462 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS [Chap. 19 



Conversely when it is known that the dosage rate or intensity must be 

 reduced by a factor/, the number of half- value layers required for a specified 

 absorbing material and energy is 



n = S3 log/ 



These formulas, in a strict sense, apply only to collimated beams of radia- 

 tion and hence will give somewhat conservative values for most conditions 

 met in practice. The requisite reduction could be achieved by distance 

 alone, for at distances large compared to the largest dimensions of the volume 

 of active material, the intensity diminishes as the distance squared. Thus 

 if I is the dose rate or intensity at distance d, the intensity at any other 

 radial distance x, neglecting absorption, is 



Materials that emit beta or alpha particles but no gamma radiation fre- 

 quently can be handled with shielding confined to each piece of apparatus. 

 This is especially useful and effective in the form of separate thick-walled 

 lucite containers tailored to fit constantly used items of glassware such as 

 beakers, flasks, serum bottles, and test tubes [1], thus enabling the operator 

 to see and handle pieces of equipment directly. To be useful, however, such 

 containers must be as completely closed as possible, preferably with screw- 

 type caps or plugs. Transfer of active liquids is made through a small hole 

 in the cap or by removing the cap temporarily. At these times no part of 

 the body, including the hands, should be above and near the level of the 

 opening since the daily tolerance dose can sometimes be received in a matter 

 of seconds from the emerging cone of intense direct and scattered radiation. 

 The container design should also be such that contamination of the outside 

 from spilling or spattering is minimized. Before reuse they should be 

 thoroughly cleaned with dilute acid. Aside from this type of shield, lucite 

 and other transparent plastics offer limitless possibilities for inexpensive and 

 efficient protection against beta-active and even low-intensity, soft gamma- 

 active materials. The many uses to which they can be put to simplify 

 operations while still providing the necessary protection will depend on the 

 ingenuity exercised in designing special shields. 



Boron, cadmium, and bismuth are the most effective materials for shielding 

 against slow neutrons. They have large resonance and thermal neutron 

 capture cross sections (boron, a ~ 500 barns; cadmium, a ~ 7,000 barns; 

 bismuth, a ~ 10 barns) and can be obtained in quantities and forms suitable 

 for fabricating shields. Boron and cadmium should in addition be backed 

 with gamma-ray absorber. Bismuth alone does not emit gamma rays in the 

 process of neutron capture. Fast neutrons present a shielding problem 



