Cowan–Reines Neutrino Experiment
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The Cowan–Reines neutrino experiment was conducted by physicists
Clyde Cowan Clyde Lorrain Cowan Jr (December 6, 1919 – May 24, 1974) was an American physicist and the co-discoverer of the neutrino along with Frederick Reines. The discovery was made in 1956 in the neutrino experiment. Reines received the Nobel Prize in ...
and
Frederick Reines Frederick Reines ( ; March 16, 1918 – August 26, 1998) was an American physicist. He was awarded the 1995 Nobel Prize in Physics for his co-detection of the neutrino with Clyde Cowan in the neutrino experiment. He may be the only scientist in ...
in 1956. The experiment confirmed the existence of
neutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...
s. Neutrinos,
subatomic particle In physics, a subatomic particle is a particle smaller than an atom. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles (for example, a baryon, lik ...
s with no
electric charge Electric charge (symbol ''q'', sometimes ''Q'') is a physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative''. Like charges repel each other and ...
and very small mass, had been conjectured to be an essential particle in
beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron ...
processes in the 1930s. With neither mass nor charge, such particles appeared to be impossible to detect. The experiment exploited a huge flux of (then hypothetical) electron
antineutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that it ...
s emanating from a nearby nuclear reactor and a detector consisting of large tanks of water. Neutrino interactions with the protons of the water were observed, verifying the existence and basic properties of this particle for the first time.


Background

During the 1910s and 1920s, the observations of electrons from the nuclear
beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron ...
showed that their energy had a continuous distribution. If the process involved only the atomic nucleus and the electron, the electron's energy would have a single, narrow peak, rather than a continuous energy spectrum. Only the resulting electron was observed, so its varying energy suggested that energy may not be conserved. This quandary and other factors led
Wolfgang Pauli Wolfgang Ernst Pauli ( ; ; 25 April 1900 – 15 December 1958) was an Austrian theoretical physicist and a pioneer of quantum mechanics. In 1945, after having been nominated by Albert Einstein, Pauli received the Nobel Prize in Physics "for the ...
to attempt to resolve the issue by postulating the existence of the neutrino in 1930. If the fundamental principle of
energy conservation Energy conservation is the effort to reduce wasteful energy consumption by using fewer energy services. This can be done by using energy more effectively (using less and better sources of energy for continuous service) or changing one's behavi ...
was to be preserved, beta decay had to be a three-body, rather than a two-body, decay. Therefore, in addition to an electron, Pauli suggested that another particle was emitted from the atomic nucleus in beta decay. This particle, the neutrino, had very small mass and no electric charge; it was not observed, but it carried the missing energy. Pauli's suggestion was developed into a proposed theory for beta decay by
Enrico Fermi Enrico Fermi (; 29 September 1901 – 28 November 1954) was an Italian and naturalized American physicist, renowned for being the creator of the world's first artificial nuclear reactor, the Chicago Pile-1, and a member of the Manhattan Project ...
in 1933. The theory posits that the beta decay process consists of four
fermion In particle physics, a fermion is a subatomic particle that follows Fermi–Dirac statistics. Fermions have a half-integer spin (spin 1/2, spin , Spin (physics)#Higher spins, spin , etc.) and obey the Pauli exclusion principle. These particles i ...
s directly interacting with one another. By this interaction, the
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
decays directly to an
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
, the conjectured
neutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...
(later determined to be an
antineutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that it ...
) and a
proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
. The theory, which proved to be remarkably successful, relied on the existence of the hypothetical neutrino. Fermi first submitted his "tentative" theory of beta decay to the journal ''
Nature Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...
'', which rejected it "because it contained speculations too remote from reality to be of interest to the reader." One problem with the neutrino conjecture and Fermi's theory was that the neutrino appeared to have such weak interactions with other matter that it would never be observed. In a 1934 paper,
Rudolf Peierls Sir Rudolf Ernst Peierls, (; ; 5 June 1907 – 19 September 1995) was a German-born British physicist who played a major role in Tube Alloys, Britain's nuclear weapon programme, as well as the subsequent Manhattan Project, the combined Allied ...
and
Hans Bethe Hans Albrecht Eduard Bethe (; ; July 2, 1906 – March 6, 2005) was a German-American physicist who made major contributions to nuclear physics, astrophysics, quantum electrodynamics and solid-state physics, and received the Nobel Prize in Physi ...
calculated that neutrinos could easily pass through the Earth without interactions with any matter.


Potential for experiment

By
inverse beta decay In nuclear and particle physics, inverse beta decay, commonly abbreviated to IBD, is a nuclear reaction involving an electron antineutrino scattering off a proton, creating a positron and a neutron. This process is commonly used in the detect ...
, the predicted neutrino, more correctly an
electron antineutrino The electron neutrino () is an elementary particle which has zero electric charge and a spin of . Together with the electron, it forms the first generation of leptons, hence the name ''electron neutrino''. It was first hypothesized by Wolfgang ...
(\bar_e), should interact with a
proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...
() to produce a
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
() and
positron The positron or antielectron is the particle with an electric charge of +1''elementary charge, e'', a Spin (physics), spin of 1/2 (the same as the electron), and the same Electron rest mass, mass as an electron. It is the antiparticle (antimatt ...
(e^+), :\bar_e + p \to n + e^+ The chance of this reaction occurring was small. The probability for any given reaction to occur is in proportion to its cross section. Cowan and Reines predicted a cross section for the reaction to be about . The usual unit for a cross section in nuclear physics is a
barn A barn is an agricultural building usually on farms and used for various purposes. In North America, a barn refers to structures that house livestock, including cattle and horses, as well as equipment and fodder, and often grain.Allen G ...
, which is and 20 orders of magnitudes larger. Despite the low probability of the neutrino interaction, the signatures of the interaction are unique, making detection of the rare interactions possible. The
positron The positron or antielectron is the particle with an electric charge of +1''elementary charge, e'', a Spin (physics), spin of 1/2 (the same as the electron), and the same Electron rest mass, mass as an electron. It is the antiparticle (antimatt ...
, the
antimatter In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding subatomic particle, particles in "ordinary" matter, and can be thought of as matter with reversed charge and parity, or go ...
counterpart of the
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
, quickly interacts with any nearby
electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
, and they annihilate each other. The two resulting coincident
gamma ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...
s () are detectable. The neutron can be detected by its capture by an appropriate nucleus, releasing a third gamma ray. The coincidence of the positron annihilation and neutron capture events gives a unique signature of an antineutrino interaction. A water molecule is composed of an oxygen and two
hydrogen atom A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral hydrogen atom contains a single positively charged proton in the nucleus, and a single negatively charged electron bound to the nucleus by the Coulomb for ...
s, and most of the hydrogen atoms of water have a single proton for a nucleus. Those protons can serve as targets for antineutrinos, so that simple water can serve as a primary detecting material. The hydrogen atoms are so weakly bound in water that they can be viewed as free protons for the neutrino interaction. The interaction mechanism of neutrinos with heavier nuclei, those with several protons and neutrons, is more complicated, since the constituent protons are strongly bound within the nuclei.


Setup

Given the small chance of interaction of a single neutrino with a proton, neutrinos could only be observed using a huge neutrino flux. Beginning in 1951, Cowan and Reines, both then scientists at
Los Alamos, New Mexico Los Alamos (, meaning ''The Poplars'') is a census-designated place in Los Alamos County, New Mexico, United States, that is recognized as one of the development and creation places of the Nuclear weapon, atomic bomb—the primary objective of ...
, initially thought that neutrino bursts from the atomic weapons tests that were then occurring could provide the required flux. For a neutrino source, they proposed using an atomic bomb. Permission for this was obtained from the laboratory director, Norris Bradbury. The plan was to detonate a "20-kiloton nuclear bomb, comparable to that dropped on Hiroshima, Japan". The detector was proposed to be dropped at the moment of explosion into a hole 40 meters from the detonation site "to catch the flux at its maximum"; it was named "El Monstro". They eventually used a
nuclear reactor A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...
as a source of neutrinos, as advised by Los Alamos physics division leader J.M.B. Kellogg. The reactor had a neutrino flux of neutrinos per second per square centimeter, far higher than any flux attainable from other
radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
sources. The source specifically was
beta minus decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron t ...
from
fission products Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the releas ...
, creating electron antineutrinos, for example in the fission production
iodine-131 Iodine-131 (131I, I-131) is an important radioisotope of iodine discovered by Glenn Seaborg and John Livingood in 1938 at the University of California, Berkeley. It has a radioactive decay half-life of about eight days. It is associated with nu ...
: ^_I -> ^_Xe^\ast + \beta^ + \bar\nu_e + 606 keV A detector consisting of two tanks of water was employed, offering a huge number of potential targets in the protons of the water. At those rare instances when neutrinos interacted with
protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' ( elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an electron (the pro ...
in the water,
neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...
s and
positron The positron or antielectron is the particle with an electric charge of +1''elementary charge, e'', a Spin (physics), spin of 1/2 (the same as the electron), and the same Electron rest mass, mass as an electron. It is the antiparticle (antimatt ...
s were created: \bar_e + p \to n + e^+ or rather \bar_ + H2O \to OH^ + n + e^+
Electron–positron annihilation Electron–positron annihilation occurs when an electron () and a positron (, the electron's antiparticle) collide. At low energies, the result of the collision is the annihilation of the electron and positron, and the creation of energetic ph ...
then occurs: e^ + e^ \to 2\gamma_ The two gamma rays created by positron annihilation were detected by sandwiching the water tanks between tanks filled with liquid
scintillator A scintillator ( ) is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the ab ...
. The scintillator material gives off flashes of light in response to the gamma rays, and these light flashes are detected by
photomultiplier A photomultiplier is a device that converts incident photons into an electrical signal. Kinds of photomultiplier include: * Photomultiplier tube, a vacuum tube converting incident photons into an electric signal. Photomultiplier tubes (PMTs for sh ...
tubes. The scintillator used was the wavelength shifter POPOP, which peaks in violet light: \gamma_ \ + C24H16N2O2 \to C24H16N2O2 \ + A \gamma_ The additional detection of the neutron from the neutrino interaction provided a second layer of certainty. Cowan and Reines detected the neutrons by dissolving
cadmium chloride Cadmium chloride is a white crystalline compound of cadmium and chloride, with the formula CdCl2. This salt is a hygroscopic solid that is highly soluble in water and slightly soluble in alcohol. The crystal structure of cadmium chloride (describe ...
, CdCl2, in the tank.
Cadmium Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...
is a highly effective neutron absorber and gives off a gamma ray when it absorbs a neutron. : + → → + The arrangement was such that after a neutrino interaction event, the two gamma rays from the positron annihilation would be detected, followed by the gamma ray from the neutron absorption by cadmium several
microsecond A microsecond is a unit of time in the International System of Units (SI) equal to one millionth (0.000001 or 10−6 or ) of a second. Its symbol is μs, sometimes simplified to us when Unicode is not available. A microsecond is to one second, ...
s later. The experiment that Cowan and Reines devised 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 A scintillator ( ) is a material that exhibits scintillation, the property of luminescence, when excited by ionizing radiation. Luminescent materials, when struck by an incoming particle, absorb its energy and scintillate (i.e. re-emit the ab ...
layers which contained 110 five-inch (127 mm)
photomultiplier A photomultiplier is a device that converts incident photons into an electrical signal. Kinds of photomultiplier include: * Photomultiplier tube, a vacuum tube converting incident photons into an electric signal. Photomultiplier tubes (PMTs for sh ...
tubes.


Results

In 1953, Cowan and Reines built a detector they dubbed "Herr Auge", "Mr. Eye" in German. They called the neutrino-searching experiment "Project Poltergeist", because of "the neutrino’s ghostly nature". A preliminary experiment was performed in 1953 at the
Hanford Site The Hanford Site is a decommissioned nuclear production complex operated by the United States federal government on the Columbia River in Benton County in the U.S. state of Washington. It has also been known as SiteW and the Hanford Nuclear R ...
in
Washington state Washington, officially the State of Washington, is a state in the Pacific Northwest region of the United States. It is often referred to as Washington State to distinguish it from the national capital, both named after George Washington ...
, but in late 1955 the experiment moved to the Savannah River Plant near
Aiken, South Carolina Aiken is the most populous city in, and the county seat of, Aiken County, South Carolina, United States. According to 2020 census, the population was 32,025, making it the 15th-most populous city in South Carolina, and one of the two largest ci ...
. The Savannah River site had better shielding against
cosmic rays Cosmic rays or astroparticles are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar ...
. This shielded location was 11 m from the reactor and 12 m underground. After months of data collection, the accumulated data showed about three neutrino interactions per hour in the detector. To be absolutely sure that they were seeing
neutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...
events from the detection scheme described above, Cowan and Reines shut down the reactor to show that there was a difference in the rate of detected events. They had predicted a cross-section for the reaction to be about and their measured cross-section was . The results were published in the July 20, 1956 issue of
Science Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...
.
This source reproduces the 1956 paper.


Legacy

Clyde Cowan died in 1974 at the age of 54. In 1995,
Frederick Reines Frederick Reines ( ; March 16, 1918 – August 26, 1998) was an American physicist. He was awarded the 1995 Nobel Prize in Physics for his co-detection of the neutrino with Clyde Cowan in the neutrino experiment. He may be the only scientist in ...
was honored with the
Nobel Prize The Nobel Prizes ( ; ; ) are awards administered by the Nobel Foundation and granted in accordance with the principle of "for the greatest benefit to humankind". The prizes were first awarded in 1901, marking the fifth anniversary of Alfred N ...
for his work on
neutrino A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...
physics Physics is the scientific study of matter, its Elementary particle, fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge whi ...
. The basic strategy of employing massive
detectors A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...
, often water based, for neutrino research was exploited by several subsequent experiments, including the Irvine–Michigan–Brookhaven detector,
Kamiokande The is a neutrino and gravitational waves laboratory located underground in the Mozumi mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida in Gifu Prefecture, Japan. A set of groundbreaking neutrino experimen ...
, the Sudbury Neutrino Observatory and the
Homestake Experiment The Homestake experiment (sometimes referred to as the Davis experiment or Solar Neutrino Experiment and in original literature called Brookhaven Solar Neutrino Experiment or Brookhaven 37Cl (Chlorine) Experiment) was an experiment headed by astr ...
. The Homestake Experiment is a contemporary experiment which detected
neutrinos A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that it ...
from nuclear fusion in the solar core. Observatories such as these detected neutrino bursts from
supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...
SN 1987A SN 1987A was a Type II supernova in the Large Magellanic Cloud, a dwarf satellite galaxy of the Milky Way. It occurred approximately from Earth and was the closest observed supernova since Kepler's Supernova in 1604. Light and neutrinos ...
in 1987, the birth of
neutrino astronomy Neutrino astronomy is a branch of astronomy that gathers information about astronomical objects by observing and studying neutrinos emitted by them with the help of neutrino detectors in special Earth observatories. It is an emerging field in as ...
. Through observations of
solar neutrinos A solar neutrino is a neutrino originating from nuclear fusion in the Sun's Solar core, core, and is the most common type of neutrino passing through any source observed on Earth at any particular moment. Neutrinos are elementary particles with ex ...
, the Sudbury Neutrino Observatory was able to demonstrate the process of
neutrino oscillation Neutrino oscillation is a quantum mechanics, quantum mechanical phenomenon in which a neutrino created with a specific lepton lepton number, family number ("lepton flavor": electron, muon, or tau lepton, tau) can later be Quantum measurement, mea ...
. Neutrino oscillation shows that neutrinos are not massless, a profound development in particle physics.


See also

* List of neutrino experiments *
Subatomic particle In physics, a subatomic particle is a particle smaller than an atom. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles (for example, a baryon, lik ...
s


References


External links


Cowan and Reines Neutrino Experiment







Cowan & Reines Experiments: Poltergeist, Hanford, Savannah River

The Neutrino with Dr. Clyde L. Cowan (Lecture on Nobel Prize winning experiment)
{{DEFAULTSORT:Cowan-Reines neutrino experiment Particle experiments