MONTE CARLO SIMULATIONS OF THE RESPONSE OF ESR DOSIMETERS TO NEUTRON BEAMS

Electron Spin Resonance (ESR) spectroscopy is extensively used in gamma photon dosimetry. It relies on the detection of the signal from free radicals (paramagnetic species) produced after sample irradiation. Organic materials, such as alanine and ammonium tartrate, are widely used for dosimeter preparation, thanks to the good photon sensitivity and tissue equivalence of such compounds. However, the low neutron cross section of their nuclei does not make them suitable for neutron dosimetry. Thanks to its very high neutron capture cross section and to the high Linear Energy Transfer of released particles, even small additions of gadolinium will yield large sensitivity enhancements of the dosimeters. However, its high atomic number strongly reduces the dosimeter tissue equivalence. Our recent results have shown, both experimentally and computationally (through Monte Carlo simulations), how the ESR response depends on additive concentration, for organic dosimeters added with gadolinium oxide and exposed to mixed fields of thermal neutrons and gamma photons. Since Monte Carlo simulations have proved to be a valuable tool for describing the neutron energy released inside the dosimeter, we have decided to extend our computational investigation to neutron beams with composite energy spectra and different irradiation setups, in order to simulate real Neutron Capture Therapy conditions. The Monte Carlo results presented here will be useful for planning our future experiments.