GAMMA RADIATIONS ARE NOT HOMOGENEOUS 73 



in the construction of radium storage vaults is quite obvious from Table 

 II-8. The data in Table II— 9 are presented to show the effect of in- 

 creased filter thickness on the coefficients of absorption of gamma radia- 

 tions after having passed through 8 mm of preliminary filtering by lead. 

 These data show that there is practically no change in the total absorp- 

 tion coefficient (ju) with increased filter thickness up to 1.5 cm for the 

 low-atomic-number elements, and therefore no hardening in the rays 

 can take place through the use of these filters. 



The absorption is very nearly proportional to the density of the filter, 

 and for comparison it can be stated that gamma radiations are approxi- 

 mately 200 times more penetrating than beta rays. 



If the total radiations from Ra(B + C), after passing through 5 cm 

 of lead, are examined it will be found that the total absorption coefficient 

 no longer decreases with increase in filter thickness, and that no further 

 hardening of the gamma radiation takes place if greater filter thicknesses 

 are used. 



In external radium therapy, where only gamma radiations are desired, 

 the metal capsule containing the radioactive material can be used to 

 remove all alpha and beta rays. Further filtration much beyond this 

 point in order to obtain harder gamma radiation is, according to many 

 authorities, of little practical value. 



In the absorption process the frequency of a scattering process in- 

 creases rapidly with hardness of the incident gamma radiation as com- 

 pared with that of the photoelectric process. For the hard gamma 

 radiations from RaC with wavelength 0.005 A a scattering process occurs 

 20 times more frequently than an absorption process. In absorbers 

 of low atomic number, such as carbon, the above ratio may be from 

 3000 to 8000 times as large. In very-low-atomic-weight material com- 

 parable to tissue or water the photoelectric process is negligible as com- 

 pared with scattering. 



Gamma Radiations Are Not Homogeneous 



None of the gamma radiators of radium emit a homogeneous gamma 

 radiation. The emissions, however, can be resolved into a few promi- 

 nent, practically homogeneous wavelength groups, each possessing an 

 appreciably different absorption coefficient, or what should be desig- 

 nated as a coefficient of reduction of intensity. For instance, Table II- 1 1 

 shows that in the decomposition of the radium products absorption 

 measurements indicate roughly four groups of more or less homogeneous 

 radiations. For comparison, the radiations are designated as soft, 

 medium, hard, and very hard. In Table 11-12 are listed the comparable 



