Anatomy of an Experiment: An Account 

 of the Discovery of the Neutrino 



By Clyde L. Cowan 



Professor of Physics, Catholic University of America 



[With 8 plates] 



The first three decades of this century saw the absohite conservation 

 laws and the theory of relativity take on dimensions extending from 

 the astronomical to the atomic. But during these years a serious 

 challenge to their general validity was also building up as a group at 

 the Cavendish Laboratory led the work of compiling the facts of 

 radioactivity. Of the three kinds of radioactivity known at that 

 time — alpha, beta, and gamma decay — the first and the last were well 

 behaved. In these, the alpha particle and the gamma ray were found 

 to carry away from the decaying nucleus just the right amount of 

 energy and momentum. Each time such a decay occurred, the energy 

 lost by the decaying nucleus Tvas to be fomid in the emitted particle. 



For beta decay the story was very different. Although, again, the 

 amount of energy lost by the nucleus was well known, the emitted beta 

 particle never carried away this amount from the decay. It was in- 

 triguing that the beta particle never had too much energy, but always 

 too little. The distribution-in-energy, called the "energy spectrum," 

 of the beta particles from any given type of decay, when collected for 

 many decays and plotted in a bar graph resulted in a plot typified by 

 figure 1. 



Energy was, apparently, being lost — disappearing from the uni- 

 verse. Otherwise, each time a beta decay occurred, the beta particle 

 would have had that energy, and all would have been plotted in a 

 single bar at the point marked "end point energy" in the figure. 



THE FABLE OF THE FRUSTRATED BULLETMAKER 



Consider an analogous (but totally mythical) situation: A maker 

 of rifle bullets compounds a new gunpowder and, of course, must test 

 it by firing a number of bullets filled with the new mixture. He 

 mounts his test rifle on a firm stand and aims it down range. The first 



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