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Plastic scintillators are widely used in radiation portal monitors because of their low cost and availability in large sizes. However, due to their low density and low effective atomic number ( Z ), they offer low intrinsic efficiency and little spectroscopic information. The addition of high- Z constituents to these plastics can greatly increase both their total stopping power and the amount of photoelectric absorption, leading to full-energy deposition and thus useful gamma spectra. In this article, we present the performance of the latest formulation of Bi-loaded plastic scintillators showing their useful spectroscopic information up to relatively high energy (~1 MeV) due to their high stopping power compared to the current commercially available plastics. These Bi-loaded plastics use 20 weight percent (wt%) Bi-pivalate (8 wt% elemental Bi) dissolved in polyvinyltoluene (PVT) matrix and conventional fast fluors (~10 ns decay time). These Bi-loaded plastics achieve up to approximately 6000 photons/MeV and have been produced in sizes up to 17 in3. The performance of these Bi-loaded plastics is also demonstrated in the existing portal monitor hardware (Rapiscan Model TSA Trainer 770) showing the possibility to provide improved sensitivity as a drop-in replacement with continued scale-up.

Plastic-based scintillator detectors have many advantages over inorganic scintillators, including mechanical ruggedness and cost. However, their range of application has generally been limited by their lack of gamma spectroscopic performance. We have been developing metal-organic doped plastic scintillators which allow for spectroscopy while maintaining the advantages of plastics. These scintillators allow for the use of plastics in many new application spaces. Using iridium based fluors, bismuth loaded plastics have demonstrated high light yields of >20,000 photons/MeV and good energy resolution (

We report on a small gamma spectrometer with directional detection, based on two modules. Each module contains an array of 128 scintillator pixels coupled to reverse-biased photodiodes, an ASIC and a microcontroller. Modules communicate via USB and are operated via an Android GUI on a WiFi-connected device. Transparent ceramic GYGAG ((Gd,Y,Ce)3(Ga,Al)5O12) garnet scintillator pixels offer energy resolution as good as 3.1% at 662 keV for individual pixels. We previously reported on an eight-module planar spectrometer, based on the same technology1. The current compact system uses two modules in a back-to-back configuration to improve its speed and accuracy in locating a gamma-emitting source in a 4-pi field of view. This dual module system (~2.5 x 2.5 x 3.5 inches) is suitable as a pager-sized RIID or spectroscopic personal radiation detector (SPRD). Multiple dual module units can be combined for improved efficiency. Spectroscopic and directional performance will be described.

Fast neutrons accessible from 14-MeV D-T neutron generators have higher transmission through high-Z materials compared to radiography X-rays due to a more uniform attenuation as a function of material Z. These neutrons can therefore image low-Z materials even when shielded by high-Z materials. The constraints in portable fast-neutron digital imaging systems include limited neutron output, limited light produced by neutron imaging scintillators, and lower resolution images due to large source spot size and required scintillator thickness. In addition, digital panel dark-noise counts can be 100x higher than the image signal, and variations across the panel can also be comparable to this signal. We will discuss recent efforts to develop a portable neutron-radiography system, including improved neutron scintillator, mitigation of digital panel noise, and new portable D-T neutron generators. We will also present MCNP efforts to model the system, including neutron scattering effects.

High energy X-rays and neutrons can provide 3-D volumetric views of large objects made of multiple materials. Lenscoupled computed tomography using a scintillator imaged on a CCD camera obtains high spatial resolution, while a surface-mounted segmented scintillator on an amorphous silicon (A-Si) array can provide high throughput. For MeV Xray CT, a new polycrystalline transparent ceramic scintillator referred to as “GLO” offers excellent stopping power and light yield for improved contrast in sizes up to a 12” field-of-view. For MeV neutron CT, we have fabricated both contiguous and segmented plates of “Hi-LY” plastic scintillator, offering light yields 3x higher than standard plastic.

Thallium bromide (TlBr) is a compound semiconductor with a band gap of 2.68 eV making it ideal for room temperature radiation detection. The high atomic numbers, 81 and 35, and the high density of 7.56 g/cm3 give it excellent gamma-ray stopping power. TlBr is a cubic material that melts congruently at a relatively low temperature (~ 480 C). These properties make it relatively easy to grow good quality crystals with high yield. As a result of improvements in the purification of TlBr mobility-lifetime product of electrons, µτe, is now on the order of 10-2 cm2/V, which is similar to that of CZT. High µτe enables the fabrication of thicker detectors with good charge collection and energy resolution. The properties of TlBr make it ideal for use in room temperature gamma radiation operation [Smith 2013]. Single carrier devices such as small pixel arrays [O'Neal 2018a] and Frisch collar devices [Hitomi, et al.2013] which were developed for CZT can also been applied to TlBr. For example, better than 1% FWHM at 662 keV has been obtained for single pixel events with small (e.g. 3 x 3 pixels, 1-mm pitch, 5-mm thick) arrays.

Thallium-bromide (TlBr) is currently under investigation for use as an alternative room-temperature semiconductor gamma-ray spectrometer. Performance of better than 1% full-width at half maximum at 662 keV can be achieved when TlBr detectors are cooled to −20 °C. The theoretical limit of energy resolution is determined by the ionization energy in semiconductor detectors, and accurately measuring it is important for determining the best possible performance. One method to determine the ionization energy of semiconductor detectors compares pulse heights obtained from the semiconductor to pulse heights from a silicon detector. Due to their higher trapping, the charge collection efficiency (CCE) of TlBr is significantly lower than it is in silicon, therefore a correction to the ionization energy must be made. In this paper, we present the theory and measurement for accurately determining the CCE using the Shockley-Ramo theorem and apply it to measurement of ionization energy in pixelated TlBr detectors. The ionization energy of two TlBr detectors is measured and found to be 4.83(8) and 5.49(10) eV for the two samples at room temperature and −20 °C, respectively.

Thallium bromide (TlBr) is an attractive material for room-temperature semiconductor radiation detection due to its high atomic number and density. Performance of better than 1% full-width at half maximum at 662 keV has been measured on $\sim 5 \times 5 \times 5$ mm3 pixelated TlBr detectors. Though most TlBr detectors show expected performance, a few detectors have high-energy tails on their photopeaks which are caused by nonflat tails on anode waveforms. The properties of these anode tails are studied and the generation of extra electrons by the movement of holes is proposed to explain the observations. The detector is operated in reverse bias (with holes drifting toward the pixelated electrode) to help confirm these observations and a mechanism for correcting preamplifier decay from long collection time digital waveforms is developed and used in the analysis.

Plastic scintillators are in wide use in radiation portal monitors because of their low cost and availability in large sizes. However, due to their low density and atomic number (Z), they offer low intrinsic efficiency and little to no spectroscopic information. The addition of high-Z constituents to these plastics can greatly increase both their total stopping power and the amount of photo-electric absorption, leading to full-energy deposition and thus spectroscopic information in plastics. In this work, we present the performance of the largest bismuth-loaded plastics to date, showing useful spectroscopic information up to relatively high energy (~1 MeV) and their high stopping power compared the current commercially available plastics. These Bi-loaded plastics are based on 20 wt% Bi-pivalate (8 wt% elemental Bi) dissolved in a polyvinytoluene (PVT) matrix and conventional fast fluors (

Inexpensive spectroscopic personal radiation detectors (SPRDs) are needed to monitor environmental radioactivity and search for sources. For gamma spectroscopy, excellent light yield, material uniformity, light yield proportionality, mechanical and environmental ruggedness can be achieved in polycrystalline ceramic oxide garnets. We are building a compact detector based on 14 cm3 of transparent ceramic garnet, formed into 256 pixels (3mm x 3mm x 6mm each) and mounted on two stacked silicon photodiode arrays. GYGAG(Ce) garnet transparent ceramics offer density = 5.8g/cm3, Zeff = 48, principal decay of

We have previously made improvements to the longevity of TlBr semiconductor gamma ray detectors by applying electrodes having the mixed semiconductor composition Tl(Cl,Br) via surface treatments in HCl, leading to a significant enhancement to the lifetime of the detectors. In order to examine the electron transport properties more closely, we have monitored the first-derivative of the cathode waveform (being proportional to velocity and number of carriers) as a function of time and the point of the gamma-interaction. The observed decay in this signal, especially at lower voltage, would naturally be interpreted as the usual trapping phenomenon. However, this phenomenon alone is not able to account for the observed waveforms, most dramatically for the case of increasing signal as the electrons approach the anode, for waveforms originating at the cathode. After detailed consideration of alternative explanations, the cathode waveform data has been interpreted in terms of a non-uniform field owing to variation in the resistivity as a function of position. We have interpreted the shape of the decay as a “built-in” resistivity profile and have further verified this interpretation by reversing the sense of the field (which as expected reverses the “sense” of the waveform). We modeled this effect in order to quantitatively deduce the resistivity profile and are currently working to relate the waveform observations to the relative orientation of the crystal growth direction and the applied electrodes.

The two-bias method typically used to calculate the mobility-lifetime product ( $\mu \tau $ ) in single-polarity charge sensing detectors, uses induced signals from cathode-side events to measure the $\mu \tau $ . This method assumes an ideal weighting potential (exactly zero through the bulk and a rapid rise to unity at the anode). When a non-ideal weighting potential (e.g., from a pixelated detector) is used, the $\mu \tau $ is systematically overestimated. In this work, we characterize this overestimation and present a simple correction factor $k$ that can be applied to pixelated electrode configurations. It was found that the correction factor is only dependent on the pixel-pitch to detector thickness ratio.

Plastic scintillators incorporating 8 weight percent elemental Bismuth offer enhanced sensitivity and distinct photopeak spectra in the 3 Bismuth plastic scintillator plate. Count rates compared to standard plastic scintillator of the same size reveal a sensitivity improvement of >5x in the

Thallium-bromide (TlBr) is currently under investigation as an alternative room-temperature semiconductor gamma-ray spectrometer due to its favorable material properties (large bandgap, high atomic numbers, and high density). Previous work has shown that 5 mm thick pixelated TlBr detectors can achieve sub-1% FWHM energy resolution at 662 keV for single-pixel events. These results are limited to $ - 20{^ \circ }{\rm C}$ operation where detector performance is stable. During the first one to five days of applied bias at $ - 20{^ \circ }{\rm C}$ , many TlBr detectors undergo a conditioning phase, where the energy resolution improves and the depth-dependent electron drift velocity stabilizes. In this work, the spectroscopic performance, drift velocity, and freed electron concentrations of multiple 5 mm thick pixelated TlBr detectors are monitored throughout the conditioning phase. Additionally, conditioning is performed twice on the same detector at different times to show that improvement mechanisms relax when the detector is stored without bias. We conclude that the improved spectroscopy results from internal electric field stabilization and uniformity caused by fewer trapped electrons.

Due to favorable material properties such as high atomic number (Tl: 81, Br: 35), high density ( g/cm ), and a wide band gap (2.68 eV), thallium-bromide (TlBr) is currently under investigation for use as an alternative room-temperature semiconductor gamma-ray spectrometer. TlBr detectors can achieve less than 1% FWHM energy resolution at 662 keV, but these results are limited to stable operation at . After days to months of room-temperature operation, ionic conduction causes these devices to fail. This work correlates the varying leakage current with alpha-particle and gamma-ray spectroscopic per- formances at various operating temperatures. Depth-dependent photopeak centroids exhibit time-dependent transient behavior, which indicates trapping sites form near the anode surface during room-temperature operation. After refabrication, similar perfor- mance and functionality of failed detectors returned.

Transparent polycrystalline ceramic scintillators based on the garnet structure and incorporating gadolinium for high stopping power are being developed for use in gamma spectrometers. Optimization of energy resolution for gamma spectroscopy involves refining the material composition for high stopping and high light yield, developing ceramics fabrication methodology for material homogeneity, as well as selecting the size and geometry of the scintillator to match the photodetector characteristics and readout electronics. We have demonstrated energy resolution of 4% at 662 keV for 0.05 cm3 GYGAG(Ce) ceramics with photodiode readout, and 4.9% resolution at 662 keV for 18 cm 3 GYGAG(Ce) ceramics and PMT readout. Comparative gamma spectra acquired with GYGAG(Ce) and NaI(Tl) depict the higher resolution of GYGAG(Ce) for radioisotope identification applications. Light yield non-proportionality of garnets fabricated following different methods reveal that the fundamental shapes of the light yield dependence on energy are not intrinsic to the crystal structure, but may instead depend on trap state distributions. With exposure to 9 MeV Brehmsstrahlung radiation, we also find that GYGAG(Ce) ceramics exhibit excellent radiation hardness.

Development of the Europium-doped Strontium Iodide scintillator, SrI2(Eu), involves advances in crystal growth, optics and readout methodology for prototype detectors. We have demonstrated energy resolution of 3% at 662 keV for a 26 cm3 SrI2(Eu) crystal, which is equivalent to the performance obtained with Cerium-doped Lanthanum Bromide of equivalent size. Compared to standard analog readout, use of a digital readout method allows improved energy resolution to be obtained with large volume SrI2(Eu) crystals. Comparative gamma spectra acquired with LaBr3(Ce) and NaI(Tl) quantitatively depict the value of the high resolution of SrI2(Eu) in discriminating closely spaced gamma lines for radioisotope identification applications.

Thallium-bromide (TlBr) is a viable alternative to CZT for room-temperature gamma ray spectroscopy because of its high atomic number, high density, and wide band-gap. Sub-1% FWHM energy resolution at 662 keV has been achieved on 5 mm thick pixelated devices, however, these results are mostly limited to stable operation at −20°C. During room-temperature operation, the devices degrade and fail, a phenomenon known as polarization. Polarization is likely caused by bulk-to-contact interactions facilitated by ionic conduction. In 2008, Hitomi et al. found that room-temperature lifetime could be extended by applying Tl electrodes. In this work we verify stable room-temperature performance of a 4.1 mm thick pixelated TlBr detector with Tl electrodes. Energy resolution, mobility-lifetime product, and resistivity are characterized and presented. Resolutions of 1.6% and 1.2% FWHM are achieved for all depths and cathode-side events only, respectively. A very high electron mobility-lifetime product is calculated: 3.1±0.2 ·10-2 cm2/V.

Due to its wide band gap (2.68 eV) and high stopping power, thallium bromide (TlBr) is being investigated as a room-temperature semiconductor gamma-ray spectrometer. When cooled to −20°C, performance of better than 1% FWHM at 662 keV has been observed on 5×5×5 mm3 pixelated TlBr detectors. The room-temperature lifetime (under continuous bias) of thick TlBr detectors operated at high fields (∼2000 V/cm) has been limited to weeks or months due to polarization caused by ionic conduction. Previous work has shown that the degradation process is limited to the surface and that surface preparation techniques can extend the lifetime of thin planar TlBr devices at room temperature. In this work, the lifetime and stability of two large 5×5×5 mm3 pixelated TlBr arrays are presented. Detector performance is compared for different surface preparation techniques and improved stability is observed. Additionally, we use depth-sensing techniques to track the depth-dependent photopeak centroids over time and conclude that once surface degradation effects are mitigated, TlBr performance can improve over time at room temperature. The improvement is likely similar to conditioning at −20°C in which the electric field stabilizes and becomes more uniform.

Thallium-bromide (TlBr) is currently under investigation as an alternative room-temperature semiconductor gamma-ray spectrometer due to its favorable material properties. Previous work has shown that 5 mm thick pixelated TlBr detectors can achieve sub 1% FWHM energy resolution at 662 keV. However, these results are mostly limited to −20 °C operation. In addition to good electron mobility, some TlBr detectors show hole mobility as high as 15–20% of the electron mobility. High hole mobility can affect depth reconstruction when single-polarity charge sensing is assumed. In this work, we use digital signal processing on the planar cathode waveforms to identify and account for the motion of holes and improve depth reconstruction at all depths for high hole mobility detectors. The hole drift only affects the cathode waveforms because the generated charge induces cathode signal at all depths. Due to the small pixel effect, the anode signal induction only occurs in a region right near the anode. As a result, the motion of holes does not significantly affect the anode signal.

Thallium bromide (TlBr) is being investigated for use as room temperature semiconductor gamma-ray spectrometers because of its wide band gap (2.68 eV) and high stopping power. When cooled to −20°C, performance of better than 1% FWHM at 662 keV has been observed on 5×5×5 mm3 pixelated TlBr detectors. Currently, each of nine anode pixels (in a 3 × 3 array) is read out individually through separate preamplifiers, limiting the number of pixels that can practically be used. VAD_UM digital application specific integrated circuit (ASIC) is currently used to read out signals from an array of 11×11 pixel anodes on each 3D CdZnTe detector. This work applies the same digital ASIC to TlBr by designing and building a compact system capable of reading out a larger detector (12mm × 12mm × 5mm as opposed to the current 5mm × 5mm × 5mm) while keeping the detector cooled for stable operation. We present an initial characterization of the system, including operating performance and power consumption used to keep the detector at various temperatures.

High energy resolution gamma-ray detectors that can be formed into relatively large sizes while operating at room temperature offer many advantages for national security applications. We are working toward that goal through the development of SrI 2 (Eu) scintillator detectors, which routinely provide 10 cm3. In this study, we have tested pure, undoped SrI 2 to gain a better understanding of the scintillation properties and spectroscopic performance achievable without activation. An undoped crystal grown from 99.999% pure SrI 2 pellets was tested for its spectroscopic performance, its light yield, and uniformity of scintillation light collection as a function of gamma-ray interaction position relative to the crystal growth direction. Undoped SrI 2 was found to provide energy resolution of 5.3% at 662 keV, and the light collection nonuniformity varied by only 0.72% over the length of the crystal. Measurements of both a 3% Eu-doped and the undoped SrI 2 crystal were carried out in the SLYNCI facility and indicate differences in their light yield non-proportionality. The surprisingly good scintillation properties of the pure SrI 2 crystal suggests that with high-purity feedstock, further reduction of the Eu concentration can be made to grow larger crystals while not adversely impacting the spectroscopic performance.

Recently discovered scintillators for gamma ray spectroscopy, single crystal SrI{sub 2}(Eu), GYGAG(Ce) transparent ceramic and Bismuth-loaded plastics, offer resolution and fabrication advantages compared to commercial scintillators, such as NaI(Tl) and standard PVT plastic. Energy resolution at 662 keV of 2.7% is obtained with SrI{sub 2}(Eu), while 4.5% is obtained with GYGAG(Ce). A new transparent ceramic scintillator for radiographic imaging systems, GLO(Eu) offers high light yield of 70,000 Photons/MeV, high stopping, and low radiation damage. Implementation of single crystal SrI{sub 2}(Eu), Gd-based transparent ceramics, and Bi-loaded plastic scintillators can advance the state-of-the art in ionizing radiation detection systems.