DISCOVERY OF THE NEUTRINO — COWAN 425 



second, virtually all would pass through as if the detector were not 

 there. Several times each hour, however, one antineutrino would 

 react with a proton — the nucleus of a hydrogen atom in one or the other 

 H2O target tank. When this occurred, a fast positron would be 

 emitted by the proton, and the proton would then be a moderately 

 fast neutron as it recoiled from the site of the event. We knew what 

 the energy spectrum of the antineutrinos coming from the reactor 

 should be, because we knew quite a bit about the various radioactive 

 fission fragment nuclei being formed in it, and we had Fermi's theory 

 to guide us from there. We knew, for instance, that about 10"^^ 

 antineutrinos should strike each square centimeter of our water target 

 per second, that the effective energy of these antineutrinos should be 

 about 3 Mev., and that the cross section presented by each proton in 

 the water hydrogen to each antineutrino would be about 10"*^ square 

 centimeters. 



After an antineutrino had reacted with a proton, the positron 

 would slow to a stop very quickly in the water, would capture an 

 electron from near where it stopped, and then the two would combine 

 to produce two 0.51 Mev. gamma rays. Suppose this happened 

 in the top water target. Then one gamma would pass into the 

 top scintillator, producing a flash of light there, while the other 

 would do the same — at the same time — in the center scintillator. A 

 pair of pulses would then be recorded by our equipment as having 

 occurred "in coincidence," and the electronics would be alerted by 

 this and start to watch for a second signal produced by the neutron. 



The neutron would leave the site of the event with a few Kev. energy, 

 and, being much heavier than the positron, would slow down much 

 more reluctantly. Nevertheless, the neutron would be of "thermal" 

 energy in about 2 microseconds and would then drift about in the 

 water until it happened close to a cadmium nucleus. Let us imagine 

 that this would be about 4 microseconds later. One of the cadmium 

 isotopes has a strong affinity for neutrons that are just drifting about 

 with little energy. The neutron is quickly captured by the cadmium, 

 and a burst of gamma rays then is emitted by the cadmium nucleus. 

 Again, some of these would pass into the top scintillator, some into the 

 center. Flashes of light would again be detected as they produced 

 pulses of electricity in the equipment. We know the total energy of 

 the cadmium gamma rays when it captures a neutron, so the total 

 light produced should be just the right amount. So also, should the 

 total electrical pulse voltage, i.e., the sum of the two electrical pulses. 



Thus, a set of four pulses (two of the right amplitude each, fol- 

 lowed in 6 microseconds by two of the right total amplitude) would 

 be fed into the electronic racks in the trailer. This particular pattern 

 is very distinctive and is not very likely to occur by accident or by 

 any other sort of nuclear interaction in the detector. Among the 



