DISCOVERY OF THE NEUTRINO — COWAN 415 



sectional area of about 10"^^ square centimeters. But this is a number 

 so small that it is impossible to visualize. We can make the com- 

 parison with an electron, however, and say that the electron is several 

 hundred billion billion times larger than a neutrino. The neutrino is 

 quite surely the smallest piece of reality that has even been seriously 

 contemplated by man. 



TO CATCH A NEUTRINO 



It is precisely this extreme penetrating power of a neutrino which 

 caused them to escape from the beta decay experiments leading to 

 Pauli's hypothesis. It is also this ability to penetrate matter which 

 sets the main problem in trying to observe a neutrino in flight from 

 the instant of its birth. In order to "observe" an entity like an ele- 

 mentary particle, the entity must react with something so as to pro- 

 duce an observable signal — say an electrical impulse. In the case of 

 a neutrino, we have seen that it will penetrate astronomical thicknesses 

 of matter before it has the opportunity to react at all. But, given 

 sufficient thickness of matter, it will react. And here is the key to the 

 detection problem. For, if instead of asking for one neutrino to 

 react with a great thickness, we can turn the question around and sup- 

 ply a reasonable thickness and ask for an astronomical number of 

 neutrinos to be incident upon it. Then we can hope to detect inter- 

 actions in this matter. 



In the years following the hypothesis of Pauli and the theory of 

 Fermi, such attempts were made, but not nearly enough radioactive 

 material was available to supply the astronomical number of neu- 

 trinos required. Attention then turned to the investigation of those 

 aspects of beta decay which were observable. Measurements of beta 

 spectra and lifetimes were refined greatly. The theory itself was re- 

 fined to account for some deviations found, and it began to yield a 

 deepening insight into the nature of the elementary particles. 



The search for the neutrino turned to indirect methods. Careful 

 measurements both of the beta particle momentum and the recoil of 

 the nucleus were made. It turned out that, within the accuracy at- 

 tainable, the two particles, nucleus and electron, recoiled from the site 

 of the decay just as if a neutrino had shot off in some other direction. 

 Thus, if a neutrino did shoot off as the theory said, the conservation 

 laws still held true. These observations of conservation of energy 

 and momentum, assuming the existence of a neutrino, became a popular 

 argument for the existence of the tiny particle. The concept of the 

 neutrino had been developed to save the conservation laws. The fact 

 that the concept then permitted their retention — as it must if the 

 algebra is worked correctly — was then taken as proof of the existence 

 of the neutrino. This circular reasoning is the sort that postulates 

 the existence of a poltergeist to explain the unattended movement of a 



