74 BIOLOGICAL EFFECTS OF RADIATION 



open-air chamber. The degree of the ionization from this source depends 

 principally upon the atomic number of the wall material. 



With a wall made of some material having the same atomic number as 

 air, the ionization in the enclosed volume due to secondary X-rays and 

 electrons should theoretically be the same as the free air in a large 

 chamber. It has likewise been demonstrated experimentally (14, 15) 

 that a wall material having effectively the same atomic number N^^f as 



o 



air will, within a fairly wide range of wave-lengths (0.1 to 1.5 A), have the 

 same effect on the ionization current as air. The effective atomic number 

 A^^ff is given by the relation 



iV.„ = >.^1 + "^NJ +■■ ■ 



where a is the fractional quantity of material of atomic number iV. 

 By suitably choosing a and N in the materials composing a thimble 

 chamber, its effective atomic number can be adjusted to 7.69, that of air. 



By a cut-and-try method, the material, shape, and position of the 

 walls and collector electrode may be adjusted so that the photoelectrons 

 from them produce the same net effect as air. It is thus possible to 

 make a chamber having a graphite shell with aluminum collector which 

 will give readings nearly proportional to the open-air chamber over a 

 wide wave-length range of the harder radiations. 



Method of Calibration. — Without going into further detail, it is evident 

 that a thimble chamber cannot be relied upon to indicate true roentgens 

 unless calibrated against a standard open-air ionization chamber. Even 

 in production manufacture, it has not been found feasible to mechanically 

 construct thimble chambers having identical characteristics without 

 resorting to individual calibrations. To cahbrate a thimble chamber, 

 the intensity of an X-ray beam, after suitable limitation by diaphragms, 

 is measured in roentgens per minute at a given point with a standard 

 free-air ionization chamber. Holding the X-ray output steady, the 

 standard chamber is removed from the beam, the center of the thimble 

 chamber placed at the exact position of the front edge of the limiting 

 diaphragm (D, Fig. 12) of the standard chamber, and the time-rate of 

 deflection noted for the thimble-chamber indicator. This must be done 

 for each quality of radiation for which the instrument is to be used. 

 Where the dependence of the deflection upon quality is small, only a few 

 calibration qualities are necessary, it then being permissible to interpolate 

 for intermediate qualities. Experience has shown that it is seldom safe 

 to extrapolate such a calibration. 



Several other sources of error must be guarded against (7, 43, 57); 

 for example, there may be some uncertainty as to the effective center of a 

 thimble chamber when placing it in a test beam. To avoid undue errors 



