Experiments with Medium-Z Materials for Shielding Against Low-Energy X Rays

Abstract

Reference to tables of mass absorption coefficients shows that a wide range of elements in the middle of the periodic table have mass absorption coefficients which exceed those of lead within certain bands of x-ray energy widely employed in diagnostic radiology. Table I presents typical data (1) illustrating these facts. These observations have stimulated experiments to compare several elements with lead as absorbers for x rays generated at voltages between 50 and 150 kVp. Early experiments were made with uniform layers of tin, iodine, and barium in elementary or compound form, since these elements are economically competitive with lead. Film was initially used as a qualitative indicator of relative x-ray transmission. In later experiments, air-wall ionization chambers have been employed which have confirmed the radiographic results. With the use of x-ray beams from diagnostic units filtered by 2.5 mm Al and operating at 90 kVp, it was established that layers of tin, iodine, or barium provided less attenuation than layers of lead with the same mass per unit area. The reasons for this result, despite the higher mass absorption coefficients, appear to be twofold: a. A considerable fraction of the x-ray photons in such beams have energies lower than the K-edges of medium-Z elements (Z = 42 to 64); for these energies µm is much lower than µm for lead. b. (Photoelectric absorption in these elements produces characteristic x rays at energies for which these materials are relatively transparent. Further experiments have been made in which the attenuation of combinations of the three elements with lead have been compared to that of lead alone. These experiments have shown that: a. Such combinations may provide greater attenuation of x-ray beams than an equal weight of lead per unit area. In other words, combinations of these elements may be produced which provide a given x-ray attenuation with less weight of absorber. In particular, such combinations have been found with weights ranging from 70 to 80 per cent of the weight of lead. b. Greater attenuation is produced when the material immediately adjacent to the protected zone is lead than when this material is a medium-Z element. c. Such combinations of elements may be incorporated into plastic materials,2 either rigid or flexible, so as to form barriers with lower weight than an equivalent lead barrier. d. These combination barriers show a weight advantage, compared with lead, for x rays generated at voltages from 50 kVp up to at least 125 kVp.