286 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



chamber, e the electron charge, and r the resolving time of the 

 detecting equipment. 



The smallest amount of ionization (instantaneous value) that can 

 be detected with the ionization chamber depends upon the current 

 sensitivity of the stage following the chamber. One roentgen per 

 hour causes an ion current of the order of 10 -13 amp/cm 3 at 1 atm. 

 With a 1,000-cm 3 chamber, the current is, therefore, 10~ 10 amp. 

 If 10~ 16 amp can be measured (this is about the practical limit of 

 most electrometrical current measuring systems), 1 firjhr can be 

 detected. The integrating method (which measures the total X-ray 

 dose or the average intensity over a long time, rather than instan- 

 taneous values of radiation intensity) is, of course, capable of 

 measuring smaller values of radiation intensity. 



Errors in ionization-chamber measurements are likely to arise if 

 either the primary particles or the secondary electrons strike the 

 walls or the electrodes. Limiting the cross section of the incident 

 beam, Fig. (5-2)1, diaphragm D, and the use of large chambers help 

 to prevent such errors. 



The ion current in a small chamber is primarily caused through 

 secondary electron emission from the walls of the chamber. If the 

 chamber is to be used for the absolute measurement of X-ray in- 

 tensity (which is based on ionization in air), the walls of the chamber 

 should be made of a material which causes per incident X ray the 

 same ionization current as air (air-equivalent chamber). 1 Such 

 materials consist usually of a mixture of graphite, aluminum, silicon, 

 or magnesium with a plastic binder. 2 



Background ionization and spurious discharges may cause further 

 errors and may reduce the ultimate sensitivity of the chamber. The 

 chamber walls are frequently contaminated to a small degree. Alpha 

 particles usually come from contaminations of the chamber walls. 

 Some authors recommend zinc, iron, steel, and aluminum to be used 

 as construction material for the walls and advise against the use of 

 copper, brass, or platinum. A thin layer of carbon black or collodion 

 applied to the inside walls of the ionization chamber frequently 

 reduces the effect of alpha rays. The effect of alpha emission from 

 the wall is reduced if the counter is filled with gas at high pressure. 

 Beta or gamma rays from the outside can be shielded by thick layers 

 of lead or similar materials. 



1 H. Fricke and O. Glasser, Fortschr. Rontgen, 33, 239 (1925). 

 2 R. Glocker and E. Kaupp, Strahlentherapie, 23, 447 (1926); see also E. 

 Miehlnickel, Ann. Physik, 20, 737 (1934). 



