Development of a Simulation Methodology for Neutron Multiplicity Analysis

The objective of this work is to benchmark the use of the MCNP-PoliMi code for the simulation of neutron multiplicity measurements and the calculation of singles (S), doubles (D), and triples (T) rate parameters. To establish this capability, measurements of five different fissile samples were performed using a Canberra JCC-51 active well coincidence counter (AWCC) operating in passive mode at the PERLA laboratory of the Joint Research Center in Ispra, Italy. These experiments were then simulated using MCNP-PoliMi. A specialized post-processing algorithm was developed to apply the AWCC system dead time effects to the simulated data and to calculate the neuron multiplicity distribution. To establish the accuracy of the simulation, the S, D, and T rates were calculated from the simulated neutron multiplicity distributions and compared to the values for the measured data. The S, D rates were predicted within 6% of the measured values. The T rates demonstrate an over-prediction bias for all of the cases that were simulated. These results demonstrate that MCNP-PoliMi is capable of accurately predicting the neutron multiplicity distributions as well as the S, and D rates, but tends to over-predict the T rates.