MEASUREMENT OF X-RAYS AND RADIUM 



47 



tion of the total energy contained in the hnes increases, although it never 

 becomes a major portion of the total. In addition, in passing through 

 various filters, the wave-lengths composing the incident radiation are 

 absorbed unequally according to known laws of absorption, so that the 

 resultant spectral energy-distribution curve may be very different in 

 general form from the initial. 



It is, therefore, evident that to measure properly the quantity or 

 intensity of an X-ray beam, the actual or effective spectral distribution 

 of the measured radiation must first 

 be known. For simplicity, the spectral 

 distribution of an X-ray beam is 

 referred to as X-ray quality. 



Effect of Voltage Wave Form. — The 

 direct determination of X-ray quality 

 is complicated by a number of factors, 

 most important of which is the fact 

 that only rarely is the exciting potential 

 of a truly steady value. Actually, the 

 applied tube voltage is usually fluctuat- 

 ing over all ranges from zero-to- 

 maximum to values steady within a few 

 per cent of maximum (ripple voltage). 

 Since, as noted, the spectral distri- 

 bution or quahty changes with voltage, 

 if a varying voltage be applied to the 

 tube, a composite spectrum results which is made up of all the 

 component simple spectra corresponding to all instantaneous values of 

 the impressed voltage. 



For example, suppose a varying voltage as represented by A in Fig. 3 

 be applied to an X-ray tube and that at time ta the voltage is a = 20 kv. ; 

 the spectral distribution is represented by curve a in Fig. 2 (53). Simi- 

 larly, at time tb the voltage is 6 = 30 kv., and the spectral distribution 

 is given by curve h in Fig. 2, and so on. If we are chiefly interested in 

 40- to 50-kv. X-rays, we note that they are present only between times 

 tc and te or less than one-third of the cycle. If, on the other hand, the 

 voltage wave form is that shown in Fig. 3, B, it is seen that the 40- to 

 50-kv. X-rays are present from t[ to t'„ or about four-fifths of the cycle. 

 For the wave form shown in Fig. 3, C, the spectral distribution lies always 

 between curves c and d (Fig. 2). Although the peak voltage is the same 

 in all three cases, a wave form such as 3, C gives a preponderance of the 

 more penetrating radiation over wave forms 3, A and 3, B. 



Relationship of Voltage to Quality.— li is clear that, since the resultant 

 or composite spectral distribution is the summation of the simple distribu- 



0.2 



Fig. 2. 



0.4 Q6 0.8 1.0 



Wavelength in Angstroms 



-X-ray continuous spectra. 



iUlrey.) 



