A proposal for a new neutron moderator material for Accelerator-Based BNCT.

Burlon Alejandro, Kreiner Andres, Minsky Daniel1 and Valda Alejandro2 1Escuela de Ciencia y Tecnolog´ıa-UNSAM. Argentina// CNEA-Argentina 2Escuela de Ciencia y Tecnolog´ıa-UNSAM - Argentina

The use of nuclear reactions induced by accelerated particle beams, as neutron sources for Boron Neutron Capture Therapy (BNCT) appears as a competitive option to nuclear reactors. This technique involves the administration and selective uptake of the capture agent 10B (who has a thermal neutron capture cross section of 3840 b) in tumor cells and a suitable thermal neutron flux. The capture reaction 10B(n,α)7Li generates the heavy fragments α and 7Li that will damage the tumor cells. BNCT aims at treating pathologies that cannot be treated satisfactorily with conventional therapy (especially brain tumors). The neutron spectrum obtained from a nuclear reaction must be optimized using a moderator-reflector assembly before its application to a patient. It is important to get an epithermal neutron beam that termalizes inside the body in order to maximize the thermal flux at the tumor position. In the frame of a production-target optimization for accelerator-based BNCT, feasibility studies for a new moderator material have been performed. This material consists in successive stacks of Al, polytetrafluoroethylene (PTFE), commercially known as Teflon, and LiF. It is easy to build and its cost is relatively low. The interaction neutron cross section in Al and PTFE presents complementary resonances above 30 keV, which reduces the energy of the fast neutrons. On the other hand, the 6Li in the LiF captures the thermal neutrons in order to avoid high doses in the skin, where the boron uptake is greater than in the rest of the healthy tissue. An exhaustive Monte Carlo simulation study has been performed evaluating the doses delivered to a cylindrical water-filled head phantom by a neutron production Li-metal target based on the 7Li(p,n)7Be reaction. A 35.8 cm diameter lead reflector and a 7.5 cm diameter Al/PTFE/LiF moderator were considered. Several moderator thickness have been studied and the figures of merit show that it is possible to reach a 98% Tumor Control Probability for a 6.5 cm deep tumor with a 34 cm thick moderator and a 20 mA proton bean at a bombarding energy of 2.3 MeV in 50 minutes treatment time.