Michael A. Reichenberger

First-order numerical optimization of fission-chamber coatings using natural uranium and thorium

Presently, fission chamber technology utilizes neutron-sensitive coatings often composed of radioisotopes that are becoming increasingly difficult to obtain. Using only natural uranium and thorium, a maximized stable device lifetime in a constant neutron flux can be obtained by optimizing the regenerative fission chamber coating. A system of coupled differential equations was developed by considering neutron-induced fission, neutron absorption, and radioactive decay of 17 radioisotopes spanning the product family tree from 232Th to 241Pu. By solving the system of coupled differential equations, the interaction rate of the regenerative fission chamber coating can be found as a function of time (or total fluence) in a constant neutron flux. The Kansas State University (KSU) TRIGA Mark II nuclear reactor was used as a basis to study the effectiveness of mixed fission chamber coatings over time. By varying the initial composition of the fission chamber coating, an optimal composition of 232Th and natural uranium is capable of maintaining stable device operation (<;5% deviation from initial response) under a constant neutron flux of 2.2 × 1013 n cm-2 s-1 up to a total neutron fluence of 7.65 × 1022 n cm-2, corresponding to over 110 years.

Growth and preparation of LiZnP and LiZnAs for solid-state neutron detectors

Most gas-filled neutron detectors are difficult to manufacture as compact detectors. Boron-based compounds, such as BP, BN and BAs have shown limited success, and thus far do not appear promising due to problems with crystal growth and materials preparation. Although some Li compounds have been quite successful as neutron detectors, such as LiF thermoluminescent dosimeters and LiI scintillators, neutron detectors from Li-based semiconductor compounds have not been explored to a similar extent as B-based semiconductors. In the present study, crystals of the filled tetrahedral compound class (AIBIICV) of Li semiconductors were grown, known as Nowotny-Juza compounds, such as LiZnP and LiZnAs for investigation as high efficiency compact neutron detectors. Equimolar portions of Li, Zn and P or As were sealed under vacuum (10−6 Torr) in quartz ampoules with a graphite lining. The loaded ampoules were placed into a compounding furnace and raised to 560 °C to form the ternary compound, LiZnP or LiZnAs, and further annealed to promote crystallization. The synthesized compound was crushed into a fine powder and sealed in a tantalum ampoule by arc welding. The ampoule was mounted in a vacuum chamber with a background argon pressure of 300mTorr. The ampoule was connected to a low-voltage high-current power source where the LiZnP or LiZnAs was melted and slowly cooled to crystallization. The bulk crystals were extracted, then cut, and polished for further electrical and physical property investigations.

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.

Enhanced Micro-Pocket Fission Detector for High Temperature Reactors - FY17 FInal Project Report

In-core fission chamber design has remained relatively unchanged for decades. Improvements in performance, overall size, and operational modes have been implemented; however, all have been based on the same design that utilizes coaxial cylinders with a high-pressure fill gas. These design considerations limit the robustness, lifetime, size, and operational performance of such sensors in high-performance Material Test Reactor (MTR) environments.

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.

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.