Monte Carlo Study to determine the Neutron Flux spectrum for a water Phantom: Preliminary Results

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

2. Introduction • In neutron therapy facility, neutron beam are used to irradiate cancer cells. • Consequently, prior knowledge of the neutron flux is required for calculation of absorption dose in human tissue. • During previous experimental studies, neutron flux 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 flux spectrum as an alternative technique. • The Monte Carlo neutron Transport code Mcnpx Was used for Simulations.

3. 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

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

5. Material and Methods • Reaction rates for bare foils inside a water phantom were calculated using Mcnpx. Fig.4. Activation foils positions inside a water phantom

6. 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.

7. 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

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

9. conclusion • The unfolded spectrum follows the same trend 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.

10. Acknowledgements • Manus & Matsci • E-HCD & KG • Immanuel Mulaudzi • Vusi Masondo THANKS TO YOU ALL ....