Nathaniel S. Edwards

Monte Carlo simulation of energy deposition by neutron reaction products in lithiated foam using dynamic path generation

Foam materials saturated with neutron sensitive compounds are of interest as a viable replacement for 3He-based neutron detectors. Previous studies have shown the feasibility of 6LiF-saturated foam detectors as a valid replacement both experimentally and theoretically. However, the random geometry of foam material has previously limited the effective theoretical considerations for such materials to generalized approximations of detector efficiency based on the effective amount of neutron-sensitive material and effective charged-particle ranges. Neutron transport and subsequent charged particle energy deposition have been simulated using novel Monte Carlo methods in order to develop reaction-product pulse-height spectra, to determine intrinsic thermal-neutron detection efficiency, and to optimize material properties for open-cell foam neutron detectors. The theoretical maximum intrinsic thermal-neutron detection efficiency of a 2-inch diameter, 27.5% 6LiF saturated, open-cell foam neutron detector was found to be 39.48% +/- 0.06% (assuming 100% charge extraction from the bulk foam) using observed material properties. The pulse-height spectra and intrinsic neutron detection efficiencies determined from Monte Carlo simulations have been confirmed by comparison to experimental pulse-height spectra. Intrinsic thermal-neutron detection efficiencies of up to 65.90% +/- 0.08% can be obtained for 2-inch diameter, 27.5% 6LiF saturated, open-cell foam by optimizing the foam strut dimensions.

Lithium foil gas-filled neutron detector using microstrip electrodes

Microstrip electrodes have been fabricated and combined with one and five suspended 6Li foils positioned within a pressurized, gas-filled chamber to create a suspended foil microstrip neutron detector. This new detector offers a mechanically and electrically robust alternative to multi-wire proportional counters. Incident neutrons are converted into charged-particle reaction products that ionize the backfill gas. Charge carriers produced from the ionization of the backfill gas drift toward their respectively-charged electrodes due to the influence of the electric field formed from the potential difference between the drift electrode and the microstrip electrode anode and cathode strips. Gas multiplication occurs as electrons approach the surface of the microstrip electrode resulting in an increase in signal amplitude. Suspended foil microstrip neutron detectors containing one and five suspended 6Li foils were simulated using MCNP6 and compared to experimental results. The measured count rates from a moderated 26-ng 252Cf source positioned 18 cm from microstrip neutron detectors equipped with one and five suspended 6Li foils were 3.25 ± 0.04 and 10.62 ± 0.14 counts per second, respectively. The intrinsic thermal neutron detection efficiency of each detector was 4.02 ± 0.04% and 14.58 ± 0.11% for one and five suspended 6Li foils, respectively.

Charge propagation through- and neutron sensitivity of- reticulated vitreous carbon foam

Several potential neutron conversion materials have been studied over the past several years due to the 3He shortage. One candidate neutron conversion material is reticulated vitreous carbon (RVC) foam which can function as a coated substrate and is also suitable for high temperate environments. However, one concern with the material is the charge carrier propagation characteristics through the bulk of RVC foam. Electron propagation through the bulk of RVC foam samples was studied by comparing the resulting count rates and peak channel locations, with and without a sample present, for samples with linear pore densities ranging from 5-100 pores per linear inch (PPI). Count rates and pulse-height spectra from charge carriers generated by collimated 5.48 MeV 241Am alpha particles were studied and are reported here. The observed count rate and pulse height peak locations indicate that some electrons are able to drift through the 5 PPI sample. The peak channel locations with and without the 5 PPI sample were 230 and 360, respectively. However, all other linear pore density samples tested yielded much lower pulse heights, indicating loss of induced charge. Additionally, the intrinsic thermal neutron detection efficiency of a 10B4C-coated 45 PPI RVC foam sample was 3.23 ± 0.05%, indicating that the 10B4C-coated sample essentially functioned similar to a thin-film-coated device. Finally, the coating thickness of the 10B4C coating layer was measured using a SEM to be 1.29 ± 0.47 μm.

Current status of aerogel as a neutron converting material

Due to the recent 3He shortage, numerous alternative neutron conversion materials have emerged as potential 3He replacements. One such material is lithium-borosilicate aerogel which, unlike substrates coated with a neutron conversion material, is composed of the neutron conversion materials (Li and B) which are contained within the aerogel structural matrix. Additionally, silica-based aerogels can be utilized in high-temperature environments due to the materials high melting temperature range between 700-1000°C. Two different types of geometries of lithium-borosilicate were produced by Aerogel Technologies™ for a feasibility study in the use of aerogel as a neutron-conversion material. Neutron sensitivity testing of the samples was performed using a moderated-252Cf source and the resulting count rates, relative to background, indicate that the samples were sensitive to, and able to detect, neutrons. Additionally, simulations were performed using MCNP6 to define the optimal composition of a lithium-borosilicate aerogel sample containing macrochannels incorporated into the bulk of the sample. The resulting maximum theoretical thermal neutron detection efficiency was 45.2% for a sample containing 50% 6Li, 5% 11B, 30% O, and 15% Si.

Purification and crystallinity results from LiZnAs and LiZnP semiconductor materials

Nowotny-Juza compounds continue to be explored as candidates for solid-state neutron detectors. Such devices would have greater efficiency, in a compact form, than present day gas-filled 3He and 10BF3 detectors. The 6Li(n,t)4He reaction yields a total Q value of 4.78 MeV, larger than 10B, an energy easily identified above background radiations. Hence, devices fabricated from semiconducting compounds containing either natural Li (nominally 7.5% 6Li) or enriched 6Li (usually 95% 6Li) may provide a semiconductor material for compact high efficiency neutron detectors. Starting material was synthesized by preparing equimolar portions of Li, Zn, and P sealed under vacuum (10-6 Torr) in quartz ampoules lined with boron nitride and subsequently reacted in a compounding furnace. A static vacuum sublimation in quartz was then performed to remove the volatile impurities from the synthesized material. Samples from the sublimation process were analyzed by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), which showed that binaries and unreacted materials were sublimed out of the ternary material. Bulk crystalline samples were grown from the purified material. Individual samples were characterized for crystallinity on a Bruker AXS D8 DISCOVER, high-resolution x-ray diffractometer with a 0.004° beam divergence. The (220) oriented LiZnAs and LiZnP samples yielded rocking curves with a 0.235° full width at half maximum (FWHM) and 0.417° FWHM respectively.

Charge collection efficiency mapping of a Frisch-collared BiI3 device

Charge collection efficiency (CCE) mapping was performed by simulating a 5.0 × 5.0 × 10.0 mm³ Frisch-collared BiI₃ gamma-ray detector with an applied bias of 4800V and comparing the results to those previously published for a 4.7 × 4.7 × 9.5 mm³ Frisch-collared CdZnTe device with an applied bias of 1200V. The BiI₃ mobility-lifetime products used for CCE mapping were μ_eτ_e = 9.5 × 10^-6 cm² V^-1 and μ_hτ_h = 1.0 × 10^-7 cm² V^-1 for electrons and holes, respectively. The simulations were performed using the modified form of the Hecht equation and the necessary weighting-potential and weighting-field distributions were simulated using the commercially available software package COULOMB©. After comparing the simulation results to CdZnTe, the applied bias and mobility-lifetime products were adjusted separately until the Frisch-collared BiI₃ device resembled the CCE of the CdZnTe Frisch-collared device. Using the previously stated mobility-lifetime products, an applied bias of 2.1 MV is necessary to achieve similar CCE as that of CdZnTe at 1200V. Likewise, an improvement of three orders of magnitude (μ_eτ_e = 9.5 × 10^-3 cm² V^-1 and μ_hτ_h = 1.0 × 10^-4 cm² V^-1) to the BiI₃ mobility-lifetime products is required to operate the device at an applied bias of 4800V and approximately match the CCE of CdZnTe at 1200V.