Theory of X-ray Quality Measurement

Abstract

Part I THE measurement of the quantity of X-radiation has been placed upon a rational physical basis. The r-unit and its application has progressed to the point where it is quite definite and theoretically justifiable. The measurement of quality, on the other hand, is still in a state of indefiniteness. A great deal of experimental work has been done, but certain general theoretical considerations seem to have been insufficiently stressed. There are at present a number of methods of designating the quality of X-radiation. Any method depending on the spectrograph must be excluded because of the practical difficulties. The methods of practical importance are the following: 1. Effective wave length by absorption in a fixed layer of Cu or Al. 2. Effective wave length by absorption in a layer of Cu or Al of such thickness that the incident energy is reduced to half by passage through the filter (half value layer). 3. Effective wave length from the absorption coefficient corresponding to the slope of the absorption curve in Cu, Al, or tissue for varying thickness of absorbing material. A general definition of effective wave length is the following: The effective wave length of a complex radiation is the wave length of the homogeneous radiation which, with the same intensity, produces the same effect as the complex. The various methods of measuring effective wave length differ in the effect to be considered. In Method I the effect is the absorption in a fixed thickness of a specified material. In Method II the effect is the absorption in the half value layer of a specified material. In Method III the effect is practically the absorption in a very thin layer of a specified material. It is possible to define the average wave length without all the indefiniteness of these methods. Some of the possible definitions of this more general type are as follows: (A) The average wave length of a complex radiation is the arithmetic average of the wave lengths present in the complex, weighted according to the intensity per unit wave length interval. (B) The average wave length of a complex radiation is the wave length corresponding to the arithmetic average of the frequencies present in the complex, weighted according to the intensity per unit frequency interval. (C) The average wave length of a complex radiation is the wave length corresponding to the average of the absorption coefficients in Cu, Al, or tissue of all the components of the complex, weighted according to the intensity per unit difference in absorption coefficient. (D) The average wave length of a complex radiation is the wave length corresponding to the arithmetic average of the frequencies present in the complex, weighted according to the number of quanta per unit frequency interval. These four definitions base the effective wave length upon qualities inherent in the composition of the complex radiation, rather than upon a specified experiment.