88 THE PHYSICS OF VIRUSES 



electrons generated by the X-rays are now slower and ionize 

 more densely, so that their effects may not be due to single 

 primary ionizations scattered at random but may have ionization 

 concentrated in tracks. 



X-rays are measured, in practice, in terms of a unit based 

 on ionization in air. This is the roentgen. Radiation dosimetry 

 in these terms is described in many accounts, notably one by 

 Evans (1949). Of somewhat more use is a unit based on energy 

 loss. The amount of radiation which will release 83 ergs per 

 gram of material is called a roentgen equivalent physical (rep) 

 and roughly matches the reading on an air dosimeter in roentgens. 

 A conversion factor from rep to primary ionizations (pi) per 

 cubic centimeter in protein is 1 rep is equivalent to 6.13 X 10^^ pi/ 

 cm^. With this conversion factor, X-ray inactivation data, taken 

 under conditions 2vhere primary ionization is the inactivating agent, 

 can be used to give inactivation volumes by the use of Eq. 3.6 

 for random volume action. 



Secondary Radiation Effects 



We can now turn to some secondary radiation effects of impor- 

 tance. By far the most important of these is the action of radia- 

 tion on water. Water is the most prevalent biological constituent, 

 and any action upon it must therefore be treated most seriously. 

 The first action on water is undoubtedly ionization. This is 

 followed, in many cases, by dissociation, so that there will now 

 be present the free radicals H and OH. These are uncharged and, 

 therefore, are subject only to the force fields of chemical valence, 

 which operate over a few Angstrom units, or less. This is in 

 contrast to ions which are rapidly brought together by direct 

 Coulomb forces and so recombine. The free radicals are chemi- 

 cally active, and by combination can release about 5 ev of 

 energy, so that they are also carriers of energy. Lea (1947) has 

 estimated that the half-life for recombination of radicals formed 

 by sparsely ionizing particles such as electrons is 2 X 10~^ sec, 

 and for densely ionizing particles (alpha particles) is 10""^ sec. 

 The rate of collision with solute molecules at a concentration of 

 1 gm/1 he estimates as 10^" per sec if the solute molecular weight 



