Abstract
This paper presents the results of neutron detection efficiency and dosimetry between a borated centrifugally tensioned metastable fluid detector (B-CTMFD) configured to detect thermal to fast neutrons versus two widely used neutron detection devices of similar form factors: moderated He-3 based Ludlum-42-49B neutron detector, and, the Fuji Electric's NSN3TM (NNSN3) pressurized nitrogen-methane filled neutron detector. The one-on-one performance comparisons were conducted using soft (Cf-252 fission) neutron spectrum 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 with Monte Carlo N-Particle transport code (MCNP) code simulations to account for three-dimensional effects and to relate the absolute detection rate with the dose rate—to derive the sensitivity factor (cpm/μSv/h). While the Ludlum 42-49B and NSN3 detectors operate at a fixed sensitivity setting, the centrifugally tensioned metastable fluid detector (CTMFD) can be (and was) operated to detect and separate the contributions from epithermal (0.02 eV–0.2 MeV) and from fast (>0.2 MeV) neutrons at various sensitivity levels by varying the tensioned metastable negative pressure (Pneg) from 0.3 MPa to 0.7 MPa. The B-CTMFD (configured for detecting the full 0.02 eV to 12 MeV neutrons >0.1 MeV neutron detection at Pneg = 0.7 MPa) offered relative sensitivity enhancements of up to ∼22× greater than Ludlum and over 5× greater than NSN3, over the 0-15 cm range of Pb shielding, and the 0–30 cm concrete shield thickness. The contribution from detecting down scattered neutrons increases with increased thickness, especially for concrete shielding. The B-CTMFD design overcomes the detection penalty (up to 60% depending on shielding type and thickness) inherent in the nonborated centrifugally tensioned metastable fluid detector (NB-CTMFD), designed only to detect fast-energy neutrons—as described in the companion (Part-1) paper. However, unlike the NB-CTMFD, which used 100% decafluorapentane (DFP) (C5H2F10), the B-CTMFD requires the use of an azeotropic mixture of DFP, methanol, and tri-methyl borate (TMB—C3H9BO3, using natural boron) in 80:4:16 proportion. The B-CTMFD was about 6 times more sensitive than NB-CTMFD under the most heavily shielded condition and, taken together, also offered 2-energy bin neutron spectroscopic enablement, together with 22-5× higher absolute efficiency and relative sensitivity compared with the nonspectroscopic Ludlum (He-3) and NSN3 (methane-nitrogen) based detectors. From an intrinsic efficiency standpoint, the B-CTMFD operating at Pneg = 0.7 MPa state demonstrated ∼103× higher intrinsic efficiency over Ludlum 42-49B and NSN3 detectors, respectively.
