Employment of MCNP in the study of TLDS 600 and 700 seeking the implementation of radiation beam characterization of BNCT facility at IEA-R1

Boron Neutron Capture Therapy, BNCT, is a bimodal radiotherapy procedure for cancer treatment. Its useful energy comes from a nuclear reaction driven by impinging thermal neutron upon Boron 10 atoms. A BNCT research facility has been constructed in IPEN at the IEA-R1 reactor, to develop studies in this area. One of its prime experimental parameter is the beam dosimetry which is nowadays made by using activation foils, for neutron measurements, and TLD 400, for gamma dosimetry. For mixed field dosimetry, the International Commission on Radiation Units and Measurements, ICRU, recommends the use of pair of detectors with distinct responses to the field components. The TLD 600/ TLD 700 pair meets this criteria, as the amount of 6Li, a nuclide with high thermal neutron cross section, greatly differs in their composition. This work presents a series of experiments and simulations performed in order to implement the mixed field dosimetry based on the use of TLD 600/TLD 700 pair. It also intended to compare this mixed field dosimetric methodology to the one so far used by the BNCT research group of IPEN. The response of all TLDs were studied under irradiations in different irradiation fields and simulations, underwent by MCNP, were run in order to evaluate the dose contribution from each field component. Series of repeated irradiations under pure gamma field and mixed field neutron/gamma field showed differences in the TLD individual responses which led to the adoption of a Normalization Factor. It has allowed to overcome TLD selection. TLD responses due to different field components and spectra were studied. It has shown to be possible to evaluate the relative gamma/neutron fluxes from the relative responses observed in the two Regions of Interest, ROIs, from TLD 600 and TLD 700. It has also been possible to observe the TLD 700 response to neutron, which leads to a gamma dose overestimation when one follows the ICRU recommended mixed field dosimetric procedure. Dose response curves were obtained for the distinct types of TLDs for pure gamma and mixed fields. This work recommends the TLD 600/TLD 700 pair methodology for mixed field dosimetry, due to present a better precision than the one based on TLD 400, however one has to be careful to avoid gamma dose super estimation. (author)