Beam Properties

Beam Properties

The neutron spectrum at the DUT (at 8 cm from the target) with 13MeV and 0.16mC proton beam has been calculated using MCNPX.

Without any moderate material, the most-fast neutron with energies above 0.1MeV has been produced: Intensities are approximately 1.5×1010 n/cm2 /sec. The thermal neutron with energies blow 1eV has been produced with 5cm thick polyethylene moderation: Intensities are approximately 0.5×1010 n/cm2 /sec. 1-MeV equivalent neutron fluence (øeq,1Mev,Si) and the neutron energy spectrum hardness parameter (Heq,1Mev,Si) for silicon have been calculated: the values are listed below. In order to adsorb the thermal neutron, cadmium cover has been used.

This graph shows the energy dependence of the neutron flux (from 1meV to roughly 10 MeV) expected, based on MCNP simulations, for three different conditions: No moderator (giving no thermal neutrons), with a bare Polyethylene moderator (which gives thermal neutrons and a reduced flux of fast neutrons), or a Cd covered moderator (to allow separation of the fast and thermal neutron effects). The fast neutron spectrum peaks near 5MeV and the thermal spectrum around 60 meV.
This figure shows the dependence of the fast and thermal neutron flux on the position of the DUT from the target from 8 through 14 cm (with the moderator in place). Fast flux falls from 8e9 n/cm2.s to 3e9 n/cm2.s and thermal flux from 3.4e9 n/cm2.s to 1.5e9 n/cm2.s over this range of distances.