Conceptual design of Passive Neutron Spectrometers using polyethylene and polymethylmethacrylate moderators

Two passive neutron spectrometers were designed using Monte Carlo methods. Each spectrometer utilizes six 20.5 cm-diameter cylindrical moderators of varying thicknesses (0.5, 4.5, 8.5, 12.5, 16.5, and 20.5 cm). One spectrometer employs polyethylene moderators, while the other uses polymethylmethacrylate. Thermal neutrons are measured using pairs of thermoluminescent chips with different ⁶Li content (TLD600 and TLD700). Responses to γ-rays of these TLDs and are approximately the same because have the same elemental composition, the same effective atomic number and similar densities. For both spectrometers, fluence response functions and absorbed doses from neutrons and secondary γ-rays were estimated for 45 monoenergetic neutron energies ranging from 10⁻⁹ to 20 MeV. As monoenergetic neutrons are moderated to thermal energies, they reach the TLDs with an energy distribution, inducing ⁶Li (n, α) reactions that define the fluence response function. Energy deposition from these neutrons and γ-rays within the TLDs allows for absorbed dose calculation. The fluence response matrices are similar for both moderator materials; however, neutron absorbed dose is consistently higher in TLD600 than in TLD700, independent of the moderator. Conversely, γ-ray absorbed dose is similar across TLD types and moderators. These spectrometers offer a smaller size and lower weight compared to a passive Bonner sphere spectrometer, while also being easier to construct and more portable.