Studies with encapsulated sources of 125I III. Microdosimetry using a non-metallic wall-less proportional counter

Abstract

A significant problem in measurement of microdosimetric spectra of low energy X rays has been the perturbing influence of the counters used in experiments. The conventional proportional counters used in microdosimetry have employed electrode structures of stainless steel. Since photoelectric cross-sections are highly sensitive to atomic number (approximately proportional to z 4, the photoelectrons, emitted from the steel structures, with correspondingly higher energies, may dominate over the softer Compton electrons emitted from the wall and gas. This is more serious for a wall-less type of counter, in which metallic supporting structures may contribute an additional source of photoelectrons entering the sensitive region. In order to perform accurate measurement with 125I photons and other soft X rays having energies less than 100 keV, a special counter was constructed having no metal parts. The construction is of the grid-defined “wall-less” type, contained inside a plastic spherical shell. The plastic was chosen to match the energy absorption coefficient of tissue at 25 keV as closely as possible; therefore, conventional tissue-equivalent A-150 was replaced by an “air-equivalent” type, C-552, which at 25 keV matches tissue to within a few percent. This material, based on the plastic polyvinylidene fluoride has additional advantages in terms of tensile strength and rigidity, for the purpose of molding delicate structures. The gas used was a propane-based tissue-equivalent type, modified to match the plastic. A human hair, coated with graphite, was used as the central collecting wire. The construction and details of this detector are described, and measurements are reported of 125I and 60 keV photons from 14 241Am, as well as high energy photons from 137Cs and 60Co. Values of Yd are compared to similar wall-less measurements with a conventional wall-less counter.