Characterization of neutron detection efficiency in a liquid organic scintillator

Abstract: Neutron detectors in the form of liquid organic scintillators are widely used for nuclear disarmament purposes. The main purpose of this thesis is to characterize the neutron detection efficiency of the organic liquid scintillator EJ-309 using Monte Carlo based simulations in the simulation program Geant4. The Monte Carlo simulation code was benchmarked using recorded light output data from a laboratory experiment and it was confirmed that the simulation code could reproduce data from this experiment. In order to characterize the efficiency of the neutron detector, the benchmarked code was used to simulate light output distributions of 241-Am-Be and Cf-252, an (α, n) and a spontaneous fission source respectively, where it was found that the two different types of sources could be distinguished. However, distinguishment between two different spontaneous fission sources was not possible, as simulations of the light output distribution of another spontaneous fission source, Pu-240, was similar to the one for Cf-252. The light output distributions from the simulations were compared to earlier research where good agreements were obtained. A quantitative study was also performed to determine how many detected neutrons were needed to properly be able to distinguish between the two sources 241-Am-Be and Cf-252, and the number was found to be 560. This value was considered low relative to the number of detected neutrons in the simulations, and hence, more quantitative studies are needed in order to verify this result. The absolute and intrinsic detection efficiency were also determined for varying threshold values of neutron energy.