MEASUREMENT OF RADIATION DOSE DISTRIBUTIONS WITH PHOTOCHROMIC MATERIALS.

Abstract

The problems of isodose measurement and depth-dose distribution studies on a microscopic scale have not been solved satisfactorily by conventional methods of dosimetry. Energy dependence of photographic emulsion response leads to large errors in dosimetry of heterogeneous fields. Although extrapolation and tissue-equivalent ionization chambers have found wide use for measuring dose distribution, they do not determine the distribution on a microscopic scale. The liquid chemical, glass, and solid-state detectors are not suitable for mapping on this scale and may tend to interfere with the radiation field. Radiochemical changes in aromatic dyestuffs have provided convenient indicators for dose distributions (1, 2), but some are energy-dependent because of the presence of halogens, sulfur, or metal salts. Usually the radiation effect is a bleaching of the dye, as in methylene blue, or a color change, as with methyl yellow. In addition to energy dependence, other difficulties are the small latitude in response, instability, and sensitivity to the presence of impurities which may lead to chain reactions. The work now in progress indicates that many of these disadvantages are absent in the utilization of the radiosynthesis of dyestuffs in colorless solid solutions of certain dye progenitors; in particular, derivatives of the triphenyl methane molecule. Stabilized forms of pararosaniline nitriles, for example, become deeply colored by intense ultraviolet irradiation of wave lengths below about 0.32 µ or with large exposures of ionizing radiations (3). The response range of about one-mil sections of typical sensitizer solutions in plastic is about 0.1 to 100 MR for Co60 gamma rays, as shown in the figure. The optical transmission density was measured, using narrow-band pass niters with the densitometer. The data for the solid curves were measured at the wave lengths of the absorption peaks of the two dyes. If the density is measured at a wave length just below the absorption peak of dye “B” (0.58 µ, instead of 0.61 µ), the D-vs-E curve would be linear over much of the range, as shown by the dashed curve on the log-log plot. The upper end of the range was limited by radiation damage to the plastic matrix. The main disadvantage of this system for dosimetry is the limitation of response to the high dose range (0.1 to 100 MR). Some important advantages are stability, quality and rate independence of response, high spatial resolution, and ease of handling in various media. Potential applications in radiation measurements include dose distribution studies in phantoms and at interfaces, sterilization processing dosimetry, and measurements of absorbed doses of mixed radiations.