Polyethylene density uncertainty in the simulation of neutron detectors

• Published in: Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment Vol. 1046; p. 167667
• Main Authors: Weinmann-Smith, Robert K., Arellano, James, Natzic, Robert A., Swinhoe, Martyn T.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 11.01.2023
• Subjects: Active well coincidence counter (AWCC); High density polyethylene (HDPE); International safeguards; Monte Carlo N-particle® (MCNP®) simulations; Neutron coincidence counting; Uncertainty; International safeguards; Uncertainty; Monte Carlo N-particle® (MCNP®) simulations; Active well coincidence counter (AWCC); High density polyethylene (HDPE); Neutron coincidence counting
• ISSN: 0168-9002; 1872-9576
• DOI: 10.1016/j.nima.2022.167667

Simulations with the Monte Carlo N-Particle® (MCNP®) code are frequently used to study neutron coincidence counters for international safeguards. Reducing sources of uncertainty enables more accurate simulations, which reduces the need for expensive or impossible measurements. Accurately specifying the density of polyethylene is crucial for accurate simulations. The density of polyethylene ranges from 0.88 to 0.97 g/cm3 and high-density polyethylene (HDPE) ranges from about 0.944 to 0.965 g/cm3. The density of a specific detector should be measured exactly for the best simulations. The density of the blue/white Active Well Coincidence Counter (AWCC) bulk polyethylene was measured to be 0.9594 +- 0.0013 g/cm3 at 1-sigma confidence. The density was calculated from repeated measurements of the various polyethylene dimensions for volume, and weighing the polyethylene for the mass. The AWCC neutron detection efficiency was simulated for each polyethylene density. The relative range of efficiencies (3σ) was 3.4% for unspecified polyethylene, 0.9% for HDPE, and 0.3% for the measured density. The impact of modeling small features such as screws was also studied and was found to be a negligible 0.2%. Measuring polyethylene density can reduce its role in uncertainty from one of the largest to one of the smallest contributors.