The Measurement of Radiation 187 



strength of the beam of radiation must be sensibly uniform 

 over the cavity.* 



In some cases, particularly in the ordinary X-ray region, 

 the behavior of small "thimble" chambers appears to deviate 

 from the foregoing analysis. On general grounds, it may be 

 presumed that the conditions attaching to the principle of 

 equivalence have not been fulfilled in these cases. Although 

 the deviations are not usually large, and the use of such chambers 

 can be avoided in practice, yet the effects are of considerable 

 intrinsic interest and have received much attention. 



The Redejxnition of the Rontgen and the Extrapolation Chamber 



With the development of the work on the measurement of 

 gamma radiation, the need was increasingly felt for a rewording 

 of the definition of the rontgen. One reason was the desirability 

 of admitting the "thimble" chamber, previously excluded by the 

 clause about avoiding wall effect, as a valid device for measur- 

 ing in rontgens, but more important was the practical necessity 

 to disentangle the fundamental dose unit from the complexities 

 surrounding the actual ionization in air in certain conditions. 



For example, because of the relatively long range of the sec- 

 ondary electrons produced by gamma radiation, the ionization 

 at any point may not bear any simple relation to the strength 

 of the radiation beam there, i.e., the energy actually communi- 

 cated to the medium at a given point may not come from energy 

 conversion of the radiation at this point, but from various points, 

 depending on the geometry of the environment. Normally, a 

 complete compensation exists, and^the energy converted is equal 

 to the energy communicated to the medium at the same place, 



* Strictly, it must be sensibly uniform throughout all that part of the medium 

 from which secondary particles can reach the cavity. One particular application 

 of the theory is to determine the specific gamma-ray dose rate of radium by measure- 

 ments with a "thimble" chamber. For a chamber wall of light elements, for ex- 

 ample, graphite, the energy conversion of this quality of radiation is the same (per 

 electron) as for air. Thus, by correcting the observed ionization in the chamber, 

 according to the quantity q (which is known), the ionization in a true "air wall" 

 chamber is deduced. The specific gamma-ray dose rate of radium determined in this 

 way is very close to 8.4 r per hour. 



