XVII. RADIOACTIVE TRACERS ()39 



derived for air, the same energy absorption per gram is used for tissue 

 as well. For many purposes the two units can be considered synony- 

 mous. One thousandth part of each of these units, the milliroentgen 

 (mr.) and millirep (mr.e.p.) are frequently used. Another unit re- 

 cently introduced, the r.h.m. (roentgens per hour at one meter), is 

 more for the comparison of source strengths than dosages {17). 



From the experience of roentgenologists and radium workers it has 

 been decided that the amount of radiation that can be absorbed by 

 the body each day without damage is 100 mr. This so-called "toler- 

 ance value" represents an absolute maximum if the worker is sub- 

 jected to it each day and a working maximum if the exposure is infre- 

 quent.* Since genetic effects probably occur at lower levels than 

 those that produce observable damage, it is well to consider 100 mr. 

 per day as a maximum even for occasional exposure and to keep the 

 average exposure to less than 10 mr. per day. On the basis of an eight 

 hour working day, tolerance is frequently given as 12.5 mr. per hour. 

 Although these levels were originally proposed for radiation received 

 by the whole l)ody it is safer to consider 100 mr. as a maximum daily 

 dose for any part of the body {2, pp. 395, 409). 



The estimation of expected radiation dosage from external sources 

 under given conditions of source strength, type and energy of radia- 

 tion, distance and shielding is not a particularly difficult problem. 

 It will differ for each kind of radiation but y rays constitute the only 

 real hazard from external sources. Since several of the quantities 

 involved are known only approximately the calculation cannot be 

 made with any great accuracy, but such accuracy is not necessary for 

 the purpose, results within 20% being adequate. To this accuracy 

 the absorption coefficient of y rays in air (mq) can be assumed constant 

 over the range 0.05 to 2 m.e.v. and the value 3.6 X 10~^ cm.~^ is used. 

 Below this range the gamma rays are easily shielded and no important 

 tracers have y rays with energy appreciably above 2 m.e.v. 



The formula for calculating the dosage rate in roentgens per hour 

 from a source of m millicuries at distance d cm. can then be put to- 

 gether as follows: 



^ ^ EefXaA 5.6Em ^, 



Dosage rate = ^^^^ = ^^ r./hr. 



E = 7-ray energy in million electron volts, e = charge on ion pair = 

 4.8 X 10"^" e.s.u., A = disintegrations per hour = 3.7 X 10^ X 



* Recently, many laboratories have reduced their operating limit to 50 mr./day. 



