Retrieval and Characterization of a Plutonium-beryllium Source at the University of Utah

Abstract

A shielded neutron irradiation facility has been constructed to contain a 74 GBq plutonium-beryllium neutron source at the University of Utah. Due to lack of information and recent use, the GEANT4 Monte Carlo Toolkit was used to perform dose assessments and validation of the designed shield and beam port for storage and operational geometries where the source was to be placed. Borated polyethylene was used as the primary shielding material with a lead-lined beam port to collimate neutrons and reduce gamma rays within the active beam profile. The lead lining was found to reduce the gamma-to-neutron ratio within the profile to 0.259 ± 0.001 and maintain an average neutron flux of 4,633.7 ± 10.2 n cm⁻² s⁻¹. The measured dose rate of the source at 30 cm from its constructed storage configuration was found to be 22.3 ± 1.1 μSv h⁻¹ and in good agreement with simulated values. This was also close to the target rate of 20 μSv h⁻¹, the public limit for whole-body dose accumulation, imposed on the design. Measurements of the source during its retrieval and placement into the shield show good agreement with the simulated results and provide a strong validation for the future use of the source in experiments at the University of Utah.