Thermalization of accelerator-produced neutrons in a concrete room

We investigated a thermalization of neutrons, which were produced by an accelerator in a concrete room, by experiments and calculations. It was clarified that the widely used simple empirical formula phi th = c x Q/S, where Q is the neutron source intensity and S is the total surface area of a room, gives about one-third the underestimated value to our experimental and calculated results and the coefficient, c, is not a constant, but dependent on the source neutron energy.