3He ionization chambers as neutron beam monitors for the NPDGamma experiment

NPDGamma is a low energy nuclear physics experiment that is in the final stages of preparation for a high precision measurement of Aγ, the parity-violating correlation between gamma-ray momentum direction and neutron spin direction in the reaction n→-n + p → d + γ. Of crucial importance to this experiment are the beam monitors, 3He ionization chambers that monitor the apparatus by observing the flux of the polarized pulsed neutron beam. In this thesis, it is verified that the beam monitors are simple, reliable, low noise detectors that are very effective at monitoring the rate of neutrons incident on them. These properties allow them to provide continuous knowledge of the beam polarization throughout an asymmetry measurement by observing the time of flight-dependent beam flux downstream of the beam polarizer. The 3He content of these beam monitors was measured, and a Monte Carlo calculation was performed to determine the average amount of energy deposited in the chambers per detected neutron. It was verified that the beam monitor signals can be interpreted to reproduce the known time of flight dependence of beam flux from the neutron source, and that the neutron beam polarization can be measured at the 2% level from direct measurements of the transmission of the beam through the beam polarizer. An absolute calibration of the beam monitors was attempted by comparing the beam monitor signals to a Monte Carlo calculation of the beam flux. This calibration, which is not important to analysis for the NPDGamma experiment, gave rise to a discrepancy that will require further measurements in order to be resolved. (author)