View source for Cowan–Reines neutrino experiment

{{Short description|Institute of Technology Experimental confirmation of neutrinos}}
[[File:Poltergeist Team 1953.jpg|thumb|Frederick Reines (far right) with Clyde Cowan (far left) and other members of Project Poltergeist]]
The '''Cowan–Reines neutrino experiment''' was conducted by physicists [[Clyde Cowan]] and [[Frederick Reines]] in 1956. The experiment confirmed the existence of [[neutrino]]s. Neutrinos, [[subatomic particle]]s with no [[electric charge]] and very small mass, had been conjectured to be an essential particle in [[beta decay]] 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]]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]] 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.<ref name = "NucEl">{{cite book |last=Stuewer |first=Roger H. |editor-last=Shea |editor-first=William R. |title=Otto Hahn and the Rise of Nuclear Physics |location=Dordrecht, Holland | publisher=D. Riedel Publishing Company |date=1983 |pages=19–67 |chapter=The Nuclear Electron Hypothesis |isbn=978-90-277-1584-5}}</ref> This quandary and other factors led [[Wolfgang Pauli]] to attempt to resolve the issue by postulating the existence of the neutrino in 1930. If the fundamental principle of [[energy conservation]] 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 [[Fermi's interaction|theory for beta decay]] by [[Enrico Fermi]] in 1933.<ref>{{cite journal | last1 = Yang | first1 = C. N. | year = 2012 | title = Fermi's β-decay Theory | journal = Asia Pacific Physics Newsletter | volume = 1 | issue = 1| pages = 27–30 | doi=10.1142/s2251158x12000045}}</ref><ref name="griffiths">
{{cite book
 |last=Griffiths |first=D.
 |year=2009
 |title=Introduction to Elementary Particles
|url=https://archive.org/details/elementarypartic00grif |url-access=limited |edition=2nd |pages=[https://archive.org/details/elementarypartic00grif/page/n328 314]–315
 |isbn=978-3-527-40601-2
}}</ref> The theory posits that the beta decay process consists of four [[fermion]]s directly interacting with one another. By this interaction, the [[neutron]] decays directly to an [[electron]], the conjectured
[[neutrino]] (later determined to be an [[antineutrino]]) and a [[proton]].<ref name="Feynman">
{{cite book
|last=Feynman|first=R.P. |author-link=Richard Feynman
|title=Theory of Fundamental Processes
|year=1962
|publisher=[[W. A. Benjamin]]
|at=Chapters 6 & 7
}}</ref> 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 (journal)|Nature]]'', which rejected it "because it contained speculations too remote from reality to be of interest to the reader.<ref name="InwardBound">{{cite book|last1=Pais|first1=Abraham|title=Inward Bound|url=https://archive.org/details/inwardboundofmat00pais_0|url-access=registration|date=1986|publisher=Oxford University Press|location=Oxford|isbn=978-0-19-851997-3|page=[https://archive.org/details/inwardboundofmat00pais_0/page/418 418]}}</ref>"

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]] and [[Hans Bethe]] calculated that neutrinos could easily pass through the Earth without interactions with any matter.<ref>{{cite journal |journal=[[Nature (journal)|Nature]] |title=The Neutrino |first1=H. |last1=Bethe |author1-link=Hans Bethe |first2=R. |last2=Peierls |author2-link=Rudolf Peierls |volume=133 |issue=532 |pages=689–690 |date=5 May 1934 |doi=10.1038/133689b0 |bibcode=1934Natur.133..689B |s2cid=4098234 }}</ref><ref name="noblp">
{{cite web |url=http://nobelprize.org/nobel_prizes/physics/laureates/1995/
 |title=The Nobel Prize in Physics 1995
 |publisher=[[Nobel Foundation|The Nobel Foundation]]
 |accessdate=2018-08-24
}}</ref>

==Potential for experiment==
By [[inverse beta decay]], the predicted neutrino, more correctly an [[Neutrino|electron antineutrino]] (<math>\bar{\nu}_e</math>), should interact with a [[proton]] ({{SubatomicParticle|Proton}}) to produce a [[neutron]] ({{SubatomicParticle|Neutron}}) and [[positron]] (<math>e^+</math>),

:<math>\bar{\nu}_e + p \to n + e^+</math>

The chance of this reaction occurring was small. The probability for any given reaction to occur is in proportion to its [[Cross section (physics)|cross section]]. Cowan and Reines predicted a cross section for the reaction to be about {{val|6|e=-44|u=cm2}}. The usual unit for a cross section in nuclear physics is a [[Barn (unit)|barn]], which is {{val|1|e=-24|u=cm2}} 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 [[antimatter]] counterpart of the [[electron]], quickly interacts with any nearby [[electron]], and they [[Annihilation|annihilate]] each other. The two resulting coincident [[gamma ray]]s ({{SubatomicParticle|Gamma}}) 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]]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==
[[File:Group portrait of the Project Poltergeist.jpg|thumb|Group portrait of the “Project Poltergeist” team searching for the neutrino; Frederick Reines holds the poster, Clyde Cowan is at far right; Los Alamos Scientific Laboratory, c. 1953]]
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]], initially thought that neutrino bursts from the [[Nuclear weapons testing#History|atomic weapons test]]s that were then occurring could provide the required flux.<ref>
{{cite journal
 |year=1997
 |title=The Reines-Cowan Experiments: Detecting the Poltergeist
 |url=http://library.lanl.gov/cgi-bin/getfile?25-02.pdf
 |journal=[[Los Alamos Science]]
 |volume=25 |page=3
}}</ref> 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".<ref name="Abbott">{{cite journal |last1=Abbott |first1=Alison |title=The singing neutrino Nobel laureate who nearly bombed Nevada |journal=Nature |date=17 May 2021 |volume=593 |issue=7859 |pages=334–335 |doi=10.1038/d41586-021-01318-y |url=https://www.nature.com/articles/d41586-021-01318-y |access-date=7 August 2023 |language=en|doi-access=free }}</ref> They eventually used a [[nuclear reactor]] as a source of neutrinos, as advised by Los Alamos physics division leader J.M.B. Kellogg. The reactor had a neutrino flux of {{val|5|e=13}} neutrinos per second per square centimeter,<ref>
{{cite book
 | author=Griffiths, David J.
 | year=1987
 | title=Introduction to Elementary Particles
 | publisher=[[John Wiley & Sons]]
 | isbn=978-0-471-60386-3
}}</ref> far higher than any flux attainable from other [[radioactive]] sources. The source specifically was [[Beta decay|beta minus decay]] from [[Nuclear fission product|fission products]], creating electron antineutrinos, for example in the fission production [[iodine-131]]:

<chem>^{131}_{53}I -> ^{131}_{54}Xe^\ast{} + \beta^{-} + \bar\nu_e + 606 keV</chem>

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]] in the water, [[neutron]]s and [[positron]]s were created:

<math>\bar{\nu}_e + p \to n + e^+</math>

or rather

<chem>\bar{\nu}_{e}  + H2O  \to OH^{-}{} + n{}  +  e^+</chem>

[[Electron–positron annihilation]] then occurs:

<chem>e^{+}{} + e^{-} \to 2\gamma_{511 keV}</chem>

The two gamma rays created by positron annihilation were detected by sandwiching the water tanks between tanks filled with liquid [[scintillator]]. The scintillator material gives off flashes of light in response to the gamma rays, and these light flashes are detected by [[photomultiplier]] tubes. The scintillator used was the [[wavelength shifter]] [[POPOP]], which peaks in violet light:

<chem>\gamma_{511 keV} \ + C24H16N2O2 \to C24H16N2O2 \ + A \gamma_{3 eV} </chem>

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]], CdCl<sub>2</sub>, in the tank. [[Cadmium]] is a highly effective neutron absorber and gives off a gamma ray when it absorbs a neutron.

:{{SubatomicParticle|Neutron}} + {{SimpleNuclide|Cadmium|108|link=yes}} → {{SimpleNuclide|Cadmium|109|m|link=yes}} → {{SimpleNuclide|Cadmium|109}} + {{SubatomicParticle|Gamma}}

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]]s later.

The experiment that Cowan and Reines devised used two tanks with a total of about 200 liters of water with about 40&nbsp;kg of dissolved CdCl<sub>2</sub>. The water tanks were sandwiched between three [[scintillator]] layers which contained 110 five-inch (127&nbsp;mm) [[photomultiplier]] tubes.

==Results==
[[File:Clyde Cowan.jpg|thumb|Frederick Reines, left, and Clyde Cowan, at the controls of the Savannah River experiment, 1956]]
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]] in [[Washington (state)|Washington state]], but in late 1955 the experiment moved to the [[Savannah River Site|Savannah River Plant]] near [[Aiken, South Carolina]].<ref>{{cite web |last1=Laboratory |first1=Los Alamos National |title=Ghost particles and Project Poltergeist |url=https://discover.lanl.gov/news/1029-ghost-particles/ |website=Los Alamos National Laboratory |access-date=6 August 2023 |language=en}}</ref><ref>{{cite journal |last1=Sutton |first1=Christine |title=Ghosts in the machine |journal=CERN Courier |date=July–August 2016 |volume=56 |issue=6 |page=17 |url=https://cds.cern.ch/record/2232603/files/vol56-issue6-p017-e.pdf}}</ref><ref>{{cite journal |last1=Alcazar |first1=Daniel Albir |title=Ghost particles and Project Poltergeist: Long-ago Lab physicists studied science that haunted them |date=18 November 2020 |url=https://www.osti.gov/biblio/1726118 |publisher=Los Alamos National Lab. (LANL), Los Alamos, NM (United States) |language=English}}</ref> The Savannah River site had better shielding against [[cosmic rays]]. 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]] 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 {{val|6|e=-44|u=cm2}} and their measured cross-section was {{val|6.3|e=-44|u=cm2}}. The results were published in the July 20, 1956 issue of [[Science (journal)|Science]].<ref>
{{cite journal
 |author1=C. L. Cowan Jr. |author2=F. Reines |author3=F. B. Harrison |author4=H. W. Kruse |author5=A. D. McGuire |date=July 20, 1956
 |title=Detection of the Free Neutrino: a Confirmation
 |journal=[[Science (journal)|Science]]
 |volume=124 |issue=3212 |pages=103–4
 |doi=10.1126/science.124.3212.103
|pmid=17796274
|bibcode = 1956Sci...124..103C }}</ref><ref>
{{cite book
 |last=Winter|first=Klaus
 |year=2000
 |title=Neutrino physics
 |url=https://books.google.com/books?id=v_tiL2NlfvMC&pg=PA38
 |page=38ff
 |publisher=[[Cambridge University Press]]
 |isbn=978-0-521-65003-8
}}<br>This source reproduces the 1956 paper.</ref>

==Legacy==
Clyde Cowan died in 1974 at the age of 54. In 1995, [[Frederick Reines]] was honored with the [[Nobel Prize]] for his work on [[neutrino]] [[physics]].<ref name="noblp"/>

The basic strategy of employing massive [[neutrino detector|detectors]], often water based, for neutrino research was exploited by several subsequent experiments,<ref name="noblp"/> including the [[Irvine–Michigan–Brookhaven (detector)|Irvine–Michigan–Brookhaven]] detector, [[Kamioka Observatory|Kamiokande]], the [[Sudbury Neutrino Observatory]] and the [[Homestake Experiment]]. The Homestake Experiment is a contemporary experiment which detected [[neutrinos]] from nuclear fusion in the solar core. Observatories such as these detected neutrino bursts from [[supernova]] [[SN 1987A]] in 1987, the birth 
of [[neutrino astronomy]]. Through observations of [[solar neutrinos]], the Sudbury Neutrino Observatory was able to demonstrate the process of [[neutrino oscillation]]. Neutrino oscillation shows that neutrinos are not massless, a profound development in particle physics.<ref name="BargerMarfatia2012">{{cite book|last1=Barger|first1=Vernon|last2=Marfatia|first2=Danny|last3=Whisnant|first3=Kerry Lewis|title=The Physics of Neutrinos|url=https://books.google.com/books?id=qso8NEr3XY8C|year=2012|publisher=Princeton University Press|isbn=978-0-691-12853-5}}</ref>

==See also==
* [[List of neutrino experiments]]
* [[Subatomic particle]]s

==References==
{{Reflist}}

==External links==
* [http://hyperphysics.phy-astr.gsu.edu/hbase/particles/cowan.html Cowan and Reines Neutrino Experiment]
* [http://hyperphysics.phy-astr.gsu.edu/hbase/particles/proton.html#c4 Decay of the Neutron]
* [http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/beta.html#c2 Beta Decay]
* [http://hyperphysics.phy-astr.gsu.edu/hbase/particles/neutrino.html#c1 Electron Neutrinos and Antineutrinos]
* [http://library.lanl.gov/cgi-bin/getfile?00326606.pdf Cowan & Reines Experiments: Poltergeist, Hanford, Savannah River]
* [https://www.youtube.com/watch?v=AYqEtm0X2Sc The Neutrino with Dr. Clyde L. Cowan (Lecture on Nobel Prize winning experiment)]

{{neutrino detectors}}

{{DEFAULTSORT:Cowan-Reines neutrino experiment}}
[[Category:Particle experiments]]