DOSIMETRY 



239 



TABLE 9-2. Sources of Irradiation of Human Beings. 



Source 



Approx Dose or Dose Rate 



Natural external background, including 

 cosmic rays 



Increase in background due to nuclear 

 testing 1945-1962 peak soon after test 



Average increase 



Internal exposure to Ra 226 and K 40 from 

 foods 



K 40 alone 



One chest X ray: 

 best 

 average 

 fluoroscopic examination 



Local dose during irradiation of tumors 



Median lethal dose, whole body 



Maximum permissable* dose rate, 

 whole body 



0.073 rem/year (widely variable) 



to many thousand times natural 

 background, depending upon loca- 

 tion 



0.1% of natural 



0.15 to 0.5 rem/yr 

 0.03 rem/yr 



0.006 rem 

 0.2 rem 

 ~ 1 rem 



3000 to 7000 rem 

 ~400 rem 



15 rem/yr (0.3/wk) 



*Recommended by the International Commission on Radiological Protection. 1 958. 



The celebrated Fricke dosimeter is based on this principle. It is an aque- 

 ous solution of 0.1 .\/-H 2 S0 4 containing 10 ~ 3 A/-FeS0 4 and a tracce of chlo- 

 ride. Upon irradiation, ferrous ( + 2) is oxidized to ferric ( + 3) iron, and the 

 amount of ferric produced is easily estimated, as FeCl, from the extent of 

 absorption of light of wave length 3040 A. Thus 1 rad of hard X or gamma 

 radiation has the chemical effect of converting 1.5 x 10~ 8 moles of Fe +2 to 

 Fe +3 per liter of solution. The system is widely used, because it is simple, 

 reproducible, accurate, and independent of dose rate (e.g., rads/hr). Its use- 

 ful range is from about 500 to several thousands of rads. This dosimeter 

 system — standard methodology, advantages and disadvantages — has been 

 described in detail elsewhere. 18 



Since biological damage can occur at much lower doses than this, recent 

 developments have been toward more sensitive aqueous dosimeters. In 

 sealed vials, chlorinated hydrocarbons liberate chlorine and change color 

 in crude field dosimeters — sensitive, but results are not too reproducible. 

 Two other recent developments will now be described very briefly. 



In the first, advantage is taken of the fact that certain molecules, such as 

 hydroxybenzoic acid, in water will fluoresce. That is, if ultraviolet light 



