80 BIOLOGICAL EFFECTS OF RADIATION 



is not yet in a very satisfactory state, and it is still an open question 

 whether or not the range of the large open-air standard can be extended 

 sufficiently to measure such radiations. The particular difficulty is intro- 

 duced by the increased relative importance of scattered radiation at the 

 higher voltage. 



The international definition of the roentgen (Paris, 1931) does not 

 mention scattered radiation except that from the "wall effect," and it is 

 only by fortunate circumstances that this omission has not caused 

 difficulty in that below 150 kv. (r.m.s.) its effect is almost neghgible. 



One of the most important uses of the roentgen at higher voltages, is 

 to permit a proper evaluation of X-ray dosage in comparison with that at 

 ordinary voltages. It is, therefore, necessary that over the whole 

 voltage range the roentgen be the measure of a quantity which can be 

 translated into actual energy absorbed by tissue. Since the roentgen is 

 proportional to the energy absorbed from the beam by air, it is necessary 

 to restrict conditions so that the absorption will be known. At tube 

 voltages below 150 kv. (r.m.s.) the absorption is largely photoelectric, 

 whereas at the higher voltages, absorption is due principally to the energy 

 removed from the beam by the recoil electrons in the Compton scattering 

 process (27) (28). 



The question then arises as to whether the definition should or should 

 not include the effect of scattered radiation (56). If it should, we are 

 confronted with the problem of how to devise an apparatus which will 

 definitely include all of it. Technical difficulties at the higher voltages — 

 above 150 kv. (r.m.s.) — seem at present to render this impossible. 

 Should we then include only a definite fraction of the scattered radiation 

 in the measurement, and if so, what shall that fraction be, and how shall 

 we define and determine it? As this does not appear feasible with our 

 present knowledge, the only alternative in making measurements is to 

 avoid, as far as possible, all effective secondary radiation, and to reword 

 the definition so as to embody the necessary conditions. 



The revised definition given above appears capable of realization at 

 high voltages without going to excessively large ionization chambers. 

 Photographs made in a Wilson cloud expansion chamber 30 cm. in 

 diameter show that for a narrow 700-kv. X-ray beam practically all of the 

 ion paths are confined within 10 cm. of the beam, and the bulk of them 

 within the geometrical beam. Furthermore, it is seen that practically 

 no ion tracks originate in the space outside the geometrical beam. This 

 shows clearly that (a) the effect of radiation scattered from the chamber 

 walls is negligible, and (6) there is no appreciable ionization due to 

 photo- or recoil electrons, produced by secondary (degraded) radiation 

 scattered out of the main beam. The presence of tracks originating 

 outside the beam indicates the entrance of a scattered quantum into that 



