Radiation dose distributions in three dimensions from tomographic optical density scanning of polymer gels .2. Optical properties of the BANG polymer gel

A newly developed method of radiation dosimetry makes use of the optical properties of polymer gels. The dose-response mechanism relies on the production of light-scattering polymer micro-particles in the gel at each site of radiation absorption. The scattering produces an attenuation of transmitted light intensity that is directly related to the dose and independent of dose rate. For the BANG polymer gel (bis, acrylamide, nitrogen, and gelatin) the shape of the dose-response curve depends on the fraction of the cross-linking monomer in the initial mixture and on the wavelength of light. At 500 nm the attenuation coefficient (mu) increases by. approximately 0.7 mm(-1) when the dose increases from 0 to 5 Gy. The refractive index of an irradiated gel shows no significant dispersion in the visible region and depends only slightly on the dose. Turbidity difference spectra are compared with theoretical spectra of efficiency factors for total scattering, derived using Mie-Debye theory, and the average sizes of the cross-linked particles produced by radiation, as a function of dose, are established. The particle sizes increase with dose and reach approximately the wavelength of red light. The dependence of the particle sizes on cross-linker fraction parallels a similar dependence of the water proton NMR transverse relaxation rate dose response.