40 



RADIATION BIOLOGY 



energy associated with the very existence of an electron and a positron, 

 i.e., the so-called "rest energy," which amounts to 0.5 Mev per particle; 

 and (c) the kinetic energy acquired by the preexisting charged particle as 



it is forced to recoil. 



The minimum photon energy re- 

 quired for pair production amounts 

 to 2 X 0.5 = 1.0 Mev, since the 

 kinetic energies (a) and (c) may be 

 negUgible. A photon energy of 1.0 

 Mev corresponds to an X-ray fre- 

 quency of 2.4 X 10"" cycles/sec and 

 to a wave length of 1.2 X 10^^" cm. 

 The minimum energy requirement 

 increases by a factor of 2 for pairs 

 produced in the proximity of an 

 electron because the electron always 

 recoils violently (item c) on account 

 of its low mass. 



"^^ 



v^».i * 



The i)henomenon of pair production 

 has a curious history. Its discovery by 

 Anderson in 1932 was wholly unex- 

 pected, but the observed facts were 

 soon found to fit in with a preexisting 

 theory by Dirac. This agreement with theory implies that pair production is the 

 logical consequence of previously established facts. Nevertheless, the logical 

 connection has not yet been presented in nonmathematical language. 



Fig. 1-28. Cloud-chamber picture of pair 

 production. {Gentner et al., 1940.) 



2 5 10 20 50 100 



PHOTON ENERGY, Mev 

 Fig. 1-29. Cross section for pair production for photons of different energies in the 

 space surrounding the nuclei of aluminum (left-hand scale) and of lead (right-hand 

 scale). Data for other materials may be obtained by expanding or contracting the 

 scale of ordinates in proportion to Z^. {Courtesy G. R. White.) 



The probability of pair production relates directly to the strength of the 

 electric forces in the space surrounding the charged particle. Therefore 

 pair production is far more efficient in the proximity of nuclei of high 



