DISCOVERY OF THE NEUTRINO COWAN 417 



a given decaying nucleus, one must find that energy in the particle 

 and show that it came from the site of the theft. If theory was correct, 

 there were plenty of these small culprits fleeing from the decaying 

 nuclei in a nuclear explosion fireball so that one could hope to catch 

 a few of them. 



Frederick Eeines and the author resolved to attempt this. As a 

 signal of the capture of an antineutrino in flight, we would ask for 

 the radioactive decay of a proton. Now protons, most familiar as 

 nuclei of ordinai-y hydrogen, are among the most stable objects 

 known — they never decay spontaneously. If one should capture an 

 antineutrino, however, it would be forced into changing into a neutron 

 by emission of a positive beta particle, a positron. 



Thus, if one detects protons emitting positrons, then one has every 

 reason to believe that an antineutrino has been captured. We calcu- 

 lated that we could provide enough protons (as hydrogen atoms) in a 

 few hundred gallons of an organic liquid so that a few hundred such 

 positrons should be produced by antineutrinos coming from a nuclear 

 fireball — if we could get the liquid close enough to the fireball. 



Two problems were raised by this conclusion, however: (1) How 

 could a few hundred positrons be detected when released in several 

 hundred gallons of liquid; and (2) how could such a detector, once 

 built, be placed close enough to the violence of a nuclear explosion and 

 survive to tell the story ? 



By "close enough," we calculated that it must be at least within 

 200 feet or so from the base of a tower on which a 20-kiloton explosion 

 was fired. Such towers are usually about 100 feet high. We set about 

 finding answers to these questions. 



For the first problem, there was already a lead. Certain organic 

 liquids had been found which when purified and then contaminated 

 with traces of particular compounds become sensitive to the passage 

 of fast electrons. They "scintillate" — they emit short bursts of light. 

 These bursts are extremely weak, but what intensity they have 

 is proportional to the range (therefore, the energy) of the electron 

 passing through them. These bui-sts of light are detected by highly 

 sensitive phototubes which in turn produce pulses of electricity. This 

 lead was partial, however, for at that time (1950) such organic liquid 

 scintillators had only been made and used in small quantities. To see 

 into several hundred gallons of it would require some additional effort. 



To test this possibility, Eeines and I (both of us were working at 

 the Los Alamos Scientific Laboratory at the time) built a large bi- 

 pyramidal brass tank, of about 1 cubic meter in volume, and mounted 

 four photomultiplier tubes at the two opposing apexes. We filled this 

 tank (now named El Monstro) with veiy pure toluene activated so 



766-746—65 31 



