X-ray attenuation in lead, aluminum, and concrete in the range 275 to 525 kilovolts.

Abstract

A great deal of work has been done in recent years to determine how the attenuation of an x-ray beam is modified by the contribution of radiation scattered in the absorber. This scattering contribution depends on the irradiated area and on the solid angle subtended by the detector at the absorbers. The limiting case of a small irradiated area and a small detector solid angle is referred to as “good” or “narrow-beam” geometry; “poor” or “broad-beam” geometry implies that the irradiated area or the detector solid angle is large. In protection studies the term “broad beam” is slightly more restrictive, being used when both the irradiated area and the detector solid angle are large enough so that any further increase in either would not affect the attenuation. It has been found that when the Compton effect contributes to the absorption mechanism, the attenuation per unit thickness under broad-beam conditions is less than with a narrow beam. This increase in penetration is due to the fact that the Compton effect is a scattering process (with some energy degradation) rather than a total absorption process and some of the scattered photons ultimately penetrate the barrier. The ratio of penetrations under broad-and narrow-beam geometries, for a given thickness of absorber, is known as the build-up factor. In protection work, the attenuations are usually expressed in terms of percentage of dose transmitted, and the build-up determined with such data is known as the dose build-up factor. In general, the build-up factor increases with depth of absorber and with decreasing atomic number of absorber, but is not a simple function of the energy of the incident photons. In shielding against x-rays from machines with peak voltages below 250 kv, lead has been widely used because of its large photoelectric absorption coefficient. (The photoelectric absorption coefficient, in cm.2/atom, varies approximately as the fifth power of the atomic number.) Since photoelectric absorption accounts for almost all the attenuation in lead at these energies, the effect of build-up is negligible. For this reason, the narrow-beam data available for lead in this energy range (1) are sufficient for protection design. At higher energies, a larger fraction of the attenuation mechanism consists of Compton scattering, which essentially involves interaction with electrons. Attenuation by this process depends, therefore, for all practical purposes, on the weight of the absorber per unit area. Thus, for protection against x-rays from machines operating at 500 kv to several million volts, lead loses most of its advantage, and concrete, having better structural properties, is commonly used. In designing a concrete shield, it is important to take account of build-up in the shield if it is to be exposed to a broad beam of radiation. Attenuation data in this energy range have recently been obtained (2) for lead and concrete and for both “good” and “bad” geometry.