218 



Applied Biophysics 



of which approximate to that of tissues. Generally water is 

 chosen, but sometimes wax. mixtures such as rice flour and 

 sodium bicarbonate, and "pressedwood" — compressed wood-pulp 

 boards — are used. Also, for the sake of standard conditions, the 



FIG. lb. IsoDOSE Curves for a Deep-Therafy Fi.eld 



Field 6 cm circle, focus-skin distance 50 cm. radiation generated by 200 kv and filtered 

 by 1 mm copper and 1 mm aluminum, HVL 1.5 mm copper (Mayneord ^) 



measurements are made in a phantom large enough to approxi- 

 mate to a semiinfinite slab. Deviations from these charts which 

 are likely to occur in practice, due to the nature of the human 

 body, are considered later. 



When a suitable distribution of fields has been chosen to give 



