540 L. H. GRAY 



The Energy Unit. The unit proposed by Gray and Read adopts 

 the amount of energy lost by the ionizing particles per unit mass of 

 water exposed to 1 r. of 7 radiation, so that on the basis of currently 

 accepted physical data : 



1 energy unit = 93 ergs/g. tissue 



This unit has the advantage that the dose received by tissues exposed 

 to hard X rays and 7 rays has the same numerical value whether ex- 

 pressed in roentgens or energy units, so that if the unit gains general 

 acceptance the doses employed in earlier X-ray and 7-ray work, ex- 

 pressed in roentgens, would be strictly comparable, without numerical 

 adjustment, with later doses expressed in energy units. 



The Roentgen-Equivalent-Physical. The roentgen-equivalent- 

 physical adopts as the unit of energy the energy lost by fast electrons 

 in producing in air ions of either sign carrying 1 electrostatic unit of 

 charge, so that the accepted data lead to: 



1 r.e.p. = 84 ergs/g. tissue 



Tissue exposed to 1 r. of 7 radiation receives a dose of 1 energy unit 

 or 1.1 r.e.p. 



2. iMeasureuieut of Ionization 



Figure 15a shows diagrammatically one method of measuring 

 ionization currents, which has proved convenient (83). The passage 

 of ionizing particles across the air volume defined b}^ the ionization 

 chamber gives rise to the formation of pairs of ions that, in the ab- 

 sence of an electric field, rapidly recombine. If the chamber wall is 

 charged to, say, 100 v. positive with respect to the collecting electrode, 

 which is at earth potential, the positively charged ions move toward 

 the collecting electrode and the negative ions toward the walls; and 

 if the electric field is sufficiently large virtually all the ions of one sign 

 will be collected by the inner electrode and none will be lost by re- 

 combination with ions of opposite sign. It is, of course, of cardinal 

 importance to an absolute measurement of dose that no appreciable 

 fraction of the ions should be lost. The ease with which this condi- 

 tion can be fulfilled varies with the nature of the gas being ionized, the 

 gas pressure, and the type of ionizing particle. Difficulties are 

 greater at high pressures than at atmospheric pressure, and increase 

 with increasing linear ion density, i.e., generally with particle type in 



