FriXy gel dosimeters for fast neutron dose in BNCT epithermal beams

BNCT thermal neutron beams, for their low penetration power, are useful only for shallow depth tumour treatments; the therapeutic benefits of neutron beams can be extended to deeper tissues (until depths of several centimeters) increasing their energy. To this purpose several BNCT facilities employ neutron beams with a relevant epithermal (0.5 eV – 10 keV) component; such beams, as far as the boron capture is concerned, produce a skin spare effect thermalizing in the tissues, but also cause secondary radiation. In fact, epithermal beams (which sometimes have a not negligible fast component) deposit energy in the traversed tissue without distinction between normal or tumour cells mainly as effect of the elastic interaction with the hydrogen nuclei, producing high LET protons. Therefore, beside the different dose components of a thermal neutron beam(gamma ray dose, proton dose from nitrogen capture, boron dose), also the dose due to elastic scattering of epithermal neutrons has to be evaluated. The well established method based on FriXy gel dosimeters, allowing the measurement of the different dose components in thermal neutron beams, has been improved in order to detect also the dose due to epithermal neutron elastic interactions. A FriXy gel, based on Fricke solution containing Xylenol Orange, changes its optical transmittance when irradiated, and its optical density is proportional to the absorbed dose. This work presents a method that employs at the same time gel dosimeters made of light and heavy water; it is possible to separate the gamma and the neutron elastic scattering doses inside the phantom comparing the optical densities and calculating with a Monte Carlo software (MCNP5) the ratio between proton and deuteron recoil energy for different incident neutron energies.