Thallium bromide (TlBr) is a promising material for semiconductor gamma-ray detectors due to its high atomic number (Tl = 81, Br = 35), high density (7.56 g/cm3), and relatively low melting point (460 degrees C). These properties give TlBr several competitive features, including high detection efficiency, good energy resolution, and lower fabrication costs, making it suitable for applications requiring compact, highly sensitive gamma-ray detection systems. Recent developments of TlBr detectors have progressed toward thicker crystals, up to 10-20 mm, configured as pixelated or virtual Frisch-grid (VFG) devices with volumes >1 cm3. Despite these advances, challenges remain in practical implementation, including material and fabrication issues that affect energy resolution, device stability, performance, and longevity. These challenges are similar to those faced by HgI2 and lead-halide perovskite detectors. This study presents results from characterizing over 100 position-sensitive 5x5x12 mm3 virtual Frisch-grid detectors fabricated by Radiation Monitoring Devices Inc. For a prototype Radioisotope Identification Device (RIID). The work identified factors limiting the performance and longevity of TlBr detectors, including stochastic response variations that limit the energy resolution to 2-3 % (at 662 keV) for most devices, performance degradation associated with defect decoration, and contact delamination. Addressing these issues advances the use of TlBr and similar materials for reliable, high-resolution gamma-ray spectroscopy.

Semiconductor crystals of 6 LiInSe 2 and 6 LiIn 0.5 Ga 0.5 Se 2 were grown through Vertical Bridgman (VB) technique using enriched lithium ( 6 Li). The materials were characterized for electronic and optical properties. Thermal neutron detectors were fabricated and characterized. Pulse height spectra were collected from an 241 AmBe neutron source and a 60 Co gamma-ray source. The measured resistivity, bandgap and mobility lifetime product of the samples are reported and compared. The thermal neutron detection efficiency of the 6 LiInSe 2 , , and 6 LiGaInSe 2 were also estimated and reported using Geant4 simulation and was compared with the fundamental calculation. Furthermore, the devices performance at elevated temperature (~50 °C) and room-temperature (RT) were also experimentally investigated and the results were compared.

Position sensitivity enables the correction of response non-uniformities in room-temperature semiconductor detectors caused by crystal defects and other factors. It can also be used to pinpoint the exact location of crystal defects responsible for the response variations. This work describes a technique for revealing and visualizing the detector regions affecting the charge collection efficiency in CdZnTe (CZT), TlBr, and CsPbBr3 detectors configured as position-sensitive virtual Frisch-grid (VFG) devices. The technique correlates the photopeak events in energy spectra with their spatial distributions inside the detectors using the position information. By selecting the events from narrow energy intervals within a photopeak, we can visualize the detector volumes with particular charge collection efficiencies, which, in turn, correlate with the locations of electrode and crystal defects. We demonstrate this technique in several examples. Columnar structures in the volume plots (position distribution maps) are consistent with signal losses near or at the anode in selected samples of CZT and TlBr. Structures exhibiting a distinct depth dependence are consistent with grain boundaries or other crystal defects.

High-temperature detectors are required for nuclear material accounting measurements for some advanced reactor designs including molten salt reactors. The Cs 2 LiLa (Br,Cl) 6 :Ce (CLLBC) scintillator is a dual-mode scintillator that shows excellent gamma detection along with the capability to discriminate gamma rays from neutrons and is available as a commercial product from Radiation Monitoring Devices Inc. (RMD). The energy resolution at room temperature (RT) is ~3.0% at 662 keV with a light yield of ~45 000 ph/MeV. The gamma-neutron pulse shape discrimination (PSD) figure of merit (FOM) is ~3.5 at RT. We investigated the gamma and neutron detection performance of CLLBC scintillators from 25 °C to 200 °C. The results showed a good light yield response for gamma events as a function of temperature while the light yield for neutron-capture events monotonically decreased from 100% to 76% at 200 °C. When integrated with a high-temperature photomultiplier, at 175 °C we obtained energy resolutions of 7.8% for 662 keV gammas and 4.2% for neutron-capture events, together with a PSD FOM of 0.74.

This work evaluates data from position-sensitive capacitive Frisch-grid (PS-CFG) TlBr detectors for correcting the gamma-ray energy spectrum based on the location of the interaction in the crystal. This enables the correction of non-uniformity in the signal response. Signals from 5×5×12 mm 3 TlBr PS-CFG detectors generate anode amplitude versus cathode-to- anode-ratio histograms, which map the interaction depth for a 32×32 grid of XY pixels. The width of the 662-keV photopeak from 137 Cs irradiation contains contributions from statistics and non-uniformity. The dependence of the signal on the interaction depth produces an asymmetric "tail" in the photopeak in the anode spectra. This work also evaluates the dependence of the non-uniformity corrections on long-term stability, while under constant 1500 V bias at room temperature after 3 and 5 months of operation. The 3D corrections compensate for some non-uniformity; however, the interaction location determined by the approach represents the centroid of the charge distribution. Variations in the sampling of non-uniformities produced by the shape of the charge distribution may not be corrected. We suspect that the current energy resolution may be limited by uncorrected non-uniformities.

Li-based semiconductor materials represent a promising alternative to 3-He and scintillation materials for thermal neutron detection and imaging instruments. Semiconductor crystals of LiInSe2, LiInP2Se6, and LiGaInSe2 (LiGa0.5In0.5Se2) were grown using natural and enriched lithium (6Li). The materials were characterized for electronic and optical properties including optical transmission, current-voltage (I-V) characteristic for resistivity, and bandgap. Thermal neutron detectors were fabricated and characterized for neutron and gamma-ray response. Pulse height spectra were collected from a moderated custom-designed 241AmBe neutron source and a 60Co gamma-ray source. The LiInSe2 samples exhibited a 2.8 eV cutoff in the optical spectrum and a resistivity of ~8×1011 Ω·cm. LiInSe2 devices exhibit a noise floor of <30 keV which operated at a field of 630 V/mm, for the 0.8-mm thick device. The Vertical Gradient Freeze (VGF) grown LiInP2Se6 samples exhibited a 2.2 eV cutoff in the optical spectrum and resistivity of ~4×1012 Ω·cm. The Chemical Vapor Transport (CVT) grown LiInP2Se6 devices exhibit a noise floor of <60 keV which operated at a field of 8,000 V/mm, for the 0.05- mm thick device. Furthermore, the long-term stability of LiInSe2 devices during multiple weeks under continuous bias was investigated.

This work presents results from analyzing position-sensitive capacitive Frisch-grid (PS-CFG) TlBr gamma-ray detectors. As a room-temperature semiconductor detector, TlBr exhibits a high atomic number, high density, and low Fano factor compared to other material. The use of the 3D position sensing technique provides information on the crystal uniformity. This technique presents the spatial variation in the histograms of the anode amplitude versus the cathode-to-anode ratio, necessary for depth correction, for detectors fabricated with 5×5×12 mm 3 TlBr crystals at room temperature operating at continuous bias (1.5 kV) over the course of nine months. This work also presents the temperature dependence of the leakage current for a TlBr PS-CFG detector over the temperature range of -20 to +60 °C, which varies from <0.2 to 40 nA, respectively, along with changes in the spectrum from the anode signal measured with 137 Cs irradiation. This work shows the utility of the 3D technique to evaluate the performance and uniformity of PS-CFG detectors for applications that require high-energy resolution gamma-ray spectroscopy, such as radionuclide identification.

The annual modulation of scintillation event rate observed by the DAMA/LIBRA experiment has been a long-standing controversy in the quest of the direct detection of dark matter. The effort to definitively confirm or refute the annual modulation has turned out to be challenging due to the lack of NaI(Tl) crystals with high enough radio-purity. Most recently, we successfully grew a 6-kg ingot free from contamination during growth, from which a 3.4-kg crystal scintillator was made. The $^{39}$K concentration in the final crystal is estimated to be 4.3$\pm$0.2~ppb, unprecedented for NaI(Tl) crystals. The $^{210}$Pb activity is estimated to be 0.34$\pm$0.04~mBq/kg via $\alpha$ counting of $^{210}$Po, among the lowest of currently-running NaI-based dark matter experiments except DAMA/LIBRA. More importantly, the techniques and protocols we have developed will further contribute to the growth of higher purity NaI(Tl) crystals for dark matter searches.

Lithium indium selenide (LiInSe2) is being developed for use as a room temperature semiconductor detector for thermal neutrons. The material has been studied for a number of applications including non-linear optics such as parametric oscillators, as anode material for lithium ion batteries, piezoelectrics, as a scintillation detector material, and as a semiconductor detector material. The recent advances of the crystal growth, material processing, and detector fabrication have led to semiconductor neutron detectors with up to 100 mm(2) active area. The theoretical thermal neutron detection sensitivity and gamma rejection ratio (GRR) are comparable to 10 atm, He-3 tubes of similar size. Detector fabrication and characterization are described and the results are discussed.

The Majorana nature of neutrinos is one of the most important questions being pursued in physics today. This has given the search for neutrinoless double-beta decay (0ν β β) a high priority among proposed nuclear physics experiments. In this work, the objective was to evaluate several crystal compositions for possible use as a scintillating bolometer in 0ν β β research. A crucial characteristic of the investigated crystals is that their compositions must include one of the few elements known to decay with double-beta emission. Therefore, we grew crystals containing molybdenum or selenium as a major constituent. Results are presented here for single-crystals of sodium molybdate and lithium indium diselenide. Scintillating bolometer data were measured at milli-Kelvin temperature and show good discrimination between alpha and beta/gamma emissions, which is crucial for background reduction. The lithium indium diselenide crystal was also used to measure the energy spectrum for the beta emission of indium decay, the first time a beta decay spectrum has been measured from self-emission in a scintillating bolometer.

Low-cost radiation detectors capable of providing efficient gamma-ray and neutron detection are essential in passive nuclear monitoring systems such as radiation portal monitors (RPMs) and detection of Special Nuclear Materials (SNM) through Active Interrogation (AI). Three low-cost detector solutions are discussed in this paper—(1) Large-area polystyrene-based plastic scintillators for AI (2) Pulse Shape Discrimination (PSD) capable plastic scintillators doped with heavy-metals for imparting gamma-ray spectroscopy (3) Composite detector technology based on inorganic scintillator submerged into the plastic scintillator, for detection of thermal and fast neutrons in addition to gamma-ray spectroscopy. Composite detectors using inorganic component such as Cs2LiYCl6 (CLYC), Cs2LiLaBrxCl6−x (CLLBC), or CsI:Tl have been developed up to 4” sizes. Neutron and gamma efficiencies are compared for samples with different inorganic loading.

Lithium indium diselenide (LISe) semiconductors, enriched to 95% 6Li, exhibits a very high neutron detection efficiency per unit thickness and the potential for fast timing. However, the range of the secondary alpha and triton particles from neutron absorption limits the spatial resolution to ~34 µm. To overcome the limited spatial resolution of LISe-based neutron imagers, we have developed a modified η-function to identify the location of neutron absorption using a double-sided strip design. The modified η-function consists of training data from the simulated response across the strip electrodes for any interaction location and emission angle of the secondary charged particles. The current machine learning technique utilized is the coarse and fine regression trees. The η-function is tested using MCNP simulations, which shows that we are approaching our targeted 5 µm spatial resolution.

Under US Department of Homeland Security sponsorship, Spectral Labs Incorporated has developed a prototype high-resolution retrofit for an existing mobile VACIS, named the High-Resolution Imaging System (HiRIS). The legacy 256 NaI detectors in the VACIS detector column were replaced with 576 CsI detectors, more than doubling the pixel count. Using SiPMs to replace conventional PMTs allowed the packing of more detectors in the same VACIS detector enclosure. Legacy analog signal-processing electronics were replaced with advanced digital signal-processing electronics. Replacing gross counting in the legacy system with multichannel analysis of the counts from each detector will allow better control of detector crosstalk. The HiRIS detector modules were installed on a VACIS truck refurbished to as-new condition. Initial testing of the HiRIS prototype demonstrates enhanced spatial resolution by a factor of two as compared to the legacy system, without any degradation in throughput capability (20 containers per hour).

Radiation is an important part of security space: It is detected either passively in search of special nuclear materials or actively to monitor or interrogate objects of interest. Systems relying on radiation require adequate detectors. The most common radiation detectors are based on scintillating materials that convert hard (gamma, x-ray or neutron) radiation into visible light registered by a photodetector.

We report on our development of large volume Cs 2 LiYCl 6 (CLYC) detectors for nuclear security applications. Three-inch diameter boules have been grown and 3-in right cylinders have been fabricated. Crystals containing either >95% 6 Li or >99% 7 Li have been grown for applications specific to thermal or fast neutron detection, respectively. We evaluated their gamma and neutron detection properties and the performance is as good as small size crystals. Gamma and neutron efficiencies were measured for large crystals and compared with smaller size crystals. With their excellent performance characteristics, and the ability to detect fast neutrons, CLYC detectors are excellent triple-mode scintillators for use in handheld and backpack instruments for nuclear security applications.

In this paper, we report on the characterization of detectors constructed at RMD using 1-inch, 1.5-inch, and 2-inch diameter right cylinders of CLYC and CLLBC crystals coupled to arrays of silicon photomultipliers (SiPM). Detectors constructed with small volume CLLBC crystals coupled to a 12 mm × 12mm total area SiPM array show an excellent energy resolution of ~3% at 662 keV and a large volume CLYC crystal when coupled to a 24 mm × 24 mm area array shows an energy resolution of ~6.6% at 662 keV. Both detectors show pulse shape discrimination capabilities.

Radiation Monitoring Devices (RMD) has developed a composite detector technology for multi-mode detection, based on incorporation of the inorganic scintillation material into a pulse shape discrimination (PSD) - capable plastic scintillator, mainly for handheld and backpack applications. The inorganic component is a dual-mode scintillator such as CLYC, CLLBC, or TLYC, capable of detecting gamma-rays and thermal neutrons. The plastic scintillator serves as an optical light guide, and provides gamma and fast neutron detection along with PSD among all the signatures. The advantage of this technology is the multi-mode detection within a single element at a lower cost compared to high quality large single crystals of inorganic scintillators for building highly efficient, large volume detectors. The technology is scalable since composite detector with multiple elements of inorganic scintillator behaves as a large volume single crystal, and the size is not limited by the crystal growth yield issues.

Lithium Indium Selenide (LiInSe2) has been under development in RMD Inc. and Fisk University for room temperature thermal neutron detection due to a number of promising properties. The recent advances of the crystal growth, material processing, and detector fabrication technologies allowed us to fabricate large detectors with 100 mm(2) active area. The thermal neutron detection sensitivity and gamma rejection ratio (GRR) were comparable to He-3 tube with 10 atm gas pressure at comparable dimensions. The synthesis, crystal growth, detector fabrication, and characterization are reported in this paper.

Properties of dual-mode scintillation detectors based on CLLBC crystals are reported. Energy resolution and light yield are measured at 2.9% (FWHM) at 662 keV and 45000 photons/MeV, respectively, for a 1-in-diameter and 1-in-long crystal. With less than 2% variation in light yield as a function of energy, CLLBC has better proportionality than LaBr 3 and NaI:Tl. Neutron peak resulting from reactions with neutrons emitted by 252 Cf (moderated) is measured at a gamma energy equivalent of 3.1 MeVee (electron energy equivalent), making pulse height discrimination between gamma-rays and neutrons easy. The material is also of effective pulse shape discrimination. The figure-of-merit for discrimination of gamma rays and thermal neutrons in CLLBC can be as high as 3.2, which is comparable to that of CLYC.

Samples of Cs2LiYCl6:Ce (CLYC) scintillator have been characterized using monoenergetic neutron beams in the energy range 4.1–5.5MeV. Four crystals with dimensions (thickness×diameter) of 1″×1″, 1″×2″, and 2″×2″ were evaluated, including one crystal with natural concentrations of Li isotopes and three that were enriched in 6Li. The intrinsic efficiency of CLYC for fast-neutron detection has been determined for the natural-Li crystal. These measurements were translated into reaction cross-sections, and show good agreement with available cross-section data for neutron interactions with the 35Cl component of CLYC. Furthermore, it is shown that the charged-particle energy released in the fast-neutron reactions on 35Cl varies linearly with the energy of the incoming neutron. These results verify the efficacy of CLYC for fast-neutron spectroscopy in a range of applications.