Neutron Transport Calculations Using Monte-Carlo Methods

Fusion reactions from direct-drive inertial confinement fusion experiments at the Laboratory for Laser Energetics (LLE) release large amounts of stored nuclear energy. Most of it is given off as energetic neutrons. Plastic scintillators are used as detectors for these neutrons. The neutrons that interact with the scintillator produce photons that are recorded using a photomultiplier tube on a fast oscilloscope. The detector has a lead shield used to block x-rays that would flood the detector with energy. The neutrons travel throughout the target area interacting with several structures, such as the chamber wall, that significantly affect the neutron transport. In order to understand the effects of the target chamber and lead shield on the detected neutron signal, a Monte-Carlo simulation program (Geant4) was used to simulate neutron interactions. Geant4, a toolkit designed to calculate particle interactions with matter, is used predominantly in high-energy, nuclear, and accelerator physics. By changing the geometric dimensions and material properties of the structures in the target area, effects that cannot be tested experimentally have been simulated. It was found that the lead shield and the target chamber have a minimal effect (< 1%) on the full width at half maximum (FWHM) of the neutron signal.