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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 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.

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

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.

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.

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.

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.