Preliminarily results of Monte Carlo study of neutron beam production at iThemba LABS University of the western cape and iThemba LABS Energy Postgraduate Conference 2013 Presenter : Mulaudzi Immanuel Supervisor: Mark Herbert Co-supervisor : Peane Maleka

Introduction At neutron beams facility of iThemba LABS, fast neutron beams are produced by bombarding two targets 9 Be and 12 C with protons beams of energy up to 200 MeV. Neutron beams produced require well-characterization in terms of their fluence spectra, since neutron interactions cross sections are energy dependent [1]. Fast neutron beams are widely used in radiation therapy for cancer treatment, radiation protection for detector calibration and basic research in nuclear physics for neutron cross section measurement [2]. In principle, these fluence spectra can either be calculated by Monte Carlo Methods or measured experimentally [2]. In this work it was decided to use the Monte Carlo Code MCNPX to investigate neutron production at IThemba LABS because it had been widely used and validated in the energy range of interest. 1. TLABS, IThemba laboratory for Accelerator based Sciences M.S Herbert, F.D. Brooks, M.S Allies, A. Buffler, M.R. Nchudu, S.A. Makupula, D.T.L Jones and K.M

Neutron beam facility Figure 1. shows neutron beam facility were fast neutron beams are produced by the 9 Be (p, n) 9 B reactions. Targets, Carbon and beryllium Target thickness 5 mm and 10 mm Emission angles 0, 4, 8, and 16 degrees. TLABS, IThemba laboratory for Accelerator based Sciences.

Monte Carlo calculations methods Monte Carlo codes MCNPX requires input for the geometry, collision physics and tallies specifications. Structure of the input files Title card {optional} Cell cards { Block 1} blank line delimiter Surface cards {Block 2} blank line delimiter Data cards {block 3} (collision physics) End MCNPX Input geometry Judith F. Briesmenister, Editor, MCNP A General Monte Carlo N-particle Tranport code LA M version 4B Manual.

Simulation of neutron production In this work Monte Carlo code MCNPX was used to investigate neutron production by different targets either 9 Be (10.0 mm) or 6 C (10.0 mm), for fast neutron beams facility at iThemba LABS. For all simulations in this work, it was decided to use pencil beam for good results. Neutron production was investigated in terms 1. Proton neutron ratio 2. Fluence spectra as a function of 2.1. Target thickness (5 mm and 10 mm) 2.2. Incident proton energy (66 MeV and 100 MeV) 2.3. Angular distribution ( 0 0 and 16 0 )

Results and analysis 1. Neutron proton ratio Neutron production ratio obtained by simulating the interaction of proton beams with two different targets beryllium ( 9 Be) and carbon ( 6 C) as presented in table1. For this work simulation were performed for equal time. Table 1. Targets, energies, neutron proton ratio, number of incident protons and produced neutrons Comparing ratios from the table1, production ratio of beryllium is higher than of carbon. Target with higher z produce more neutron than with lower z. Increase in the incident proton energy also increase the production of neutron on the target of same thickness.

Results and analysis cont.… fluence spectra as a function of energy Figure 2. (a) and (b) shows the effects of protons beams energy on the 9 Be and 6 C targets of 10 mm thickness at zero degree emission angle with respect to the beam axis. Each spectrum has high energy peak results from ground transition state and the first excited state. The low energy continuum on the spectrums results from the breakup reaction and secondary neutrons.

Results and analysis cont.…… 2.2 fluence spectra as a function of angles Figure.3. shows the effects of emission angle on the fluence spectra of 9 Be (10 mm) target. Neutron fluence decrease with increase emission angle especially around on the high energy peak. Zero degrees has high energy peak but it decrease at sixteen degrees and shift to lower energy due to more neutrons with high energy at zero degrees. Spectrum shows similar results for 6 C (10 mm).

Results and analysis cont.… fluence spectra as a function of thickness Figure 4. shows the effect of target thickness on the fluence spectra of 6 C targets. For thicker target high energy peak is broadened and its average energy shifted towards lower neutrons energy. There is a greater energy loss of protons in the thick target. Increase in target thickness also increases neutron fluence. Spectrum shows similar results for 9 Be targets.

Conclusions Target with higher z has high neutron production ratio than with low z, this is because of higher neutron proton ratio of 9 Be than of 6 C target. Neutron production was increased by Increasing energy of incoming proton beam, this results in narrow high energy peak on the spectrum. High energy peak is a results of neutron from ground transition state and the first excited state. Neutron fluence was found to be decreasing as increase in emission angles because most of neutrons with high energy are on zero degrees. Results obtained shows that, the greater energy loss of protons on a thick target results in broader peak shifted to lower energy region. In future work the effect of collimation on a spectra will be investigated

Acknowledgement SUNHARP iThemba LABS UWC Team members - Andrew Esau - Vusimusi Masondo - Mathews Makhubela