IONIZING RADIATION AND VIRUSES 87 



and to an electron wliich escapes with high kinetic energy. 

 The atomic excitation energy for biological atoms (H, C, N, O, 

 P, and S) is usually around 400 ev, whereas the escaping elec- 

 tron carries the rest of the energy, which is not often less than 

 30,000 ev. So the overwhelming majority of the effect is in the 

 escaping electron, a fast, charged particle, behaving exactly as 

 already described. Specific studies by Guild (1952) show that 

 characteristic absorption in phosphorus is not favored in bio- 

 logical action, at least to a first approximation. So as far as 

 photoelectric absorption is concerned. X-rays merely act to put 

 fast electrons into the specimen. Their big advantage is that the 

 photons penetrate, so that the electrons are really released in the 

 middle of the system. 



Compton recoil is slightly more complex. The photon does 

 not die, but suffers a change in wavelength. A fast electron is 

 nevertheless produced here also. The longer- wavelength photon 

 speedily becomes absorbed by nearby atoms, again with the 

 ejection of an electron. These scattered photons can have low 

 energy, and so the photoelectrons have low energy. This rather 

 complicates X-ray action. Photoelectric absorption is pre- 

 dominant in biological material below about 50 kv, and Compton 

 recoil is predominant above "200 kv. In either event, the major 

 physical action of X-rays is the production of fast electrons. 



The energy of these fast electrons does not exceed the equiva- 

 lent energy of the photons, which is roughly that of the X-ray 

 tube. Actually, a wide distribution of energies is present in X-ray 

 bombardment. This is not a serious complication since, as has 

 already been pointed out, fast, charged particles lose energy by 

 reason of their field, and this depends on velocity for any one 

 particle. Now the light electron very readily approaches the 

 velocity of light, after which it effectively becomes heavier, as it 

 gains energy, without changing speed and ionizes in just the 

 same way as if it had less energy. For electrons above about 

 200,000 ev, the rate of energy loss is nearly constant. This impor- 

 tant fact means that very little difference in biological effect is to 

 be expected for high- or medium-energy X-rays. For very-low- 

 voltage X-rays, below 25 kv, this ceases to be true since the 



