. This is a factor of two more precise than previous measurements made using either of the methods. While the results do not solve the mystery of why the bottle and beam methods disagree, they bring scientists closer to an answer.
"This new result provides an independent assessment to help settle the neutron lifetime puzzle," says Brad Filippone, the Francis L. Moseley Professor of Physics and a co-author of the new study. The methods continue to disagree, he explains, because either one of the methods is faulty or because something new is going on in the physics that is yet to be understood.
"When combined with other precision measurements, this result could provide the much-searched-for evidence for the discovery of new physics," he says.
The results can also help to solve other long-standing mysteries, such as how matter in our infant universe first congealed out of a hot soup of neutrons and other particles. "Once we know the neutron lifetime precisely, it can help explain how atomic nuclei formed in the early minutes of the universe," says Filippone.
Blind tests
In 2017 and 2018, the UCNtau team performed two bottle experiments at the Los Alamos National Laboratory (LANL). In the bottle method, free neutrons are trapped in an ultracold, magnetized bottle about the size of a bathtub, where they begin to decay into protons. Using sophisticated data analyses methods, researchers can count how many neutrons remain over time. (In the beam method, a beam of neutrons decays into protons, and the protons are counted not the neutrons.)
Over the span of the experiments, the UCNtau collaboration counted 40 million neutrons.
To remove any possible biases in the measurements, caused by researchers consciously or unconsciously skewing results to match expected outcomes, the collaboration split into three groups that worked in a blind fashion. One team was led by Caltech, another by Indiana University, and another by LANL. Each team was given a fake clock, so that the researchers would not actually know how much time had elapsed.
"We made our clocks purposely a little off by an amount that somebody knew but then kept secret until the end of the experiment," says co-author Eric Fries (Ph.D. '22), who led the Caltech team and performed the research as part of his Ph.D. thesis.
"This makes the experiment more reliable because there's no chance of conscious or unconscious bias in fitting the results to match the expected neutron lifetime," adds Filippone. "Thus, we don't know the actual lifetime until we correct for this at the very end during the 'unblinding.'"
Trapping the zippy neutrons
One challenge in the study of stray neutrons is that they can easily bind to atoms, says Filippone. He notes that atomic nuclei in the experimental apparatus can readily "eat up the neutrons like Pac-Man." As a result, the researchers had to create a very tight vacuum in the chamber to keep out unwanted gases.
They also had to dramatically slow down the neutrons so that they can be trapped by magnetic fields and counted.
"We have to cool these neutrons down through various steps," says Filippone. "The key step at the end is to make the neutrons interact with a solid frozen chunk of deuterium [a heavier version of hydrogen] about the size of a birthday cake, which causes the neutrons to lose energy."
Once the experiments were done and the data were collected, each of the three teams used different approaches to analyze the data. Fries and the Caltech team used machine learning methods to help count the neutrons. "The tricky part is to look at the individual data points and say, yes, that is in fact a neutron," says Fries.
When all three teams unblinded their results, they found a remarkable level of agreement. "We all dealt with the data differently but came up with nearly the same answer, with differences that were less than the overall statistical error," says Fries.
In the end, the neutron lifetime was calculated to a precision better than 400 parts per million, making it the most precise result yet. Future experiments are underway to help further refine measurements made using the beam method and to ultimately determine whether systematic errors or new physics are behind the neutron-lifetime mystery.
The paper is titled, "An improved neutron lifetime measurement with UCNtau."
Improved neutron lifetime measurement with UCNτ, arXiv:2106.10375 [nucl-ex] arxiv.org/abs/2106.10375
Journal information: Physical Review Letters
Provided by California Institute of Technology
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Why did James Chadwick use berryllium to discover neutrons?
In Rutherford's alpha particle scattering experiment he used gold foil because it's malleable and can be drawn into very thin sheet. Why did James Chadwick use Berryllium while discovering neutrons then? He could have used Gold as well.
experimental-physics
1 $\begingroup$ This question would perhaps (also) be a good fit for History of Science and Mathematics . $\endgroup$ – Danu Commented Feb 9, 2016 at 12:13
2 Answers 2
Chadwick didn't discover the neutron on purpose, of course. After the discovery of the nucleus by Rutherford in 1911, alpha particles were used to probe its structure. These kind of experiments were pioneered by Rutherford himself (as an example, he discovered in 1917 that the nitrogen nucleus contains hydrogen nuclei, i.e. protons). In 1930, Bothe and Becker had discovered that when hit by a stream of alpha particles, Beryllium emitted some kind of electrically neutral radiation. At first, it was thought that it was regular gamma radiation. Then in 1932 Irène and Frédéric Joliot-Curie showed that the properties of the radiation emitted by Beryllium couldn't be explained in terms of gamma rays, so Chadwick set up an experiment to confirm Joliot-Curie's observations, and he succeeded to do so.
Now let's have a look at the neutron separation energies ($S_{n}$) of the first, let's say, ten stable nuclei of the periodic table (see for example here ).
Helium: 20577 keV
Litium: 7251 keV
Beryllium: 1665 keV
Boron: 11454 keV
Carbon: 18721 keV
Nitrogen: 10553 keV
Oxygen: 15664 keV
Fluorine: 10432 keV
Neon: 16865 keV
As you can see, Beryllium has the lowest neutron separation energy amongst the lightest stable nuclei. Gold has
Gold: 8072 keV
I'm not able to check the data about each and every stable element of the periodic table, but according to the plots in this document (see page 110), there are no light stable elements with $S_{n}$ lower than that of Beryllium. Light elements were used for those kind of experiments (in order to probe the nuclear structure, it's easiest to start from the bottom). It's easy to guess why the first element to be found to release neutrons was Beryllium.
In 1930 two German physicists Walther Bothe and Herbert Becker dicovered that when alpha particles were directed at beryllium some form of ionising radiation was produced, which they thought was gamma rays. Later Irene Joliot-Curie (Marie Curie's daughter) and her husband discovered that this radiation could eject protons from paraffin wax. Chadwick's advance was to measure the energies of the incident alpha particles and the produced protons very carefully, and show from this that the radiation couldn't be gamma rays as the energy required would be too high. The only solution was that the radiation was a neutral particle, which of course was the neutron.
So Chadwick used beryllium because he already knew it worked. The next question is why Bothe and Becker used beryllium ...
$\begingroup$ If you look at the publication: (doi: 10.1007/BF01390908) they didnt only use beryllium, but could detect "$\gamma$-Radiation" in "Li, Be, B, F, Mg, Al" (i havent read the whole publication, there might be some practical advantage of Be) $\endgroup$ – Bort Commented Feb 9, 2016 at 12:50
$\begingroup$ I think that an interesting point is that Bothe and Becker had actually produced neutrons but did not realise that they had done so. That is why Chadwick is credited with the discovery of the neutron. $\endgroup$ – Farcher Commented Feb 9, 2016 at 12:58
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April 1, 2016
11 min read
A Puzzle Lies at the Heart of the Atom
Two precision experiments disagree on how long neutrons live before decaying. Does the discrepancy reflect measurement errors or point to some deeper mystery?
By Geoffrey L. Greene & Peter Geltenbort
Luckily for life on Earth, most matter is not radioactive. We take this fact for granted, but it is actually somewhat surprising because the neutron, one of the two components of atomic nuclei (along with the proton), is prone to radioactive decay. Inside an atomic nucleus, a typical neutron can survive for a very long time and may never decay, but on its own, it will transform into other particles within 15 minutes, more or less. The words “more or less” cover a disturbing gap in physicists' understanding of this particle. Try as we might, we have not been able to accurately measure the neutron lifetime.
This “neutron lifetime puzzle” is not just embarrassing for us experimentalists; resolving it is vital for understanding the nature of the universe. The neutron decay process is one of the simplest examples of the nuclear “weak” interaction—one of nature's four fundamental forces. To truly understand the weak force, we must know how long neutrons live. Furthermore, the survival time of the neutron determined how the lightest chemical elements first formed after the big bang. Cosmologists would like to calculate the expected abundances of the elements and compare them with astrophysical measurements: agreement would confirm our theoretical picture, and discrepancy could indicate that undiscovered phenomena affected the process. To make such a comparison, however, we need to know the neutron lifetime.
More than 10 years ago two experimental groups, one a Russian-led team in France and the other a team in the U.S., attempted separately to precisely measure the lifetime. One of us (Geltenbort) was a member of the first team, and the other (Greene) was a member of the second. Along with our colleagues, we were surprised and somewhat disturbed to find that our results disagreed considerably. Some theoreticians suggested that the difference arose from exotic physics—that some neutrons in the experiments might have transformed into particles never before detected, which would have affected the different experiments in divergent ways. We, however, suspected a more mundane reason—perhaps one of our groups, or even both, had simply made a mistake or, more likely, had overestimated the accuracy of its experiment. The U.S. team recently completed a long, painstaking project to study the most dominant source of uncertainty in its experiment in hopes of resolving the discrepancy. Rather than clearing up the situation, that effort confirmed our earlier result. Similarly, other researchers later confirmed the findings of Geltenbort's team. This discrepancy has left us even more perplexed. But we are not giving up—both groups and others continue to seek answers.
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Timing Neutrons
In theory, measuring the neutron lifetime should be straightforward. The physics of nuclear decay are well understood, and we have sophisticated techniques for studying the process. We know, for instance, that if a particle has the possibility of transforming into a lower-mass particle or particles while conserving such characteristics as charge and spin angular momentum, it will. Free neutrons display this instability. In a process called beta decay, a neutron breaks up into a proton, an electron and an antineutrino (the antimatter counterpart of the neutrino), which collectively sum to a slightly lower mass but the same total charge, spin angular momentum and other conserved properties. These conserved properties include “mass-energy,” meaning that the daughter particles carry the difference in mass in the form of kinetic energy, the energy of motion.
We cannot predict exactly when a particular neutron will decay because the process is a fundamentally random quantum phenomenon—we can say only how long neutrons live on average. Thus, we must measure the average neutron lifetime by studying the decay of many neutrons.
Investigators have employed two experimental methods—one called the “bottle” technique and the other the “beam” approach. Bottle experiments confine neutrons in a container and count how many are left after a given time. The beam method, in contrast, looks not for the disappearance of neutrons but rather for the appearance of the particles into which they decay.
The bottle approach is particularly challenging because neutrons can pass easily through matter and thus through the walls of most containers. Following a suggestion first explicitly made by Russian physicist Yuri Zel'dovich, experimentalists who use the bottle approach—as Geltenbort and his colleagues in France do—get around the problem by trapping extremely cold neutrons (that is, those with a very low kinetic energy) within a container of very smooth walls. If the neutrons are slow enough and the bottle smooth enough, they reflect from the walls and hence remain in the bottle. To achieve this effect, the neutrons must move at speeds on the order of just a few meters per second, as opposed to the roughly 10 million meters per second neutrons travel when emitted during nuclear fission, for instance. These “ultracold” neutrons are so slow that you could “outrun” them. The most accurate bottle experiment to date took place at the Institut Laue-Langevin (ILL) in Grenoble, France.
Unfortunately, no bottle is ever perfect. If neutrons occasionally leak out of the bottle, we will attribute this loss to beta decay and will get the wrong lifetime. We must therefore be sure to correct our calculations so as to count only those particles that actually undergo beta decay.
To make that correction, we use a clever technique. The number of neutrons lost through the walls of the bottle depends on the rate at which neutrons bounce against the walls. If the neutrons are slower or the bottle is bigger, the bounce rate, and thus the loss rate, will go down. By varying both the size of the bottle and the energy (velocity) of the neutrons in successive trials, we can extrapolate to a hypothetical bottle in which there are no collisions and thus no wall losses. Of course, this extrapolation is not perfect, but we do our best to account for any error this calculation introduces.
In the beam method—used by Greene and others at the National Institute of Standards and Technology (nist) Center for Neutron Research—we send a stream of cold neutrons through a magnetic field and a ring of high-voltage electrodes that traps positively charged particles. Because neutrons are electrically neutral, they pass right through the trap. If, however, a neutron decays within the trap, the resulting positively charged proton gets “stuck.” Periodically we “open” the trap and expel and count the protons. In principle, the proton trapping and detection are nearly perfect, and we must make only very small corrections for the possibility that we missed decays.
Where Could We Go Wrong?
To be useful, a measurement must be accompanied by a reliable estimate of its accuracy. A measurement of a person's height that has an uncertainty of one meter, for example, is much less meaningful than a measurement that has an uncertainty of one millimeter. For this reason, when we make precision measurements we always report an experimental uncertainty; an uncertainty of one second, for instance, would mean our measurement had a high probability of being no more than a second shorter or a second longer than the true value.
Any measurement has, in general, two sources of uncertainty. Statistical error arises because an experiment can measure only a finite sample—in our case, a finite number of particle decays. The larger the sample, the more reliable the measurement and the lower the statistical error.
The second source of uncertainty—systematic error—is much more difficult to estimate because it arises through imperfections in the measurement process. These flaws may be something simple, like a poorly calibrated meter stick used to measure a person's height. Or they can be more subtle, like a sampling bias—in a telephone poll, for example, one might overly rely on calls to land lines rather than to cell phones and thus fail to capture a truly representative population sample. Experimentalists go to great lengths to reduce these systematic errors, but they are impossible to eradicate completely. The best we can do is carry out a detailed study of all imaginable sources of error and then estimate the lingering effect each might have on the final result. We then add this systematic error to the statistical error to give a best estimate of the overall reliability of the measurement. In other words, we put great effort into estimating the “known unknowns.”
Of course, our great fear is that we have overlooked an “unknown unknown”—a systematic effect that we do not even know we do not know—hidden within the experimental procedure. While we go to extreme pains to explore all possible uncertainties, the only way to overcome this type of additional error with real confidence is to perform another, completely independent measurement using a totally different experimental method that does not share the same systematic effects. If two such measurements agree within their quoted uncertainties, we have confidence in the results. If, on the other hand, they disagree, we have a problem.
For the measurement of the neutron lifetime we have two such independent methods: the beam and the bottle. The most recent result from the beam experiment at nist gave a value for the neutron lifetime of 887.7 seconds. We determined the statistical uncertainty in our estimate to be 1.2 seconds and the systematic uncertainty 1.9 seconds. Combining those errors statistically gives a total uncertainty of 2.2 seconds, which means that we believe the true value of the neutron lifetime has a 68 percent probability of being within 2.2 seconds of the measured value.
The bottle experiment at ILL, on the other hand, measured a neutron lifetime of 878.5 seconds with a statistical uncertainty of 0.7 second, a systematic uncertainty of 0.3 second and a total uncertainty of 0.8 second.
These are the two most precise neutron lifetime experiments of each type in the world, and their measurements differ by approximately nine seconds. Such a time span may not sound like a lot, but it is significantly larger than the calculated uncertainties for both experiments—the probability of obtaining a difference of this size by chance alone is less than one part in 10,000. We must therefore seriously consider the possibility that the discrepancy results from an unknown unknown—we have missed something important.
Exotic Physics
An exciting explanation for the difference could be that it actually reflects some exotic physical phenomenon not yet discovered. A reason to think such a phenomenon might exist is that although the bottle and beam methods disagree, other beam studies show good agreement among themselves, as do other bottle studies.
Imagine, for example, that in addition to the regular beta decay, neutrons decayed via some previously unknown process that does not create the protons sought in beam experiments. The bottle experiments, which count the total number of “lost” neutrons, would count both the neutrons that disappeared via beta decay as well as those that underwent this second process. We would therefore conclude that the neutron lifetime was shorter than that from “normal” beta decay alone. Meanwhile the beam experiments would dutifully record only beta decays that produce protons and would thus result in a larger value for the lifetime. So far, as we have seen, the beam experiments do measure a slightly longer lifetime than the bottles.
A few theorists have taken this notion seriously. Zurab Berezhiani of the University of L'Aquila in Italy and his colleagues have suggested such a secondary process: a free neutron, they propose, might sometimes transform into a hypothesized “mirror neutron” that no longer interacts with normal matter and would thus seem to disappear. Such mirror matter could contribute to the total amount of dark matter in the universe. Although this idea is quite stimulating, it remains highly speculative. More definitive confirmation of the divergence between the bottle and beam methods of measuring the neutron lifetime is necessary before most physicists would accept a concept as radical as mirror matter.
Much more likely, we think, is that one (or perhaps even both) of the experiments has underestimated or overlooked a systematic effect. Such a possibility is always present when working with delicate and sensitive experimental setups.
Why the Neutron Lifetime Matters
Figuring out what we missed will of course give us experimentalists peace of mind. But even more important, if we can get to the bottom of this puzzle and precisely measure the neutron lifetime, we may be able to tackle a number of long-standing, fundamental questions about our universe.
First of all, an accurate assessment of the timescale of neutron decay will teach us about how the weak force works on other particles. The weak force is responsible for nearly all radioactive decays and is the reason, for instance, that nuclear fusion occurs within the sun. Neutron beta decay is one of the simplest and most pure examples of a weak force interaction. To calculate the details of other, more complex nuclear processes involving the weak force, we must first fully understand how it operates in neutron decay.
Discerning the exact rate of neutron decay would also help test the big bang theory for the early evolution of the cosmos. According to the theory, when the universe was about one second old, it consisted of a hot, dense mixture of particles: protons, neutrons, electrons, and others. At this time, the temperature of the universe was roughly 10 billion degrees—so hot that these particles were too energetic to bind together into nuclei or atoms. After about three minutes, the universe expanded and cooled to a temperature where protons and neutrons could stick together to make the simplest atomic nucleus, deuterium (the heavy isotope of hydrogen). From here other simple nuclei were able to form—deuterium could capture a proton to make an isotope of helium, two deuterium nuclei could join together to create heavier helium, and small numbers of larger nuclei formed, up to the element lithium (all the heavier elements are thought to have been produced in stars many millions of years later).
This process is known as big bang nucleosynthesis. If, while the universe was losing heat, neutrons had decayed at a rate that was much faster than the universe cooled, there would have been no neutrons left when the universe reached the right temperature to form nuclei—only the protons would have remained, and we would have a cosmos made almost entirely of hydrogen. On the other hand, if the neutron lifetime were much longer than the time required to cool sufficiently for big bang nucleosynthesis, the universe would have an overabundance of helium, which in turn would have affected the formation of the heavier elements involved in the evolution of stars and ultimately life. Thus, the balance between the universal cooling rate and the neutron lifetime was quite critical for the creation of the elements that make up our planet and everything on it.
From astronomical data we can measure the cosmic ratio of helium to hydrogen, as well as the amounts of deuterium and other light elements that exist throughout the universe. We would like to see if these measurements agree with the numbers predicted by big bang theory. The theoretical prediction, however, depends on the precise value of the neutron lifetime. Without a reliable value for it, our ability to make this comparison is limited. Once the neutron lifetime is known more precisely, we can compare the observed ratio from astrophysical experiments with the predicted value from theory. If they agree, we gain further confidence in our standard big bang scenario for how the universe evolved. Of course, if they disagree, this model might have to be altered. For instance, certain discrepancies might indicate the existence of new exotic particles in the universe such as an extra type of neutrino, which could have interfered in the process of nucleosynthesis.
One way to resolve the difference between the beam and bottle results is to conduct more experiments using methods of comparable accuracy that are not prone to the same, potentially confounding systematic errors. In addition to continuing the beam and bottle projects, scientists in several other groups worldwide are working on alternative methods of measuring the neutron lifetime. A group at the Japan Proton Accelerator Research Complex (J-PARC) in Tokai is developing a new beam experiment that will detect the electrons rather than protons produced when neutrons decay. In another very exciting development, groups at ILL, the Petersburg Nuclear Physics Institute in Russia, Los Alamos National Laboratory, the Technical University of Munich and the Johannes Gutenberg University Mainz in Germany plan to use neutron bottles that confine ultracold neutrons with magnetic fields rather than material walls. This is possible because the neutron, though electrically neutral, behaves as though it is a small magnet. The number of neutrons accidentally lost through the sides of such bottles should be quite different from that of previous measurements and thus should produce quite different systematic uncertainties. We fervently hope that, together, continuing bottle and beam experiments and this next generation of measurements will finally solve the neutron lifetime puzzle.
Geoffrey L. Greene is a professor of physics at the University of Tennessee, with a joint appointment at the Oak Ridge National Laboratory's Spallation Neutron Source. He has been studying the properties of the neutron for more than 40 years.
Peter Geltenbort is a staff scientist at the Institut Laue-Langevin in Grenoble, France, where he uses one of the most intense neutron sources in the world to research the fundamental nature of this particle.
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Physicists confront the neutron lifetime puzzle
To solve a long-standing puzzle about how long a neutron can “live” outside an atomic nucleus, physicists entertained a wild but testable theory positing the existence of a right-handed version of our left-handed universe. They designed a mind-bending experiment at the Department of Energy’s Oak Ridge National Laboratory to try to detect a particle that has been speculated but not spotted. If found, the theorized “mirror neutron” — a dark-matter twin to the neutron — could explain a discrepancy between answers from two types of neutron lifetime experiments and provide the first observation of dark matter.
“Dark matter remains one of the most important and puzzling questions in science — clear evidence we don’t understand all matter in nature,” said ORNL’s Leah Broussard, who led the study published in Physical Review Letters .
Neutrons and protons make up an atom’s nucleus. However, they also can exist outside nuclei. Last year, using the Los Alamos Neutron Science Center, co-author Frank Gonzalez, now at ORNL, led the most precise measurement ever of how long free neutrons live before they decay, or turn into protons, electrons and anti-neutrinos. The answer — 877.8 seconds, give or take 0.3 seconds, or a little under 15 minutes — hinted at a crack in the Standard Model of particle physics. That model describes the behavior of subatomic particles, such as the three quarks that make up a neutron. The flipping of quarks initiates neutron decay into protons.
“The neutron lifetime is an important parameter in the Standard Model because it is used as an input for calculating the quark mixing matrix, which describes quark decay rates,” said Gonzalez, who calculated probabilities of neutrons oscillating for the ORNL study. “If the quarks don't mix as we expect them to, that hints at new physics beyond the Standard Model.”
Oak Ridge National Laboratory’s Leah Broussard shows a neutron-absorbing "wall" that stops all neutrons but in theory would allow hypothetical mirror neutrons to pass through. Credit: Genevieve Martin/ORNL, U.S. Dept. of Energy
From left, ORNL’s Matthew Frost and Leah Broussard used a neutron scattering instrument at the Spallation Neutron Source to search for a dark matter twin to the neutron. Credit: Genevieve Martin/ORNL, U.S. Dept. of Energy
To measure the lifetime of a free neutron, scientists take two approaches that should arrive at the same answer. One traps neutrons in a magnetic bottle and counts their disappearance. The other counts protons appearing in a beam as neutrons decay. It turns out neutrons appear to live nine seconds longer in a beam than in a bottle.
Over the years, perplexed physicists have considered many reasons for the discrepancy. One theory is that the neutron transforms from one state to another and back again. “Oscillation is a quantum mechanical phenomenon,” Broussard said. “If a neutron can exist as either a regular or a mirror neutron, then you can get this sort of oscillation, a rocking back and forth between the two states, as long as that transition isn’t forbidden.”
The ORNL-led team performed the first search for neutrons oscillating into dark-matter mirror neutrons using a novel disappearance and regeneration technique. The neutrons were made at the Spallation Neutron Source, a DOE Office of Science user facility. A beam of neutrons was guided to SNS’s magnetism reflectometer . Michael Fitzsimmons, a physicist with a joint appointment at ORNL and the University of Tennessee, Knoxville, used the instrument to apply a strong magnetic field to enhance oscillations between neutron states. Then the beam impinged on a “wall” made of boron carbide, which is a strong neutron absorber.
If the neutron does in fact oscillate between regular and mirror states, when the neutron state hits the wall, it will interact with atomic nuclei and get absorbed into the wall. If it is in its theorized mirror neutron state, however, it is dark matter that will not interact.
So only mirror neutrons would make it through the wall to the other side. It would be as if the neutrons had gone through a “portal” to some dark sector — a figurative concept used in the physics community. Yet, the press reporting on past related work had fun taking liberties with the concept, comparing the theorized mirror universe Broussard’s team is exploring to the “Upside Down” alternate reality in the TV series "Stranger Things." The team’s experiments were not exploring a literal portal to a parallel universe.
“The dynamics are the same on the other side of the wall, where we try to induce what are presumably mirror neutrons — the dark-matter twin state — to turn back into regular neutrons,” said co-author Yuri Kamyshkov, a UT physicist who with colleagues has long pursued the ideas of neutron oscillations and mirror neutrons . “If we see any regenerated neutrons, that could be a signal that we’ve seen something really exotic. The discovery of the particle nature of dark matter would have tremendous implications.”
Matthew Frost of ORNL, who received his doctorate from UT working with Kamyshkov, performed the experiment with Broussard and assisted with data extraction, reduction and analysis. Frost and Broussard performed preliminary tests with help from Lisa DeBeer-Schmitt , a neutron scattering scientist at ORNL.
Lawrence Heilbronn, a nuclear engineer at UT, characterized backgrounds, whereas Erik Iverson, a physicist at ORNL, characterized neutron signals. Through the DOE Office of Science Scientific Undergraduate Laboratory Internships Program, Michael Kline of The Ohio State University figured out how to calculate oscillations using graphics processing units – accelerators of specific types of calculations in application codes – and performed independent analyses of neutron beam intensity and statistics, and Taylor Dennis of East Tennessee State University helped set up the experiment and analyzed background data, becoming a finalist in a competition for this work. UT graduate students Josh Barrow, James Ternullo and Shaun Vavra with undergraduates Adam Johnston, Peter Lewiz and Christopher Matteson contributed at various stages of experiment preparation and analysis. University of Chicago graduate student Louis Varriano, a former UT Torchbearer , helped with conceptual quantum-mechanical calculations of mirror-neutron regeneration.
The conclusion: No evidence of neutron regeneration was seen. “One hundred percent of the neutrons stopped; zero percent passed through the wall,” Broussard said. Regardless, the result is still important to the advancement of knowledge in this field.
With one particular mirror-matter theory debunked, the scientists turn to others to try to solve the neutron lifetime puzzle. “We’re going to keep looking for the reason for the discrepancy,” Broussard said. She and colleagues will use the High Flux Isotope Reactor, a DOE Office of Science user facility at ORNL, for that. Ongoing upgrades at HFIR will make more sensitive searches possible because the reactor will produce a much higher flux of neutrons, and the shielded detector at its small-angle neutron scattering diffractometer has a lower background.
Because the rigorous experiment did not find evidence of mirror neutrons, the physicists were able to rule out a far-fetched theory. And that takes them closer to solving the puzzle.
If it seems sad that the neutron lifetime puzzle remains unsolved, take solace from Broussard: “Physics is hard because we’ve done too good a job at it. Only the really hard problems – and lucky discoveries – are left.”
The title of the paper is “Experimental Search for Neutron to Mirror Neutron Oscillations as an Explanation of the Neutron Lifetime Anomaly.”
DOE’s Office of Science and ORNL’s Laboratory Directed Research and Development Program supported the work. The study used resources of the Spallation Neutron Source, a DOE Office of Science user facility at ORNL.
UT-Battelle manages ORNL for the Department of Energy’s Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science .
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Nine seconds. An eternity in some scientific experiments; an unimaginably small amount in the grand scheme of the universe. And just long enough to confound nuclear physicists studying the lifetime of the neutron.
The neutron is one of the building blocks of matter, the neutral counterpart to the positive proton. Like many other subatomic particles, the neutron doesn’t last long outside of the nucleus. Over the course of about 15 minutes, it breaks apart into a proton, an electron, and a tiny particle called an anti-neutrino.
But how long the neutron takes to fall apart presents a bit of a mystery. One method measures it as 887.7 seconds, plus or minus 2.2 seconds. Another method measures it as 878.5 seconds, plus or minus 0.8 second. At first, this difference seemed to be a matter of measurement sensitivity. It may be just that. But as scientists continue to perform a series of ever-more-precise experiments to evaluate possible issues, the discrepancy remains.
This persistence leads to the possibility that the difference is pointing to some type of unknown physics. It could be revealing an unknown process in neutron decay. Or it could be pointing to science beyond the Standard Model scientists currently use to explain all of particle physics. There are a number of phenomena that the Standard Model doesn’t fully explain and this difference could point the way towards answering those questions.
To unravel this strange disparity, the Department of Energy’s (DOE) Office of Science is working with other federal agencies, national laboratories, and universities to nail down the duration of the neutron lifetime.
A Fundamental Quantity
Nuclear physicists first started studying the neutron lifetime because of its essential role in physics. “There are some fundamental quantities in nature that seem to be always important,” said Geoff Greene, University of Tennessee professor and physicist at DOE’s Oak Ridge National Laboratory. He’s been researching the neutron lifetime for much of his lifetime – about 40 years. “Theories come and go, but the neutron lifetime seems to remain a central parameter in a variety of things.”
The neutron is a useful guide to understanding other particles. It’s the simplest particle that is radioactive, which means that it regularly breaks down into other particles. As such, it provides a lot of insight into the weak force, the force that determines if neutrons turn into protons or not. Often, this process releases energy and causes the nuclei to break apart. The interactions of the weak force also play an important role in nuclear fusion, where two protons combine.
The neutron lifetime may also provide insight into what happened just moments after the Big Bang. In the few seconds after protons and neutrons formed but before they joined together into elements, there was a precise bit of timing. The universe was cooling rapidly. At a certain point, it got cool enough that protons and neutrons almost instantaneously joined to form helium and hydrogen . If neutrons decayed a little faster or slower into protons, it would have vast effects on that process. There would be a very different balance of elements in the universe; it’s likely that life wouldn’t exist.
“It’s one of those fortuitous accidents of nature that we have chemical elements at all,” said Greene.
Scientists would like to have a solid number for the neutron lifetime to plug into these equations. They need the uncertainty of the lifetime down to less than a second. But getting this certainty is more difficult than it initially seemed. “The neutron lifetime is one of the least well-known fundamental parameters in the Standard Model,” said Zhaowen Tang, a physicist at DOE’s Los Alamos National Laboratory (LANL).
Individual experiments have been able to reach this level of precision. But the incongruity between different types of experiments is preventing scientists from nailing down a specific number.
Discovering a Discrepancy
Finding out there was a difference at all arose from physicists’ desire to be comprehensive. Using two or more methods to measure the same quantity is the best way to guarantee an accurate measurement. But scientists can’t put timers on neutrons to see how fast they fall apart. Instead, they find ways to measure neutrons before and after they decay to calculate the lifetime.
Beam experiments use machines that create streams of neutrons. Scientists measure the number of neutrons in a specific volume of the beam. They then send the stream through a magnetic field and into a particle trap formed by an electric and magnetic field. The neutrons decay in the trap, where the scientists measure the number of protons left in the end.
“The beam experiment is a really hard way to do a precision measurement,” said Shannon Hoogerheide, a physicist at the National Institute of Standards and Technology (NIST), who has collaborated with DOE scientists. “The beam measurement requires not one, but two absolute measurements.”
In contrast, bottle experiments trap ultra-cold neutrons in a container. Ultra-cold neutrons move much slower than regular ones – a few meters per second compared to the 10 million meters per second from fission reactions. Scientists measure how many neutrons are in the container at the beginning and then again after a certain period of time. By examining the difference, they can calculate how fast the neutrons decayed.
“The bottle experiment measures the survivors, the beam experiment measures the dead,” said Greene. “The bottle experiment sounds easy but actually is very hard. On the other hand, the beam experiment sounds hard and is hard.”
A beam experiment at NIST in 2005 (with support from DOE) and a bottle experiment in France not long after first revealed the difference in measurement. Since then, experiments have tried to reduce the space between the two by minimizing as many uncertainties as possible.
Greene and his collaborators took new measurements in 2013 at NIST that helped them recalculate the 2005 beam experiment even more accurately . By that point, scientists had completed five bottle and two beam experiments. Greene was convinced that previous beam experiments had missed one of the biggest sources of uncertainty – precisely counting the number of neutrons in the beam. They improved their measurement of this variable to make it five times more accurate. But eight years of hard work left them with almost the exact same gap in results.
Physicists working on bottle experiments faced their own struggles. One of the biggest challenges was to keep the neutrons from getting lost from interactions with the material the container is made of. A leak changes the number of neutrons at the end and throws off the lifetime calculation.
To solve this problem, the most recent bottle experiment at LANL (which was supported by the Office of Science) eliminated physical walls. Instead, the nuclear physicists used magnetic fields and gravity to hold the neutrons in place. “I was in the camp of, if we do that, we might get a neutron to live longer and agree with the beam lifetime,” said Chen-Yu Liu, an Indiana University professor who led the experiment. “That was my personal bias.”
But the difference remained. “That was a big shock to me,” she said, describing the result published in 2018. The odds of that difference happening from random chance are less than 1 in 10,000. But it could still be caused by a flaw in the experiments.
Hunting Down the Root Cause
Scientists face two types of uncertainties or errors in experiments: statistical or systematic. Statistical errors come from not having enough data to draw solid conclusions. If you can get more data, you can reliably lower those errors. Systematic errors are fundamental uncertainties with the experiment. Many times, they’re far from obvious. The two types of neuron lifetime experiments have vastly different potential systematic errors. The experiments would be a great check on each other if the results matched. But it makes it devilishly hard to figure out why they don’t.
“The hardest thing about measuring the neutron lifetime is that it’s both too short and too long,” said Hoogerheide. “It turns out 15 minutes is a really awkward time to measure in physics.”
So nuclear scientists are continuing work to collect more data and minimize systematic errors.
“One of the things that I find most fun about my field is the exquisite attention to detail required and how deeply you have to understand every aspect of your experiment in order to make a robust measurement,” said Leah Broussard, a nuclear physicist at ORNL.
At NIST, Hoogerheide, Greene, and others are running a new beam experiment that walks through each possible issue in as comprehensive a way as possible. Unfortunately, each tweak affects the others, so it’s two steps forward, one step back.
Other efforts are looking into new ways to measure the neutron lifetime. Researchers from Johns Hopkins University and the U.K.’s Durham University supported by DOE figured out how to use data from NASA to measure the neutron lifetime . Based on neutrons coming off of Venus and Mercury, they calculated a lifetime of 780 seconds with an uncertainty of 130 seconds. But because the data collection wasn’t designed for this purpose, the uncertainty is too high to resolve the lifetime difference. At LANL, Tang is setting up an experiment that’s a cross between the bottle and beam experiments. Instead of measuring protons at the end, it will measure electrons.
Exotic Possibilities Await
There’s also the possibility that the difference is revealing a gap in our knowledge of this fundamental particle.
“We cannot leave any stones unturned,” said Tang. “There are so many examples of people who have seen something, just chucked something to a mistake, not worked on it hard enough, and someone else did and they got the Nobel Prize.”
One theory is that the neutron is breaking down in a way that scientists simply aren’t aware of. It may break down into different particles than the familiar proton, electron, and anti-neutrino combination. If it does, that would explain why neutrons are disappearing in the bottle experiments but the corresponding number of protons aren’t showing up in the beam experiments.
Other ideas are even more radical. Some theorists proposed that neutrons are breaking up into gamma rays and mysterious dark matter . Dark matter makes up 75 percent of the matter in the universe, yet as far as we know only interacts with regular matter via gravity. To test this theory, a group of scientists at LANL did a version of the bottle experiment where they measured both neutrons and gamma rays. But the proposed gamma rays didn’t materialize, leaving scientists with no evidence for dark matter from neutrons.
Mirror matter is another possible concept that sounds like science-fiction. In theory, the “missing” neutrons could be turning into mirror neutrons, perfect copies that exist in an opposite universe. Having evolved in a different way from our universe, this mirror universe would be much colder and dominated by helium. While some nuclear scientists such as Greene think that this is “implausible,” others are interested in testing it just in case.
“It’s relatively unexplored territory. It’s very compelling for me because I’ve got a great source of neutrons in my backyard,” said Broussard, referring to the Spallation Neutron Source and High Flux Isotope Reactor, both DOE Office of Science user facilities at ORNL.
To test this theory, Broussard is analyzing data from an experiment that mimics the beam lifetime experiments, but adjusted to catch a sign of the neutron’s potential invisible partner. By shooting a neutron beam through a specific magnetic field and then stopping it with a material that halts normal neutrons, she and her colleagues should be able to detect whether or not mirror neutrons exist.
Whatever results this experiment delivers, the work to understand the neutron lifetime will continue. “It’s very telling that there are so many attempts to precisely measure the neutron lifetime. That tells you the emotional reaction of scientists to a discrepancy in the field – ‘I want to explore this!’” said Broussard. “Every scientist is motivated by the desire to learn, the desire to understand.”
The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information please visit /science .
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Measuring the Neutron Lifetime with Record-Breaking Precision
National Institute of Standards and Technology, Gaithersburg, MD, USA
Outside of atomic nuclei, neutrons decay quickly into other particles, with an average lifetime 𝜏 n of approximately 15 minutes. Obtaining a precise value for 𝜏 n has potentially far-reaching consequences for our understanding of the Universe, as it offers a way to test important details of the laws of physics that are relevant to particle physics, astronomy, and cosmology. Now, with an experiment at the Los Alamos Neutron Science Center (LANSCE) in New Mexico, the UCN 𝜏 Collaboration has achieved the most precise measurement of 𝜏 n to date [ 1 ]. At 877.75 s, with a total uncertainty of 0.039%, this measurement has less than half the uncertainty of the previous best experiments—one conducted by the same group [ 2 ] and one from a European collaboration [ 3 ].
The new measurement was made using a so-called bottle experiment, in which a trap (the “bottle”) is filled with a known number of ultracold neutrons (UCNs). By counting how many neutrons remain after a certain interval, the neutron decay rate, and therefore the average neutron lifetime, can be calculated. One of the challenges in performing this measurement is that the neutron lifetime is long. Although 15 minutes might seem like a manageable interval, achieving a sufficient number of decays to reach the required statistical precision in a practical amount of time requires the confinement of large numbers of particles. Further challenges include slowing down, or moderating, the neutrons (reactors or spallation sources generate neutrons with several-hundred-MeV energies, whereas trapping requires neutrons with energies of a few hundred neV), controlling neutron losses, and evening out fluctuations in the stored UCN energy spectrum.
These challenges have been managed to some extent during earlier measurements conducted at LANSCE, but the new experiment reduces the uncertainty deriving from these effects to an unprecedentedly low level. The researchers use a setup that is similar to their previous experiments, in which UCNs at approximately 180 neV are polarized so that they seek magnetic-field minima before being fed into a magnetogravitational trap—an open-topped bowl made of permanent magnets arranged in a so-called Halbach array (Fig. 1 ). The UCN sample is then “cleaned,” meaning the particles with the highest energy are either captured by boron-10-coated ZnS surfaces or scattered out of the trap. This cleaning process is important to ensure the trap contains only the lowest-energy UCNs, which have little chance of escaping during the storage period. Any escaping neutrons could lead to an error in the determination of 𝜏 n . After cleaning, the UCNs are stored in the trap for 20–1550 s and then counted.
The new study differs from previous experiments in the addition of a buffer volume between the neutron source and the trap in which the UCNs are held before the storage period begins. This buffer flattens the energy distribution of the UCNs by minimizing the effects of source fluctuations, and it serves as an additional cleaning stage.
A further difference comes at the end of the storage period. Whereas other UCN bottle experiments dumped the UCNs out of the trap so that they could be counted at a separate detector, the UCN 𝜏 Collaboration utilizes an in situ detector that can be lowered into the trap in stages. Not only does this technique eliminate the possibility of incurring losses while the UCNs are transported to the detector, but it also allows the team to map out the energies and trajectories of the UCNs by taking measurements at different heights. This additional information provides a way of checking for systematic particle losses.
The UCN 𝜏 researchers also improved the way they analyze their data. In their latest study, they use three independent, blinded analyses to reduce the contributions of all systematic effects, of abnormal running conditions and unphysical data points, and of statistical biases. Each analysis determines 𝜏 n by two different methods: a “paired” analysis, which averages the short- and long-storage-time runs, and a “global” analysis, which uses a maximum likelihood fit over all acquired data. The results of the blinded analyses are compared, and the data are unblinded only when the three 𝜏 n values agree to within 0.1 s. The final result for 𝜏 n comes from an unweighted average of the central values from the three analyses.
As a result of these experimental and analytical improvements, the researchers have been able to reduce the effect of all known systematics to the level where the few corrections that must be applied are smaller than the experiment’s overall uncertainty. The resulting precision could help to shape our most fundamental theories. For example, the neutron lifetime is one of the inputs used to calculate the abundance of helium-4 in the early Universe due to big bang nucleosynthesis (BBN) [ 4 ]. Combining BBN calculations with astronomical observations therefore offers a powerful probe of new physics.
An analysis of 𝜏 n in combination with other properties of neutron beta decay can also be used to test the standard model. For example, such an analysis can help to reduce the uncertainty in the value of the weak axial vector coupling constant, which governs processes involving charged weak interactions including BBN, neutron star formation, solar fusion, and (anti)neutrino detection. The standard model predicts that this parameter, together with the vector coupling constant, fully describes such interactions, while some beyond-the-standard-model theories require additional scalar and tensor terms, which would in turn affect the value of 𝜏 n . A combined analysis can also determine an important parameter describing the weak interaction called the quark mixing matrix element V u d . The current best value for V u d comes from observing a set of nuclear beta decays involving complex nuclei, but such derivations demand nuclear-structure corrections. These corrections are unnecessary for free-neutron decay, so extracting this parameter from 𝜏 n may be more reliable. The UCN 𝜏 Collaboration’s new result means that free-neutron determinations of V u d are nearly competitive with those from superallowed nuclear decays, giving physicists yet another handle in their quest for new physics.
As impressive and important as this result is, it should be noted that the improved precision is insufficient to resolve a lingering discrepancy affecting neutron-lifetime measurements. Bottle experiments like that conducted by the UCN 𝜏 Collaboration, in which the remaining neutrons are counted, represent just one way of measuring 𝜏 n . This value can also be measured by observing the products of neutron beta decay in a cold-neutron beam. On average, these beam experiments yield a value for 𝜏 n about 8 s longer than that indicated by bottle experiments [ 5 ]. On its own, the UCN 𝜏 Collaboration’s new result doesn’t close this gap (the uncertainty in both types of experiments was already much smaller than 8 s). Eliminating the discrepancy will require new complementary measurements such as those utilizing the beam method, measurements that combine both beam and bottle methods, or even novel space-based techniques [ 6 ]. Fortunately, many experiments are underway or in development to do just that [ 7 ].
F. M. Gonzalez et al. (UCNτ Collaboration), “Improved neutron lifetime measurement with UCN 𝜏 ,” Phys. Rev. Lett. 127 , 162501 (2021) .
R. W. Pattie et al. , “Measurement of the neutron lifetime using a magneto-gravitational trap and in situ detection,” Science 360 , 627 (2018) .
A. Serebrov et al. , “Measurement of the neutron lifetime using a gravitational trap and a low-temperature Fomblin coating,” Phys. Lett. B 605 , 72 (2005) .
R. H. Cyburt et al. , “Big bang nucleosynthesis: Present status,” Rev. Mod. Phys. 88 , 015004 (2016) .
G. L. Green and P. Geltenbort, “A puzzle lies at the heart of the atom,” Sci. Am. 314 , 36 (2016).
D. J. Lawrence et al. , “Space-based measurements of neutron lifetime: Approaches to resolving the neutron lifetime anomaly,” Nucl. Instrum. Methods Phys. Res. A 988 , 164919 (2021) .
D. Dubbers and B. Märkisch, “Precise measurements of the decay of free neutrons,” Annu. Rev. Nucl. Part. Sci. 71 , 139 (2021) .
About the Author
Shannon Hoogerheide is a staff physicist in the Neutron Physics Group at the National Institute of Standards and Technology (NIST). Her research focuses on precision measurements of fundamental physics using neutrons, including experiments to measure the neutron lifetime and the spin rotation of polarized neutrons. She is also involved in neutron interferometry, detector development, and neutron irradiations and calibrations. She received a B.S. in Physics from Calvin College and an A.M. and a Ph.D. in Physics from Harvard University, where she worked on measurements of the electron and positron magnetic moments. Prior to joining the Neutron Physics Group, she was a National Research Council Postdoctoral Fellow in the Atomic Spectroscopy Group at NIST where she worked with highly charged ions.
Improved Neutron Lifetime Measurement with UCN τ
F. M. Gonzalez et al. ( UCN τ Collaboration)
Phys. Rev. Lett. 127 , 162501 (2021)
Published October 13, 2021
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Neutron Scattering for Experimental Research
Mason jiang march 18, 2013, submitted as coursework for ph241 , stanford university, winter 2013, introduction.
An image taken at the ISIS neutron research facility in Oxfordshire, UK. (Source: .)
Neutrons are subatomic particles with no net electric charge, unlike other particles such as protons and electrons, which have an intrinsic electric charge. It is this distinction that makes the interaction between neutrons and matter particularly interesting when compared to proton-matter or electron-matter interactions. While the latter interactions are dictated by Coulomb forces involving strong charge repulsion or attraction, charge-free neutrons can move through matter undeterred by these factors. This translates to neutrons generally possessing the ability to penetrate deeper into matter than other subatomic particles. When contemplating the application value of this trait, one can imagine that by evaluating the properties of neutrons scattering from matter, information can be gathered about the bulk characteristics of that matter that cannot be gained from the scattering of other subatomic particles with shorter penetration depths. This opens up a whole variety of possible physics knowledge that can be acquired from studying neutron-matter interactions and indeed there is an expansive field in which neutron scattering is utilized as a versatile experimental technique. This report provides a brief overview of the neutron scattering technique and its many applications.
Neutron Research Facilities - Where Do the Neutrons Come From?
Before diving into how neutrons are used to learn about different characteristics of matter, it should be discussed where the neutrons used for neutron scattering actually come from. There are currently around 30 active neutron research facilities scattered around the globe, each one containing the means to generate thermal neutrons for experimental research. These neutrons are usually produced by either research reactors specifically engineered for releasing neutrons or by a process known as spallation, both of which will now be described.
In essence, research reactors are just like most other nuclear reactors and depend highly on the process of fission. In these fission processes, a nucleus, usually U-235 or Pu- 239 [1], captures a neutron and then splits into fragments including neutrons with kinetic energy, beta radiation, and photons. Each fission reaction produces on average 2.5 neutrons [1], one of which ends up being used to incite further fission, a half of which is produced delayed and is essential for reactor control, and the last one actually collected for experimental research.
As an alternative to the use of research reactors, some neutron research facilities that have a high-powered particle accelerator can produce neutrons in a process called nuclear spallation. Here, accelerated high-energy protons are directed toward a dense and high-mass- number target such as tungsten, uranium, tantalum, or mercury whereby the collision leaves the nuclei of the target in a highly excited state. The array of products that emerge from this interaction includes various high-energy neutrons and protons that proceed to collide with other yet-to-be excited target nuclei along with lower energy evaporation neutrons [2] that end up being collected for scattering experiments. In terms of yield, for each proton-target nuclei collision, an average of 20 neutrons are produced [3] with the majority of those being evaporation ones.
For neutron production in both the cases of research reactors and spallation, the immediate low-energy neutrons outputted still possess too much energy for practical experimental usage. Thus, a moderation stage is necessary in order to slow them down to become so-called thermal neutrons. Typically this involves surrounding the neutron source with a large volume of an apt moderator such as heavy water, water, or beryllium [4] where fast neutrons will enter and gradually lose their energy in a series of collisions with the nuclei of the moderators. It should be mentioned that at many neutron research facilities today, there are also cold neutron sources in which the moderator used is liquid hydrogen, which allows neutrons to cool to cryogenic temperatures for sub-room temperature experiments. After tens of collisions per neutron [4], the resulting thermal neutrons leak out of the moderator through a designated beam tube and are ready to be utilized for scattering experiments.
Neutron Scattering Experimental Technique
The experimental scattering of neutrons can basically be divided into two categories: elastic neutron scattering (also known as neutron diffraction) and inelastic neutron scattering. Both techniques have their own unique purposes and methods of implementation. Here, basic overviews of these techniques will be provided.
Elastic Neutron Scattering (Neutron Diffraction)
To start, the basis of elastic neutron scattering is fundamentally no different than that of other diffraction processes such as the various forms of light diffraction. Just like all other quantum particles, neutrons can be described from both a particle and wave-like perspective. In this case, the wave-like behavior of neutrons allows it to experience the phenomenon of diffraction typically associated with light, which is more naturally thought of from a wave point of view. Given the usual energies of the neutrons produced at neutron research facilities, the consequent neutron wavelengths fall in the Angstrom range, which happens to be the scale of atomic separation in matter. This means that thermal neutrons impinging on matter can produce diffraction patterns after scattering from atomic layers as governed by the well-known Bragg's law, which is generally brought up in descriptions of x-ray diffraction.
Since thermal neutrons can so intimately be affected by matter atomically through diffraction, they can reveal a wealth of information about the crystal structure of the target material. This is very analogous to the link between x-ray diffraction patterns and the determination of a material's crystal structure. However, it must be understood that despite the similarities, the information that can be extracted from neutron diffraction is complementary rather than overlapping with that attained from x-ray diffraction. This is a result of the fact related earlier in the introduction that neutron-matter interactions are inherently very different from other types of particle/radiation-matter interactions. In this case, when encountering matter, x- rays more directly intermingle with the electron clouds surrounding atoms whereas neutrons, not affected by the charged electrons, interact with the atomic nuclei. This significant difference also leads to the selection of materials that can be studied for neutron diffraction based on varying scattering cross sections. While x-rays experience stronger diffraction patterns from materials with large atomic number atoms due to the presence of more electrons, neutron diffraction patterns are actually more sensitive to materials with small atomic number atoms (lighter materials). Although, for neutrons, there is more of a dependence on the type of isotope of the material studied rather than a linear dependence on atomic number.
Given the concept of neutron diffraction, the experimental point of view can now be established for the technique. First, after neutrons are produced either from research reactors or through spallation, their wavelength properties must be defined so that experiments can be accurate and effective. Typically, this is performed using monochromators and filters at research reactors to self-select the neutron wavelength to be used or using the so-called time of flight technique at spallation sources in which individual incident neutrons of different energies are sorted and labeled based on their varying velocity levels and then filtered later on with the aid of that information. After this, the neutron beam is directed to be incident on a sample of choice sitting on a rotating stage (to change the orientation of the sample) and the scattered beam is collected by a detector, also able to rotate in order to find the angle of the scattered beam relative to the sample. As an important side-note, it's worth mentioning that samples used in any neutron scattering experiment must be either powders or relatively large single crystals in order to match the typical diameters of neutron beams. Single crystals, for instance, need to at least be on the order of 1 cubic-millimeter in size to be studied in most cases [5]. This is a disadvantage compared to x-ray scattering experiments, which do not have such strict requirements on sample size. The resulting neutron diffraction data is usually represented in a plot of scattered neutron intensity as a function of scattering angle, very much like with x-ray diffraction.
A very basic schematic of an inelastic neutron scattering experiment.
Inelastic Neutron Scattering
The inelastic scattering of neutrons, as the name of the technique implies, involves processes in which neutrons incident on a sample experience an exchange of energy with the matter, resulting in a shift in energy/wavelength of the exiting neutrons relative to the incident ones. Ruminating on the origin of energy exchanges between neutrons and matter, one realizes that the collisions between neutrons and atomic nuclei can lead to atomic motion within material lattices and if certain quantized lattice vibrations (phonons) are resonantly excited, then energy will be transferred from the neutrons to the matter. By detecting the energy shifts of the scattered neutrons, then, much knowledge can be acquired about the fundamental atomic and molecular motions taking place in various materials. This is the very basis for all experiments involving inelastic neutron scattering and clearly differs from the concept behind neutron diffraction.
Experimentally, inelastic neutron scattering is more involved and demanding than neutron diffraction. To begin, since experiments involving this sort of scattering search for shifts in energy that may be quite subtle and small, it becomes extremely important to have fine energy resolution. This translates to the need for either good quality monochromatization of the neutron beam or accurate time-of-flight stamping of the produced neutrons in order to confidently establish the wavelength/energy parameters of the experiment. Given this, an inelastic neutron scattering experiment can be carried out with a schematic very much like that shown in Fig. 2 and with the results bundled into a function called the dynamic structure factor S(k,w) where k is basically a reflection of the change in momentum of the incident neutrons in the material under investigation and w is the energy change of the material. Finally, typical presentations of inelastic neutron scattering data show contour plots of material energy change (w) as a function of neutron momentum shift (k). These images directly reveal atomic motion and activity in matter that can be mapped in the form of a phonon dispersion curve, which will also be briefly described in the next section.
Applications
Based on the fundamental aspects behind both types of neutron scattering, there are many different applications for the techniques. For neutron diffraction, most of the applications focus on obtaining static crystal structure information. On the other hand, for inelastic neutron scattering, the applications are geared toward understanding dynamic crystal activity. Both types of scattering thus provide separate important information and justify the use of neutrons for experimental research.
To begin, recall that one of the notable properties of neutron diffraction is that in comparison to x-ray diffraction, neutrons are more sensitive to low atomic number atoms due to interactions with atomic nuclei as opposed to electron clouds. This leads to the special ability for neutrons to probe the structure of compounds that have typically inaccessible low atomic number compounds such as hydrogen. For instance, classes of compounds containing hydrogen such as transition metal hydrides have been successfully characterized structurally with the aid of elastic neutron scattering [7]. Following along the same lines of characterizing materials with low atomic number atoms and the ability to probe hydrogen-containing matter, neutron scattering applications in biology is particularly notable. Since many biological specimens and elements such as proteins contain hydrogen and other low atomic number atoms, the use of neutron scattering techniques is fitting. As an example, small-angle neutron scattering (SANS) is a widely-used neutron diffraction technique in which a highly-collimated neutron beam is focused onto a biological sample, often in some sort of solution, and the collected scattered light provides a wealth of information on the structural properties of the sample [8].
Another application of neutron diffraction is in a technique called neutron reflectometry, which is specifically used for measuring the structures of thin, flat films. Here, a once again highly-collimated neutron beam reflects off an extremely flat-surfaced thin film and the characteristics of the reflected beam are gathered as a function of the neutron wavelength and/or the incident angle. The shape and profile of the reflected neutron beam ends up revealing detailed information about the structure of the film including parameters for roughness, thickness, density, etc. In the end, neutron diffraction is a very useful scattering technique if attempting to accurately determine the static structural properties of a particular material, especially if it contains low atomic number atoms.
In contrast to the applications of neutron diffraction, as mentioned earlier, inelastic neutron scattering specializes in providing dynamic atomic information due to energy exchanges between neutrons and materials under investigation. This leads to many possibilities for systems that can be studied using this type of scattering. All applications of inelastic neutron scattering rely on acquiring momentum and energy information with very fine resolution so instruments associated with this type of scattering must have those characteristics. One common form of inelastic neutron scattering is called triple-axis spectrometry, whose associated spectrometer instrument allows detectors to gather scattered neutrons at any point in both energy and momentum space that are physically accessible to the instrument. This allows for a complete construction of the previously mentioned dynamic structure factor function S(k,w) for a particular material. Another form of inelastic neutron scattering, called time-of-flight scattering, is familiar since it involves the same concepts mentioned earlier for keeping track of the specific energies of neutrons produced in research reactors or through spallation. With this technique, the position and time of each incident neutron on a sample is accurately tagged and fixed along with the same coordinates of the neutrons after scattering off the material. With this information at hand, a simple mathematical transformation yields both the momentum and energy transfers of each neutron, which also builds S(k,w).
A phonon dispersion curve mapped out for GaAs. (Source: )
Since inelastic neutron scattering can so accurately describe the atomic dynamics of a material in terms of energy and momentum, the most natural description of such components that springs to mind is a phonon dispersion curve. Phonons, as briefly discussed earlier, are representations of quantized vibration modes of atoms in a material's lattice. More specifically, they are coherent movement of atoms in a lattice rather than just random thermal motion. Different phonon modes will end up being associated with different energies/frequencies depending on the momentum shifts of the lattice atoms. This translates to an ability to map out the energies of various phonon modes in a material (w) as a function of the shifts in momentum of the atoms (k), most commonly known as phonon dispersion curves. Given the energy and momentum resolution of inelastic neutron scattering experiments, they end up being used quite frequently for mapping out these phonon dispersion relations for various materials [9]. Fig. 3 provides an example of a phonon dispersion plot (for gallium arsenide here) with the different curves corresponding to different phonon modes, the y-axis being energy, the x-axis being momentum, and the labels above the x-axis just standing as conventions for naming different points in momentum space. Plots such as this contain a great deal of information describing the activity of atoms in a particular material and their dynamics and relation to each other.
As an experimental technique, neutron scattering provides the opportunity to learn plenty of information about atomic structure and dynamics. All of this traces back to the fundamentally unique interaction between neutrons and matter. It is no wonder that there are so many neutron research facilities around the globe with active research reactors and spallation-based sources.
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Review Article
Published: 09 April 2021
Tests of fundamental quantum mechanics and dark interactions with low-energy neutrons
Among the known particles, the neutron is special because it provides experimental access to all of the four fundamental forces and a wide range of hypothetical interactions. Despite being unstable, free neutrons live long enough to be used as test particles in interferometric, spectroscopic and scattering experiments probing low-energy scales. Recognized already in the 1970s, fundamental concepts of quantum mechanics can be tested in neutron interferometry using silicon perfect single crystals. Besides enabling tests of uncertainty relations or Bell inequalities, neutrons offer the opportunity to observe the effects of gravity and hypothetical dark forces acting on extended matter wavefunctions. Such tests gained importance in the light of recent discoveries of inconsistencies in the understanding of cosmology and the incompatibility between quantum mechanics and general relativity. Experiments with low-energy neutrons are, thus, indispensable tools for probing fundamental physics and represent a complementary approach to particle colliders. In this Review, we discuss the history and experimental methods used at this low-energy frontier of physics and overview the current bounds and limits on quantum mechanical relations and dark energy interactions.
The neutron is an excellent probe of various interactions, because it is sensitive to all four fundamental forces and hypothetical beyond-standard-model interactions.
Matter-wave interferometry with neutrons offers several advantages, such as macroscopic beam separation, individual control of the sub-beams and long interaction and coherence times at room temperature.
The neutron as a single-particle quantum system is almost ideal to study fundamental concepts of quantum mechanics, such as entanglement, weak values or uncertainty relations, because it can be prepared, manipulated and detected with high efficiency and accuracy.
Tensions between observation and the standard model of Big Bang cosmology and the ongoing quest to understand the nature of the dark sector are a strong motivation to search for new physics.
Neutrons, with their vanishing electric charge and large mass, are ideal test particles, allowing for the most sensitive measurements at sub-micrometre distances for several hypothetical dark sector models.
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Acknowledgements
The authors thank their co-workers and collaborators for their long-term efforts and support, and especially wish to thank the Institut Laue-Langevin (ILL), in Grenoble, France, for ongoing support and hospitality at instruments S18 and PF2. This work was supported by the Austrian Science Fund (FWF) project nos. P 30677, P 27666 and P 33279. Y.H. is partly supported by KAKENHI.
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What happens when neutron stars collide?
New simulations show that neutrinos created during these cataclysmic events are briefly out of thermodynamic equilibrium with the cold cores of the merging stars.
Volume rendering of density in a simulation of a binary neutron star merger. New research shows that neutrinos created in the hot interface between the merging stars can be briefly trapped and remain out of equilibrium with the cold cores of the merging stars for 2 to 3 milliseconds. Credit: Provided by David Radice / Penn State . Creative Commons
June 18, 2024
By Sam Sholtis
UNIVERSITY PARK, Pa. — When stars collapse, they can leave behind incredibly dense but relatively small and cold remnants called neutron stars. If two stars collapse in close proximity, the leftover binary neutron stars spiral in and eventually collide, and the interface where the two stars begin merging becomes incredibly hot. New simulations of these events show hot neutrinos — tiny, essentially massless particles that rarely interact with other matter — that are created during the collision can be briefly trapped at these interfaces and remain out of equilibrium with the cold cores of the merging stars for 2 to 3 milliseconds. During this time, the simulations show that the neutrinos can weakly interact with the matter of the stars, helping to drive the particles back toward equilibrium — and lending new insight into the physics of these powerful events.
A paper describing the simulations, by a research team led by Penn State physicists, appeared in the journal Physical Reviews Letters .
“For the first time in 2017, we observed here on Earth signals of various kinds, including gravitational waves, from a binary neutron star merger,” said Pedro Luis Espino, a postdoctoral researcher at Penn State and the University of California, Berkeley, who led the research. “This led to a huge surge of interest in binary neutron star astrophysics. There is no way to reproduce these events in a lab to study them experimentally, so the best window we have into understanding what happens during a binary neutron star merger is through simulations based on math that arises from Einstein’s theory of general relativity.”
Neutron stars get their name because they are thought to be composed almost entirely out of neutrons, the uncharged particles that, along with positively charged protons and negatively charged electrons, make up atoms. Their incredible density — only black holes are smaller and denser — is thought to squeeze protons and electrons together, fusing them into neutrons. A typical neutron star is only tens of kilometers across but has about one-and-a-half times the mass of our Sun, which is about 1.4 million kilometers across. A teaspoon of neutron star material might weigh as much as a mountain, tens or hundreds of millions of tons.
“Neutron stars before the merger are effectively cold, while they may be billions of degrees Kelvin, their incredible density means that this heat contributes very little to the energy of the system,” said David Radice, assistant professor of physics and of astronomy and astrophysics in the Eberly College of Science at Penn State and a leader of the research team. “As they collide, they can become really hot, the interface of the colliding stars can be heated up to temperatures in the trillions of degrees Kelvin. However, they are so dense that photons cannot escape to dissipate the heat; instead, we think they cool down by emitting neutrinos.”
According to the researchers, neutrinos are created during the collision as neutrons in the stars smash into each other and are blasted apart into protons, electrons and neutrinos. What then happens in those first moments after a collision has been an open question in astrophysics.
To try to answer that question, the research team created simulations requiring massive amounts of computing power that model the merger of binary neutron stars and all of the associated physics. The simulations showed for the first time that, however briefly, even neutrinos can be trapped by the heat and density of the merger. The hot neutrinos are out of equilibrium with the still cool cores of the stars and can interact with the matter of the stars.
“These extreme events stretch the bounds of our understanding of physics and studying them allows us to learn new things,” Radice said. “The period where the merging stars are out of equilibrium is only 2 to 3 milliseconds, but like temperature, time is relative here, the orbital period of the two stars before the merge can be as little as 1 millisecond. This brief out-of-equilibrium phase is when the most interesting physics occurs, once the system returns to equilibrium, the physics is better understood.”
The researchers explained that the precise physical interactions that occur during the merger can impact the types of signals that could be observed on Earth from binary star mergers.
“How the neutrinos interact with the matter of the stars and eventually are emitted can impact the oscillations of the merged remnants of the two stars, which in turn can impact what the electromagnetic and gravitation wave signals of the merger look like when they reach us here on Earth,” Espino said. “Next-generation gravitation-wave detectors could be designed to look for these kinds of signal differences. In this way, these simulations play a crucial role allowing us to get insight into these extreme events while informing future experiments and observations in a kind of feedback loop.”
In addition to Espino and Radice, the research team includes postdoctoral scholars Peter Hammond and Rossella Gamba at Penn State; Sebastiano Bernuzzi, Francesco Zappa and Luís Felipe Longo Micchi at Friedrich-Schiller-Universität Jena in Germany; and Albino Perego at Università di Trento in Italy.
Funding from the U.S. National Science Foundation; the U.S. Department of Energy (DOE), Office of Science, Division of Nuclear Physics; the Deutsche Forschungsgemeinschaft; and the European Union Horizon 2020 and Europe Horizon initiatives supported this research. Simulations were performed on Bridges2, Expanse, Frontera and Perlmutter supercomputers. The research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy. The authors acknowledged the Gauss Centre for Supercomputing e.V. for funding this project by providing computing time on the GCS Supercomputer SuperMUC-NG at the Leibniz Supercomputing Centre.
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A study of the neutron skin of nuclei with dileptons in nuclear collisions.
1. Introduction
2. nuclear structure, 3. quasi-real photon density, 4. photoproduction, author contributions, data availability statement, conflicts of interest.
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Xu, K.; Chen, B. A Study of the Neutron Skin of Nuclei with Dileptons in Nuclear Collisions. Symmetry 2024 , 16 , 1195. https://doi.org/10.3390/sym16091195
Xu K, Chen B. A Study of the Neutron Skin of Nuclei with Dileptons in Nuclear Collisions. Symmetry . 2024; 16(9):1195. https://doi.org/10.3390/sym16091195
Xu, Ke, and Baoyi Chen. 2024. "A Study of the Neutron Skin of Nuclei with Dileptons in Nuclear Collisions" Symmetry 16, no. 9: 1195. https://doi.org/10.3390/sym16091195
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The discovery of the neutron and its properties was central to the extraordinary developments in atomic physics in the first half of the 20th century. Early in the century, Ernest Rutherford developed a crude model of the atom, [1]: 188 [2] based on the gold foil experiment of Hans Geiger and Ernest Marsden. In this model, atoms had their mass ...
James Chadwick
James Chadwick (born October 20, 1891, Manchester, England—died July 24, 1974, Cambridge, Cambridgeshire) was an English physicist who received the Nobel Prize for Physics in 1935 for the discovery of the neutron. Chadwick was educated at the University of Manchester, where he worked under Ernest Rutherford and earned a master's degree in 1913.
James Chadwick: The Man Behind the Neutron
Discovery of the Neutron. In 1932, Chadwick made a fundamental discovery in the domain of nuclear science. Chadwick was fascinated by an experiment done by Frdric and Irne Joliot-Curie that studied the then-unidentified radiation from beryllium as it hit a paraffin wax target. The Curies found that this radiation knocked loose protons from ...
May 1932: Chadwick Reports the Discovery of the Neutron
The discovery of neutron quickly changed scientists' view of the atom, and Chadwick was awarded the Nobel Prize in 1935 for the discovery. Scientists soon realized that the newly discovered neutron, as an uncharged but fairly massive particle, could be used to probe other nuclei.
Discovery of the Neutron
Discovery of the Neutron. It is remarkable that the neutron was not discovered until 1932 when James Chadwick used scattering data to calculate the mass of this neutral particle. Since the time of Rutherford it had been known that the atomic mass number A of nuclei is a bit more than twice the atomic number Z for most atoms and that essentially ...
PhysicsLAB: Famous Experiments: The Discovery of the Neutron
Famous Experiments: The Discovery of the Neutron. In 1920, Ernest Rutherford postulated that there were neutral, massive particles in the nucleus of atoms. This conclusion arose from the disparity between an element's atomic number (protons = electrons) and its atomic mass (usually in excess of the mass of the known protons present).
The discovery of the neutron and its consequences (1930-1940)
The discovery of the neutron. In 1930, at the Physikalisch-Technische Reichsanstalt, in Berlin-Charlottenburg, Walther Bothe and Herbert Becker showed [1] that beryllium (Be), boron (B), fluorine (F), and lithium (Li) atoms, when bombarded by α-particles emitted from polonium (Po), produce a radiation of great penetrating power.
Chadwick's Discovery of the Neutron
The experiments showed that the maximum velocity of the protons ejected from paraffin wax was about 3·3 x 10 9 cm. per second. This is therefore the maximum velocity of the neutrons emitted from beryllium bombarded by α-particles of polonium.
PDF James Chadwick
Several photographs were taken: some β-particle tracks -presumably recoil electrons - were observed, but nothing unexpected.** The first real step towards the discovery of the neutron was given by a very beautiful experiment of Mme. and M. Joliot-Curie, who were also investigating the properties of this beryllium radiation.
A Science Odyssey: People and Discoveries: Chadwick discovers the neutron
Chadwick repeated their experiments but with the goal of looking for a neutral particle -- one with the same mass as a proton, but with zero charge. His experiments were successful. He was able to ...
James Chadwick and the Discovery of the Neutron
Chadwick's Discovery. The next person known to have been experimenting on the gamma ray theory was James Chadwick himself. He performed many experiments, stating that the radiation his German colleagues talked about contained uncharged particles of about the mass of a proton. The particles were called neutrons and his theory spread quickly ...
Chadwick's Discovery of The Neutron
Chadwick's Experiment and Discovery of the Neutron. Chadwick conducted the same experiment using beryllium and paraffin block but provided a different interpretation. He claimed that this unknown radiation was actually neutral particles - neutrons. By applying the law of conservation of momentum and conservation of energy, Chadwick determined ...
Physicists announce the world's most precise measurement of neutron
The experiment, known as UCNtau (for Ultra Cold Neutrons tau, where tau refers to the neutron lifetime), has revealed that the neutron lives 14.629 minutes with an uncertainty of 0.005 minutes ...
Physicists make most precise measurement ever of neutron's ...
Physicists have measured the lifetime of the neutron more precisely than ever before. The average time it takes for the subatomic particle to decay is 877.75 seconds, according to an experiment ...
Why did James Chadwick use berryllium to discover neutrons?
In Rutherford's alpha particle scattering experiment he used gold foil because it's malleable and can be drawn into very thin sheet. Why did James Chadwick use Berryllium while discovering neutrons...
A Puzzle Lies at the Heart of the Atom
A Puzzle Lies at the Heart of the Atom Two precision experiments disagree on how long neutrons live before decaying. Does the discrepancy reflect measurement errors or point to some deeper mystery?
Physicists confront the neutron lifetime puzzle
Physicists confront the neutron lifetime puzzle. To solve a long-standing puzzle about how long a neutron can "live" outside an atomic nucleus, physicists entertained a wild but testable theory positing the existence of a right-handed version of our left-handed universe. They designed a mind-bending experiment at the Department of Energy ...
The Mystery of the Neutron Lifetime
The Mystery of the Neutron Lifetime. From left, ORNL staff Matthew Frost and Leah Broussard work at the Magnetism Reflectometer at the Spallation Neutron Source, used for a search for mirror neutrons. Nine seconds. An eternity in some scientific experiments; an unimaginably small amount in the grand scheme of the universe.
Measuring the Neutron Lifetime with Record-Breaking Precision
The new measurement was made using a so-called bottle experiment, in which a trap (the "bottle") is filled with a known number of ultracold neutrons (UCNs). By counting how many neutrons remain after a certain interval, the neutron decay rate, and therefore the average neutron lifetime, can be calculated.
Neutron Scattering for Experimental Research
The experimental scattering of neutrons can basically be divided into two categories: elastic neutron scattering (also known as neutron diffraction) and inelastic neutron scattering. Both techniques have their own unique purposes and methods of implementation. Here, basic overviews of these techniques will be provided.
Tests of fundamental quantum mechanics and dark interactions ...
Experiments with low-energy neutrons are, thus, indispensable tools for probing fundamental physics and represent a complementary approach to particle colliders.
What happens when neutron stars collide?
New simulations of binary neutron star mergers show that hot neutrinos created at the interface of the merging stars are briefly trapped and remain out of equilibrium with the cold cores of the stars for 2 to 3 milliseconds. A paper describing the simulations, developed by a research team led by Penn State physicists, appeared in the journal Physical Reviews Letters.
A Study of the Neutron Skin of Nuclei with Dileptons in Nuclear ...
Specifically, a neutron skin covers the nucleus, while protons, carrying positive charges, are distributed internally. To investigate the electric charge distribution, we propose calculating the photoproduction of dileptons, which can be measured in experiments [54,55,56].
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The discovery of the neutron and its properties was central to the extraordinary developments in atomic physics in the first half of the 20th century. Early in the century, Ernest Rutherford developed a crude model of the atom, [1]: 188 [2] based on the gold foil experiment of Hans Geiger and Ernest Marsden. In this model, atoms had their mass ...
James Chadwick (born October 20, 1891, Manchester, England—died July 24, 1974, Cambridge, Cambridgeshire) was an English physicist who received the Nobel Prize for Physics in 1935 for the discovery of the neutron. Chadwick was educated at the University of Manchester, where he worked under Ernest Rutherford and earned a master's degree in 1913.
Discovery of the Neutron. In 1932, Chadwick made a fundamental discovery in the domain of nuclear science. Chadwick was fascinated by an experiment done by Frdric and Irne Joliot-Curie that studied the then-unidentified radiation from beryllium as it hit a paraffin wax target. The Curies found that this radiation knocked loose protons from ...
The discovery of neutron quickly changed scientists' view of the atom, and Chadwick was awarded the Nobel Prize in 1935 for the discovery. Scientists soon realized that the newly discovered neutron, as an uncharged but fairly massive particle, could be used to probe other nuclei.
Discovery of the Neutron. It is remarkable that the neutron was not discovered until 1932 when James Chadwick used scattering data to calculate the mass of this neutral particle. Since the time of Rutherford it had been known that the atomic mass number A of nuclei is a bit more than twice the atomic number Z for most atoms and that essentially ...
Famous Experiments: The Discovery of the Neutron. In 1920, Ernest Rutherford postulated that there were neutral, massive particles in the nucleus of atoms. This conclusion arose from the disparity between an element's atomic number (protons = electrons) and its atomic mass (usually in excess of the mass of the known protons present).
The discovery of the neutron. In 1930, at the Physikalisch-Technische Reichsanstalt, in Berlin-Charlottenburg, Walther Bothe and Herbert Becker showed [1] that beryllium (Be), boron (B), fluorine (F), and lithium (Li) atoms, when bombarded by α-particles emitted from polonium (Po), produce a radiation of great penetrating power.
The experiments showed that the maximum velocity of the protons ejected from paraffin wax was about 3·3 x 10 9 cm. per second. This is therefore the maximum velocity of the neutrons emitted from beryllium bombarded by α-particles of polonium.
Several photographs were taken: some β-particle tracks -presumably recoil electrons - were observed, but nothing unexpected.** The first real step towards the discovery of the neutron was given by a very beautiful experiment of Mme. and M. Joliot-Curie, who were also investigating the properties of this beryllium radiation.
Chadwick repeated their experiments but with the goal of looking for a neutral particle -- one with the same mass as a proton, but with zero charge. His experiments were successful. He was able to ...
Chadwick's Discovery. The next person known to have been experimenting on the gamma ray theory was James Chadwick himself. He performed many experiments, stating that the radiation his German colleagues talked about contained uncharged particles of about the mass of a proton. The particles were called neutrons and his theory spread quickly ...
Chadwick's Experiment and Discovery of the Neutron. Chadwick conducted the same experiment using beryllium and paraffin block but provided a different interpretation. He claimed that this unknown radiation was actually neutral particles - neutrons. By applying the law of conservation of momentum and conservation of energy, Chadwick determined ...
The experiment, known as UCNtau (for Ultra Cold Neutrons tau, where tau refers to the neutron lifetime), has revealed that the neutron lives 14.629 minutes with an uncertainty of 0.005 minutes ...
Physicists have measured the lifetime of the neutron more precisely than ever before. The average time it takes for the subatomic particle to decay is 877.75 seconds, according to an experiment ...
In Rutherford's alpha particle scattering experiment he used gold foil because it's malleable and can be drawn into very thin sheet. Why did James Chadwick use Berryllium while discovering neutrons...
A Puzzle Lies at the Heart of the Atom Two precision experiments disagree on how long neutrons live before decaying. Does the discrepancy reflect measurement errors or point to some deeper mystery?
Physicists confront the neutron lifetime puzzle. To solve a long-standing puzzle about how long a neutron can "live" outside an atomic nucleus, physicists entertained a wild but testable theory positing the existence of a right-handed version of our left-handed universe. They designed a mind-bending experiment at the Department of Energy ...
The Mystery of the Neutron Lifetime. From left, ORNL staff Matthew Frost and Leah Broussard work at the Magnetism Reflectometer at the Spallation Neutron Source, used for a search for mirror neutrons. Nine seconds. An eternity in some scientific experiments; an unimaginably small amount in the grand scheme of the universe.
The new measurement was made using a so-called bottle experiment, in which a trap (the "bottle") is filled with a known number of ultracold neutrons (UCNs). By counting how many neutrons remain after a certain interval, the neutron decay rate, and therefore the average neutron lifetime, can be calculated.
The experimental scattering of neutrons can basically be divided into two categories: elastic neutron scattering (also known as neutron diffraction) and inelastic neutron scattering. Both techniques have their own unique purposes and methods of implementation. Here, basic overviews of these techniques will be provided.
Experiments with low-energy neutrons are, thus, indispensable tools for probing fundamental physics and represent a complementary approach to particle colliders.
New simulations of binary neutron star mergers show that hot neutrinos created at the interface of the merging stars are briefly trapped and remain out of equilibrium with the cold cores of the stars for 2 to 3 milliseconds. A paper describing the simulations, developed by a research team led by Penn State physicists, appeared in the journal Physical Reviews Letters.
Specifically, a neutron skin covers the nucleus, while protons, carrying positive charges, are distributed internally. To investigate the electric charge distribution, we propose calculating the photoproduction of dileptons, which can be measured in experiments [54,55,56].