Monte Carlo for the NCD phase of the Sudbury Neutrino Observatory

Abstract

In the third phase of the Sudbury Neutrino Observatory (SNO) experiment an array of 3He proportional counters was added deployed in the heavy-water volume of the SNO detector. This Neutral-Current Detection (NCD) Array detected the neutrons from the neutral-current interaction of 8B solar neutrinos with deuterium. Before we can determine the neutrino flux we must separate the neutron-capture pulses from pulses due to alpha particles and instrumental backgrounds. We have created a unique, detailed simulation of the current pulses from the proportional counters that includes energy straggling, ion drift, electron diffusion, space charge, and electronics effects. We have conducted extensive studies to determine the accuracy of the simulation. In the solar neutrino analysis the NCD Monte Carlo is used to determine the energy spectrum of the alpha background, as well as the applicable systematic effects. In the near future it will be used to fit the data pulses to separate neutron-capture and alpha pulses. With this pulse-shape analysis method the differences between pulse characteristics can be associated directly with the physical mechanisms of track formation and charge motion in the counter gas.