Physics Department Seminar
Dr. Evan Rand
Applied Physicist
Canadian Nuclear Laboratories
Tuesday, March 19, 2019


High-performance computation and the simulation of (pesky) cosmic-ray and gamma-ray induced secondary neutrons

Simulation is now an integral part, and undeniably vital scientific-tool, in all areas of physics. The most widely applied simulation method within the nuclear and particle physics community is the Monte Carlo method. Monte Carlo simulation allows researchers to “solve” complex problems which are analytically intractable or simply too time-consuming or costly to perform in the laboratory. In this talk I present two such cases on the simulation of secondary neutrons and illustrate how high-performance computation can be applied to investigate quantities that are typically too difficult to extract from experimentation.
The first case examines cosmic-ray induced neutrons within the DEAP-3600 experiment, a state-of-the-art dark matter detector comprising of 3600 kg of liquid argon. The detector is located 2 km below the surface of the earth at the SNOLAB facility in Sudbury, Ontario. At this depth, background events originating from cosmic-rays are significantly reduced, therefore enabling the measurement of extremely rare interactions; such as the interactions theorized between ordinary matter and dark matter. To achieve this level of sensitivity, the detector is housed within a large ultra-pure water tank which serves as a radiation shield and a Cerenkov veto detector for cosmic-ray muons. Although rare, secondary neutrons are produced within the water tank and surrounding rock via cosmic-ray muon interactions. These secondary neutrons can mimic dark matter interactions within liquid argon, and therefore understanding their impact is paramount. In this work we characterize the efficiency of the DEAP-3600 cosmic-ray muon veto system and calculate the probability of these secondary neutrons to enter the liquid argon detector.

The second case studies the dynamic behaviour of heavy-water reactors through the lens of photoneutrons, secondary neutrons produced via photonuclear interactions between deuterium and high-energy gamma-rays from the beta decay of fission products. The production of these neutrons are “delayed” with respect to the initial fission event and generate a time-dependence in the neutron flux. Understanding and predicting this time-dependence and the dynamic behaviour of reactors in general is critical for reactor safety. In this work we apply the Geant4 toolkit and recent nuclear data to calculate the photoneutron yield from the thermal fission of 233,235U and 239Pu within a quasi-infinite bath of D2O. Large discrepancies were observed between the simulation results and the recommended 235U photoneutron yields used in reactor kinetics calculations and applications. The results raise concerns on the validity of the yields derived from experimental data. In this talk I will present our results and discuss possible explanations for the observed disagreement with historical data.