Investigation of the pulsed neutron source method measuring the fission decay properties of a subcritical assembly with low-Z reflectors

Abstract

The pulsed neutron source (PNS) method measuring the fission decay properties of a subcritical system with reflectors was analyzed numerically. Using the Monte Carlo code JMCT, a spherical subcritical assembly with a low atomic number (low-Z) reflector was modeled, and the neutron and photon transport process initiated by an instantaneous neutron source was simulated. The time dependence of the leakage photon count rate and the neutron populations in the fissile region and the reflector was obtained. It is indicated that the neutron population in the assembly is dominated by that in the reflector after a relatively short period, the length of which is mainly dependent on the reflector's thickness. That is to say, the neutron population in the assembly is more affected by the reflector than by the fissile region. The leakage photon count rate is approximately proportional to the neutron population in the fissile region during the specific time window of hundreds of nanoseconds after the source pulse. The fission decay properties of the fissile region can therefore be approximately derived from the leakage photon count rate based on the PNS method. The reason for the existence of the above time window was analyzed, and the effective time range of leakage photon count rate was discussed.