Monte Carlo calculation of fast neutron spectra inside a lead hollow cylinder

A simulation study was carried out in order to investigate the enhancement of the fast neutron spectra in a sample irradiated near the core of a pool type nuclear reactor The irradiation device consisted of a lead hollow cyli nder where a thin aluminium sample holder containing the sample to be irrad iated is placed. Calculations were performed considering water or beryllium + water between the simulated fission neutron source and the irradiation d evice; the thickness of the the wall of the lead cylinder was varied up to 10 cm. The fast neutron spectra in the sample position were calculated and the average fast neutron energy, the total neutron fluence, phi (E> 0.1 MeV ), the fast neutron fluence, Phi (E>1 MeV), and the conversion factor, C = phi/Phi, were determined. It was found that the lead cylinder surrounding t he sample induces art appreciable enhancement of the fast neutron spectra. This effect can be described using a gain factor defined at each point, as the ratio between the fast neutron fluences with and without the irradiatio n device, G = Phi/Phi(t = 0). The gain of the neutron fluence increases fro m 2 to 4.5 when the lead thickness varies between 2 and 10 cm.