240 BIOLOGICAL EFFECTS OF IONIZING RADIATIONS 



impinges on them, they absorb it, turn some of the energy into heat energy, 

 and re-emit the rest as light in the visible region. A sensitive photocell de- 

 tects this re-emitted light, and the photoelectric current is amplified and 

 recorded. The reduced form of the fluorescent material, benzoic acid, does 

 not fluoresce. Irradiation causes oxidation. The intensity of the fluorescence 

 is a function of dose. Of the order of 1 rad can be accurately measured. 



In the second, advantage is taken of the fact that the electrochemical po- 

 tential of an electrode, measured against some suitable reference electrode, 

 is dependent upon the ratio of the concentrations of oxidized to reduced 

 form present in the solution. For instance, Ag/Ag + in H 2 S0 4 is one redox 

 system which has been shown to be practical. Irradiation produces Ag + and 

 the voltage of the cell (Ag in H 2 S0 4 solution vs a mercury-mercurous sulfate 

 reference electrode) decreases as the concentration of Ag + is increased by 

 the radiation. Measurement of voltage vs time or irradiation thus gives a 

 continuous measurement of absorbed dose. When done carefully, a fraction 

 of a rad can be measured. This is the only continuous-recording and re- 

 useable dose-measuring instrument known. 



However, biological damage is not subject to such reliable, quantitative 

 measurement. Measurement of biological damage, by its very nature, has so 

 far had to be a quantity such as the LD 50 (lethal dose 50 ). The LD 50 is that 

 dose in rads which will kill 50 per cent of the cells or organisms irradiated 

 (see Fig. 9-4). Further, since irradiation damage is often not immediate, but 

 may set in only after days or even years, in the case of mammals an arbitrary 

 limit of 50 per cent killed within 30 days after exposure has been accepted 

 by workers in this field as a further specification of the LD 50 . 



3 alive 3 dead 



Figure 9-4. LD 50 : 50 per cent Lethality, Measured at Some Constant but Arbitrarily 

 Chosen Time After Exposure (30 days for man). 



As a general rule, the LD 50 (30 days) for mammals is 200 to 1000 rads; 

 for man (whole-body irradiation), it is about 400 rads (equivalent to 400 

 rems if the radiation is X or gamma) (of course there are no good statistical 

 data to support this number!). For lower animals it is higher: frog, 700; 

 bacteria, 10,000; insects, 60,000; paramecia 300,000 rads. 



The LD 50 is a useful measure also of the effectiveness of partial-body ir- 

 radiation. In some cases one simply makes a suspension and estimates the 

 number of cells left living in the tissue irradiated. 



