Abstract

A simulation study was carried out in order to investigate the enhancement of the fast neutron spectra in a sample irradiated near the core qf a pool type nuclear reactor. The irradiation device consisted of a lead hollow cylinder where a thin aluminium sample holder containing the sample to be irradiated is placed. Calculations were performed considering water or beryllium + water between the simulated fission neutron source and the irradiation device; the thickness of 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. Φ(E>0.1 MeV), the fast neutron fluence, Φ(E>1 MeV), and the conversion factor, C = Φ/Φ, were determined. It was found that the lead cylinder surrounding the sample induces an 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 irradiation device, G =Φ/Φ(t =0). The gain of the neutron fluence increases from 2 to 4.5 when the lead thickness varies between 2 and 10 cm.

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