72 APPLIED RADIOACTIVITY 



TABLE II-8 

 Absorption by Lead of Gamma Rays from Radium C 



Ionization 



* A 1 . 0-cm lead filter removes all beta raya 



TABLE II-9 



Coefficients of Absorption 



Gamma radiation 

 Value of m after having traversed an 8-mm lead fore-filter 



Filter 



0-2.5 mm 



2.5-5.0 mm 



5-10 mm 



10-15 mm 



For gamma as well as x-rays the photoelectric absorption in high- 

 atomic-number elements is approximately proportional to the fourth 

 power of the atomic number of the filter and to the cube of the wave- 

 length of the incident radiation. The total absorption coefficient n is 

 therefore made up of two parts, the coefficient of photoelectric absorp- 

 tion t, plus the coefficient of diffusion or scattering <r, so that /jl = r + a. 



A useful table of transmission characteristics in the form of the rela- 

 tive amounts of radiation transmitted by various metal filters, complied 

 by E. H. Quimby, is reproduced in detail in Table II-5. Table II— 8 

 and Table II-9 show what may be expected in the way of absorption 

 by different practical filters. The protective value of 20 cm of lead used 





