MEASUREMENT OF IONIZING RADIATIONS 



163 



gases and equal to the most probable ratio in air, 36/32.5 = 1.11. We 

 may conclude, therefore, that the use of the tissue-equivalent chamber 

 (both gas and wall) offers the advantage of requiring a minimal number of 

 uncertainties when appUed to a great variety of neutron fluxes and retains 

 its basic usefulness in 7-ray measurements. The vahies of D [as expressed 

 by Eq. (12)] for this idealized tissue exposed to a monoergic flux of 10^ 

 n/cm^ is shown in Fig. 2-8 as a function of neutron energy. 



c 



a. 5 



u 

 a> 



(/) 

 (/} 



t 3 



a. 

 oa 

 O 



in 



CD 



< 



>- 



oc 

 u 



I 



2 3 4 5 6 7 



NEUTRON ENERGY, Mev 

 Fig. 2-8. Energy absorbed in tissue as a function of fast neutron fiux of various 

 neutron energies. 



In conclusion, the cavity ionization chamber with walls and gas closely 

 following the atomic composition of the system under investigation 

 appears at this time to possess definite advantages as a means of measur- 

 ing doses under a wide variety of conditions. The accuracy of the dose 

 estimates thus obtained depends, however, on the accuracy of the physical 

 constant W of the gases used in the cavity, as it applies to all types of 

 ionizing particles arising from neutron interactions. Although a properly 

 constructed tissue wall chamber should in principle prove adequate for 

 the measurement of dose in suitably conducted experiments involving 

 exposure to slow and thermal neutrons, no experimental attempts of this 

 sort have been described thus far. There exist, instead, numerous pub- 

 Ucations (Mitchell, 1947; Snyder, 1950; Tait, 1949; Capron, Faes and 

 Tavernier, 1949) dealing with calculations correlating dose to thermal- 

 neutron flux and only limited experimental data (Capron et at., 1948) con- 

 cerning the variation of thermal-neutron flux as a function of depth in 

 phantoms of sizeable dimensions. The main object of these reports has 

 been the establishment of maximum permissible levels of exposure for 

 personnel in atomic energy laboratories. In practice, personnel exposures 

 to thermal neutrons are being estimated by means of photographic nuclear 



