182 Applied Biophysics 



use of the "free-air" chamber. A narrow beam of radiation, 

 accurately defined by a diaphragm, is passed through a large 

 chamber of air and out through a hole in the far end, completely 

 avoiding the walls. A iniiform electric field between two parallel 

 plates on either side of the beam collects the ions as fast as they 

 are formed. A measurement is made of the current to a small, 

 separately insulated section near the middle of one plate. The 

 length of this section and the cross-sectional area of the beam 

 define an effective "ionized volume" of air, so the ionization 

 current per cubic centimeter of air may be deduced — that is, 

 the dose rate in rontgens per second.* 



The details of such a measurement call for very careful 

 attention, but an intercomparison of the various national stand- 

 ards in 1931 showed that there was agreement to within -^%. 



The ''Thimble" Chamber 



Parallel with these developments was the gradual emergence 

 of the small ionization chamber, the so-called "thimble" cham- 

 ber, the theory of which will be referred to below. The "free- 

 air" chamber is clearly a special laboratory instrument and, 

 further, is inapplicable to the measurement of the diffuse radia- 

 tion produced when a beam enters matter. It was realized that 

 the difficulty of the wall-effect of a "thimble" chamber would 

 not arise if the material of the walls themselves behaved like 

 air in its interaction with the radiation. It was hoped that a 

 chamber with walls, the effective atomic number of which, in 

 relation to the photoelectric process, was the same as that of 

 air, would give readings exactly paralleling those of the "free- 

 air" chamber for any quality, i.e., that it would be "wave length 

 independent." Unfortunately this is not strictly borne out in 

 practice, the precise reasons for the discrepancy still not being 

 fully understood. 



However, by suitable choice of such factors as the materials 



* If the cross section of the beam at the defining diaphragm is used in the 

 calculation of the ionized volume, tlien the dose rate so deduced refers to the 

 strength of the beam at the diaphragm. 



