Photon Flux Monitor for a Mono-energetic Gamma Ray Source

A novel photon flux monitor has been designed and tested for use at the Duke University High Intensity Gamma Source, where the photon beam produced is essentially mono-energetic but it is not tagged. Direct counting of the number of photons using a high-efficiency detector is not possible because of the high photon fluxes expected. Therefore, a direct counting detector with a low, accurately known efficiency was required. The photon flux monitor based on a five scintillator paddle system detects recoil electrons and positrons from photoelectric, Compton and pair-production processes. It has been designed to be insensitive to gain and detector threshold changes and to be usable for photon energies above 5 MeV. It has been calibrated using direct counting with a NaI detector and its efficiency has been shown to be well predicted by a GEANT4 simulation. Results of measurements, calibration and calculations required to characterize the 5-paddle photon flux monitor are presented. The photon flux monitor has met its design specifications of being able to determine the number of photons incident on it during the live time of a measurement to within a systematic error of 2%. A paper based on the work for this thesis has been published in the Nuclear Instruments and Methods in Physics Research Journal. [Pyw09]