Monte Carlo Study to determine the Neutron Fluence spectrum for a water Phantom: Preliminary Results University of the Western Cape Energy Postgraduate Conference 2013 Presenter: Makhubela Mathews Supervisor: Mark Herbert Co-Supervisor: Andrew Esau

Introduction In neutron therapy facility, neutron beam are used to irradiate cancer cells. Consequently, prior knowledge of the neutron fluence is required for calculation of absorption dose in human tissue. During previous experimental studies, neutron fluence spectra inside a water phantom were experimentally measured at iThemba Labs. The current study was undertaken to investigate the feasibility of using neutron threshold spectrometry to determine the neutron fluence spectrum as an alternative technique. The Monte Carlo neutron Transport code Mcnpx Was used for Simulations.

Material and Methods Seven foils shown in table.1 were chosen as activation detectors due to the availability of cross section data in Mcnpx libraries[1] [1] Zhonglu Wang, Nolan E. Herbert, and Eric Burgett, Fermilabs Neutron Therapy Facility Neutron Spectrum Determination by Threshold Foils. International Workshop ON Fast Neutron Detectors University of Cape Town, South Africa, April 3-6,2006

Material and Method Activation reaction selection criteria Fig.2. cross section plots for activation foils Fig.1. example of a decay scheme

Material and Methods Reaction rates for bare foils placed at different position in a water phantom were calculated using Mcnpx. Fig.4. Activation foil positions inside a water phantom

Response Matrix Calculations Each foil is characterized by a response as a function of incident energy, The response for each foil was calculated at different positions in a water phantom. The result were binned into 43 bines from 5.23 to 67Mev these was done using the FT special treatment card which bines the responses by incident energy groups.

Neutron spectrum unfolding The spectrum adjustment code Maxed[2] solves the inverse of the matrix shown in previous slide to obtain the spectral distribution. Maxed uses a priori information to choose the solution which best fits the activation data from a infinite number of solution. Herbert et al measurement were use as estimates for the a priori information. [2] M.REGGINATO, The ”Few-Channel” Unfolding Programs in the UMG package: MXD_fc33,GRV_FC33 and IQU_FC33,UMG package, Version3.3-release

Results Fig.5:Comparison of the unfolded spectrum and measurement, Measured Default spectrum.

conclusion The unfolded spectrum agree well with Measurement. Although it is clear that the shape of the final spectrum depends on the shape of the a priori spectrum. Introducing high energy threshold reaction in our simulations might improve the quality of the spectrum.

Acknowledgements Manus & Matsci E-HCD & KG Immanuel Mulaudzi Vusi Masondo