Sec. 19.7] THE SAFE HANDLING OF RADIOACTIVE MATERIALS 



463 



similar to that attending gamma radiation in that large thicknesses are 

 necessary and scattered and secondary radiation can be serious. The 

 neutrons first must be slowed to near the thermal velocities and then absorbed 

 in a thin layer of boron, cadmium, or bismuth. Shielding materials for fast 

 neutrons therefore should be low in atomic weight. Large barriers con- 

 structed of concrete or water are customarily employed because they are the 

 least expensive and easy to fabricate. For smaller shields, for example, to 

 shield Ra-Be sources, paraffin or graphite is more convenient. 



Table 41. Gamma-ray Absorption Coefficients of Shield Materials [6] 



Table 42. Half-valce Layers of Shielding Materials for Gamma Rays 16] 



* Poured concrete with limestone aggregate, density 2.7. 



19.7. Radiation Monitoring. Rigid control and assessment of radiation 

 exposure can be ensured only through diligent surveys of radiation intensity 

 in all areas and equipment to which personnel have access and through the 

 continuous indication of each individual's total exposure. In view of the 



