420 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



view the problem just once more to see if we could possibly use the neu- 

 trinos emitted by a fission reactor rather than those from a fission 

 explosion. We knew that the flux of neutrinos from even the largest of 

 reactors would be thousands of times less than that from an explosion, 

 while the background noise from neutrons and gamma rays would be 

 about the same with the available shielding. Nevertheless, we sat late 

 into the evening going over every estimate. Then the thought struck ! 



We were planning to force protons to undergo beta decay by absorp- 

 tion of antineutrinos. This decay would be the emission of a positron 

 as the proton was changed into a neutron. The positron, being an 

 antielectron, would be captured quickly by one of the ordinary elec- 

 trons in the atoms of the liquid, both positron and electron would 

 vanish, and two 0.51 Mev. (million electron volts) gamma rays would 

 be produced. These gamma rays were to constitute our signal, as 

 they, in turn, bounced off other electrons in the liquid, making it 

 scintillate. The neutron, we knew, would also bounce around in the 

 liquid as it struck protons and lost its energy to them, then would drift 

 about for many microseconds before finally being captured by a proton 

 to form deuteriimi, or heavy hydrogen. The neutron-proton capture 

 would release a gamma ray of 2.2 Mev., but we had planned to use this 

 gamma ray only as an independent signal to increase the detection 

 efficiency somewhat. 



Suddenly, we realized that if we could manage to dissolve a cadmium 

 salt in our liquid, then the neutron would be captured more quickly 

 (as cadmium has a much greater "cross section" for neutron capture 

 than has hydrogen), and we could mark a neutrino signal by two 

 characteristic bursts of gamma radiation which followed one another 

 by a few microseconds : First, the two 0.51 Mev. gammas from posi- 

 tron-electron annihilation, then a burst of gammas totaling about 9 

 Mev. as the neutron was captured by cadium. This unique set of sig- 

 nals would provide us with a powerful discrimination against the 

 backgrounds from a reactor. It would then be possible to use the 

 much weaker but calmer neutrino fluxes emitted by a reactor. Instead 

 of detecting a burst of neutrinos in a second or two coming from the 

 fury of a nuclear explosion, we would now be able to watch patiently 

 near a reactor and catch one every few hours or so. And there are 

 many hours available for w^atching in a month — or a year. 



A new plan and a first try 



We called a meeting of our group the following day and set about 

 devising a plan for work near a reactor. The road ahead now looked 

 much clearer, and we felt that we were finally closing in on our quarry. 



During the winter of 1952 we built two cylindrical detectors, each 

 about 30 inches high and 28 inches in diameter. We mounted 90 photo- 

 multipliers around the curved walls of each and filled them with 



