Equivalent half-value thicknesses and mean energies of filtered X-ray bremsstrahlung spectra.

Abstract

For a complete description of an X-ray brems-strahlung spectrum the spectral photon fluence needs to be known. Spectrometry is too expensive and time-consuming, however, for many routine applications of X rays. Therefore half-value thicknesses are often used (in connection with tube voltage and filtration) to describe the beam quality. The half-value thickness is defined as the thickness of a given material to reduce the exposure rate to half its initial value. This concept has two advantages: (1) the measurement is relatively simple, using equipment which is normally available to a hospital physicist, and (2) the measurement yields a single number instead of a complex spectrum. Aluminium and copper are commonly used as half-value thickness materials, aluminium for the lower and copper for the higher energy X rays. In a large intermediate energy range either material may be used. If data from different sources are to be compared it is therefore frequently necessary to convert half-value thicknesses from values in Al to values in Cu and vice versa. Equivalent half-value thicknesses for monochromatic radiations may be calculated easily from tabulated cross sections (Storm and Israel, 1970). Equivalent half-value thicknesses for polychromatic radiations depend, however, on the shapes of the spectra. Conversion tables which have been published to date (Clarkson, 1972; Wachsmann and Drexler, 1976) can therefore be only of an approximate nature.