MEASUREMENT OF X-RAYS AND RADIUM 81 



region with the subsequent collision with an air atom and the production 

 of measurable ions. 



The revised definition can probably be realized by the use of (a) 

 suitable diaphragms to stop secondary X-rays and secondary electrons 

 from the filters and diaphragm edges from reaching the measuring 

 volume, (6) a narrow and well-defined X-ray beam, and (c) materials of 

 low atomic number for the walls and electrodes of the chamber. If the 

 diameter of the beam is sufficiently small in comparison with the distance 

 in which the beam loses, say, half its energy, then the fraction of the 

 degraded radiation which is absorbed in the measuring volume directly, 

 or scattered back from the surroundings, may be neglected. Since the 

 energy absorbed from the beam is transferred largely to recoil electrons 

 and thence to ions, it may be determined directly from the measured ion 

 current, which gives the number of ion pairs. It is ob^dous that the 

 inclusion of an appreciable amount of scattered radiation would render 

 such a determination very uncertain at best. 



There is no definite mformation at present to indicate that a suitable 

 thimble chamber can be constructed to measure accurately X-rays above 

 250 kv., although certain work indicates that gamma radiation can only 

 be measured in roentgens with a thimble chamber. There is also some 

 evidence that ultra-high-voltage radiations may be measured with the 

 drumhead type of chamber. These questions are still too controversial 

 to warrant a discussion here. 



RADIUM MEASUREMENT 



The measurement of gamma rays from radium has been largely 

 of a comparative and not absolute nature. Since radium dosage has been 

 expressed in terms of geometrical factors and the amount of Ra element 

 employed, it has been impossible to correlate dosages and biological effects 

 of gamma rays as compared with X-rays. In many biological experi- 

 ments this has naturally retarded the proper choice as to which radiation 

 should be used. 



The difficulties in the measurement of gamma rays in roentgens are 

 somewhat analogous to those for ultra-high voltages, only accentuated 

 (13, 27, 29, 30). For gamma rays, the avoidance of scattering becomes 

 exceedingly difficult and moreover there is no certainty as to hoAv much, 

 if any, of the secondary scattering effects should be included in the 

 measurement. It is not easy to segregate a narrow well-defined beam 

 of gamma rays without any scattered radiation whatever, for passage 

 through an ionization chamber. Use of a broad beam again introduces 

 uncertainties due to scattering, and would require an excessively large 

 chamber for its measurement. 



