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Presented are design considerations for a potential detection and measurement technique that could provide operational awareness of high power microwave (HPM) directed energy weapon exposure for force health protection applications, leveraging thermoacoustic (TA) wave generation as the field interaction mechanism. The HPM electromagnetic frequency (EMF) regime, used in applications in both the counter-materiel and non-lethal counter-personnel design space, presents real-time personnel exposure warning challenges due to the potentially wide variation in time and frequency domain characteristics of the incident beam. As with other EM-thermal interactions, the thermoacoustic wave effect provides the potential to determine EM energy and power deposition without the need to measure ambient field intensity values or overload-sensitive EMF survey equipment. Following measurement of relevant EM, thermal, and elastic material property values, a carbon-filled polytetrafluoroethylene (CF-PTFE) lossy dielectric medium subject to pulsed HPM was computationally modeled using the commercial finite element method multi-physics simulation software package COMSOL. The simulation was used to explore the impacts of various material properties on TA signal output as a function of simulated incident field power density, EM frequency, and pulse length, thereby informing the selection of system components for the further development of a full TA-based HPM detection chain.

Detection of microwave-induced thermoacoustic (TA) wave generation was evaluated as a potential technique for detection of high power microwave (HPM) directed energy exposure. Even when HPM is employed for counter-materiel effects, incidental but still potentially harmful personnel exposure is possible. Real-time detection of ongoing exposure with potentially unknown time and frequency domain characteristics is a critical first step in preventing acute health effects by alerting and then enabling the timely use of electromagnetic frequency energy shielding, such as structures and vehicles. Leveraging the TA effect as a field interaction mechanism, a lossy dielectric polymer subjected to pulsed HPM was tested using a planar sample geometry with thin film piezoelectric sensors used to capture the resulting TA output. The piezoelectric signal was analyzed in both the time and frequency domain to determine empirical relationships between incident microwave beam properties and signal components. This analysis was coupled with an empirically-based single term Cole-Cole model approximation fit for the complex permittivity. The results were used to identify appropriate signal conditioning and processing techniques needed to convert the TA response into a useful form for personnel exposure applications. These results also served as a comparison point for multi-physics finite element method computational modeling of the electromagnetic response of a simplified three-layer tissue model.

Neutrons from an atmospheric nuclear explosion can be detected by sensors in orbit. Current tools for characterizing the neutron energy spectrum assume a known source and use forward transport to recreate the detector response. In realistic scenarios the true source is unknown, making this an inefficient, iterative approach. In contrast, the adjoint approach directly solves for the source spectrum, enabling source reconstruction. The time–energy fluence at the satellite and adjoint transport equation allow a Monte Carlo method to characterize the neutron source’s energy spectrum directly in a new model: the Space to High-Altitude Region Adjoint (SAHARA) model. A new adjoint source event estimator was developed in SAHARA to find feasible solutions to the neutron transport problem given the constraints of the adjoint environment. This work explores SAHARA’s development and performance for mono-energetic and continuous neutron energy sources. In general, the identified spectra were shifted towards energies approximately 5% lower than the true source spectra, but SAHARA was able to capture the correct spectral shapes. Continuous energy sources, including real-world sources Fat Man and Little Boy, resulted in identifiable spectra that could have been produced by the same distribution as the true sources as demonstrated by two-dimensional (2D) Kolmogorov–Smirnov tests.

We report on the acquisition and modeling of the transient response of a commercial silicon (Si) solar cell using a benchtop pulsed X-ray source. The solar-cell transient output to the X-ray pulses was acquired under the dark and steady-state light illumination to mimic the practical operation of a solar cell under different light illumination levels. A solar-cell circuit model was created to develop a fundamental understanding of the transient current/voltage response of solar cell at read-out circuit level. The model was validated by a good agreement between the simulation and experimental results. It was found that the solar-cell resistance ( $R$ ) and capacitance ( $C$ ) depend on the light illumination, and the resulting variation in $RC$ time constant significantly affects the solar-cell transient response. Thus, the solar cell produced different transient signals under different illumination intensities in response to the same X-ray pulse. The experimental data acquired in this work proves the feasibility of using solar panels for prompt detection of nuclear detonations, which also builds a practical mode of X-ray detection using a low-cost self-powered detector.

Measurements of thermal neutrons and gamma rays using multiple ${\mathrm{ Cs}}_{2}{\mathrm{ LiYCl}}_{6}:{\mathrm{ Ce}}^{3+}$ (CLYC) crystals with photomultiplier tube (PMT) and silicon photomultiplier (SiPM) are compared. The first set of measurements used three single-sided crystals (two made from 95% enriched 6Li and one made with 99% enriched 7Li) mated with the PMT and SiPM to compare the energy resolutions and figures of merit (FOM) for the pulse shape discrimination of gamma rays and neutrons. All crystals and photomultipliers were able to resolve full-energy photopeaks for 137Cs and 60Co and distinguish gamma-ray interactions from neutron interactions ( $\text {FOM} > 1$ in all cases) while operating in a thermal neutron environment. Measurements made with the PMT had better energy resolution, and switching to the SiPM degraded the energy resolution by an average of 34.0± 0.7% (at 662 keV). The second set of measurements used a CLYC crystal with two optical windows to enable simultaneous measurements with the PMT and SiPM. Once again, both photomultipliers were able to distinguish gamma rays from neutrons, and the PMT had the best energy resolution. However, in this case, the SiPM was not able to resolve the 137Cs full-energy photopeak, and neither photomultiplier could resolve any photopeaks if 60Co was used simultaneously. The degradation in energy resolution was due to the relative splitting of scintillation light intensity between each photomultiplier depending on the position of the interaction site within the crystal. Each pair of waveforms recorded from the PMT and SiPM, corresponding to a single interaction within the double-sided crystal, were combined to improve the energy resolution at 662 keV from 15.9 ± 0.9% to 12.7 ± 0.3% and the FOM from 1.850 ± 0.004 to 2.117 ± 0.003 when compared to the data just from the PMT.

In this study, we evaluated the feasibility of applying solar photovoltaic (PV) panels as sensors of nuclear and electromagnetic radiation that includes neutrons, x-rays and gamma-rays, and optical radiation emanating from a nuclear explosion. We investigated the steady-state and transient response of both a commercial silicon (Si) and a perovskite solar cell to different radiation types. Solar cell current-voltage characteristics and short-circuit current (I sc ) response under steady-state x-ray illumination were measured. The fast transient radiation pulse from a nuclear detonation was mimicked by using a fast switching, nanosecond pulsed laser source and the transient response of the solar cells was captured on an oscilloscope. Subsequently, the transient response of Si solar cells to pulsed x-rays generated by a mechanical x-ray chopper was measured. A 2 MeV neutron beam chopper was built at the fast neutron beam facility of a research reactor to produce time-modulated neutrons and evaluate the solar cell transient response to a neutron pulse. Our steady-state measurements demonstrated good response of solar cells to x-rays and neutrons. The pulsed radiation measurements indicated that the solar cells are able to detect a fast transient radiation and produce a proportional measurable output signature.

With the increasing cost and decreasing availability of 3He, there have been many efforts to find alternative neutron detection materials. Lithium calcium aluminum fluoride (LiCAF) enriched to 95% 6 Li doped with europium was evaluated here as a replacement material for 3 He. Wafers 0.5 cm thick, consisting of LiCAF crystals in a rubberized matrix, were embedded with wavelength shifting fibers (WSF) and mated to silicon photo-multipliers (SiPMs) to measure the photon response in a flux of neutrons from a DD neutron generator. Excellent discrimination was realized between neutrons and gammas, and both pulse-height discrimination and pulse-shape analysis were explored. A Figure of Merit (FoM) of 1.03 was achieved. By applying pulse-shape analysis, a simple neutron count output was generated by utilizing a low-pass filter to suppress fast pulses from the SiPM output and subsequently applying a threshold to the remaining signal. Custom electronics were built to bias the SiPMs, then amplify, filter, discriminate, and digitize the LiCAF/WSF scintillation photons, resulting in a digital pulse that can easily be counted with any microcontroller or field programmable gate array. A significant advantage of LiCAF is that it can be fabricated into any shape/size (when embedded in a rubberized matrix), and the light output and transparency is sufficient to allow for thicker scintillators which enable detection of both thermal and epithermal neutrons. This work demonstrated that Eu:LiCAF is capable of discriminating gammas from neutrons and is a potential replacement material for 3 He, especially for nuclear security applications and neutron spectroscopy .

When doped with copper ions, lithium borate materials are candidates for use in radiation dosimeters. Copper-doped lithium tetraborate (Li2B4O7) crystals have been widely studied, but little is known thus far about copper ions in lithium triborate (LiB3O5) crystals. In the present investigation, Cu+ ions (3d10) were diffused into an undoped LiB3O5 crystal at high temperature. These ions occupy both Li+ and interstitial positions in the crystal. A photoluminescence (PL) band peaking near 387 nm and a photoluminescence excitation (PLE) band peaking near 273 nm verify that a portion of these Cu+ ions are located at regular Li+ sites. After an irradiation at room temperature with x rays, electron paramagnetic resonance (EPR) spectra show that Cu+ ions at Li+ sites have trapped a hole and converted to Cu2+ ions (3d9) while Cu+ ions at interstitial sites have trapped an electron and converted to Cu0 atoms (3d104s1). Two distinct Cu2+ trapped-hole spectra are formed by the x rays: one due to isolated Cu2+ ions with no nearby defects and the other due to perturbed Cu2+ ions. When the x-ray-irradiated crystal is heated above room temperature, a thermoluminescence (TL) peak appears at 120 °C with a maximum in the emitted light near 630 nm. EPR shows that this TL peak occurs when trapped electrons are thermally released from interstitial Cu0 atoms. Thermal quenching above room temperature prevents the electron-hole recombination at Cu2+ ions from contributing to the TL emission.

Optically stimulated luminescence (CW-OSL) is observed from Ag-doped lithium tetraborate (Li 2 B 4 O 7 ) crystals. Photoluminescence, optical absorption, and electron paramagnetic resonance (EPR) are used to identify the defects participating in the OSL process. As-grown crystals have Ag + ions substituting for Li + ions. They also have Ag + ions occupying interstitial sites. During a room-temperature exposure to ionizing radiation, holes are trapped at the Ag + ions that replace Li + ions and electrons are trapped at the interstitial Ag + ions, i.e., the radiation forms Ag 2+ (4d 9 ) ions and Ag 0 (4d 10 5s 1 ) atoms. These Ag 2+ and Ag 0 centers have characteristic EPR spectra. The Ag 0 centers also have a broad optical absorption band peaking near 370 nm. An OSL response is observed when the stimulation wavelength overlaps this absorption band. Specifically, stimulation with 400 nm light produces an intense OSL response when emission is monitored near 270 nm. Electrons optically released from the Ag 0 centers recombine with holes trapped at Ag 2+ ions to produce the ultraviolet emission. The OSL response is progressively smaller as the stimulation light is moved to longer wavelengths (i.e., away from the 370 nm peak of the absorption band of the Ag 0 electron traps). Oxygen vacancies are also present in the Ag-doped Li 2 B 4 O 7 crystals, and their role in the OSL process as a secondary relatively short-lived electron trap is described.

An intense green luminescence is observed from single crystals of LiGaO2 doped with copper. Czochralski-grown undoped crystals are wrapped in thin copper foil and then held at 900 °C for 1 h in a flowing nitrogen atmosphere. Large concentrations of Cu+ ions enter the crystals during this process and occupy Li+ sites. These copper-diffused crystals are characterized with optical absorption, photoluminescence (PL), photoluminescence excitation (PLE), thermoluminescence (TL), and electron paramagnetic resonance (EPR). An optical absorption band peaking near 350 nm is assigned to the Cu+ ions at Li+ sites and represents an excitation from a 3d10 ground state to a 3d94s1 excited state. A broad PL emission from these excited Cu+ ions has a peak near 523 nm and the related PLE band has a peak near 356 nm (this PLE band links the emission to the optical absorption band). Illuminating a Cu-diffused crystal at room temperature with 325 nm laser light converts a portion of the Cu+ ions to Cu2+ ions. EPR spectra from these 3d9 ions are easily seen at low temperatures and their angular dependence is used to determine the g matrix and the 63Cu hyperfine matrix. Subsequent heating produces a TL peak near 122 °C with a maximum in its spectral dependence near 535 nm. Correlated EPR measurements show that this TL peak occurs when trapped electrons are thermally released from unintentionally present transition-metal ions (most likely Fe) and recombine with holes at the Cu2+ ions.

Pulse shape discrimination of neutrons and gammas is demonstrated using a FemtoDAQ, a low-cost digitizer that uses commercial off-the-shelf components. This digitizer is paired with a CLYC scintillator coupled to a photomultiplier tube to concurrently detect neutrons and gammas. Python code was written to analyze the detector waveforms to determine the energy deposited and distinguish neutron and gamma events within the CLYC crystal. The energy of each waveform is determined by the summation of the discrete amplitudes recorded by the digitizer for each pulse. This method is compared to a traditional multichannel analyzer operating with commercial software for validation and shown to produce the same energy spectrum . Pulse shape discrimination is accomplished by measuring and summing the amplitudes of the prompt portion of each waveform (first 80 ns) and the delayed portion (following 500 ns) and then calculating a ratio of the delayed region to the total. This technique was able to clearly distinguish thermal neutron events from gamma events with a figure of merit of 1.42.

A portable ten-layer neutron spectrometer has been developed using a novel material, LiCAF. The spectrometer consists of rubber matrix wafers with embedded europium doped lithium calcium aluminum fluoride (Eu:LiCAF) crystals and crosshatched wave shifting fibers (WSF). Layers of high density polyethylene (HDPE) separating each Eu:LiCAF wafer serve as neutron moderating material. Neutrons entering the spectrometer react in the Eu:LiCAF crystal, creating scintillation photons . The photons travel down the WSFs to silicon photomultipliers (SiPMs) that convert the photons to an electrical signal. Custom electronics are used to gather, amplify, process, and readout the signal. A library of several neutron response curves was created in Geant4 and MCNP. The experimental output of the spectrometer was unfolded using a maximum entropy algorithm, MAXED. The spectrometer was tested using two separate DD generators. These two tests resulted in unfolded neutron spectra with average neutron energies of 2.71 and 2.78 MeV and chi-squared values of 0.88 and 1.51 χ 2 /D.O.F. respectively, when fit to a 2.45 MeV monoenergetic energy spectrum .

A large optically stimulated luminescence (CW-OSL) response has been observed from bulk lithium aluminate (LiAlO2) crystals that have been copper-diffused. After introducing the copper, an optical absorption band peaking at 277 nm is present and is assigned to Cu+ (3d10) ions at Li+ sites. Photoluminescence (PL and PLE) from copper-diffused samples shows an emission band near 359 nm and an excitation band near 273 nm. These PL bands are also assigned to the Cu+ ions. After an irradiation at room temperature with x-rays, the copper-diffused crystals give an intense OSL signal when stimulated with light in the 425–500 nm region. The OSL emitted light has a peak near 357 nm and thus is directly associated with recombination at a Cu ion. Electron paramagnetic resonance (EPR) shows that the x-rays produce two distinct Cu2+ (3d9) centers, both at Li+ sites. One of the trapped-hole centers is a Cu2+ with no nearby defects and the other is a Cu2+ with a neighboring Li+ vacancy. Monitoring the EPR spectra before and after OSL shows that only the isolated Cu2+ ions (i.e., those with no nearby defect) participate in the OSL process. The electron traps participating in the OSL are shown with EPR to be transition-metal ions that are unintentionally present. Specifically, Fe2+ ions replacing Li+ ions, possibly with a nearby lithium vacancy, convert to Fe+ (3d7) ions when they trap an electron during irradiation. These electrons are optically released by the OSL exciting light and migrate to the holes trapped as isolated Cu2+ ions, where electron–hole recombination gives the characteristic Cu+ emission. By combining OSL, PL, and EPR, we establish the general OSL mechanism in copper-diffused LiAlO2 crystals.

Electron paramagnetic resonance (EPR) is used to identify the primary hole trap in undoped lithium aluminate (LiAlO2) crystals. Our interest in this material arises because it is a candidate for radiation detection applications involving either optically stimulated luminescence (OSL) or thermoluminescence (TL). During an x-ray irradiation at room temperature, holes are trapped at oxygen ions adjacent to lithium vacancies. Large concentrations of these lithium vacancies are introduced into the crystal during growth. With the magnetic field along the [001] direction, the EPR spectrum from these trapped-hole centers consists of eleven lines, evenly spaced but with varying intensities, caused by nearly equal hyperfine interactions with two 27Al nuclei (I=5/2, 100% abundant). The g matrix is determined from the angular dependence of the EPR spectrum and has principal values of 2.0130, 2.0675, and 2.0015. These g shifts strongly support the model of a hole in a p orbital on an oxygen ion. The adjacent lithium vacancy stabilizes the hole on the oxygen ion. A sequence of pulsed thermal anneals above room temperature shows that the EPR spectrum from the holes trapped adjacent to the lithium vacancies disappears in the 90–120 °C range. The thermal decay of these hole centers directly correlates with an intense TL peak near 105 °C. Signals at lower magnetic field in the 9.4 GHz EPR spectra suggest that the electron trap associated with this TL peak at 105 °C may be a transition-metal-ion impurity, most likely Fe, located at a cation site. Additional less intense TL peaks are observed near 138, 176, and 278 °C.

Excited states in Re186 with spins up to J=12 were investigated in two separate experiments using W186(d,2n) reactions at beam energies of 12.5 and 14.5 MeV. Two- and threefold γ-ray coincidence data were collected using the CAESAR and CAGRA spectrometers, respectively, each composed of Compton-suppressed high-purity germanium detectors. Analysis of the data revealed rotational bands built on several two-quasiparticle intrinsic states, including a long-lived Kπ=(8+) isomer. Configuration assignments were supported by an analysis of in-band properties, such as gK-gR values. The excitation energies of the observed intrinsic states were compared with results from multi-quasiparticle blocking calculations, based on the Lipkin-Nogami pairing approach, that included contributions from the residual proton-neutron interactions.

Charge pumping and drain current-gate voltage ( I D - V G ) measurements are used to investigate degradation and environmental effects in X-ray and proton-irradiated graphene transistors. X-ray irradiation initially degrades mobility due to hole trapping in the oxide, and induces oxygen-related p-type doping that is evident in the increase in ID once trapped holes anneal out; after long times, interface trap buildup dominates device response. Both the drain current and charge pumping current decrease initially after proton irradiation due to displacement damage and dedoping. However, continued degradation indicates further damage to the graphene layer itself, likely from oxygen-related defect creation in the graphene, induced water and gas molecules between the graphene layer and underlying substrate, or some combination of both. These results indicate that the environment can significantly affect the radiation response of graphene devices, and that the long-term response of the device may be quite different from the initial response due to defect creation, charge migration and annealing.

Electron paramagnetic resonance (EPR) is used to investigate the effects of ionizing radiation on Ag-doped lithium tetraborate (Li2B4O7) crystals. Two similar, yet distinct, trapped-hole centers (Ag2+ ions substituting for Li+ ions) are produced by 60 kV x rays. One Ag2+ ion, labeled Center A, has no nearby defects and the other Ag2+ ion, labeled Center B, has a neighboring impurity which is most likely a Ag+ ion substituting for a Li+ ion. The production and thermal decay properties of the two Ag2+ ions are described and their g matrices and 107Ag and 109Ag hyperfine matrices are obtained from the EPR angular dependences. The principal values of the g matrices are similar for the two centers, but the hyperfine principal values differ significantly (Center B has smaller values than Center A). There are also differences in the directions of the principal axes for the two centers. Together, these results imply (1) that the unpaired spin is less localized for Center B and (2) that the ground-state positions of the neighboring oxygen ions are different for Centers A and B. This explains why the peaks of the Ag2+ charge-transfer photoluminescence bands associated with Centers A and B occur at different wavelengths (502 and 725 nm, respectively). An isochronal pulsed thermal anneal shows that these radiation-induced Ag2+ ions serve as the recombination site for the intense thermoluminescence peak observed near 152 °C.

Using extended X-ray absorption fine structure (EXAFS) spectroscopy, we find that Cr substitutes primarily in the Li þ site as a dopant in lithium tetraborate Li2B4O7 glasses, in this case 98.4Li2B4O7e1.6Cr2O3 or nominally Li1.98Cr0.025B4O7. This strong preference for a single site is nonetheless accompanied by site distortions and some site disorder, helping explain the optical properties of chromium doped Li2B4O7 glasses. The resulting O coordination shell has a contraction of the CreO bond lengths as compared to the LieO bond lengths. There is also an increase in the O coordination number.

Two broad photoluminescence (PL) bands peaking near 502 and 725 nm are observed at room temperature in Ag-doped Li 2 B 4 O 7 crystals pre-irradiated at room temperature with X-rays. Their respective excitation bands peak near 297 and 325 nm. The requirement that the crystal be pre-irradiated establishes that these emissions are not related to Ag + ions. Electron paramagnetic resonance (EPR) helps assign models to the PL bands. EPR spectra show that two substitutional Ag 2+ trapped-hole centers and one interstitial Ag 0 trapped-electron center are produced at room temperature by the X-rays. A majority of the Ag 2+ ions have no neighboring defects (and are referred to as Center A) while the remaining Ag 2+ ions have a nearby defect (and are referred to as Center B). The PL bands are assigned to these two Ag 2+ ions (the 502 nm emission to Center A and the 725 nm emission to Center B). A charge-transfer mechanism is responsible for the observed luminescence, i.e., an electron from a neighboring oxygen ion, upon excitation, moves to the Ag 2+ ion and produces a Ag + ion with an adjacent O − ion. This excited-state complex quickly decays radiatively and the original Ag 2+ and O 2− ions are restored. Optical absorption related to Ag 2+ and Ag 0 defects are observed in the ultraviolet and visible after the Ag-doped crystals are X-ray irradiated at room temperature.

Radiation effects on graphene field effect transistors (GFETs) with hexagonal boron nitride (h-BN) thin film substrates are investigated using 60Co gamma-ray radiation. This study examines the radiation response using many samples with varying h-BN film thicknesses (1.6 and 20 nm thickness) and graphene channel lengths (5 and 10 μm). These samples were exposed to a total ionizing dose of approximately 1 Mrad(Si). I-V measurements were taken at fixed time intervals between irradiations and postirradiation. Dirac point voltage and current are extracted from the I-V measurements, as well as mobility, Dirac voltage hysteresis, and the total number of GFETs that remain properly operational. The results show a decrease in Dirac voltage during irradiation, with a rise of this voltage and permanent drop in Dirac current postirradiation. 1.6 nm h-BN substrate GFETs show an increase in mobility during irradiation, which drops back to preirradiation conditions in postirradiation measurements. Hysteretic changes to the Dirac voltage are the strongest during irradiation for the 20 nm thick h-BN substrate GFETs and after irradiation for the 1.6 nm thick h-BN GFETs. Failure rates were similar for most GFET types during irradiation; however, after irradiation, GFETs with 20 nm h-BN substrates experienced substantially more failures compared to 1.6 nm h-BN substrate GFETs.

Solar neutrons have been detected aboard the International Space Station (ISS), using lithium tetraborate and boron carbide detector elements. We find that evidence of a solar neutron flux, as detected in a neutron calorimeter following subtraction of the proton background, with an energy of about 2 to 4 MeV. This solar neutron flux is likely no more than 250 to 375 neutrons cm−2sec−1, with a lower bound of 50–75 neutrons cm−2sec−1 at one au.

Single crystals of thorium dioxide ThO2, grown by the hydrothermal growth technique, have been investigated by ultraviolet photoemission spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and L3, M3, M4, and M5 X-ray absorption near edge spectroscopy (XANES). The experimental band gap for large single crystals has been determined to be 6 eV to 7 eV, from UPS and IPES, in line with expectations. The combined UPS and IPES, place the Fermi level near the conduction band minimum, making these crystals n-type, with extensive band tailing, suggesting an optical gap in the region of 4.8 eV for excitations from occupied to unoccupied edge states. Hybridization between the Th 6d/5f bands with O 2p is strongly implicated. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Carbon nanotube field-effect transistors with Al2O3 gate dielectrics have been characterized before and after 1 MeV electron irradiation. Pre-irradiation interface trap densities determined by charge pumping measurements were on the order of 1011 cm-2eV-1. Following irradiation to fluences of 1016 e/cm2 and 1017 e/cm2, the measured interface trap densities decreased significantly for all devices. This result is largely attributed to the removal of molecular adsorbates that act as trap sites on the nanotubes (de-doping) while the devices are irradiated under vacuum. Negative shifts in the transistor I-V curves with irradiation indicate radiation-induced hole trapping in the gate dielectric, with some recovery after room temperature annealing. No significant changes were observed in the Raman spectra following irradiation, indicating the intrinsic structure of the carbon nanotubes remained unchanged. These results suggest that the radiation response of these devices is dominated by charge trapping in the oxide layers and at the carbon nanotube surfaces. Furthermore, the charge pumping technique can be used as a tool for probing surface adsorbates in carbon nanotube devices, and elucidating their role in the device radiation response.

Electron paramagnetic resonance (EPR) is used to identify the electron and hole traps responsible for thermoluminescence (TL) peaks occurring near 100 and 200 °C in copper-doped lithium tetraborate (Li2B4O7) crystals. As-grown crystals have Cu+ and Cu2+ ions substituting for lithium and have Cu+ ions at interstitial sites. All of the substitutional Cu2+ ions in the as-grown crystals have an adjacent lithium vacancy and give rise to a distinct EPR spectrum. Exposure to ionizing radiation at room temperature produces a second and different Cu2+ EPR spectrum when a hole is trapped by substitutional Cu+ ions that have no nearby defects. These two Cu2+ trapped-hole centers are referred to as Cu2+-VLi and Cu active 2 + , respectively. Also during the irradiation, two trapped-electron centers in the form of interstitial Cu0 atoms are produced when interstitial Cu+ ions trap electrons. They are observed with EPR and are labeled Cu A 0 and Cu B 0 . When an irradiated crystal is warmed from 25 to 150 °C, the Cu active 2 + centers have a partial decay step that correlates with the TL peak near 100 °C. The concentrations of Cu A 0 and Cu B 0 centers, however, increase as the crystal is heated through this range. As the crystal is further warmed between 150 and 250 °C, the EPR signals from the Cu active 2 + hole centers and Cu A 0 and Cu B 0 electron centers decay simultaneously. This decay step correlates with the intense TL peak near 200 °C.

This study combines electron microscopy equipped with energy dispersive spectroscopy to probe major element composition and autoradiography to map plutonium in order to examine the spatial relationships between plutonium and fallout composition in aerodynamic glassy fallout from a nuclear weapon test. A sample set of 48 individual fallout specimens were interrogated to reveal that the significant chemical heterogeneity of this sample set could be described compositionally with a relatively small number of compositional endmembers. Furthermore, high concentrations of plutonium were never associated with several endmember compositions and concentrated with the so-called mafic glass endmember. This result suggests that it is the physical characteristics of the compositional endmembers and not the chemical characteristics of the individual component elements that govern the un-burnt plutonium distribution with respect to major element composition in fallout.

The neutron-capture reaction is fundamental for identifying and analyzing the γ-ray spectrum from an unknown assembly because it provides unambiguous information on the neutron-absorbing isotopes. Nondestructive-assay applications may exploit this phenomenon passively, for example, in the presence of spontaneous-fission neutrons, or actively where an external neutron source is used as a probe. There are known gaps in the Evaluated Nuclear Data File libraries corresponding to neutron-capture γ-ray data that otherwise limit transport-modeling applications. In this work, we describe how new thermal neutron-capture data are being used to improve information in the neutron-data libraries for isotopes relevant to nonproliferation applications. We address this problem by providing new experimentally-deduced partial and total neutron-capture reaction cross sections and then evaluate these data by comparison with statistical-model calculations.

The local structure of Mn-doped Li2B4O7(001) was investigated using extended X-ray absorption fine structure (EXAFS) at the Mn K edge and electron paramagnetic resonance (EPR). The location of the Mn dopant in a lithium tetraborate crystal is consistent with occupation of a site with strong oxygen coordination. The MnO bond lengths are similar to those observed with Mn doping of the icosahedral based boron carbide where Mn is in a substitutional dopant in one of the cage sites. From EXAFS, the manganese does not appear to greatly alter the overall tetragonal form of lithium tetraborate, with the dopant most likely substituting for one of the two B sites and with placement of some of the Mn in a Li site still possible. The EPR spectra agree with the literature examined resolving multiple Mn species in the crystal lattice.

Partial $\ensuremath{\gamma}$-ray production cross sections and the total radiative thermal-neutron capture cross section for the $^{185}\mathrm{Re}(n,\ensuremath{\gamma})^{186}\mathrm{Re}$ reaction were measured using the Prompt Gamma Activation Analysis facility at the Budapest Research Reactor with an enriched $^{185}\mathrm{Re}$ target. The $^{186}\mathrm{Re}$ cross sections were standardized using well-known $^{35}\mathrm{Cl}(n,\ensuremath{\gamma})^{36}\mathrm{Cl}$ cross sections from irradiation of a stoichiometric ${}^{\text{nat}}{\text{ReCl}}_{3}$ target. The resulting cross sections for transitions feeding the $^{186}\mathrm{Re}$ ground state from low-lying levels below a cutoff energy of ${E}_{c}=746\phantom{\rule{4.pt}{0ex}}\text{keV}$ were combined with a modeled probability of ground-state feeding from levels above ${E}_{c}$ to arrive at a total cross section of ${\ensuremath{\sigma}}_{0}=111(6)\phantom{\rule{4.pt}{0ex}}\text{b}$ for radiative thermal-neutron capture on $^{185}\mathrm{Re}$. A comparison of modeled discrete-level populations with measured transition intensities led to proposed revisions for seven tentative spin-parity assignments in the adopted level scheme for $^{186}\mathrm{Re}$. Additionally, 102 primary $\ensuremath{\gamma}$ rays were measured, including 50 previously unknown. A neutron-separation energy of ${S}_{n}=6179.59(5)\phantom{\rule{4.pt}{0ex}}\text{keV}$ was determined from a global least-squares fit of the measured $\ensuremath{\gamma}$-ray energies to the known $^{186}\mathrm{Re}$ decay scheme. The total capture cross section and separation energy results are comparable to earlier measurements of these values.

Integrated circuits are inherently complicated and made more by increasing transistor quantity and density. This trend potentially enhances concomitant effects of high-energy ionizing radiation and local or impressed electromagnetic interference (EMI). The reduced margin for signal error may counter any gain in radiation hardness from smaller device dimensions. Isolated EMI and ionizing radiation studies on circuits have been extensively conducted over the past 30 years. However, little focus has been placed on the combined effects. To investigate the effect of combined EMI and ionizing radiation, two complementary metal-oxide-semiconductor inverter technologies (CD4069 and SN74AUC1G04) were analyzed for their static performance in response to both EMI and gamma radiation up to 132 krd(Si). The combined EMI and gamma radiation environment, compared to the isolated effects of each, produced the most severe degradation in inverter performance for both device technologies.

We have compared the photovoltaic charging of the (1 0 0) surface termination for Cu doped and undoped Li 2 B 4 O 7 . While the surface charging at the (1 0 0) surface of Li 2 B 4 O 7 is significantly greater than observed at (1 1 0) surface, the Cu doping plays a role in reducing the surface photovoltage effects. With Cu doping of Li 2 B 4 O 7 , the surface photovoltaic charging is much diminished at the (1 0 0) surface. The density of states observed with combined photoemission and inverse photoemission remains similar to that observed for the undoped material, except in the vicinity of the conduction band edge.

We review the pyroelectric properties and electronic structure of Li2B4O7(110) and Li2B4O7(100) surfaces. There is evidence for a pyroelectric current along the [110] direction of stoichiometric Li2B4O7 so that the pyroelectric coefficient is nonzero but roughly 103 smaller than along the [001] direction of spontaneous polarization. Abrupt decreases in the pyroelectric coefficient along the [110] direction can be correlated with anomalies in the elastic stiffness C33D contributing to the concept that the pyroelectric coefficient is not simply a vector but has qualities of a tensor, as expected. The time dependent surface photovoltaic charging suggests that surface charging is dependent on crystal orientation and doping, as well as temperature.

This paper presents the design and test results of a standalone computational radiation sensor system based on a single chip solution. A low-power sensor front end with a charge sensitive amplifier and an event driven analog-to-digital converter is integrated on the same chip as a dedicated microcontroller to process and bin the data from the neutron detector diode heterojunction according to pulse height. This combination effectively implements a single chip multichannel analyzer with the capability to do further processing of the data in software. The design was fabricated in a 0.18 μm CMOS technology with field tests demonstrating the validity of the approaches taken. The total system power consumption is 24 μW.

We find a pyroelectric current along the 〈110〉 direction of stoichiometric Li2B4O7 so that the pyroelectric coefficient is nonzero but roughly 10−3 smaller than along the 〈001〉 direction of spontaneous polarization. Abrupt decreases in the pyroelectric coefficient along the 〈110〉 direction can be correlated with anomalies in the elastic stiffness C33D contributing to concept that the pyroelectric coefficient is not simply a vector but has qualities of a tensor, as expected. The time dependent surface photovoltaic charging suggests that an inverse piezoelectric effect occurs at the (110) surface but not the (100) surface. Both effects along the 〈110〉 direction or at the (110) surface are distinct the conventional as a bulk pyroelectric effect.

The shallow Li 1s core level exhibits a surface-to-bulk core level shift for the stoichiometric Li 2 B 4 O 7 (1 1 0) surface. Angle-resolved photoemission spectroscopy was used to indentify Li 1s bulk and surface core level components at binding energies −56.5±0.4 and −53.7±0.5 eV, respectively. We find photoemission evidence for surface states of Li 2 B 4 O 7 (1 1 0) that exist in the gap of the projected bulk density of states. The existence of surface states is consistent with the large surface-to-bulk core level shift for the Li 1s core.

Al0.27Ga0.73N/GaN HFETs were electron irradiated at ~ 80 K. The gate leakage and transistor current were measured and compared to theoretical tunneling models. The results are consistent with previous work but explicitly show that radiation produces point defects in the AlGaN that are positively charged at low temperature.

AlGaN/GaN HFET's were cooled to ~85 K and irradiated to a fluence of 1012 n/cm2 (1 MeV equivalent) and gate currents measured. The observed increased gate leakage current was studied by curve-fitting a 4-parameter thermionic trap assisted tunneling model to the experimental measurements. The model parameters were constrained and a least-squares fitting routine was applied to best fit the gate current's voltage and temperature dependence. The results were used to provide an interpretation of the physical changes to the HFET gate following neutron irradiation. The fitting showed that Schottky barrier lowering plays a minor role in post irradiation gate leakage, and trap density and mean trap energy were the dominant post irradiation gate leakage parameters. The ~25% increase in trap density and ~7 mV increase in mean trap energy indicated that neutron elastic collisions produce additional non-native defects within the AlGaN gate region.

Low temperature neutron irradiated Al0.27Ga0.73N/GaN heterostructures reveal a complex temperature dependent displacement damage formation process. This process results in differences in drain currents at low (80 K) versus high (294 K) temperatures. Irradiation increases the gate and drain currents at 80 K, and decreases the drain current at room temperature. These effects saturate at ~ 3 times 1010 n/cm2 indicating complexing with a native impurity. After a room temperature anneal, the effect on the gate current persists and the drain current partially recovers. A two-step persistent interface trap formation model is presented that explains these results. This model is further supported by CV measurements at 80 K and 294 K after annealing.

The effects of low energy (0.45 MeV) electron radiation on the gate and drain currents of Al0.27Ga0.73N/GaN HFETs are investigated using IV and CV measurements. Following irradiation, the gate and drain currents increase at low temperatures and reach a saturation level. The gate leakage currents do not fully account for the drain current increase. Following a room temperature anneal, the gate and drain currents return to pre-irradiation levels. These results are explained by the buildup of positive charge in the AlGaN layer at low temperature and traps formed via a complexing precursor in the AlGaN layer near the interface. The positive charge increases the carrier concentration in the 2DEG and hence the drain current. The traps act as trap-assisted-tunneling centers that increase the gate leakage current.

The spallation neutron source at the Los Alamos Neutron Science Center Weapons Neutron Research facility was used to populate excited states in $^{186}\mathrm{Re}$ via $(n,2n\ensuremath{\gamma})$ reactions on an enriched $^{187}\mathrm{Re}$ target. Gamma rays were detected with the GErmanium Array for Neutron Induced Excitations spectrometer, a Compton-suppressed array of 18 HPGe detectors. Incident neutron energies were determined by the time-of-flight technique and used to obtain $\ensuremath{\gamma}$-ray excitation functions for the purpose of identifying $\ensuremath{\gamma}$ rays by reaction channel. Analysis of the singles $\ensuremath{\gamma}$-ray spectrum gated on the neutron energy range $10\ensuremath{\le}{E}_{n}\ensuremath{\le}25\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$ resulted in five transitions and one level added to the $^{186}\mathrm{Re}$ level scheme. The additions include the placement of three $\ensuremath{\gamma}$ rays at 266.7, 381.2, and 647.7 keV which have been identified as feeding the $2.0\ifmmode\times\else\texttimes\fi{}{10}^{5}\phantom{\rule{4pt}{0ex}}\mathrm{yr}$, ${J}^{\ensuremath{\pi}}=({8}^{+})$ isomer and yield an improved value of $148.2(5)\phantom{\rule{4pt}{0ex}}\mathrm{keV}$ for the isomer energy. These transitions may have astrophysical implications related to the use of the Re-Os cosmochronometer.

Singly ionized oxygen vacancies are produced in $\mathrm{LiAl}{\mathrm{O}}_{2}$ crystals by direct displacement events during a neutron irradiation. These vacancies, with one trapped electron, are referred to as ${\mathrm{V}}_{\mathrm{O}}^{+}$ centers. They are identified and characterized using electron paramagnetic resonance (EPR) and optical absorption. The EPR spectrum from the ${\mathrm{V}}_{\mathrm{O}}^{+}$ centers is best monitored near 100 K with low microwave power. When the magnetic field is along the [001] direction, this spectrum has a $g$ value of 2.0030 and well-resolved hyperfine interactions of 310 and 240 MHz with the two $^{27}\mathrm{Al}$ nuclei that are adjacent to the oxygen vacancy. A second EPR spectrum, also showing hyperfine interactions with two $^{27}\mathrm{Al}$ nuclei, is attributed to a metastable state of the ${\mathrm{V}}_{\mathrm{O}}^{+}$ center. An optical absorption band peaking near 238 nm is assigned to ${\mathrm{V}}_{\mathrm{O}}^{+}$ centers. Bleaching light from a Hg lamp converts a portion of the ${\mathrm{V}}_{\mathrm{O}}^{+}$ centers to ${\mathrm{V}}_{\mathrm{O}}^{0}$ centers (these latter centers are oxygen vacancies with two trapped electrons). The ${\mathrm{V}}_{\mathrm{O}}^{0}$ centers have an absorption band peaking near 272 nm, a photoluminescence band peaking near 416 nm, and a photoluminescence excitation band peaking near 277 nm. Besides the oxygen-vacancy EPR spectra, a holelike spectrum with a resolved, but smaller, hyperfine interaction with one $^{27}\mathrm{Al}$ nucleus is present in $\mathrm{LiAl}{\mathrm{O}}_{2}$ after the neutron irradiation. This spectrum is tentatively assigned to doubly ionized aluminum vacancies.

The effective Debye temperature of ytterbium and gallium in Yb:GaN thin films has been obtained using X-ray photoemission spectroscopy. The vibrational motion normal to the surface results in a dimunition of photoemission intensities from which we have estimated the effective Debye temperatures of 221 ± 30 K and 308 ± 30 K for Yb and Ga, respectively. The difference between the measured values for Yb and Ga suggests that the Debye temperatures are influenced by the local environment. The smaller effective surface Debye temperature for Yb correlates to a soft, strained surface, possibly due to an increased Yb―N bond length as compared to the Ga―N bond length.

A physical model was created for the resulting fireball of a nuclear detonation. The model focuses on the growth of the fireball as well as its temperature. The physical model was created for a 100 kt detonation, with 70% x-ray yield. A MATLAB code was created to generate these models, with yield and % x-ray yield as adjustable variables. Both temperature and radii were calculated as a function of time. The fireball was then input into DIRSIG for 3D modeling with an airfield scene, with the fireball modeled at key points in its development.

Researchers at Lawrence Livermore National Laboratory (LLNL) have started digitizing technical films spanning the above ground atmospheric nuclear testing operations conducted by the United States from 1950 through the 1960s. This technical film test data represents unique information that can be use as a primary validation data source for nuclear effects codes that are used by national researchers for assessments on nuclear force management, nuclear detection and reporting, and nuclear forensics mission areas. Researchers at the Air Force Institute of Technology (AFIT) have begun employing modern digital image processing and computer vision techniques to exploit this data set and determine specific invariant features of the early dynamic fireball growth. The focus of this paper is to introduce the methodology used for three dimensional sparse reconstructions of nuclear fireballs. Also discussed are the the difficulties associated with the technique.

Semiconductor neutron detectors (SCND) devices have undergone dramatic improvements over the last ten years, with reported single detector thermal intrinsic efficiencies approaching 30%. The proposed spectrometer design consists of several alternating layers of SCND's and neutron absorbers/reflectors. A 3-D array of SCND's is embedded into a hydrogenous media, creating a single detection layer that moderates fast neutrons to thermal energies, where they are subsequently detected. The absorption/separation layers, consisting of 1 mm of cadmium and 2 mm of bismuth, provide increased separation of detector layer responses to mono-energetic incident neutrons. The GEANT4 toolkit was used to extensively model and optimize the proposed design. The devices in each detection layer were arranged in a radial-arm array; simulations of this configuration resulted in an azimuthal angular resolution of 0.75 degrees, while polar angular resolution was found to be dependent on the number of detection layers present. A response library was built by simulating mono-energetic neutrons, ranging from 0.01 eV to 12 MeV, incident to the face of the first detection layer. This library was then used with a maximum likelihood expectation maximization (MLEM) algorithm to unfold the spectrometer response to several mono- and poly-energetic neutron sources. The unfolded spectra exhibited excellent agreement with the simulated sources, demonstrating discrimination between different neutron source types, but also between sources of the same type.