Performance of NB-CTMFD Detector Versus Ludlum 42-49B, and Fuji NSN3 Detectors for Hard (Am-Be) and Soft (Cf-252 Fission) Energy Spectra Neutron Sources Within Lead/Concrete Shielded Configurations

Abstract

This paper presents the results of neutron detection efficiency and dosimetry between a nonborated centrifugally tensioned metastable fluid detector (NB-CTMFD) configured to detect fast (>0.1 MeV) versus two widely used combined thermal and fast neutron detection devices of similar form factors: moderated He-3 based Ludlum-42-49BTM neutron detector, and, the Fuji NSN3TM pressurized nitrogen-methane filled neutron detector. The one-on-one performance comparisons were conducted using hard (Am-Be) and soft (Cf-252 fission) neutron spectra isotope neutron sources positioned behind various levels of lead and concrete shielding ranging in thickness from 0 cm (unshielded) to up to 30 cm. The comparisons were conducted together with Monte Carlo N-Particle (MCNP) code simulations to account for 3-D effects and to relate the detection sensitivity with the dose rate. The MCNP model results for the neutron energy spectrum were validated versus experimental measurements using the H*TMFD. While the Ludlum and NSN3 detectors operate at a fixed sensitivity setting, the NB-CTMFD detector could be operated at various sensitivity levels by varying the tensioned metastable negative pressure (Pneg) from 0.3 MPa to 0.7 MPa. The NB-CTMFD (configured for fast: >0.1 MeV neutron detection at Pneg = 0.7 MPa) offered relative sensitivity enhancements of up to 15x greater than the Ludlum and ∼5x greater than the NSN3 detector for low shielding thicknesses. For larger thicknesses, the advantage factor for NB-CTMFD unit reduces with increased fast neutron removal via down-scattering, which benefits the Ludlum and NSN3 detectors. Our companion paper compares performance with a Cf-252 spectrum using a borated CTMFD (covering dual energy bins: thermal-epithermal and fast energy ranges) wherein, it is demonstrated that the advantage factor for the CTMFD (if borated) remains elevated at the 5–10x higher level for low and large shielding thicknesses and soft and hard neutron spectra.