492 L. H. GRAY 



A. PLACE OF IONIZING RADIATIONS IN NATURE AND 



RESEARCH 



1. Ionizing Radiations 



Electrons that have passed without loss of energy through a 

 medium having some degree of periodicity in its molecular arrange- 

 ment may, by the diffraction pattern of the emergent beam, yield 

 information concerning the structure of the medium. This very im- 

 portant biophysical application of electron beams is closely allied 

 to the study of X-ray diffraction. More frequently, electrons lose 

 energy when they are deflected from their path, and this energy 

 which is transferred to the medium is very effective in bringing about 

 both chemical and biological changes. In this chapter we shall dis- 

 cuss certain features of the loss of energy by electrons and other 

 ionizing particles that have an important bearing on the interpreta- 

 tion of these changes, and describe techniques and procedures for their 

 study. 



a particles and swiftly moving electrons, or /3 particles, as they are 

 usually called, are capable of ionizing and exciting every kind of atom 

 through which they pass. The particles are referred to as ionizing 

 radiations because they are generally detected and measured by 

 means of the ionization they produce, but it is important at the outset 

 not to overlook the possible biological significance of the excited 

 molecules, which in fact, outnumber the molecules that are ionized. 

 Neutrons are incapable of interacting either with atoms as a whole 

 or with individual electrons, and are therefore not primary ionizing 

 particles. In the course of their passage through matter of all kinds, 

 however, neutrons give rise to ionizing particles by collision with 

 atomic nuclei that they either set in motion or disintegrate. When 

 matter as rich in hydrogen as the living cell is exposed to a beam of 

 fast neutrons about nine-tenths of the total ionization is produced 

 by hydrogen nuclei or protons, recoiling from the simple impact of a 

 neutron. From the point of view of biophysical research, fast 

 neutrons are therefore in a broad sense analogous to X rays. Each is 

 a nonionizing radiation that gives rise to an ionizing radiation 

 throughout the bulk of the irradiated medium. The energy of the 

 X-ray quantum is converted into electronic energy and that of the 

 neutron predominantly into the energy of recoil protons, which, in 

 the classification of ionizing particles to be discussed presently, occupy 

 a position between electrons and a particles. 



