introduction-weak-interaction-volume-one

and did not appear to decay, it was necessary to use some inverse reactio n
in which the neutrino took part . The process
e + p >v + n , (3 .2 .1 )
in which a proton captures an electron from the inner K shell and become s
a neutron with the emission of a neutrino was known to occur . Hence, by
crossing symmetry (T invariance and the Feynman rules) ,
3 + p --- > e * n (3 .2 .2 )
should also take place . It was thia reaction which Cowan et al . attempted to
detect . Their source of antineutrinos was the Savannah River nuclear powe r
station, which produced an estimated flux of 1 0 17 m - 2 11 eV antineutrinos .
If the process (3 .2 .2) occured, then the resultant positron woul d
produce an ionization trail in a p-terphenyl liquid scintillato rl , and ,
when it came to rest, would be annihilated by a negative electron ,
producing two gamma rays, each with an energy of about 0 .5 heV . Furthermore ,
it was discovered that the placing of a layer of Cd C 1 2
solution withi n
the scintillator resulted in the probable capture of the slow neutron b y
an atomic nucleus, with the emission of three or four gamma rays carrying a
total energy of around 9 .1 hell . These would be observed as coinciden t
flashes in the surrounding scintillator .
of Cd C1 2
Cowan et al .
sandwiched a tank of water containing a low concentratio n
between two liquid scintiliator tanks, and enclosed the whol e
assembly within an array of about 500 photomultiplier tubes . They
expected the antineutrino from the nuclear reactor to interact with
hydrogen proton in the layer of water according to (3 .2 .2) . The flas h
of light from the neutron capture gamma rays was estimated to occur abou t
10 r s after that from the positron annihilation . There were about 10 ton s
of liquid in the scintillators, and hence a considerable number o f
spurious events took place, due to cosmic rays and fast neutrons direc t
from the reactor, but these were usually revealed either by an erro r
1 . Scintillators are substances which emit pulses of light when charged
particles pass through then . The resultant light is usually directed int o
photomultiplier tubes, where it is converted into electrical signals so
that it may be analysed by electronic equipment . See e .g . Birks : Th e
Theory and Practice of Scintillation Counting, Pergamon 1964 .
a