Simulating Long Neutron Beamlines Using Duct Source Variance Reduction in Geant4

Abstract: The European Spallation Source, ESS, is set to become the world's brightest spallation neutron source. Spallation neutron sources present unique challenges with regards to radiation shielding due to the creation of a significant number of high energy neutrons during nuclear reactions in the spallation target. These can reach up to the energy of the driving proton beam. Performing Monte Carlo simulations of deep-penetration shielding problems is highly CPU intensive, and with beamlines at ESS reaching up to 150 m in length, it is a necessity to develop methods to utilize the available computing power as efficiently as possible. In this thesis a source biasing technique called a duct source is implemented in Geant4. The technique reduces variance by uniformly generating particles along the length of the beamline and altering the particle weights to keep the simulation physically valid. In addition to angular biasing for anisotropic neutron sources, energy biasing is introduced in order to better study the high energy component of the spectrum. Figures of neutron currents in beamline sections show that the duct source effortlessly transports neutrons arbitrary distances down a guide. A model problem consisting of a 77 m long beamline with multiple sections of shielding show that while analog simulations are effectively unable to populate areas outside the shielding, the duct source - particularly with energy biasing enabled - can provide neutron population in virtually the entire geometry within an acceptable time frame. Additionally, the duct source is coupled with an algorithm for automatic weight window generation, and simulations show the techniques to be complementary, with the duct source pushing particles down the beamline and the weight windows pushing them laterally toward the edges of the geometry.