26 RADIATION BIOLOGY 



corresponding to photon energies of more than 1 Mev, have been separated from 

 continuous spectra by means of crystal spectrometers. The intensity obtained 

 in this way is quite low and the method serves only for purpose of analysis. 



Radioactive materials often prove very convenient as sources of 7 rays, 

 especially because the spectrum of this radiation consists of a single or at 

 most of a few frequencies. For data on radioactive sources see Sect. l-2c 

 of this chapter, Way et at. (1950), and the Isotopes Catalogue (1949). 



2. ACTION OF RADIATIONS ON ATOMS AND MOLECULES 



The action of radiations on matter can be analyzed into a succession of 

 separate "elementary processes," each of which involves only one or a few 

 atomic particles. For example, individual electrons spring out from air 

 molecules here and there in the space exposed to X rays (see Sect. 4-3d, 

 Fig. 1-74). All air molecules located in equivalent positions with respect 

 to the X-ray source have an equal chance of experiencing the loss of an 

 electron. Not only the ejection of electrons but many other types of 

 elementary processes occur as random events among the atoms of a por- 

 tion of matter exposed to radiation. 



Atoms exhibit a remarkable statistical behavior in a great variety of phe- 

 nomena. For example, the radioactive transformation in which an a particle 

 springs out of a radium atom may take place with equal chance in each atom of a 

 sample during any given interval of time. Even though all conceivable care is 

 exerted in placing all atoms of a material under identical experimental conditions, 

 the individual atoms act erratically in statistical fashion. Therefore the laws 

 which describe the behavior of atoms are necessarily formulated in statistical 

 language. For example, each radium atom has a probability of 2.7 X 10"^^ of 

 ejecting an a particle during each 1-sec interval. This is to say that (1) out of 

 a very large number N of atoms, 2.7 X lO'^^A'^ disintegrate, on the average, 

 every second; (2) all remaining atoms are equally likely to disintegrate in the 

 next second, and, all atoms being identical, there is no conceivable way of pre- 

 dicting which atoms will disintegrate next; and (3) the probability of disintegra- 

 tion of any one atom remains constant in the course of time and does not increase 

 in relation to the duration of previous survival of the atom. The value of the 

 probability of disintegration is subject to no external influence. 



Statistical laws are encountered in various fields of knowledge. The observed 

 randomness of events is usually understood to result from circumstances in pre- 

 ceding operations which have been disregarded either deliberately or because of 

 practical circumstances. For example, the dealing of a certain "hand" in a 

 game of cards depends on the details of the mechanism of shuffling. These 

 details are disregarded, with the implication that it would be actually possible to 

 keep track of all details and thereby to predict the result of the subsequent deal. 



However, the randomness of atomic events does not appear to reflect cir- 

 cumstances that are controllable in principle, but it is regarded as a primitive 



