238 



BIOLOGICAL EFFECTS OF IONIZING RADIATIONS 



different radiations. One should bear in mind that the LET is not constant 

 while the radiation energy is being absorbed by tissue because with every bit 

 of energy lost there remains less to lose. 



For several types of irradiation, approximate values of relative biological 

 effectiveness (rbe) — i.e., damage per unit dose, relative to 250 kvp X rays — 

 can be written down, and can be used if the reservation be kept in mind that 

 they are rules-of-thumb, only approximate. Column 4 in Table 9-1 lists 

 such values. In general, the higher the energy of the impinging radiation, 

 the less energy it loses per unit length in tissue, and hence the longer it will 

 take a source to deliver a unit of absorbed radiation. In summary, the bio- 

 logical damage is given as: 



rads x rbe = rems ^ reps x rbe 



Several ways of receiving 1 rem of damage are depicted in Figure 9-3. For 

 workers, such as radiologists, who must necessarily be exposed, it is recom- 

 mended (by international agreement) that the whole-body dose be kept to 

 less than 0.3 rem per week in the blood-forming organs, the gonads, and the 

 eyes; less than 0.6 rem per week for surface irradiation. Relaxation to 1.5 

 rem per week is permitted if the radiation is of low penetrating power or if 

 only limited parts of the body are irradiated. Table 9-2 gives the number 

 of rems received under different conditions. 



100 ergs of I Mev beta 

 I rad) 



5 ergs of I Mev alpha 

 (0.05 rad) 



10 ergs of I Mev neutron 

 (0.1 rad) 



I rem of 

 damage 



100 ergs of X or 

 gamma (I rad) 



10 ergs of 0.1 Mev 

 protons (0.1 rad) 



Figure 9-3. Some Ways of Receiving 1 rem of Damage. 



Dose Measurement 



In air the dose received is measured accurately by the ionization chamber 

 and scintillation counters described in Chapter 5. However, in aqueous 

 solutions or in tissue, the situation is quite different. Here the primary tar- 

 get is water, and it breaks up not into ions but into free radicals (H + OH); 

 these react and produce new chemicals. 



