MEASUREMENT OF X-RAYS AND RADIUM 45 



THE MEASUREMENT OF X-RAY QUALITY 



Early X-ray Measurements. — Since the discovery of X-rays it was 

 known in a general way that the strength or intensity was proportional 

 to the electron current passing through the tube, and the penetration 

 or hardness was at least remotely related to the potential between the 

 tube electrodes. As experimental technique and tube construction 

 developed, it was found that current and voltage alone were insufficient 

 for describing an X-ray beam, and more elaborate means were required. 

 Later, as knowledge of the subject increased, the ideal method given 

 below was established — only to be left in abeyance because of the practical 

 difficulties involved. 



It was early realized that voltage alone did not accurately specify 

 the penetration of X-rays because of two principal reasons: (a) the 

 uncertainty in measuring the voltage, and (6) the wide variations in 

 voltage wave form. Consequently, the penetration of X-rays soon came 

 to be measured in terms of their relative absorption in different metals. 

 Best known of such devices was Benoist's penetrometer (4) of 1902, 

 which consisted of a thin silver disk surrounded by 12 numbered alumi- 

 num sectors of which the thickness varied in equal steps up to 12 mm. 

 Laid on a photographic plate and placed in the X-ray beam, that sector 

 was noted through which the transmitted radiation was the same as for 

 the silver center (as shown by the darkening of the emulsion). The 

 thickness of the matching sector obviously increases with the hardness or 

 penetration of the radiation. It is interesting to note (see below) that 

 present-day methods of measuring quality differ from early mainly in 

 the matter of refinement. 



Various effects known to be related to the intensity of an X-ray beam 

 will be briefly noted. Attempts have been made to utilize all of them 

 for practical measurements, although most of them have proved to be 

 unfeasible. The ideal method for measuring the intensity or energy 

 of an X-ray beam is with a thermal or heat-measuring device such as a 

 calorimeter which completely converts all radiation into heat (35). This 

 is the only method known to give results independent of the radiation 

 quality. Unfortunately, the energy available in an ordinary X-ray beam 

 of 1 cm. 2 cross section is of the order of 10~^ to 10~^ cal./sec, and to meas- 

 ure so small an amount of energy is exceedingly difficult even in the best- 

 equipped physical laboratory. 



Photographic methods are likewise extremely difficult because of the 

 complicated laws of photographic blackening, and the involved technique 

 necessary to control adequately the emulsion exposure and the conditions 

 for development (6, 5). The action of X-rays on selenium is similar 

 to that of light, but the utilization of selenium cells in accurate intensity 



