94 



RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



3.32 log 250 = 8. From Table 3-5, since the gamma energy is less than 

 0.5, it can be calculated that about 0.43 X 8 = 3.4 in. steel is required. 

 Generally, shields are constructed of lead or steel bricks that can be con- 

 veniently arranged so as to give 2, 4, or 6 in. of protection and thus pro- 

 vide adequate shielding for most purposes. For example, it requires less 

 than 4 in. lead to reduce the intensity of 100 mc Na^^ to 6.25 mr/hr at 

 36 in. from the source; Na-^ has one of the highest /^/-per-milhcurie 

 values, about 25. References (3, 26, 42, 43) may be consulted for various 



Table 3-5. Half-value Layers for Gamma Radiation 



(From Simon Kinsman and Donald J. Nelson, Jr., eds., "Radiological Health Hand- 

 book," Radiological Health Training Section, Environmental Health Center, Cin- 

 cinnati, Ohio, March, 1953.) 



tables and graphs that are useful for the estimation of gamma shielding 

 requirements. 



In practice, the external hazard will usually be at the time of opening 

 the original shipment and during the manipulations necessary to prepare 

 it for use in the biological system. In the biological experiment most 

 work involving less than 1 mc can be handled by the use of distance to 

 reduce the radiation intensity, the so-called tongs-and-tweezers technique. 

 A great many operations can be performed using the various tongs avail- 

 able from laboratory supply houses. It is advisable to cover the tips or 

 jaws with rubber or asbestos, which provides a firm grip on the object 

 being handled. 



The operations can be carried out either by close shielding of the indi- 

 vidual containers or by the use of barriers. The former is practical for 



