164 RADIATION BIOLOGY 



emulsions properly treated with deuterium, lithium, and boron. Fairly- 

 reproducible correlation has been reported between flux and the number 

 of tracks developed (Titterton and Hall, 1950). 



Experiments dealing with the biological effects of thermal neutrons 

 have not been many and they invariably have been reported also in terms 

 of neutron fluxes measured mostly by the foil technique (Tittle, 1951) fol- 

 lowed by conversion to dose (in roentgen eciuivalents physical, rep) 

 through relations similar to Eq. (11) of this section. 



The factors responsible for this prolonged infancy of neutron dosimetry 

 are many and cannot be discussed in detail here. It will be remarked, 

 however, that they seem to originate mainly from the pressing needs of 

 personnel protection, which has fostered the development of neutron- 

 measuring techniques characterized more by economy and sensitivity 

 than by adherence to principles of fundamental significance in radiobiol- 

 ogy. The fact that much basic radiobiological information remains to 

 be obtained by direct exposure of organisms to the more easily available 

 sources of X and 7 rays has favored this tendency. 



Ionizing Particles. In principle, the measurement of dose due to par- 

 ticle flux by means of the cavity ionization chamber is of simple attain- 

 ment. In view of what has been said before, the expression for the dose 

 D is obviously 



D =F -eZ^ Jr.-W' pm (13) 



where the symbols F, e", Jm, W, and p,„ retain their previous meaning. 

 In practice, however, large errors may arise in the measurement of J if 

 proper attention is not paid to extrapolation of the cavity to zero volume 

 at the point of interest. This can be done in many instances by means of 

 the Failla extrapolation chamber (Failla, 1937) or with very thin-walled 

 chambers of minimal dimensions as shown by Blomfield and Spiers (1946) 

 and by Neary (1946). 



Calculation of the dose is usually complex and hardly practicable. 

 For the case of a well-defined beam, containing N{E) particles per square 

 centimeter of energy between E and E + dE, striking a biological object 

 of thickness Ax small compared to the range of the least energetic particle, 

 the dose D is given by the expression 



N(E)^dE (14) 



where dE/dx is the rate of energy loss per unit path by the particles in the 

 object in units of ergs per gram per square centimeter. It is obvious that, 

 except for monoergic beams, the evaluation of the integral requires long 

 and tedious calculations besides a knowledge of N{E). To avoid this 

 inconvenience, the estimate of Z>,„ can be made by ionization measure- 



