PRINCIPLES OF RADIOLOGICAL PHYSICS 27 



fact of nature. Thus the electromagnetic disturbance which propagates away 

 from an X-ray tube truly appears to disturb all air molecules evenly; the equal 

 disturbance eventuates in the ejection of electrons from a few random molecules, 

 but no lasting effect results in the other molecules. 



The uncontrolled random behavior of atoms is regarded altogether differently 

 from the uncontrolled behavior of other objects. The "biological variability" 

 of a population usually appears to stem from a complexity of circumstances 

 which seem too difficult to control in practice even though controllable in princi- 

 ple. Therefore biological variability is looked upon as a practical obstacle to be 

 overcome. The variability of the reactions of different atoms is taken at face 

 value as an inhei'ent property of the atoms rather than as the expression of hidden 

 determining influences. 



Since exposure to radiation leads to observable effects which are local- 

 ized in separate molecviles or atoms or even in portions of atoms, it is con- 

 venient to study the various kinds of localized effects separately. These 

 local effects may be called "elementary processes" and represent the 

 component elements of the complex action of radiation on matter. 



The important elementary processes will be outlined here, following a 

 summary of data on the properties of atoms. 



2-1. SUMMARY OF INFORMATION ON ATOMS AND MOLECULES 



2-la. The Structure of Atoms. Each kind of atom is identified by two 

 whole numbers, namely, the atomic number Z and the mass number A. 

 The mass number is the integer nearest to the atomic weight. An atom 

 consists of the following components, when it is electrically neutral: 



(1) A nucleus which bears a positive charge ecjual to Z times e [see 

 Eq. (2)] and includes practically the whole mass of the atom. The 

 nucleus is a compact aggregate of Z protons and A — Z neutrons. Its 

 radius does not exceed 10~^'- cm. The aggregate holds together stiffly 

 owing to strong attractive forces which operate only between particles 

 very close to one another, i.e., no more than a few times 10""^^ cm apart. 

 These forces are still known only imperfectly and are called "nuclear 

 forces." 



(2) A cloud of Z electrons which occupy a volume with an approximate 

 radius of 1 A = 10^* cm. The cloud holds together owing to the electric 

 attraction exerted by the nuclear charge upon the electrons. This 

 attraction predominates over the repulsion between electrons because of 

 the magnitude of the nuclear charge. 



For example, ordinary sodium consists of a single kind of atoms with 

 atomic weight 22.997. These atoms have the atomic number Z = 11 and 

 the mass number A = 23. Their nuclei consist of 11 protons and of 

 23 — 11 = 12 neutrons. The body of each atom consists of 11 electrons 

 which move about the nucleus. Ordinary nitrogen consists of two kinds 

 of atoms, namely, the "isotopes" of atomic weight 14.008 and 15.005. 



