A potential application to the study of microscopic energy deposition in asolid by means of heavy charged-particle induced photochromic alterations in a tissue-equivalent matrix

A theoretical study was carried out to investigate the feasibility of using the radiation-induced colour decay of photochromic molecules embedded in a polymer matrix as a probe for studying the microscopic energy deposition o f heavy charged particles (HCPs) in a tissue-equivalent solid. The theoreti cal treatment makes use of the radial dose distribution function as derived from gas-phase physics, together with the effects of the increase in tempe rature and of matrix degradation on the colour-decay kinetics of the photoc hromic molecules, according to empirical models derived for the solid state . Bearing in mind the non-stochastic nature of the model, the use of gas-ph ase physics at the level of radiation interaction, and the fact that some e mpirical quantities used have been established macroscopically, all factors which signify that extra caution is required in the interpretation of the results, it is shown that when the optimum information retrieval time (afte r track formation) is considered the technique may be able to resolve diffe rences in the energy deposition pattern by different HCPs in the nanometre range (I-IO nm; material's mass density 1.2 g cm(-3)) from the track axis. Most importantly, though, the present study aims to erect a theoretical fra mework for the possible application of the technique and to highlight those aspects which are likely to be critical to its practical usage, such as pa rticle type and energy range, and spatial scale and magnitude of the expect ed effect together with its dependence on time, the physical characteristic s of the matrix, and the kinetic behaviour of the type of photochromic mole cule studied. Furthermore, it establishes a rationale for interpreting the experimentally observed (if available) colour changes in the HCP track in t erms of the microscopic distribution of energy deposition in it.