The Emission and Transmission of X and Gamma Radiation

Abstract

A survey of the published data from the N.P.L. and some sixteen other research centres in this country and abroad, on the rates of X-ray emission from X-ray generators of nine different types operating at voltages from 200 kV to 2,000 kV, shows that the X-ray output of a tube in röntgens (r) is predictable at any practicable voltage under standard conditions of operation, whether for “reflection” or “transmission” X-ray targets. It further appears that the maximum X-ray output from a present-day 2-million volt tube is comparable with the gamma-ray output from about 16,000 grammes of radium. The paper next deals with a method of calculating the progressive X-ray absorption by materials, based on Kramers' formula for X-ray emission and a graphical method of solution, utilising theoretical data for the components of the absorption coefficients of a “narrow” beam of X rays, at a series of selected wavelengths in the continuous spectrum. The derived transmission curves for different exciting voltages are illustrated by the case of lead filtration, particularly in its application to ensuring adequate protection for X-ray workers, on the basis of the accepted tolerance dosage-rate of 10-5 r/sec. The good agreement of such calculated lead protection values with experimental observations, since published elsewhere for a range of exciting voltages up to 1 million, suggests the feasibility of forecasting the degree of lead protection required for an even higher range of X-ray voltages. A somewhat similar procedure is followed in the case of a narrow beam of gamma rays from radium B and C, utilising published data for the energy distribution and absorption coefficients of the several components of the line spectrum of the rays. The calculated transmission curve, which again fits in well with experimental observations carried out at the Laboratory, corresponds, incidentally, to an X-ray beam excited by about 3,000 kV. By similar means, the “lead equivalents” of various materials can be calculated for a wide range of X-ray voltages, the method being exemplified for a barium-sulphate concrete, the lead-equivalent curve of which, in common with other protective materials, increases with the voltage to a maximum at about 90 kV, followed by a minimum at 200–300 kV, afterwards rising steadily up to very high voltages. Experimental observations which we have conducted with 50 to 400 kV X rays are in satisfactory agreement with such computed curves, and appear to justify the calculations for higher voltages. Gamma-ray measurements are in similar agreement with calculation, the results finding a place on the X-ray lead-equivalent curves at between 3,500 and 4,500 kV, depending on the thickness of the specimen.