Cowan–Reines neutrino experiment
The Cowan–Reines neutrino experiment was conducted by physicists
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.[1] 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 theory for beta decay by Enrico Fermi in 1933.[2][3] The theory posits that the beta decay process consists of four fermions directly interacting with one another. By this interaction, the neutron decays directly to an electron, the conjectured
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.[6][7]
Potential for experiment
By inverse beta decay, the predicted neutrino, more correctly an electron antineutrino (), should interact with a proton (
p
) to produce a neutron (
n
) and positron (),
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 6×10−44 cm2. The usual unit for a cross section in nuclear physics is a barn, which is 1×10−24 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 annihilate each other. The two resulting coincident gamma rays (
γ
) 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
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
At those rare instances when neutrinos interacted with
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, CdCl2, in the tank. Cadmium is a highly effective neutron absorber and gives off a gamma ray when it absorbs a neutron.
n
+109m→ 109
Cd
Cd
+
γ
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 microseconds 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 layers which contained 110 five-inch (127 mm) photomultiplier 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
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 6×10−44 cm2 and their measured cross-section was 6.3×10−44 cm2. The results were published in the July 20, 1956 issue of Science.[14][15]
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.[7]
The basic strategy of employing massive
See also
References
- ISBN 978-90-277-1584-5.
- .
- ^
Griffiths, D. (2009). Introduction to Elementary Particles (2nd ed.). pp. 314–315. ISBN 978-3-527-40601-2.
- ^
W. A. Benjamin. Chapters 6 & 7.
- ISBN 978-0-19-851997-3.
- S2CID 4098234.
- ^ a b c "The Nobel Prize in Physics 1995". The Nobel Foundation. Retrieved 2018-08-24.
- ^ "The Reines-Cowan Experiments: Detecting the Poltergeist" (PDF). Los Alamos Science. 25: 3. 1997.
- . Retrieved 7 August 2023.
- ^
Griffiths, David J. (1987). Introduction to Elementary Particles. ISBN 978-0-471-60386-3.
- ^ Laboratory, Los Alamos National. "Ghost particles and Project Poltergeist". Los Alamos National Laboratory. Retrieved 6 August 2023.
- ^ Sutton, Christine (July–August 2016). "Ghosts in the machine" (PDF). CERN Courier. 56 (6): 17.
- ^ Alcazar, Daniel Albir (18 November 2020). "Ghost particles and Project Poltergeist: Long-ago Lab physicists studied science that haunted them". Los Alamos National Lab. (LANL), Los Alamos, NM (United States).
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: Cite journal requires|journal=
(help) - ^
C. L. Cowan Jr.; F. Reines; F. B. Harrison; H. W. Kruse; A. D. McGuire (July 20, 1956). "Detection of the Free Neutrino: a Confirmation". PMID 17796274.
- ^
Winter, Klaus (2000). Neutrino physics. ISBN 978-0-521-65003-8.
This source reproduces the 1956 paper. - ISBN 978-0-691-12853-5.