Cowan–Reines neutrino experiment

The neutrino experiment, also called the Cowan and Reines neutrino experiment, was performed by Clyde L. Cowan and Frederick Reines in 1956. This experiment confirmed the existence of the antineutrino—a very small and neutrally charged subatomic particle.

History

In the 1930s, through the study of beta decay, it was apparent that a third particle, one of nearly no mass and with neutral charge existed and was not observed.

This was due to a continuous spread of kinetic energy and momentum values for electrons emitted in beta decay. The only way this was possible was if there was a particle of neutral charge and almost no mass (or possibly no mass) produced in the decay.

Potential for experiment

In beta decay the predicted particle, the electron antineutrino ( ${\bar {\nu }}_{e}$ ) - should interact with a proton to produce a neutron and positron - the antimatter counterpart of the electron.

{\bar {\nu }}_{e}+{\mbox{p}}\rightarrow {\mbox{n}}+{\mbox{e}}^{+}

The positron quickly finds an electron, and they annihilate each other - a process known as annihilation. The two resulting gamma rays ( $\gamma$ ) are easily detectable.

The setup

In this experiment, they used a nuclear reactor to create a neutrino flux of 5×10¹³ neutrinos per second per square centimeter,^[1] far higher than any attainable flux from other radioactive sources.

The neutrinos would then interact (as shown above) with protons in a tank of water, creating neutrons and positrons. Each positron would create a pair of gamma rays when it annihilated with an electron.

The gamma rays were detected by placing a scintillator material in a tank of water. The scintillator material gives off flashes of light in response to the gamma rays and the light flashes are detected by photomultiplier tubes.

However, this experiment wasn't conclusive enough, so they came up with a second layer of certainty.

They would detect the neutrons by placing cadmium chloride into the tank. Cadmium is a highly effective neutron absorber (and so finds use in nuclear control rods) and gives off a gamma ray when it absorbes a neutron.

{\mbox{n}}+^{108}{\mbox{Cd}}\rightarrow ^{109}{\mbox{Cd}}*\rightarrow ^{109}{\mbox{Cd}}+\gamma

The arrangement was such that the gamma ray from the cadmium would be detected 5 microseconds after the gamma ray from the positron, if it were truly produced by a neutrino.

The results

They performed the experiment preliminarily at Hanford, but later moved the experiment to the Savannah River Plant near Augusta, Georgia where they had better shielding against cosmic rays. This shielded location was 11 m from the reactor and 12 m underground.

They 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.

After months of data collection, they had accumulated data on about three neutrinos per hour in their detector. To be absolutely sure that they were seeing neutrino events from the detection scheme described above, they shut down the reactor to show that there was a difference in the number 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. Their results were published in 1956.

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

References

^ Griffiths, David J. (1987). Introduction to Elementary Particles. Wiley, John & Sons, Inc. ISBN 0-471-60386-4.