Cowan–Reines neutrino experiment

With the Cowan - Reines neutrino experiment succeeded the first direct observation of neutrinos (more precisely, electron anti-neutrinos ), uncharged elementary particle of very small mass. The experiment was conducted in 1956 by Clyde L. Cowan, Frederick Reines and employees.

Background

During the 1910s and 1920s, it became apparent through studies of beta decay that this addition to an electron a further particles of very small mass and no electric charge is emitted, which was never observed. The observed energy spectrum of the electrons is continuous. Substituting energy conservation ahead, this is only possible if the beta decay no two-body, but a three-body decay is a two -body decay would be a monochromatic line and not a continuous energy spectrum produce (see kinematics ( Teilchenprozesse ) ). This and other reasons led Wolfgang Pauli in 1930 to demand the existence of the neutrino. In 1950, the Los Alamos National Laboratory began under the name poltergeist project a project that finally prove " ghostly " particles directly.

Principle of the experiment

The detected electron Antineutrino to react with a proton, a neutron, and a positron - would result - the opposite of the electron.

The positron meets an electron after a short time, after which both are annihilated by annihilation. The two resulting photons are detected. The neutrons can be detected by the gamma radiation that is produced when it is captured by a suitable core. The coincidence of both events, pair annihilation and neutron capture, would be a clear indication of an antineutrino interaction.

Most of the hydrogen atoms which are bonded to water molecules that have a single proton as a nucleus. These protons were used in Pure ' and Cowans experiment as a target for anti-neutrinos.

Design and implementation

J. M. B. Kellogg had proposed the experimenters to use a nuclear reactor as antineutrino source. The anti- neutrino flux density was per second and square centimeter, much more than could have been reached with a radioactive preparation.

The anti- neutrinos interacted then, as described above, with the protons in a water tank, which neutrons and positrons were produced. (Almost) every positron produced in the annihilation with an electron, two photons of 511 keV each. This provoked in large to the tank placed scintillators flashes of light, which in turn could be detected by photo -multiplier. The resulting neutrons are moderated by scattering on the protons of water and detected after reaching thermal energy by the water contains dissolved cadmium chloride. Cadmium absorbs thermal neutrons very effectively and are one or at the capture reaction (usually) several photons () from:

Because of the duration of the moderation process, the photons of the cadmium had a few microseconds after the pair of photons resulting from the annihilation of the positron occur in the scintillation detectors, if all the photons originated from the reaction of the same anti-neutrinos. For the suppression of unwanted coincidences, which are caused by muons from cosmic radiation, a third scintillation detector in anticoincidence was used with the other two detectors.

The experiment was first conducted in the Hanford Site, but later moved to the Savannah River Site in South Carolina because there was shielding against cosmic rays better. The location of the experimental setup in the Savannah River site was 11 m from the reactor core and was located 12 m below the ground. Two tanks were used with a volume of 200 liters of water and about 40 kg dissolved cadmium chloride. The water tanks are surrounded by three scintillator observed with 110 photo electron multipliers of 127 mm in length.

After several months of measurement time, the measurement data indicated a rate of three anti- neutrino reactions per hour in the detector. To make sure that it was indeed events from the mechanism described above, the reactor was shut down in order to prove that there is a difference in the number of recorded events. In further control experiments, the water was replaced by, inter alia, heavy water or affects the delay between the detector signals by changing the cadmium concentration.

Result

Cowan and Reines had expected a cross section of the reaction of some. The measurement revealed. The results were published in the issue dated 20 July 1956, the journal Science.

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