DOSIMETRY 



237 



Two other units are of importance. The roentgen (r), the earliest unit of 

 dose, refers to absorption by dry air, and specifically is that amount of X or 

 gamma radiation which, when absorbed, increases the energy of dry air at 

 STP (0°C, 1 atm pressure) by 83 ergs/g. The rep (roentgen equivalent 

 physical) was originally defined as the tissue-equivalent of the roentgen, but 

 with conversion difficulties being as they are, it is best defined here as that 

 amount of X or gamma radiation which, when absorbed, increases the 

 energy of soft tissue by 83 to 93 ergs/g. 



In Chapter 5 the density of ions produced along the paths of alpha, beta, 

 gamma or X, and neutrons was described (refer to Figure 5-1). It is logical 

 that the biological effectiveness of a unit of absorbed radiation should in- 

 crease with increasing density of ionization. Density of ionization is ex- 

 pressed quantitatively as the linear energy transfer (LET). Therefore, the 

 relative biological effectiveness of one unit (i.e., 1 rad) of absorbed radiation 

 (of different kinds) should be proportional to the LET. For instance, slow 

 alphas ( 4 2 He ++ ) do twenty times the damage of X rays of equivalent dose 

 absorbed. Table 9-1 lists some average LET values for various energies of 



TABLE 9-1. Linear Energy Transfer (LET) in Thousands (Kev) or Millions (Mev) of Electron- 

 Volts Absorbed per Micron (10~ 4 cm) of Track for Some Atomic "Bullets." 

 Accepted Values of Relative Biological Effectiveness (rbe).* 



*From Report of the International Commission on Radiological Units and Measurements (ICRU), 

 Handbook 78, National Bureau of Standards, Washington, D.C., 1 959, p. 50; and Ref. 19, p 1 74. 



** rems //rems\ 

 ■/ \ dose/-, 



dose> 



250 kvp X rays 



