THE FORMULATION OF SUBSTITUTE MATERIALS WITH PREDETERMINED CHARACTERISTICS OF RADIATION ABSORPTION AND SCATTERING

Abstract

A comprehensive study of the substitute materials used in clinical radiation dosimetry has shown that many of the existing products give poor simulation for both photon and electron interactions• Those materials with known composition were classified according to the errors in their attenuation and energy absorption coefficients, stopping and angular scattering powers compared to those for the material being simulated Large discrepancies were found at low photon energies, with lung and bone substitutes giving poor results. The existing selection procedures were evaluated and two new techniques were evolved, namely, the BASIC DATA METHOD and the EXTENDED Vx) METHOD. The first procedure was based on the attenuation and absorption quantities, while the second method used an extension of the popular, but misused, concept of effective atomic number. A thorough analysis of the dependence of photon and electron interactions on atomic number was made so that the effective atomic number data could be manipulated more accurately. Computer programs based on the new procedures were written and, using a library of some 1040 materials, produced 77 new formulations including muscle, fat, lung, bone, skin, breast, liver, thyroid and air substitutes. Techniques were developed for the manufacture and quality testing of 35 of these new materials. The results of a series of 'narrow-beam photon attenuation measurements, nominally from 10 keV to 1 MeV, verified the high precision of the selection procedures and provided useful data on the contaminants present in some of the base materials. Finally, the applications of the new substitutes in practical dosimetry were investigated.