American physicist who was awarded the 1995 Nobel Prize for Physics
for his discovery 40 years earlier, together with his colleague Clyde
L. Cowan, Jr., of the subatomic particle called the neutrino, a tiny
lepton with little or no mass and a neutral charge. Reines shared the
Nobel Prize with physicist Martin Lewis Perl, who also discovered a
fundamental particle, the tau.
The neutrino was first postulated in the 1930s by Wolfgang Pauli and
later named by Enrico Fermi, but because of its minuscule size, it eluded
detection for many years. In the early 1950s Reines and Cowan set out
to detect the particle, first at the Hanford Engineer Works in Richland,
Wash., and then at the Savannah River laboratories in South Carolina.
In their experiment a nuclear reactor emitted neutrinos into a 400-litre
(105-gallon) preparation of water and cadmium chloride. When a neutrino
collided with a hydrogen nucleus (i.e., a proton), the interaction created
a positron and a neutron. The positron was slowed by the liquid solution
and destroyed by an electron, creating photons that were recorded by
scintillation detectors. The neutron was likewise slowed and destroyed
by a cadmium nucleus, creating photons that were recorded microseconds
after the first set of photons. The separate recordings of the two impacts,
therefore, gave proof of the existence of the neutrino. Reines subsequently
built other neutrino detectors underground and helped pioneer the field
of neutrino astronomy.
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