MEASUREMENT OF X-RAYS AND RADIUM 79 



voltage (constant-potential equivalent) ; b. filter (including tube wall) ; 

 c. target-skin distance; d. area, diameter, and number of fields; e. roent- 

 gens per minute in air at position of field; /. total number of roentgens 

 delivered; g. duration of exposure; h. interval between exposures; i. size, 

 shape, and position of irradiated volume. 



Comparison of X-ray Generators. — For general guidance, Table 7 

 gives approximate outputs of several standard X-ray machines illus- 

 trated diagrammatically in Fig. 1, as measured at 50 cm. from the tube 

 target (20, 32, 49, 50, 52). With the exception of constant-potential 

 excitation, large variations may occur between different generators of any 

 one group and the mean values given in the table and subject to variations 

 which may become as large as ± 10 per cent. 



MEASUREMENT OF VERY LOW- AND VERY HIGH-VOLTAGE X-RAYS 



GRENZ RAYS (3 TO 15 KV.) 



It has been adequately demonstrated that the general principles 

 involved in the measurement of 70- to 200-kv. X-rays may, with certain 

 precautions, be extended to the accurate measurement of 3- to 15-kv. 

 X-rays. The principal correction to be applied in the use of a standard 

 for 10-kv. radiation is that for absorption of the radiation between dia- 

 phragm and collector. This may amount to 40 to 140 per cent in using 

 10- to 6-kv. (peak) unfiltered radiation. Figure 16, B shows one form 

 of such chamber (48) which by the use of the guarded field reduces the 

 diaphragm-to-collector distance to a minimum (about 5 cm.) while still 

 fulfilling the requirements of a free-air chamber. This particular form of 

 chamber is adequate for radiations up to 90 kv. and by thus overlapping 

 the range of the larger chamber, permits a direct comparison between the 

 two, using radiations of such high voltage (90 kv.) as to require very small 

 air-absorption corrections (but less than 2 per cent). 



Secondary chambers of the thimble type have been adapted to the 

 measurement of 3- to 15-kv. X-rays, although their accuracy is question- 

 able in many cases (18, 21, 26). Wall effects become relatively large and 

 even the thinnest of wall materials absorbs a large percentage of the 

 radiation, thus preventing its contribution to the ionization within 

 the chamber. These two difficulties make it necessary to calibrate the 

 secondary chamber for the exact radiation for which it will be used, and 

 it has been shown that it is unsafe to interpolate between the radiation 

 qualities used in the calibration. The accurate use of such thimble 

 chambers is so beset with uncertainties that it is usually more satisfactory 

 to use a small open-air standard for biological X-ray measurements at 

 such low voltages (48). 



Ultra-high-voltage X-rays. — At the time of writing, it may be said that 

 the measurement of X-rays excited by potentials above 250 kv. (peak) 



