AB-BNCT Beam Shaping Assembly based on 7Li(p,n)7Be reaction Optimization

D.M. MINSKY; A.J. KREINER; A.A. VALDA

Buenos Aires

Congreso; 14th International Congress on Neutron Capture Therapy; 2010

International Society for NeutronCapture Therapy

Presentación Oral  (D.M. Minsky)Introduction: In the framework of Accelerator Based – Boron Neutron Capture Therapy (AB-BNCT) the 7Li(p,n)7Be reaction is the optimal reaction for neutron production. Since it is endothermic, the resulting neutron energy spectrum can be chosen to be as soft as desired by approaching the reaction threshold. On the other hand larger proton beam energies lead to larger yields. The optimal energy will be a balance between the low primary neutron energies and the moderating possibilities associated with a larger neutron yield. A numerical optimization of a beam shaping assembly (BSA) based on this reaction has been done varying the dimensions of the BSA and the proton energy. Proton energies from 2 up to 2.8MeV have been analysed.  Materials and Methods: Neutron energy spectra and angular distributions for the 7Li(p,n)7Be reaction on different target materials have been calculated based on the newest double differential cross sections and stopping powers available. A simple cross section model was adopted for near threshold energies in order to avoid numerical problems. Two reaction channels have been considered: one going to the ground state of 7Be with a 1.88MeV threshold and the other to its first excited state at 429keV whose threshold is 2.37 MeV. All calculations have been done with an analytical and numerical mathematical code and the generation of matrices and MCNP input cards has been setup with PERL scripting. Although these PERL scripts are capable of generating sources for both thick and thin targets and for different target materials, only a thick lithium fluoride target was used in the optimization. A Snyder head phantom has been considered and the BSA was optimized for deep tumor locations. The moderator consists of a cuboid formed by successive layers of PTFE, aluminum and lithium carbonate surrounded by a lead reflector. A thin acrylic layer has been added for further moderation and an enriched 6Li layer was used at the beam port for thermal neutron shielding. Several parameters were scanned in the process of optimization: the target to port distance (from 15cm to 45 cm in 5cm steps), the target to the back of the moderator distance (1cm, 5cm and 10cm), the moderator width (from 5cm up to 30cm in 5cm steps) and the proton energy (2.0MeV to 2.8MeV in 0.1MeV steps); this renders more than a thousand configurations. All the numerical simulations have been performed with the MCNP 5.140 code. The inputs for MCNP, MCNP control, the tallies collection and the analysis were done with PERL, AWK and Bash scripts. For the selection of the best configuration, the one with better depth range of treatable tumors was chosen with the constraint that the treatment time must be lower than 45 minutes for a beam of 30mA.