Eric Lukosi

Theoretical Maximum Efficiency for a Linearly Graded Alphavoltaic Nuclear Battery

This paper presents a study on the optimization of the amount of energy deposited by alpha particles in the depletion region of a silicon carbide (SiC) alphavoltaic cell using Monte Carlo models. Three Monte Carlo codes were used in this study: SRIM/TRIM, GEANT4, and MCNPX. The models examined the transport of 5.307-MeV alpha particles emitted by 210Po. Energy deposition in a 1-μm depletion region of SiC was calculated using an isotropic alpha source for a spherical geometry using GEANT4, and a monodirectional alpha source for a slab geometry using both SRIM/TRIM and GEANT4. In addition, an isotropic point source was modeled using GEANT4 and MCNPX for a slab geometry. These geometries were optimized for the maximum possible alphavoltaic energy efficiency. The models, which match very well, indicate that the maximum theoretical energy conversion efficiency, which was optimized for a SiC alphavoltaic cell, is [approximately]3.6% for the isotropic alpha source on a slab geometry and 2.1% for both the monodirectional alpha source on a slab geometry and the isotropic alpha source at the center of a sphere. This study provides a useful guide governing the upper limit of expected efficiency for an alphavoltaic cell using a linearly graded single junction SiC transducer.

Theoretical Maximum Efficiencies of Optimized Slab and Spherical Betavoltaic Systems Utilizing Sulfur-35, Strontium-90, and Yttrium-90

Monte Carlo simulations have been used for calculating the energy deposition of beta particles in the depletion region of a silicon carbide (SiC) betavoltaic cell along with the corresponding theoretical efficiencies. Three Monte Carlo codes were used in the study: GEANT4, PENELOPE, and MCNPX. These codes were used to examine the transportation of beta particles from 90Y, 90Sr, and 35S. Both the average beta energy from each source and the entire spectrum were modeled for calculating maximum theoretical energy deposition in both a spherical and slab geometry. A simulated depletion region was added in postprocessing containing the maximum energy deposited per micrometer. The calculated maximum efficiencies with the slab configuration model are approximately 1.95%, 0.30%, and 0.025% using monoenergetic average energy and 1.54%, 0.25%, and 0.019% using an energy spectrum for 35S, 90Sr, and 90Y, respectively. These efficiencies when using the spherical configuration model are 2.02%, 0.31%, and 0.023% using the monoenergetic average energy and 1.10%, 0.17%, and 0.013% using an energy spectrum for 35S, 90Sr, and 90Y, respectively.

Investigation of non-uniformity and inclusions in 6LiInSe2 utilizing laser induced breakdown spectroscopy (LIBS)

Impurity analysis and compositional distribution studies have been conducted on a crystal of LiInSe2, a compound semiconductor which recently has been shown to respond to ionizing radiation. IR microscopy and laser induced breakdown spectroscopy (LIBS) revealed the presence of inclusions within the crystal lattice. These precipitates were revealed to be alkali and alkaline earth elemental impurities with non-uniform spatial distribution in the crystal. LIBS compositional maps correlate the presence of these impurities with visual color differences in the crystal as well as a significant shift of the band gap. Further, LIBS revealed variation in the ratio of I-III-VI2 elemental constituents throughout the crystal. Analysis of compositional variation and impurities will aid in discerning optimal synthesis and crystal growth parameters to maximize the mobility-lifetime product and charge collection efficiency in the LiInSe2 crystal. Preliminary charge trapping calculations have also been conducted with the Monte Carlo N-particle eXtended (MCNPx) package indicating preferential trapping of holes during irradiation with thermal neutrons.

Microfluidic Alpha Spectrometry of UOX PWR UNF in a Molten Salt

Abstract This paper presents the computational performance of microfluidic channels (MFCs) within a semiconductor detector to monitor changes in the elemental and isotopic composition of a UOX pressurized water reactor used nuclear fuel dissolved in a KCl/LiCl molten salt. The results indicate that the use of MFC limits alpha energy loss sufficiently enough to use energy windowing techniques in spectral analysis. It was found that elemental and isotopic changes as low as 1% may be feasible, with the time to detection (TTD) ranging from seconds to hours. The TTD is inversely dependent on the number of MFCs within the sensor and the activity of the element/isotope undergoing concentration transients.

Semiconducting lithium indium diselenide: Charge-carrier properties and the impacts of high flux thermal neutron irradiation

This paper reports on the charge carrier properties of several lithium indium diselenide (LISe) semiconductors. It was found that the charge collection efficiency of LISe was improved after high flux thermal neutron irradiation including the presence of a typically unobservable alpha peak from hole-only collection. Charge carrier trap energies of the irradiated sample were measured using photo-induced current transient spectroscopy. Compared to previous studies of this material, no significant differences in trap energies were observed. Through trap-filled limited voltage measurements, neutron irradiation was found to increase the density of trap states within the bulk of the semiconductor, which created a polarization effect under alpha exposure but not neutron exposure. Further, the charge collection efficiency of the irradiated sample was higher (14–15 fC) than that of alpha particles (3–5 fC), indicating that an increase in hole signal contribution resulted from the neutron irradiation. Finally, it was observed that significant charge loss takes place near the point of generation, producing a significant scintillation response and artificially inflating the W-value of all semiconducting LISe crystals.This paper reports on the charge carrier properties of several lithium indium diselenide (LISe) semiconductors. It was found that the charge collection efficiency of LISe was improved after high flux thermal neutron irradiation including the presence of a typically unobservable alpha peak from hole-only collection. Charge carrier trap energies of the irradiated sample were measured using photo-induced current transient spectroscopy. Compared to previous studies of this material, no significant differences in trap energies were observed. Through trap-filled limited voltage measurements, neutron irradiation was found to increase the density of trap states within the bulk of the semiconductor, which created a polarization effect under alpha exposure but not neutron exposure. Further, the charge collection efficiency of the irradiated sample was higher (14–15 fC) than that of alpha particles (3–5 fC), indicating that an increase in hole signal contribution resulted from the neutron irradiation. Finally, it wa...

Neutron Imaging with Timepix Coupled Lithium Indium Diselenide

The material lithium indium diselenide, a single crystal neutron sensitive semiconductor, has demonstrated its capabilities as a high resolution imaging device. The sensor was prepared with a 55 μ m pitch array of gold contacts, designed to couple with the Timepix imaging ASIC. The resulting device was tested at the High Flux Isotope Reactor, demonstrating a response to cold neutrons when enriched in 95% 6 Li. The imaging system performed a series of experiments resulting in a <200 μ m resolution limit with the Paul Scherrer Institute (PSI) Siemens star mask and a feature resolution of 34 μ m with a knife-edge test. Furthermore, the system was able to resolve the University of Tennessee logo inscribed into a 3D printed 1 cm 3 plastic block. This technology marks the application of high resolution neutron imaging using a direct readout semiconductor.

Investigation of a Lithium Indium Diselenide detector for neutron transmission imaging

The development of a thermal neutron imaging sensor constructed with semiconducting lithium indium diselenide is presented. Both a computational and experimental investigation were conducted. In the computational investigation, it is shown that the imaging potential of Lithium Indium Diselenid (LISe) is excellent, even when using a large pixel pitch through the use of super sampling. In the experimental investigation, it was found that a single pixel LISe detector using detector super sampling shows a spatial variation in the count rate, which is a clear sign of imaging capability. However, a good image was not obtained in the first experiment and may be caused by a variety of experimental conditions. Finally, a search is still underway to find a suitable contact metal with good mechanical adhesion for wedge bonding.

Improving neutron detection in semiconducting 6LiInSe2

Chalcopyrite crystals of 6LiInSe2 have recently been shown to respond to gamma and thermal neutron radiation. Thus far, large crystals have been prepared although the charge collection efficiency has not been sufficient for high energy resolution. In an effort to improve energy resolution needed for gamma spectroscopy as well as pulse shape discrimination for mixed gamma neutron fluxes, the precipitate concentration within the 6LiInSe2 crystal have been studied. The precipitate volume greatly affects the energy resolution in the pulse height spectrum. Further, the charge mobility varies greatly with holes being preferentially trapped by these precipitates or some other defect site within the crystal.

∆E/∆E Measurements of Energetic Ions Using CVD Diamond Detectors

Experimental and computational results of a ΔE/ΔE diamond detection system are presented. The ΔE/ΔE detection system was evaluated using energetic proton and iron beams striking thick polyethylene targets at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL). The measured data for diamond sensor A show good agreement with the Geant4 simulation. In addition, simulations have demonstrated the ability to identify hydrogen isotopes using a diamond detection system.

Crystal growth and characterization of selected high-performance scintillators for national security applications (Conference Presentation)

Scintillators are important materials for radiation detection applications such as homeland security, geological exploration, and medical imaging. Scintillators for nuclear nonproliferation applications in particular must have excellent energy resolution in order to distinguish the gamma-ray signatures of potentially dangerous radioactive sources, such as highly enriched uranium or plutonium, from non-threat radioactive sources such as radioactive tracers used in medical imaging. There is an established need for scintillators with energy resolution in the 1-2% range at 662 keV. However, there are challenges surrounding the development of this new generation of high light yield/high resolution scintillators; for example, the high cost of production due to low crystal yield and slow growth process, and crystal inhomogeneity. We will discuss efforts focused on developing recently discovered high performance scintillators K(Sr,Ba)2I5:Eu, Cs4(Ca,Sr)I6:Eu and Cs2Hf(Cl,Br)6 that have potential for meeting nuclear security needs. Growth parameters for these materials have been optimized, allowing the growth of excellent quality single crystals measuring up to one-inch in diameter via the vertical Bridgman technique at translation rates between 1 and 5 mm/h. These scintillators materials have excellent properties with light yields between 30,000 and 120,000 ph/MeV, and energy resolutions between 2.3 and 4.6% at 662 keV.