68 BIOLOGICAL EFFECTS OF RADIATION 



2. The diameter of its aperture should be approximately equal to the 

 diameter of the focal spot of the X-ray tube, but always such as to limit 

 the beam entering the chamber to one of uniform intensity of cross 



section. 



3. The aperture should preferably be cylindrical on account ot the 

 ease of construction and accuracy of measurement. A covering material 

 such as cellophane shall not be used. 



4. The distance of the chamber diaphragm from the measurmg 

 electrode shall not be unnecessarily large, in order that the correction for 

 air absorption of the rays along the path of the beam between the dia- 

 phragm and the electrode shall be a minimum. 



5. Precautions shall be taken to avoid a measurable amount of 

 scattered electrons or secondary radiation originating at the rim of the 

 chamber diaphragm from reaching the measuring volume. For example, 

 a second larger diaphragm whose rim is not struck by the direct beam of 

 radiation may be placed between the chamber diaphragm and measurmg 



electrode. 



c. The ionization current is preferably measured by a null method so 

 that a distorting electric field does not exist between the guard and meas- 

 uring electrodes. Radioactive compensators should be of the constant 



capacity type. 



B. Chambers for soft radiation : For the field of diagnostics and super- 

 ficial therapy (soft radiation), a chamber smaller than for the hard 

 radiation may be used to advantage. This should, of course, agree with 

 the larger chamber over the greatest possible quality range. 



C. Chambers for ultra-soft and super-hard radiation: While pre- 

 Uminary studies have indicated the probability that the standard ioniza- 

 tion chamber employed for soft and hard radiation may be satisfactorily 

 modified for use with ultra-soft and super-hard radiation, sufficient experi- 

 ence has not yet been gained to justify international agreement thereon. 



CALIBRATION OF X-RAY DOSEMETERS 



{Roentgenometers, Dosemeters, Irradiometers, Intensimeters, Etc.) 



Small Ionization Chambers.— (a) Small ionization chambers such as are 

 frequently used in practice should be so designed and constructed as to be 

 independent of quality within the spectral range for which they are 

 intended. (6) Such chambers shall be equipped with suitable arrange-, 

 merits for checking the constancy of their cahbration. (c) There shall be 

 no ionization in the instrument extraneous to that in the small chamber 



itself. r 11 



Calibration of Small Ionization Chambers.— (a) Calibrations ot small 



chambers may be carried out by either a direct-replacement method; or a 



