Design of an Extended Range Long Counter Using Super Monte Carlo Simulation.

We have designed an extended range neutron long counter on the basis of work optimized using SuperMC code. The problem of the existing traditional long counters is that their response function falls rapidly above 5 MeV. We proposed a new designed by adding two layers of converter material inside the polyethylene moderator. The relatively low density chromium and high density lead metals convert high energy neutron by (n, xn) spallation reaction. This produces more neutrons of lower energies, which have higher probability of being detected by thermal 3He-counter. The response function at lower neutron energies was improved by inserting small polyethylene cylinder in front of 3He counter. In this design we achieved to extent the flat response function of the long counter from few keV up to 150 MeV. The total fluctuation of response curve is less than ±9% over the entire energy range. The designed long counter is suitable to be used as neutron monitor for monitoring neutron fluence at high-energy neutron source.