MEASUREMENT OF IONIZING RADIATIONS 



179 



in the calibration, its validity must be experimentally determined. Cor- 

 rection must likewise be made for air density in the thimble chamber at 

 time of calibration ; this correction is obtained from the atmospheric tem- 

 perature and pressure in the same manner as for the standard chamber. 



Finally, since this thimble chamber must often be used at locations 

 irradiated not only by the source but also by scatter from surrounding 

 materials, it is well to determine its response as a function of direction ; it 

 will be found that usually the thimble chamber is least sensitive around 

 the stem and that, if irradiated, the stem may be a source of spurious 

 ionization current. 



Measurement of Very-low-energy X Rays. Experience has adequately 

 demonstrated that the general principles involved in the measurement of 

 70- to 200-kev photons may, with certain precautions, be extended to the 

 accurate measurement of 3- to 15-kv X rays. The principal correction 

 to be applied in the use of a standard is that for air absorption of the 

 radiation between diaphragm and collector. This may amount to as 

 much as 140 per cent when using 6- to 10-kv unfiltered radiation. Fig- 

 ure 2-12B shows one form of such chamber (Taylor and Stoneburner, 



GRENZ RAYS 



TO ELECTROMETER 

 (A) 



ELEC. (B) 



Fig. 2-12. (A) National Bureau of Standards guarded-field standard ionization 

 chamber (50 to 200 kv); (B) guarded-field standard ionization chamber for Grenz 

 rays (3 to 90 kv). 



1932) which, by the use of the guarded field (Failla, 1929; Taylor and 

 Singer, 1930), reduces the diaphragm-to-collector distance to a minimum 

 (about 5 cm) while still fulfilling the requirements of a standard chamber. 

 This particular form of chamber is adequate for radiations up to 90 kev 

 and, by thus overlapping the range of the larger chamber (Fig. 2-12 A), 

 permits a direct comparison between the two, using radiation of such 

 high voltage (90 kev) as to require very small air-absorption corrections 

 (less than 2 per cent). 



