IONIZING RADIATIONS 3 



radiation to the atomic and molecular structures of the tissue. It is 

 hard, of course, to separate cleanly the job of this panel from that of 

 our colleagues the chemists. Very roughly, we quit when we have given 

 the energy to an atom or even an electron whose mean velocity is not 

 too much greater than that of the thermal motion, and which has settled 

 down to a recognizable state which may persist through at least a few 

 atomic collisions. Our story is much longer to tell than to watch, I am 

 afraid, by a factor of some 10^*. 



Ionizing Radiations 



The characteristic physical tool of the radiobiologist is actually an 

 ionizing particle. Excluding the ultraviolet region, which from its high 

 molecular specificity is the proper province of the photochemist, most 

 of the effects of radiations of every kind, from x-rays to neutrons, are 

 due to ionizing particles. This does not mean that most of the effects 

 are due to ionization itself; quite the contrary. But the initial transfer 

 of energy comes to most atoms through the more or less close approach 

 of a charged particle, whose electrostatic repulsion or attraction for the 

 nucleus itself and for the electrons of the atomic shells is the mechanism 

 of energy transfer. The disturbed atoms are very frequently ionized, 

 and because of its adaptability to electrical measurement it is the 

 ionization which is for the physicist the most conspicuous effect of the 

 passage of the particle. 



In the materials of interest, the electrons of the atomic shells move 

 with rather low velocities. Even the fastest electrons in the main 

 atoms of biological materials have energies of only a few thousand 

 electron volts; by and large they are much slower. If the velocity of 

 the incoming fast particle is considered (not its energy, but its velocity 

 matters), calculations on classical mechanical lines are adequate. One 

 simply considers the hyperbolic orbit of the motion of the incident 

 particle in the inverse-square electrostatic force field of the atomic 

 electron; the whole collision, which is an intimate one, more or less head- 

 on, will be over and done before the atomic electron has had a chance to 

 move in its slow orbital motion about the nucleus. As long as the 

 energy transfer in collision is large compared to the energy by which the 

 electron is bound to its atom, the effect of the atomic binding forces can 

 be neglected safely. The incident particle may be appreciably deflected. 

 The diagram of Fig. 1 sketches the mechanical relations in such a 

 collision (1). 



Such close collisions are often called knock-on collisions. They may 

 be handled with high accuracy. The spectrum of secondary electrons 



