MEASUREMENT OF X-RAYS AND RADIUM 



69 



method depending upon the inverse square law when its vahdity has been 

 proved. (6) Approximately the same portion of the test beam should 

 be used for both standard chamber and small chamber. This should be 

 chosen from a portion of the main beam which is uniform, (c) Walled 

 chambers of the thimble type shall be completely enveloped by the test 

 beam, (d) Filters necessary in the caUbration of small chambers shall be 

 so placed in the X-ray beam that the effect of secondary radiation 

 therefrom is not included in the measurements. 



Reports on the Calibration of Small Chambers. — In reports on the above 

 types of instruments, the participating laboratories will normally include 

 the following: (a) The equivalent in roentgens of the overall (or other 

 desired) reading of the scale for each quahty of test beam used. (6) 

 Data on the quality of each test beam used (in accordance with the 

 specifications given in the last paragraph, (c) The temperature and 

 pressure at which the cahbration was conducted, together with correction 

 factors for other temperatures and pressures, (d) Data on the irradiation 

 (Dosisleistung, ionometric intensity) which was used for the calibration, 

 hence for which the calibration is valid, (e) Data on the means provided 

 to check the secular constancy of the sensitivity of the instrument. 



TYPES OF STANDARD CHAMBERS 



Standard ionization chambers of the open-air type may have various 

 forms, all of which should be equally reliable when properly constructed. 



^f^l'h^H" 



■kwwwws. 



G C G 



End 



Side 



Fig. 1.3. — Barrel or cylindrical ionization chamber. 



The simple parallel-plate type has already been discussed in sufficient 

 detail so that the others may be but briefly described— further details 

 being obtainable in the original publications. 



The barrel or cylindrical chamber (Fig. 13) is practically the same in 

 principle as the parallel-plate type (3). Here the high-potential electrode 

 is a closed cylinder H which surrounds the collector electrode C and the 

 guards G, all of which are rods of small diameter which may be placed 

 either coaxially or eccentrically (parallel to the axis). The electric field 

 is not uniform as for the parallel-plate chamber, but is radial. To insure 

 a field strength sufficient for saturation at a given distance from the 



