reines cowan experiment

Jump to navigation

Search form

• Department Chair's Message
• Department Video
• Contact Physics
• Visiting Physics & Astronomy
• Community Education & Outreach
• Giving to Physics
• Faculty Honors & Awards
• Graduate Student Awards
• Undergraduate Student Awards
• Inclusion in UCI Physics and Astronomy
• Code of Collegial Conduct
• Advocacy and Wellness
• Black Lives Matter
• Underrepresented Genders in Physics and Astronomy (UNITY)
• Special Events
• UCI S-STEM Physics Scholars Program
• 2018 Department Letter
• Research Staff
• Postdoctoral Researchers
• Graduate Students
• Administrative Staff
• Astrophysics
• Biological Physics
• Condensed Matter Physics
• Medical Physics
• Particle Physics
• Plasma Physics
• Eddleman Quantum Institute
• Prospective Undergraduates
• Research Opportunities
• Community & Events
• Courses and More
• PhD Program
• New Astrophysics Courses
• Astronomy & Astrophysics Qualifying Exam
• Physics Grad Caucus
• Seminars & Colloquia
• Public Events
• Calendar View
• Reines Lecture Series
• Instructor Resources
• Department Committees
• Facilities & Computing
• Administrative Services
• Key Requests

The singing UCI Nobel Laureate who nearly bombed Nevada

From desert to gold mine — Frederick Reines was a larger-than-life physicist who did larger-than-life experiments.

Chasing the Ghost: Nobelist Fred Reines and the Neutrino Leonard A. Cole World Scientific (2021)

In the early 1950s, the physicist Frederick Reines and his colleague Clyde Cowan designed an experiment to detect neutrinos, the tiniest and most elusive of subatomic particles. Theorists were convinced that neutrinos must exist — and that they would be untraceable. And Reines liked nothing better than a challenge.

The experiment was to take place in the Nevada desert. A flux of neutrinos would be created by detonating a 20-kiloton nuclear bomb, comparable to that dropped on Hiroshima, Japan, a few years earlier. A deep hole would be dug 40 metres away from the detonation site, into which a detector would be dropped at the moment of explosion to catch the flux at its maximum.

Eminent physicists enthused about the plan. It was approved by Reines’s employer, the government-funded Los Alamos laboratory in New Mexico. Work began on the detector, nicknamed El Monstro, and on the construction of the shaft. At the last minute, Reines and Cowan transferred the experiment to a nuclear reactor, but not because of environmental or safety concerns. They had worked out that although the reactor would deliver a flux of neutrinos three orders of magnitude lower than that from the bomb, it offered a better option for distinguishing signal from noise.

So they did the work instead at the Savannah River nuclear reactor in South Carolina, and Reines and Cowan became the first scientists to detect neutrinos. In 1995, Reines won a share of the Nobel Prize in Physics. (Cowan had died by then.)

The idea of including a nuclear bomb in a basic-research protocol might sound outlandish, but in Chasing the Ghost, his biography of Reines, Leonard Cole reminds us that attitudes were different then. He also reminds us of this when describing Reines’s work on nuclear bombs during and after the Second World War, and his creation of a neutrino laboratory deep in a gold mine in South Africa in the 1960s, in defiance of academic sanctions against the apartheid state. (However, Cole is hardly an independent voice, being Reines’s admiring younger cousin.)

The neutrino-research community has mushroomed over the decades, as it has become clear that these elementary particles are key to understanding the physics of the Universe. Reines was probably its most rambunctious member. Cole relies on many written sources in his reconstruction of Reines’s life. He also interviewed scientists, many now in their nineties, who worked with him. He builds a picture of a larger-than-life figure who conducted larger-than-life experiments. The man who rises off the page is an inspiring, supportive colleague and an entertainingly boisterous companion, who whistled and sang his way through life.

Los Alamos Reines was born in New Jersey in 1918, into a family of Eastern European Jewish immigrants. He was a self-confident high achiever from his youth, declaring in his high-school yearbook his ambition “to be a physicist extraordinary”. He was also a gifted gymnast and musician, who considered a career as an opera singer.

As the Second World War raged, he studied the newly identified phenomenon of nuclear fission, a subject so sensitive that publication of his PhD thesis was delayed until after the war. He joined the Los Alamos laboratory in 1944 to work on the Manhattan Project, the US effort to develop an atomic bomb led by some of the world’s top physicists. Most left Los Alamos after the war; not Reines. He continued to work on the radiation emissions of nuclear bombs — above-ground atomic tests were still taking place, in Nevada and on remote atolls in the Pacific Ocean — and on the development of the even more powerful hydrogen bomb.

After the success of the Manhattan Project, the Los Alamos laboratory continued to be well funded, and investigations into fundamental physics were encouraged. Reines’s neutrino fixation began when he took a year’s sabbatical from his daily responsibilities and found a like-minded colleague in Cowan.

Neutrinos were nicknamed ghost particles because of their uncanny properties. They are the most abundant particles in the Universe — around 100 trillion pass harmlessly through your body every second. They are created in many natural processes, including nuclear reactions in the Sun, explosions of stars and processes that generate radioactivity in Earth’s core. They are also made by particle accelerators and nuclear power plants.

But with no charge and a vanishingly small mass, they can be detected only indirectly, when they interact with another particle. Detectors are made from liquids that generate a suitable signal, such as a flash of light, during interactions, and electronics that convert the signal to an electrical pulse. Neutrino interactions are exceedingly rare, however, so the detectors have to be very large, and physicists very patient.

Project poltergeist As Cole describes, Reines and Cowan began what they named Project Poltergeist at a plutonium-producing reactor in Hanford, Washington. The results were inconclusive. Undeterred, they transported their bulky yet delicate equipment — including a detector tank containing more than 1 tonne of scintillation fluid — in a crawling convoy of 5 oversized trucks, travelling 2,600 kilometres to the more powerful Savannah River reactor, which they expected to deliver a larger flux of neutrinos. In June 1956, they declared their success.

Reines left Los Alamos in 1959 for the Case Institute in Cleveland, Ohio. Seven years later, he moved to the University of California, Irvine. At Case, he began a search for natural-source neutrinos, for which he had to build an underground laboratory, sheltered from interfering cosmic radiation. He targeted the East Rand gold mine near Johannesburg, South Africa, which coiled more than 3.5 kilometres below Earth’s surface.

Cole describes the extraordinary effort to create a lab there, and the harsh and dangerous conditions for the miners, who — unlike the scientists — wore no protective gear beyond a hard hat. It took six months of blasting to create the required horizontal space. They had to transport newly built equipment from the Case Institute, including 20 tonnes of scintillation fluid in 50 containment tanks, which proved a logistical nightmare. But by 1965, the lab was detecting atmospheric neutrinos.

It made diplomatic waves. The apartheid regime in South Africa was widely distrusted. Some other African countries alleged that it intended to test nuclear weapons underground. As that issue faded, problems arose at home, writes Cole. In 1968, colleagues at Irvine openly challenged academic collaboration in an explicitly racist country. Reines countered that, in his view, science transcended politics. Still, he wound down research at the mine soon afterwards, and continued his neutrino studies at US sites.

Cole doesn’t dig deeply enough into these issues, so it is hard understand exactly how they were resolved. He also scatters descriptions of the relevant physics rather unsatisfactorily across chapters.

Nonetheless, Chasing the Ghost nicely describes how successful the derring-do attitude of individual researchers can be. Reines variously comes across as endearing, admirable and irritating. He could alarm his team by tugging on cables to test electronics as he whistled his way through a lab, and was quick to over-interpret results. But he was a hands-off, respectful lab chief who addressed his team with old-fashioned formality as ‘Mr’ (they seem to have all been men) — even as he ignored their rights to holidays. Those interviewed all tell how they fell under his spell, and worked hard to please him. One anecdote has him on one knee, singing an aria at a party. Unlike his neutrinos, it seems, Reines was always an unmistakable presence.

https://www.nature.com/articles/d41586-021-01318-y

• Collections

• APS Journals

Landmarks —Detecting the Elusive Neutrino

APS has put the entire Physical Review archive online, back to 1893. Focus Landmarks feature important papers from the archive.

The neutrino lived for a quarter of a century as a theoretical suggestion before its discovery in the 1950s, as reported in two Physical Review articles. Identifying the elusive particle required a detection system of unprecedented size, allied with ingenious data analysis, to pull a tiny signal out of a noisy background. The discovery of the neutrino, with roots in the wartime Manhattan project, marked the growing importance of “big science” projects that would increasingly dominate particle physics.

In 1930 Wolfgang Pauli proposed that an undetected particle–he called it the “neutron”–is emitted along with a positron in radioactive beta decay. This new particle would explain some puzzling aspects of the nuclear decay data. Enrico Fermi later called Pauli’s particle the “neutrino” to distinguish it from the now-familiar neutron, discovered in 1932. The neutrino was thought to rarely interact with other particles and so would be very difficult to detect.

In the early 1950s, Frederick Reines and Clyde Cowan of the Los Alamos National Laboratory in New Mexico designed a detector to identify neutrinos in the intense outburst of particles and radiation given off in a nuclear bomb test. But they abandoned that plan when they came up with a more sensitive scheme that could work with the smaller but steadier neutrino emission from a reactor.

Reines and Cowan focused on the reaction in which a neutrino hits a proton, creating a neutron and a positron. (In fact, the incoming particle is an antineutrino, but theorists didn’t yet know whether the neutrino and the antineutrino were distinct particles). A tank containing 300 liters of water supplied an abundance of protons. Any positron generated in the water would rapidly slow down and annihilate with an electron, producing a pair of detectable gamma rays.

But those detections would be enormously outnumbered by positrons created by other reactor emissions, as well as cosmic rays. In order to also detect the neutrons from neutrino-proton reactions, Reines and Cowan dissolved cadmium chloride in the water–at up to 40 kilograms per hundred liters. The water slowed down fast neutrons, allowing them to be captured by cadmium nuclei into an excited state that decayed by emission of an MeV photon. Only when this nuclear gamma ray followed an annihilation pair by a few microseconds would the experiment register a neutrino-induced event.

Using this “delayed coincidence” method at the Hanford Site in Washington state, Reines and Cowan announced a “probable” detection of the neutrino in 1953 in the Physical Review . They then moved to a more powerful reactor at Savannah River, South Carolina, where they used two 200-liter detection tanks surrounded by an even larger detection apparatus, and with better shielding of other reactor emissions. After amassing 100 days of running time, they announced in Science neutrino detections at a rate of 3 per hour [1] following up with a detailed account in the Physical Review in 1960.

Accepting a share of the 1995 Nobel Prize in physics (Cowan died in 1974), Reines remarked that before his and Cowan’s work, a “big” physics experiment might use a one-liter detector. It was their background in weapons research, suggests Francis Halzen of the University of Wisconsin, Madison, that emboldened Reines and Cowan to tackle such a large increase in experimental scale. Halzen, principal investigator of the planned IceCube neutrino observatory at the South Pole, recalls a conversation in which Reines emphasized how important it was that he approached neutrino detection with a theorist’s frame of mind–because a seasoned experimenter would have said there was no chance of success.

–David Lindley

David Lindley is a freelance science writer in Alexandria, Virginia.

• C. L. Cowan, Jr., F. Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire, “Detection of the Free Neutrino: A Confirmation,” Science 124, 103 (1956)

More Information

1995 Nobel Prize in physics information

IceCube experiment

Detection of the Free Antineutrino

F. Reines, C. L. Cowan, Jr., F. B. Harrison, A. D. McGuire, and H. W. Kruse

Phys. Rev. 117 , 159 (1960)

Published January 1, 1960

Detection of the Free Neutrino

F. Reines and C. L. Cowan, Jr.

Phys. Rev. 92 , 830 (1953)

Published November 1, 1953

Subject Areas

Related articles.

Gamma-Ray Burst Tightens Constraints on Quantum Gravity

An analysis of the brightest gamma-ray burst ever observed reveals no difference in the propagation speed of different frequencies of light—placing some of the tightest constraints on certain violations of general relativity. Read More »

Flavor Profiling the Highest-Energy Neutrinos

A way to determine the flavors of ultrahigh-energy cosmic neutrinos observed by future detectors could help scientists understand the origin of these elusive particles. Read More »

First Direct Detection of Electron Neutrinos at a Particle Collider

Electron neutrinos produced by proton–proton collisions at the LHC have been experimentally observed. Read More »

Sign up to receive weekly email alerts from Physics Magazine .

1956 – First discovery of the neutrino by an experiment

In this experiment, for which they were awarded a Nobel Prize in Physics in 1995, Clyde L. Cowan and Frederick Reines used a nuclear reactor, expecting to produce neutrino fluxes on the order of 10 12 to 10 13 neutrinos per second per cm 2 , far higher than any attainable flux from other radioactive sources. The neutrinos would then interact with protons in a tank of water, creating neutrons and positrons. Each positron would create a pair of gamma rays when it annihilated with an electron. The gamma rays were detected by placing a scintillator material in a tank of water. The scintillator material gives off flashes of light in response to the gamma rays and the light flashes are detected by photomultiplier tubes.

However, this experiment wasn’t conclusive enough, so they came up with a second layer of certainty. They would detect the neutrons by placing cadmium chloride into the tank. Cadmium is a highly effective neutron absorber (and so finds use in nuclear control rods) and gives off a gamma ray when it absorbs a neutron. The arrangement was such that the gamma ray from the cadmium would be detected 5 microseconds after the gamma ray from the positron, if it were truly produced by a neutrino.

They performed the experiment preliminarily at Hanford, but later moved the experiment to the Savannah River Plant near Augusta, Georgia where they had better shielding against cosmic rays. This shielded location was 11m from the reactor and 12m underground. They used two tanks with a total of about 200 liters of water with about 40 kg of dissolved CdCl2. The water tanks were sandwiched between three scintillator layers which contained 110 five-inch (127 mm) photomultiplier tubes.

After months of data collection, they had accumulated data on about three neutrinos per hour in their detector. To be absolutely sure that they were seeing neutrino events from the detection scheme described above, they shut down the reactor to show that there was a difference in the number of detected events. They had predicted a cross-section for the reaction to be about 6×10 -44 cm 2 and their measured cross-section was 6.3×10 -44 cm 2 . Their results were published in 1956.

You are using an outdated browser. Please upgrade your browser to improve your experience and security.

Enhanced Page Navigation

• Press release: The 1995 Nobel Prize in Physics
• The Nobel Prize in Physics 1995
• The Nobel Prize in Physics 1995 - Advanced information
• Award ceremony speech

Press release

English swedish.

11 October 1995

The Royal Swedish Academy of Sciences has decided to award the 1995 Nobel Prize in Physics for pioneering experimental contributions to lepton physics with one half to Martin L. Perl , Stanford University, Stanford, California, USA for the discovery of the tau lepton and with one half to Frederick Reines , University of California, Irvine, California, USA for the detection of the neutrino .

Discoveries of two of nature’s sub-atomic particles rewarded

Mankind seeks his place in nature. He endeavours to find answers to philosophical and physical questions alike. The home of mankind, the Universe, was created in a Big Bang. “What does this Universe consist of?” – “What are the smallest constituents of the Universe and what are their properties?” – “What can they tell us of the history of the Universe and of its future?” etc. This year’s laureates have in this search made lasting contributions: They have discovered two of nature’s most remarkable subatomic particles.

Martin L. Perl and his colleagues discovered, through a series of experiments between 1974 and 1977, at the Stanford Linear Accelerator Center (SLAC) in the USA, that the electron has a relative some 3 500 times heavier, which is called the tau .

Frederick Reines made pioneering contributions during the 1950s together with the late Clyde L. Cowan, Jr., which led to their being able to demonstrate experimentally the existence of the antineutrino of the electron.

Martin Perl ‘s discovery of the tau was the first sign that a third “family” of fundamental building blocks existed. Some years later a further building block was discovered – one of the family’s two quarks, the bottom quark. Not until 18 years later was its other quark, the top quark, discovered. The existence of the third family is very important for physicists’ confidence in the present theoretical model for understanding the properties of nature’s smallest constituents. This is called the standard model. Without a third family, the model would have been incomplete and unable to admit what is termed the Charge and Parity (CP) violation, a violation of a fundamental principle of symmetry which, among other things, regulates particle decay ( Nobel Prize to Cronin and Fitch 1980 ). If a fourth family of quarks and leptons is discovered, this may mean that the standard model must be revised and more extensive reconstruction within elementary-particle physics commenced.

Frederick Reines ‘ and Clyde L. Cowan’s first observation of neutrinos was a pioneering contribution that opened the doors to the region of “impossible” neutrinoexperiments. Nowadays we are attempting to capture neutrinos in cosmic radiation that may originate in the sun or in supernovas (exploding stars). Because of the reluctance of neutrinos to react with atomic nuclei and thus allow themselves to be captured, very large detector volumes are required for these experiments. While Reines and Cowan in the 1950s managed with about half a cubic metre of water in their detector, large-scale experiments in the 1990s use many thousand cubic metres. Some experiments have even used surrounding sea or ice as their detector volume.

Nature’s building blocks and their family structure The smallest of nature’s structures to have been studied so far are twelve types of matter particles – six quarks and six leptons. They each have their anti-particle, a sort of “mirror image” of the particle. (The name of a particle also includes its anti-particle.) As well as these quarks and leptons, there are other types of subatomic particles called force particles , since they are responsible for three of our known forces, strong force , electromagnetic force and weak force . Gravitational forces operate outside the scope of these. The most essential difference between quarks and leptons is that leptons are not affected by the strong force.

A remarkable property of matter particles is that they exhibit “family affiliation”. They come in three families, each consisting of two quarks and two leptons (Fig. 1). In many ways the three families behave as copies of one another. “Is there a fundamental principle to justify the existence of just three families?” is one of the unanswered questions of physics.

The quarks of the first family are “up quarks” and “‘down quarks”. Its lepton members are the electron and the electron-neutrino. The two quarks build protons and neutrons, which in turn form atomic nuclei and hence over 99% of all the earth’s matter. The small remainder is electrons. The electron-neutrino can be imagined very roughly as an electron deprived of charge and mass. Whether an insignificant amount of mass nevertheless remains, is another unanswered question. It was this electron-neutrino that Reines and his colleague Cowan, both then employed at the Los Alamos Scientific Laboratory, managed to capture.

The discovery of the tau During the 1960s many research groups were carrying out experiments one aim of which was to discover new charged particles, including new leptons. One approach was to search for the new particles in the products of decay of the particles that were available, e.g. kaons. Another way was to attempt to produce them in an accelerator, e.g. in collisions between high-energy electrons and a target. Martin Perl was a member of a team performing such an experiment at SLAC in 1966, but no new charged leptons were found. In 1973 a new machine started operation at SLAC – an electron-positron collider called SPEAR. A collider such as this is a lepton-hunter’s dream since the mechanism for possible production of new charged leptons (X + , X – ) is simple and easy to interpret:

electron + positron -> X + + X –

The SPEAR collider offered Perl an exceptional opportunity to continue his earlier hunt for new leptons, this time in a new and earlier inaccessible energy region of about 5 GeV (5 thousand million electron volts). After only a year came the first hint that something exciting was in the offing – something that could be signalling the production of a new type of lepton. The next year Perl and his co-workers published the first results. But a few more years were to pass before they could be certain that they had in fact discovered a lepton. The new lepton was designated with the Greek letter tau, standing for the first letter of the word triton, third.

Perl’s and co-workers’ experiment The experiment recorded frontal collisions between electrons and their antiparticles, positrons. A large cylindrical detector placed in a magnetic field surrounded the collision area. The detector consisted of many components including a number of wire spark chambers together with shower counters constructed of lead scintillators and a few proportional chambers. The first indication of a possible new phenomenon was that the research team observed 24 events of the type

electron + positron -> electron + antimuon + i.p.

electron + positron -> positron + muon + i.p.

where i.p. stands for invisible particles; those that left no trace in the detector. Thus only one electron (or positron) and an antimuon (or muon) with the opposite sign on its charges were detected. Applying the law of conservation of energy, Perl and his co-workers found that they had produced at least two invisible particles.

One possible interpretation of these events was that a pair of heavy leptons, later termed tau particles, had been produced first:

electron + positron -> tau + antitau

But these were expected to decay very rapidly and the observed electrons and muons were therefore interpreted as products of decay from reactions:

tau -> electron (or muon) + neutrinos

antitau -> antimuon (or positron) + neutrinos

The invisible particles were neutrinos, which with their notorious lack of sensitivity to their surroundings disappeared without visible trace (Fig. 2). However they made themselves felt when the energy balance was to be accounted for. They had taken with them a respectable proportion of the energy (cf. below).

Perl’s and co-workers’ hypothesis was tested in a new series of observations over many years. It gradually became clear that the tau had passed the test and thereby met all the possible requirements of a heavier relative to the electron and the muon. Like these, the tau also has its very own neutrino – the tau neutrino.

Fig. 2. Interpretation of a typical electron-muon trace from the SPEAR detector. The two heavy leptons decay within millimetres of the point of collision and cannot be seen directly. The neutrinos are also invisible. Only the charged particles e and µ are detected.

Energy conservation law cue for neutrino’s entrance The neutrino hypothesis is some 40 years older. The neutrino “was born” as a hypothetical particle in a letter written in 1930 by Wolfgang Pauli (Nobel Prize 1945). At that time it was known that many atomic nuclei ended their lives by emitting an electron. This process, termed beta decay, caused researchers many headaches, among others that one of the sacred laws of physics – the law of conservation of energy – appeared not to apply. To restore order in the statute book of physics Pauli offered what he called a “desperate solution” – the nucleus did not emit the electron alone. It was accompanied by another subatomic particle which lacked electrical charge and reacted very little with its environment. The small particle, which came to be called the neutrino, took part of the energy with it and disappeared without trace into nothingness. The energy balance proved to be as expected as long as account was taken of the proportion the neutrino had removed.

Pauli thought he had done “a frightful thing”, as he called it, by proposing a particle that could never be discovered. It took three decades and the ingenuity of Reines and Cowan to bring the neutrino to light.

The discovery of the neutrino Pauli’s neutrino hypothesis may have been “frightful” but it was also extremely attractive. It saved the energy conservation law and simultaneously solved many other riddles. The neutrino hypothesis was used by Enrico Fermi (Nobel Prize 1938) in a masterly manner to formulate a theory for one of the natural forces, the weak force. This splendid theory lent great credibility to the hypothesis that the neutrino is created simultaneously with the electron every time a nucleus disintegrates through beta decay. But how to produce conclusive proof that the neutrino existed? Researchers Hans Bethe (Nobel Prize 1967) and Rudolf Peierls had evaluated the probability of stopping neutrinos produced in the beta decay of radioactive nuclei and found that it was so minimal that a target several light years thick would be needed to capture these neutrinos efficiently. When the first nuclear reactors were built during the 1940s, Fermi was one of those who realised that the reactors could serve as intensive neutrino sources. It was estimated that the reactors would be able to give a neutrino flow of about 10 12 -10 13 per second and cm 2 . This was many orders of magnitude greater than what was obtained from radioactive sources.

In 1953 Reines and Cowan proposed a reactor experiment to capture neutrinos. The reaction to be studied was

antineutrino + proton -> neutron + positron.

Despite the great intensity of the neutrinos the reactor delivered, such a low counting speed was expected for this reaction that the attempt appeared to be bordering on the impossible. Reines and Cowan realised the importance of detecting both the neutron and the positron to reduce the risk of erroneous interpretation. After a first trial at the Hanford reactor, Reines and Cowan went to work at the Savannah River Plant.

The target in the Reines-Cowan experiment consisted of approximately 400 litres of water containing cadmium chloride placed between large liquid scintillation detectors. The course of events for the reaction sought is as follows (cf. formula above): The neutrino collides with a proton in the water and creates a positron and a neutron. The positron is slowed down by the water and destroyed together with an electron (matter meets antimatter), whereupon two photons (light particles) are created. These are recorded simultaneously in the two detectors (Fig. 3). The neutron also loses velocity in the water and is eventually captured by a cadmium nucleus, whereupon photons are emitted. These photons reach the detectors a microsecond or so later than those from the destruction of the positron and give proof of neutrino capture.

There were struggles with the low counting speed and high background. During the experiment a few events were recorded per hour. Nevertheless Reines and Cowan succeeded in a feat considered to border on the impossible: They had raised the neutrino from its status as a figure of the imagination to an existence as a free particle.

Martin L. Perl Born 1927, New York, NY, USA. American citizen. Doctor’s degree in physics 1955, Columbia University. Perl is a member of the National Academy of Sciences, USA.

Martin L. Perl, Professor Stanford Linear Accelerator Center Stanford University Stanford, CA 94305 USA

Frederick Reines Born 1918 Paterson, New Jersey, USA. American citizen. Doctor’s degree in physics 1944, New York University. Reines is a member of the National Academy of Sciences, USA, and a foreign member of the Russian Academy of Sciences.

Frederick Reines, Professor Department of Physics University of California at Irvine Irvine, CA 92717 USA

Nobel Prizes and laureates

Nobel prizes 2023.

Explore prizes and laureates

The Detection of the Neutrino, 1956 / The Nobel Prize In Physics, 1995

Ghost particles and Project Poltergeist

Lab physicists once studied science that haunted them.

October 29, 2021

By Danny Alcazar, archivist, National Security Research Center

Although Frederick Reines excelled academically at literature and history and was passionate about music and singing, a teacher saw his potential in science.

That teacher offered Reines encouragement — and a key to the science lab so he could work whenever he wanted. Reines’ grades in science began to steadily improve by high school and his response to the yearbook query about his principal ambition was, “To be a physicist extraordinaire.”

Years later, when Reines first contemplated an experiment to detect the neutrino in 1951, this particle was still a poltergeist — meaning it was a fleeting yet haunting ghost in the world of physical reality, according to the Lab’s report “The Reines-Cowan Experiments: Detecting the Poltergeist.” The neutrino’s properties had been deduced, but only theoretically — someone still had to demonstrate its reality.

Nearly 45 years later, Reines was awarded the 1995 Nobel Prize in Physics for detecting the so-called ghost particles of nuclear reactions and thus broadening scientists’ understanding of physics fundamentals.

Manhattan Project era

Reines joined the then-secret lab in Los Alamos in 1944 to help create the world’s first atomic bombs. He worked under famed physicists Richard Feynman and Hans Bethe.

By 1947, Reines first thought of pursuing evidence for neutrinos (a subatomic particle with no electric charge and a very small mass), which prompted him to ask for a sabbatical-in-residence. Decades later, he recalled, “In 1951, following the [nuclear] tests in Eniwetok Atoll in the Pacific, I decided I really would like to do some fundamental physics. … I moved to a stark empty office, staring at a blank pad for several months searching for a meaningful question worthy of a life’s work. The months passed and all I could dredge up out of the subconscious was the possible utility of a bomb for the direct detection of neutrinos.”

Seeing ghosts (particles)

Reines was on a quest to prove the neutrino really existed. Drawing inspiration from past experiences with explosives, he decided to attempt to observe the elusive neutrino and convinced his Los Alamos colleague Clyde Cowan to be his collaborator.

Knowing that atomic explosions emit lots of neutrons that then decay, they first considered using a nuclear bomb. An atomic explosion would provide an excellent source for neutrinos and a chance that the “ghost particle” might become visible. This involved building a sensitive detector and placing it close to an atomic bomb. But how could a detector be built that would be placed 100 meters from “the most violent man-made explosion” and survive?

After much experimentation, Reines and Cowan decided to use the nuclear reactor at Hanford, Washington. This would give them control of nuclear power and allow the experiment to be repeatable. There, they would have the ability to make changes to the atomic nuclei that would yield colossal numbers of neutrinos. Reines and Cowan were then able to detect neutrinos emitted from the reactor by recording their interactions with protons in liquid scintillator.

Project Poltergeist

In 1953, Reines and Cowan built a small prototype detector named Herr Auge (German for Mr. Eye) as part of Project Poltergeist — named for the neutrino’s ghostly nature. This was the first major experimental development to produce statistically significant results.

Sometime into their experiments, they were certain they had observed a free neutrino, but by 1955, Reines and Cowan moved their operation to the Savannah River Plant, which had five fission reactors.

Reines and Cowan were confronted with a colossal challenge: capturing the “most anti-social of particles.” In the summer of 1956, they did.

Nobel Prize in Physics

Not long after their discovery, Cowan left the Los Alamos Lab, followed by Reines, both to pursue teaching.

It wasn’t until 1995 that Reines received the Nobel Prize in Physics for his and Cowan’s experimental work in detecting the first neutrino, called the electron anti-neutrino. Cowan, however, did not share the prize because it is not awarded posthumously (Cowan died in 1974).

Reines’ Nobel Prize is currently on display at the Los Alamos Historical Museum’s Bethe House on Bathtub Row in downtown Los Alamos.

An official replica of the prestigious prize is on loan from the Lab’s Bradbury Museum and is on display in the National Security Research Center (NSRC), which is the Lab’s classified library. The official replicas look identical to the originals and are made by the same craftsmen, but are gold-finished bronze rather than solid gold.

Related Stories

Browse by topic.

• Awards and Recognitions
• Climate Science
• Environmental Stewardship

More Stories

Subscribe to our newsletter.

Sign up to receive the latest news and feature stories from Los Alamos National Laboratory

• Previous Article
• Next Article

The Discovery and Nondiscovery of Neutrinos: The Reines Cowan Experiment and the 17Kev Neutrino

• Article contents
• Figures & tables
• Supplementary Data
• Peer Review
• Reprints and Permissions
• Cite Icon Cite
• Search Site

Allan Franklin; The Discovery and Nondiscovery of Neutrinos: The Reines Cowan Experiment and the 17Kev Neutrino. AIP Conf. Proc. 19 June 2007; 917 (1): 12–23. https://doi.org/10.1063/1.2751935

Download citation file:

• Ris (Zotero)
• Reference Manager

In this paper I discuss the first observation of the neutrino by Frederick Reines, Clyde Cowan, and their collaborators. I also discuss how the physics community decided to reject the claim that a new, heavy, 17‐keV neutrino had been observed. Both are examples of good science.

Citing articles via

Publish with us - request a quote.

Sign up for alerts

• Online ISSN 1551-7616
• Print ISSN 0094-243X
• For Researchers
• For Librarians
• For Advertisers
• Our Publishing Partners  
• Physics Today
• Conference Proceedings
• Special Topics

pubs.aip.org

• Privacy Policy
• Terms of Use

Connect with AIP Publishing

This feature is available to subscribers only.

Sign In or Create an Account

Physical Review Journals Archive

Published by the american physical society, a proposed experiment to detect the free neutrino, f. reines and c. l. cowan, jr., phys. rev. 90 , 492 – published 1 may 1953.

• Citing Articles (50)
• Received 24 February 1953

DOI: https://doi.org/10.1103/PhysRev.90.492.2

©1953 American Physical Society

Authors & Affiliations

• Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico

References (Subscription Required)

Vol. 90, Iss. 3 — May 1953

Access Options

• Buy Article »
• Log in with individual APS Journal Account »
• Log in with a username/password provided by your institution »
• Get access through a U.S. public or high school library »

Authorization Required

Other options.

• Buy Article »
• Find an Institution with the Article »

Download & Share

Sign up to receive regular email alerts from Physical Review Journals Archive

• Forgot your username/password?
• Create an account

Article Lookup

Paste a citation or doi, enter a citation.

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

• View all journals
• Explore content
• About the journal
• Publish with us
• Sign up for alerts
• Published: July 2006

To him who waits

Nature Physics volume  2 ,  page 425 ( 2006 ) Cite this article

1413 Accesses

1 Citations

Metrics details

An Erratum to this article was published on 01 August 2006

It is 50 years since Frederick Reines and Clyde Cowan detected the neutrino 1 , directly and definitively. Wolfgang Pauli had postulated the existence of such a particle in 1930. On 15 June 1956, having been notified by telegram of its discovery, Pauli wrote to Reines and Cowan, “Thanks for the message. Everything comes to him who knows how to wait.”

Ray Davis also knew how to wait. Davis, who died on 31 May this year, was the pioneer of experiments with solar neutrinos. With John Bahcall, who died last year, they were the originators of the 'solar neutrino problem' — a mystery that took more than 30 years to solve.

Davis graduated from Yale in 1942 with a PhD in physical chemistry. In the post-war years, he joined the newly created Brookhaven National Laboratory and began work on the recoil of nuclei following neutrino emission. By the early 1950s, he was following up the suggestion by Bruno Pontecorvo 2 that neutrinos could be detected through their capture by chlorine nuclei, forming argon nuclei that could be removed and counted. “No one else appeared interested”, said Davis 3 ; “it seemed a natural and timely experiment for me to work on.” But his detection attempt, using a 3,800-litre tank of carbon tetrachloride at one of the Savannah River reactors, seemingly failed. In fact, fission reactors are sources of antineutrinos, not neutrinos, and chlorine capture only works for neutrinos: Davis had found the first proof that neutrinos and antineutrinos are not the same 4 .

Pontecorvo had also mooted that the chlorine–argon detection method could be used for solar neutrinos, and by the late 1950s developments in modelling the reactions that power the Sun suggested that its neutrino flux might be detectable. Davis's 3,800-litre tank was, however, drawing a blank. But flaws were found in some of the theoretical assumptions, other nuclear reaction rates in the process were pinned down — and the situation was worse than ever. The flux of solar neutrinos seemed unmeasurable even with the largest conceivable detector.

Nevertheless, in 1964 Davis published a proposal for a new solar neutrino detector 5 . In a companion paper 6 , Bahcall reported the theoretical breakthrough that would make it feasible: the rate at which chlorine nuclei capture the neutrinos from boron-8 produced in the Sun was in fact 20 times as high as previously calculated, owing to the existence of a 'superallowed' transition from the ground state in chlorine-37 to the 5-MeV 'isotopic analogue state' in argon-37. Davis got the go-ahead to build a 378,000-litre detector in the Homestake gold mine, South Dakota.

The experiment at Homestake began in 1967, but the results from what Davis called “our first good run” suggested that the solar neutrino flux was much lower than expected. Already concerned, Davis wrote jokingly to Willy Fowler, “We are ready now, turn on the Sun.” When the data were formally published 7 in 1968, with Bahcall's latest numbers 8 in Physical Review Letters , the pattern was set: the observations from Homestake would never muster more than a third of the predicted flux of neutrinos from the Sun. The solar neutrino problem was born.

Davis was a careful, dedicated experimenter. He checked, refined, and checked again every aspect of the process, hammering down the background and confirming the neutrino deficit. He continued to collect data at Homestake for 30 years, under the auspices of University of Pennsylvania following his retirement from Brookhaven in 1984, until the mine closed in the late 1990s. Despite a matching effort, led by Bahcall, to refine the theory, the data remained stubbornly below the prediction; the solar neutrino problem wouldn't go away.

Then in 1989 came the first confirmation of Davis's result, from the Kamiokande detector in Japan and supported by data from two other detectors, SAGE in Russia and GALLEX in Italy. But within two years, there were data from helioseismological studies that verified the theory. They couldn't both be right — but, in fact, they were. The deadlock was broken in 2001, by data from the Sudbury Neutrino Observatory (SNO), in the Inco nickel mine in Ontario. SNO confirmed the Homestake deficit of electron neutrinos, one of the three types of neutrino. But, combined with the data from Kamiokande, the results indicated that the number of neutrinos of all types emanating from the Sun matched Bahcall's model: the neutrinos just seemed to change type en route. At last, a 1969 postulation 9 by Pontecorvo (him again) and Vladimir Gribov was in the spotlight: neutrinos could oscillate between their three types, electron, muon and tau; the standard model of particle physics needed updating.

Following the SNO announcement, John Bahcall was quoted in The New York Times saying, “I feel like dancing I'm so happy.” Nearly four decades after their first foray into the Homestake mine, the problem thrown up by the pioneering efforts of Davis and Bahcall was solved. In 2002, Davis was a deserving recipient of the Nobel Prize: it is a shame his long-time friend and collaborator didn't share it with him.

Cowan, C. L. Jr et al. Science 124 , 103–104 (1956).

Article   ADS   Google Scholar  

Pontecorvo, B. Chalk River Laboratory Report PD-205 (1946).

http://nobelprize.org/physics/laureates/2002/davis-lecture.pdf

Davis, R. Jr Phys. Rev. 97 , 766–769 (1955).

Davis, R. Jr Phys. Rev. Lett. 12 , 303–305 (1964).

Bahcall, J. N. Phys. Rev. Lett. 12 , 300–302 (1964).

Davis, R. Jr, Harmer, D. S. & Hoffman, K. C. Phys. Rev. Lett. 20 , 1205–1209 (1968).

Bahcall, J. N., Bahcall, N. & Shaviv, G. Phys. Rev. Lett. 20 , 1209–1212 (1968).

Gribov, V. & Pontecorvo, B. Phys. Lett. B 28 , 493–496 (1969).

Download references

Rights and permissions

Reprints and permissions

About this article

Cite this article.

To him who waits. Nature Phys 2 , 425 (2006). https://doi.org/10.1038/nphys357

Download citation

Issue Date : July 2006

DOI : https://doi.org/10.1038/nphys357

Share this article

Anyone you share the following link with will be able to read this content:

Sorry, a shareable link is not currently available for this article.

Provided by the Springer Nature SharedIt content-sharing initiative

This article is cited by

Erratum: editorial.

Nature Physics (2006)

Quick links

• Explore articles by subject
• Guide to authors
• Editorial policies

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.