Cowan pure neutrino experiment

With the Cowan pure neutrino experiment the first direct observation of neutrinos (more precisely electron antineutrinos ), uncharged elementary particles of very small mass, was achieved. The experiment was performed in 1956 by Clyde L. Cowan , Frederick Reines, and coworkers.

background

During the 1910s and 1920s, studies of beta decay revealed that, in addition to an electron, another particle with very low mass and without an electrical charge was emitted, which, however, had never been observed. The observed energy spectrum of the electrons is continuous. Assuming conservation of energy , this is only possible if the beta decay is not a two-body but a three-body decay: a two-body decay would produce a monochromatic line and not a continuous energy spectrum (see kinematics (particle processes) ). This and other reasons led Wolfgang Pauli to claim the existence of the neutrino in 1930. Around 1950 the Los Alamos National Laboratory began a project called the poltergeist project to finally detect the “ghostly” particle directly.

Principle of the experiment

The electron antineutrino to be detected should react with a proton , whereby a neutron and a positron - the counterpart of the electron - would have to be created. ${\displaystyle {\bar {\mathrm {\nu } }}_{e}}$ ${\displaystyle \mathrm {p} }$ ${\displaystyle \mathrm {n} }$ ${\displaystyle \mathrm {e} ^{+}}$

${\displaystyle \mathrm {\bar {\nu }} _{e}+\mathrm {p} \rightarrow \mathrm {n} +\mathrm {e} ^{+}}$

After a short time, the positron meets an electron, whereupon both are annihilated by pair annihilation. The two resulting photons are detectable. The neutron can be detected by the gamma radiation that is produced when it is captured by a suitable nucleus. The coincidence of both events, pair annihilation and neutron capture, would be a clear indication of the antineutrino-proton reaction sought.

Most of the hydrogen atoms that are bound in water molecules have a single proton as their nucleus. These protons served as targets for the antineutrinos in Reines' and Cowan's experiment .

Construction and implementation

J. M. B. Kellogg had suggested the experimenters use a nuclear reactor as an antineutrino source. The antineutrino flux density was 5 · 10 ¹³ / (s · cm ² ), much more than could have been achieved with a radioactive preparation.

The antineutrinos then interacted with the protons in a water tank as described above, producing neutrons and positrons. (Almost) every positron produced two photons of 511 keV each during pair annihilation with one electron . These caused flashes of light in large scintillators placed around the tank , which in turn could be detected by photoelectron multipliers . The resulting neutrons were moderated by scattering on the protons of the water and, after reaching thermal energy, detected by cadmium chloride dissolved in the water. The cadmium - isotope 113 absorbs thermal neutrons very effective and available at the capture reaction one or (usually) more photons ( ) from: ${\displaystyle \gamma }$

${\displaystyle n+\,^{113}\mathrm {Cd} \rightarrow \,^{114}\mathrm {Cd} +\gamma }$

Because of the duration of the moderation process, the photons of the cadmium had to appear in the scintillation detectors a few microseconds later than the photon pair created by the pair annihilation of the positron, if all photons were originally due to the reaction of the same antineutrino. To suppress unwanted coincidences caused by muons from the secondary cosmic radiation , a third scintillation detector was used in anticoincidence with the other two detectors.

The experiment was initially set up in the Hanford Site , but later moved to the Savannah River Site in South Carolina, as better shielding against cosmic rays was possible there. The location of the experimental setup in the Savannah River Site was 11 m from the reactor core and 12 m underground. Two tanks with a total capacity of 200 liters of water and about 40 kg of dissolved cadmium chloride were used. The water tanks were enclosed by three scintillator layers, which were observed with 110 photomultiplier tubes 127 mm in length.

After a few months of measurement, the measurement data indicated a rate of three antineutrino reactions per hour in the detector. In order to ensure that the events from the mechanism described above were indeed involved, the reactor was shut down to prove that there was a difference in the number of events recorded. In further control experiments, the water was replaced by heavy water or the delay between the detector signals was influenced by changing the cadmium concentration.

Result

Cowan and Reines had expected a cross-section of the reaction of around 6 · 10 ⁻⁴⁴ cm ² . The measurement resulted in 6.3 · 10 ⁻⁴⁴ cm ² . The results were published in the July 20, 1956 issue of Science journal .

Clyde Cowan died in 1974; Frederick Reines was awarded the Nobel Prize in 1995 for his work on neutrino physics.

See also

• Homestake experiment (an experiment from the 1970s that detected neutrinos formed from nuclear fusion in the interior of the sun)

Individual evidence

1. ↑ The Reines-Cowan Experiments: Detecting the Poltergeist . In: Los Alamos Science . 25, 1997, p. 3.
2. ^ David J. Griffiths: Introduction to Elementary Particles . John Wiley & Sons , 1987, ISBN 0-471-60386-4 .
3. ↑ ^{a b} J. Bleck-Neuhaus: Elementary Particles. 2nd edition, 2012, ISBN 978-3-642-32578-6 , pp. 246-247.
4. ↑ C. L Cowan Jr., F. Reines, FB Harrison, HW Kruse, A. D McGuire: Detection of the Free Neutrino: a Confirmation . In: Science . 124, No. 3212, July 20, 1956, pp. 103-4. bibcode : 1956Sci ... 124..103C . doi : 10.1126 / science.124.3212.103 . PMID 17796274 .
5. ↑ Klaus Winter: Neutrino physics . Cambridge University Press , 2000, ISBN 978-0-521-65003-8 , pp. 38 ff . ( online - source is based on 1956 edition).
6. ^ The Nobel Prize in Physics 1995 . The Nobel Foundation . Retrieved June 29, 2001.