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We report on our development of large volume Cs2LiYCl6 (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% 6Li or >99% 7Li 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.

Tl2LiYCl6:Ce (TLYC), a new cerium doped-thallium based, dual mode gamma and neutron elpasolite scintillation crystal, has been grown and evaluated at RMD. Energy resolution of 4.2% at 662 keV (FWHM) is measured for samples of this material. From comparison with a 137Cs spectrum collected with NaI:Tl, a gamma-ray induced light yield of 26,000 ph/MeV is estimated for TLYC. The material also shows better proportionality of response than both LaBr3:Ce and NaI:Tl in the energy range between 32 keV to 1275 keV. Single thermal neutron interactions produce a peak measured at a gamma equivalent energy of 1.9 MeVee, corresponding to a (neutron induced) light yield of approximately 47,000 ph/n. Decay times obtained from gamma-ray interactions in TLYC are measured at about 57 ns, 431 ns, and 1055 ns, with slightly shorter values measured for neutron interactions. These differences allow for gamma-neutron pulse shape discrimination (PSD) and a PSD Figure-of-Merit (FOM) of 2 is measured with TLYC.

We report on a new family of scintillators - Lithium alkaline halides , developed based on the alkaline halides by introducing lithium for dual mode gamma-neutron detection. Many different compositions were grown, among which ${\rm LiSr}_{2}{\rm I}_{5}$ (LSI), ${\rm LiCa}_{2}{\rm I}_{5}$ (LCI), ${\rm LiSr}_{2}{\rm Br}_{5}$ (LSB) activated with divalent Europium show good gamma and neutron detection properties. LSI shows the main emission at ${\sim}$ 497 nm under X-ray excitation. It also shows good proportionality, which in combination with the light yield as high as 60000 photons/MeV, results in an energy resolution of 3.5% at 662 keV. The electron or gamma equivalent energy (GEE) of the thermal neutron peak due to the $^6{\rm Li}$ neutron capture is 4.1 MeV, which amounts to a very high neutron light yield of ${\sim}$ 245000 photons. The decay times for neutrons are faster compared to that for gamma-rays, hence we achieved good pulse shape discrimination (PSD) between gamma and neutron events. Our initial studies on the effects of Eu concentration on the properties of LSI show that 3%–4% Eu concentration is optimal for the best performance in terms of gamma and neutron light yields and pulse shape discrimination. LCI shows the main emission at ${\sim}$ 475 nm under X-ray excitation and a very high gamma light yield of ${\sim}$ 90000 photons/MeV. The measured energy resolution is 6% at 662 keV. The electron equivalent energy for neutron detection has been measured to be around 3 MeV, which gives a neutron light yield of ${\sim}$ 270000 photons. The measured decay times for neutrons are faster compared to gamma decays and the PSD between the gamma-rays and neutrons is not as good as LSI. LSB shows two emissions at ${\sim}$ 410 and 475 nm under X-ray excitation. The measured light yield is ${\sim}$ 32000 ph/MeV gamma-ray with an energy resolution of 6% at 662 keV. The electron equivalent energy of the $^6{\rm Li}$ capture peak was measured to be 3.3 MeV.

This paper reports on a new Ce-doped Tl-based scintillator , Tl 2 LaCl 5 (TLC), for gamma-ray detection. 10 mm diameter crystals have been successfully grown using the vertical Bridgman method . The emission peak of TLC is detected at 383 nm under X-ray excitation. The light yield of TLC is 76,000 ph/MeV. The samples show excellent energy resolution of 3.4% (FWHM) at 662 keV. The non-proportionality is less than 1%, from 32 keV to 1275 keV. The major scintillation decay time is 36 ns.

Samples of Cs2LiYCl6 :Ce (CLYC) scintillator have been characterized using monoenergetic neutron beams in the energy range 4.1–5.5 MeV. 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 6 Li. 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 35 Cl 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.

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 $\times $12mm total area SiPM array show an excellent energy resolution of $\sim 3$% at 662 keV and a large volume CLYC crystal when coupled to a 24 mm $\times $24 mm area array shows an energy resolution of $\sim 6.6$% at 662 keV. Both detectors show pulse shape discrimination capabilities.

CeBr3 is a new fast scintillator with timing and energy resolution similar to LaBr3:Ce. The excellent timing resolution, measured here to be 93 ps at 511 keV for 1 cm3 crystals, naturally lends itself to direct electronic measurements of excited nuclear state lifetimes in the nanosecond and sub-nanosecond range. To demonstrate this, half-lives of a 1.40 ns isomer in 152Sm and 537 ps isomer in 177Hf were directly measured using the delayed coincidence technique. With its fast timing, high light output and high efficiency, the possibility of CeBr3 as a time-of-flight PET detector was also explored through γ - ray time-of-flight measurements.

Cs 2 LiYCl 6 (CLYC) is one of the most interesting new scintillators developed during the last fifteen years. Its use as a thermal neutron scintillator was discovered in 1999 at the Delft University of Technology in Netherlands. In 2003, CLYC was selected by Radiation Monitoring Devices (RMD) for further studies and ultimately resulted in a commercial product. RMD's investigation of CLYC showed that it is a very good gamma ray detector with excellent 4% or better energy resolution, making it an ideal choice for dual mode detectors. Recently, Bubble Technology Industries of Canada discovered the possibility of fast neutron detection with CLYC, increasing its versatility even further. In this paper, we present the progress of CLYC’s development since its inception, with a focus on the work conducted at RMD. Our initial results with small crystals are presented, followed by data collected with samples of consecutively larger volumes and quality. We discuss the improvements in the energy resolution from 7% to 3.6%, as well as volume from 1 cm 3 to 100 cm 3 . In addition, we address basic properties, gamma ray and neutron detection, pulse shape discrimination, and recently discovered fast neutron detection.

Single-crystal strontium iodide (SrI2) doped with relatively high levels (e.g., 3 - 6 %) of Eu2+ exhibits characteristics that make this material superior, in a number of respects, to other scintillators that are currently used for radiation detection. Specifically, SrI2:Eu2+ has a light yield that is significantly higher than LaBr3:Ce3+ -a currently employed commercial high-performance scintillator. Additionally, SrI2:Eu2+ is characterized by an energy resolution as high as 2.6% at the 137Cs gamma-ray energy of 662 keV, and there is no radioactive component in SrI2:Eu2+ - unlike LaBr3:Ce3+ that contains 138La. The Ce3+-doped LaBr3 decay time is, however, faster (30 nsec) than the 1.2 sec decay time of SrI2:Eu2+. Due to the relatively low melting point of strontium iodide (~515 oC), crystal growth can be carried out in quartz crucibles by the vertical Bridgman technique. Materials-processing and crystal-growth techniques that are specific to the Bridgman growth of europium-doped strontium iodide scintillators are described here. These techniques include the use of a porous quartz frit to physically filter the molten salt from a quartz antechamber into the Bridgman growth crucible and the use of a bent or bulb grain selector design to suppress multiple grain growth. Single crystals of SrI2:Eu2+ scintillators withmore » good optical quality and scintillation characteristics have been grown in sizes up to 5.0 cm in diameter by applying these techniques. Other aspects of the SrI2:Eu2+ crystal-growth methods and of the still unresolved crystal-growth issues are described here.« less

Cs2LiYCl6:Ce (CLYC) has been demonstrated to be sensitive to thermal neutrons via the 6Li(n, α )t reaction, and recently to fast neutrons via the 35Cl(n,p) reaction. The scintillation properties of CLYC have been investigated in more detail to further understand its capabilities. Pulses from thermal neutron, fast neutron, and γ -ray induced excitations were captured, digitized over a 16 μ s time range, and analyzed to identify the scintillation mechanisms responsible for the observed shapes. Additionally, the timing resolutions of CLYC crystals of different sizes were measured in coincidence with a fast CeBr3 scintillator. The effect of high count rates on fast neutron energy resolution and pulse-shape discrimination was investigated up to 45 kHz.

This paper describes detectors comprised of Cs2LiYCl6:Ce (CLYC) scintillators coupled to silicon photomultipliers (MPPC, Hamamatsu). CLYC has been developed for dual gamma ray and thermal neutron detection. MPPCs are compact detectors with a very thin profile (~2 mm), but also a small active area (6 mm × 6 mm). The combination of both can create a compact replacement of a He-3 tube. Three different crystal sizes were tested: a 5 mm × 5 mm × 5 mm cube, a 1 cm × 1 cm × 4 cm cuboid, and a O1 in × 1 in cylinder. All three detectors showed a well-resolved thermal neutron peak with energy resolution of 5.4%, 7%, and 11% (FWHM), respectively. The degradation in larger crystals is due to a mismatch between the coupled crystal face and the active area of the light detector. All detectors showed the 662 keV gamma ray peak from a Cs-137 source, indicating capability for limited gamma ray spectroscopy, with energy resolutions of 10%, 17%, and 26%, respectively. The capability for pulse shape discrimination was shown as well. The efficiency of the detectors was measured against a 10 atm He-3 tube (9 cm3 volume). The best absolute efficiency was shown by the cylinder detector, next by the He-3 tube (almost the same as the cylinder detector), and finally by the cuboid detector. The latter showed the best neutron count per cm3, twice as high as the two other detectors.

Up to 1-in-diameter, Eu-doped CsBa2I5 and BaBrI crystals have been grown using the vertical Bridgman method. Scintillation properties of the crystals up to ~ 1 × 1 × 1 cm3 have been studied under gamma-ray irradiation. Systematic concentration studies have been performed for both these alkaline earth halides with Eu2+ doping levels ranging from 0% to 10% to optimize growth of large crystals. We have achieved excellent energy resolution of ~ 2.6% at 662 keV for a 2 × 2 × 1-mm3 sample of CsBa2I5:2%Eu. However, in this paper, the emphasis is on the larger crystals that can be used practically in nuclear nonproliferation applications. Our measurements of a 1.0 × 1.0 × 1.0-cm3 sample of CsBa2I5:3%Eu, processed from a 1-in-diameter crystal, shows very good energy resolution of 3.9% at 662 keV, high light output of ~ 80 000 photons/MeV, and a principal decay time of ~ 900 ns. The second investigated composition, BaBrI:5%Eu, 1.0-cm-diameter × 1.0-cm-thick sample shows high light output of 71 000 photons/MeV and energy resolution of 4.3% at 662 keV. A decay time of 480 ns was measured for this sample, which is shorter than most Eu2+ doped materials of this size. The emission in these materials is due to d → f transitions of Eu2+ and occurs over a single band between 400-500 nm. It peaks at ~ 430 nm for CsBa2I5:Eu and at ~ 413 nm for BaBrI:Eu. Since both compositions show high proportionality over a wide range of energies, we expect to see improvements in the energy resolution of large crystals, as better quality of 1-in-diameter crystals are grown.

This paper discusses our initial investigation of fast neutron detection with a Cs2LiYCl6 (CLYC) scintillator. CLYC has been already developed for dual mode detectors (thermal neutrons and gamma rays). Described are results collected under mono-energetic irradiation from a Van de Graaff generator and a continuous irradiation from a 252Cf source. There are two reactions in which fast neutrons are captured 35Cl(n,p)35S and 6Li(n,t)α; both have been observed. They produce a proton and a pair of α/t particles, respectively. The response to mono-energetic fast neutrons due to the 35Cl(n,p) reaction produces a peak and due to the 6Li(n,t) reaction a continuum in the energy spectra. The relation between the peak (continuum) position and the excitation energy is linear within evaluated energy range. This allows for fast neutron spectroscopy. The α/β ratios for both reactions were found to be different and somewhat dependent on the excitation energy. The decay times under the proton excitation slightly differ from these under the α/t excitation, while both are significantly different from the gamma ray excited curves. This is important for pulse shape discrimination. Using continuous excitation from 252Cf initial efficiency was estimated for the 35Cl(n,p) reaction. For a 1-inch right cylinder crystal the measured intrinsic efficiency is at least 0.06%, while calculated is 0.5%. The discrepancy is due to experimental inaccuracies. The relative detection efficiency scales linearly with the volume. A convolution of the theoretical 252Cf fast neutron distribution and the 35Cl(n,p) cross-section curve was found to match the experimental data.

Cerium doped Tl 2 LiYCl 6 (TLYC) is a new, Tl-based elpasolite scintillation detector. TLYC crystals with 0%, 0.5%, 3%, 5%, and 9% cerium concentrations have been grown. Emission spectra of TLYC with these different Ce concentrations peak between 430 and 445 nm. Energy resolution values of 6.8%, 5.2%, 4.2%, 3.9%, and 4.4% at 662 keV (FWHM) are measured for TLYC doped with 0%Ce, 0.5%Ce, 3%Ce, 5%Ce, and 9%Ce concentrations, respectively. Comparison with a 137Cs spectrum collected with a NaI:Tl results in gamma-ray induced light yields of 18,000 ph/MeV for 0% Ce, 27,000 ph/MeV for 0.5%Ce, 27,000 ph/MeV for 3%Ce, 30,000 ph/MeV for 5%Ce, and 28,000 ph/MeV for 9%Ce.

– In this presentation, we will discuss crystal growth and scintillation properties of large diameter SrI 2 :Eu2+ single transparent crystals. The crystals were grown successfully using the vertical Bridgman technique. Crystals with different diameters from 1 inch to 2 inches are discussed. SrI 2 :Eu2+, a scintillator discovered a half century ago by Hofstadter, was recently re-discovered as an outstanding material for gamma ray-spectroscopy. Its key properties include good non-proportionality, high light yield (from 85,000 to 120,000 photons/MeV) and excellent energy resolution (2.8 % at 662 keV). Due to its good properties our immediate focus has been on scaling up and development of this material for commercial use. Other properties of SrI 2 :Eu include Z eff of 50, a mono-exponential decay with a time constant of 1μs to 5 μs (depending on crystal size and dopant concentration), an emission band at 410-450 nm (2.7-3.0 eV). SrI 2 also shows no intrinsic radioactivity unlike lanthanide compositions.

In recent years, a number of materials from the elpasolite crystal family have been under development for either or both gamma ray and neutron detection. The scintillators show good energy resolution and thermal neutron detection efficiency. The latter is achieved due to the fact, that the selected compositions contain Li-6 ions. In order to effectively and reliably register both types of radiation, it is necessary to separate them through particle identification schemes. This can be accomplished using either pulse height or/and pulse shape discrimination, with the latter being more reliable. In this paper, we summarize our work and provide current status of pulse shape discrimination in the selected elpasolite scintillators. These include Cs2LiYCl6 (CLYC), Cs2LiLaCl6(CLLC), Cs2LiLaBr6(CLLB), and Cs2LiYBr6(CLYB).

New and emerging scintillators, such as DPA (Diphenylanthracene) and CLYC (Cs2LiYCl6:Ce), coupled with Solid-State Photomultipliers (SSPM) provide a lightweight, low voltage, potentially radiation hard spectrometer for real-time monitoring of space weather conditions. This paper presents the results from proton radiation-hardness tests conducted for candidate scintillators and for individual cells of 1 cm2 SSPMs. Decreases from original light outputs were observed for samples of DPA and CLYC by 4% and 28% respectively with a dose of 1 kGy . Following 1 kGy exposure to the SSPMs, the dark current increased by a factor of ~ 32 for a reverse bias of 30 V. The effects of room temperature annealing on the dark current were also monitored. Decreases in quantum efficiency (22% for 420 nm and 7% for 535 nm) and light response (33% for 420 nm and 26% for 535 nm) were observed as a possible result of the formation of surface recombination-generation centers. The energy resolution of a LYSO scintillator sample coupled with a SSPM and measured for a 22Na gamma source (E = 511 kev) decreased from 15% pre-irradiation to 34% post 1 kGy irradiation.

This paper summarizes the initial investigation of large diameter (2-inch) Cs2LiYCl6:Ce (CLYC) crystals grown at RMD. Although the crystals had some cracks, the tested sample provided adequate results: It produced a clear thermal neutron peak, a difference in time profiles under gamma and neutron excitation was observed, and pulse shape discrimination (PSD) based on two integration windows and their ratios was achieved. The PSD provided an excellent separation between gamma and neutron events. The discrimination ratio was better than 1:1000 (based on 15,000 events). We have also tested a relatively smaller sample with excellent crystal quality for the energy resolution. The results surpassed those previously obtained. The recorded energy resolution at 662 keV was 4.7% (FWHM) using a standard bialkali PMT. This is better than the previously measured 5.1% value. A 4.3% energy resolution was obtained with a super bialkali PMT.

Phase-pure transparent LSO ceramic was obtained, with its scintillation properties rivaling those of single crystalline LSO. The transparent LSO ceramic was prepared by a nano-technology approach. The densities of the LSO ceramic increased with increasing sintering temperature. Further SEM examination confirmed the decrease of the porosity during densification of the ceramic. Pores were segregated at grain boundaries after sintering. Hot isostatic pressing (HIPing) was employed to further densify the ceramic and eliminate porosity after sintering. Transparent polycrystalline LSO ceramics were obtained after the final HIP. XRD examination confirms single monoclinic LSO phase. A light output as high as 30,100 ph/MeV was obtained using a 22Na excitation source. LSO ceramic showed an energy resolution of 15% (FWHM) at 662 keV (137Cs source) and a fast scintillation decay of 40 ns due to the 5d \ura 4f transition of Ce3+. The excellent scintillation and optical properties make LSO ceramic a promising candidate for future gamma-ray spectroscopy as well as medical imaging applications.

We have grown and investigated 1 inch diameter (CLYC) crystals for gamma and neutron detection. The samples provided excellent results. For example, 5.1plusmn0.1% energy resolution was obtained at 662 keV (5.5% at 511 keV). The light output of 4500plusmn350 photoelectrons/MeV (PMT, Hamamatsu R6233) was measured. The samples also showed excellent plusmn1.2% non-proportionality in the 14.4 to 1274 keV range. This suggests a possibility for even better energy resolution with a superior photodetector. The intrinsic energy resolution of investigated crystals was estimated to be 2.1%. The neutron detection was also confirmed. The neutron peak was observed at about 3.2 MeV (gamma equivalent) and its resolution was 2.9plusmn0.1%. Gamma-neutron pulse shape discrimination was also achieved.

Although LSO is one of the most successful scintillator developments for medical diagnostics in the last two decades, good single crystals are not commercially available in any quantity. Consequently, we explored the feasibility of developing a ceramic version of the material, which requires a considerably lower temperature to consolidate the material to essentially crystalline density. Consolidation of the ceramic was achieved by hot pressing at temperatures up to 1700degC and pressure of 8000 psi. Hot pressing causes a loss of oxygen resulting in strong coloration of the ceramic, which had to be removed by heating (ldquobleachingrdquo) in air at approximately 1100degC. The resultant specimens were colorless and highly translucent, but the anisotropic nature of the crystal structure precluded the achievement of full transparency. The scintillation performance of the resulting ceramics was characterized and compared with that of high light-output LSO single crystals. The scintillation efficiency as measured by energy spectra generally fell in the range of 50-60% of that of the crystals. While this would be adequate for PET applications, the limited transparency provides a barrier to such use. An alternative application would be in signal integrating techniques, such as CT, where it could provide an alternative to GOS but with higher speed. Here, the problem of afterglow assumes major importance. The afterglow is a function of many factors, including conditions of excitation. Further work on improving the LSO ceramics is considered.

In this paper we present initial results on mixed LuI3-YI3-GdI3 scintillators for gamma and neutron detection. The scintillation properties were investigated and compared to the results obtained for the binary compositions. Small samples, few millimeters in size, were tested. Under X-ray excitation each crystal exhibits bright green emission in the 425 to 750 nm spectral range. The exact peak position depends slightly on the material composition. The emission is due to d-f transitions on the Ce3+ ion. The scintillation under gamma excitation is fast and decays with about 30 ns time constant. The rise time varies slightly and it can be as fast as 0.5 ns. The light output of the investigated samples varied from 115,000plusmn13,000 ph/MeV for LuI3 to 68,000plusmn8,000 ph/MeV for LuI3-YI3 composition. The energy resolution at 662 keV was difficult to estimate since the samples lacked in crystal quality. LuI3-GdI3 composition was also tested for the detection of thermal neutrons from a moderated 252Cf source. The neutron peak appears at 81 keV when compared to 60 keV peak in 241Am spectrum recorded under the same conditions. The light output per a single detected neutron is about ~6,350 photons.

The recent development of fast scintillating crystals with high light output (such as LaBr3, LYSO, and LSO) has made it possible to incorporate time-of-flight information into whole-body positron emission tomography scanners (TOF-PET). The development of fast scintillators has continued, and scintillators such as CeBr3 and the six crystals we report on here are candidates for future TOF-PET scanners. We have performed preliminary tests on six prototype scintillating crystals composed of YI3 (2% Ce), (Lu,Gd)I3 (2% Ce), Lul3 (10% Ce), Lul3 (5% Ce), Lul3 (2% Ce), and Ce(Br,Cl)3. The crystals were coupled to a PMT to observe 511 keV photons produced by positron captures in a 22Na point source. A LaBr3 coincidence detector was used in order to make timing resolution measurements. The prototype samples had a range of timing resolutions from 160 ps to 250 ps (FWHM). Ce(Br,Cl)3 was found to have the best timing resolution, with 160 ps, followed by Lul3 (2% Ce) with 180 ps. These results are competitive with the 160 ps we observed for a pair of LaBr3 crystals in coincidence with each other. The 2% Ce doped Lul3 sample was found to have the best overall energy response, with an energy resolution (FWHM) of 6% at 511 keV. Energy resolution and light yield measurements are presented for each of the crystals.

In this paper we report on the crystal growth and characterization of a new class of inorganic scintillators based on the rare earth iodides, in particular LuI 3 , YI 3 and GdI 3 , doped with trivalent cerium. Single crystals of LuI 3 :Ce 3+ , YI 3 :Ce 3+ and GdI 3 :Ce 3+ were grown by the vertical Bridgman technique in evacuated silica ampoules. In some cases, tantalum or graphite crucibles were used to minimize wetting of the ampoule. X-ray excited optical luminescence spectra of LuI 3 :Ce 3+ , YI 3 :Ce 3+ and GdI 3 :Ce 3+ exhibit a broad band due to Ce 3+ emission, peaking in the 500–550 nm region. LuI 3 :Ce 3+ , YI 3 :Ce 3+ and GdI 3 :Ce 3+ show high light yields up to 100,000 photons/MeV and fast principle decay time constants of

LaBr3:Ce and CeBr3 are novel inorganic scintillators for gamma-ray detection and spectroscopy. Both materials exhibit exceptional scintillation properties, such as very high light output, fast response and excellent energy resolution. Custom equipment was built for crystal growth and handling of the moisture- and oxygen-sensitive materials. Crystal growth runs of large diameter crystals of LaBr3:Ce and pure CeBr3 were conducted in vertical multizone-zone Bridgman systems due to their operational simplicity and the potential for reduced thermal shock to the crystals. Procedures were developed to prepare clean charges, inhibit the formation of oxyhalides, and reduce the residual stresses and cracks in the crystals.

Nuclear non-proliferation applications and monitoring of special nuclear materials (SNMs) prefer low-cost detector solutions that provide efficient gamma-ray and neutron detection in combination with neutron/gamma pulse shape discrimination (PSD). Among the inorganic and organic detector choices, recently developed plastic scintillators can be fabricated in large sizes at low cost and show good neutron detection efficiency in combination with excellent neutron/gamma PSD. Unfortunately, these plastic scintillator have low density and effective Z limiting gamma-ray detection to gross counting. In this paper we report on the characterization of metal-loaded plastic scintillators that provide gamma-ray spectroscopy as well as fast neutron detection, whereby discrimination between gamma-rays and neutrons is accomplished using pulse shape analysis. In particular, we report on the investigations of tin-, bismuth-, and lead-loaded plastic scintillators for nuclear non-proliferation.

In recent years, a number of new inorganic scintillating materials have been discovered that offer simultaneous thermal neutron and gamma ray detection. One of the first such scintillators is Cs2LiYCl6 (CLYC) [1], which offers (1) efficient thermal neutron detection (2 x higher cross section than 3He at 10 atmospheres); and (2) excellent separation between gamma and neutron events (>10−7). However, the cost of the new single-crystal scintillators is still relatively high, compared to traditional scintillators, such as NaI and CsI. Significant progress in the development of organic scintillators with neutron and gamma sensitivity has also been recently achieved [2]. They have very low production cost, but have relatively low detection and photopeak efficiencies due to the low density and low-Z constituents. They also do not provide thermal neutron detection.

Single crystals of LaBr 3 :Ce and LaCl 3 :Ce have been grown by the vertical Bridgman technique. Crystals of these scintillators are used in the fabrication of high-resolution, gamma-ray spectrometers. The LaBr 3 :Ce and LaCl 3 :Ce crystals we have grown have high light outputs of ∼80,000 and 50,000 photons/MeV, respectively, and fast principal decay constants of < 30 ns. The emission wavelength for the LaBr 3 :Ce and LaCl 3 :Ce scintillators are λ max = 360 and 350 nm, respectively. Both materials have excellent energy resolutions of ∼3% FWHM for 662 keV photons at room temperature. In this paper, we will report on our results to date for vertical Bridgman crystal growth and characterization of Ce-doped LaBr 3 and LaCl 3 crystals. We will also describe the handling and processing procedures developed for these oxygen and moisture sensitive materials.

LaCl 3 :Ce and LaBr 3 :Ce scintillators are characterized by a high light yield and very fast nanosecond primary emission. To study scintillation mechanisms in these materials, we have measured low-temperature thermoluminescence. Both crystals exhibited prominent glow peaks below 100 K: LaCl 3 :10%Ce sample showed multiple glow peaks with two major ones at 65 and 66 K, whereas the LaBr 3 :0.5%Ce sample showed two major peaks at 65 and 75 K. Assuming first-order kinetics, the traps responsible for those peaks can be described by the following parameters: E =0.168 eV, s =1×10 12 s −1 ; E =0.135 eV, s =1×10 9 s −1 for LaCl 3 :Ce and E =0.144 eV, s =1×10 10 s −1 ; E =0.166 eV, s =1×10 10 s −1 for LaBr 3 :Ce. The lifetimes of listed traps at RT are in the range of radiative lifetime of Ce 3+ emission (0.5–50 ns).

We are developing a high-resolution, high-efficiency positron emission tomography (PET) detector module with depth of interaction (DOI) capability based on a lutetium oxyorthosilicate (LSO) scintillator array coupled at both ends to position-sensitive avalanche photodiodes (PSAPDs). In this paper we present the DOI resolution, energy resolution and timing resolution results for complete detector modules. The detector module consists of a 7 x 7 matrix of LSO scintillator crystals (1 x 1 x 20 mm3 in dimension) coupled to 8 x 8 mm2 PSAPDs at both ends. Flood histograms were acquired and used to generate crystal look-up tables. The DOI resolution was measured for individual crystals within the array by using the ratio of the signal amplitudes from the two PSAPDs on an event-by-event basis. A measure of the total scintillation light produced was obtained by summing the signal amplitudes from the two PSAPDs. This summed signal was used to measure the energy resolution. The DOI resolution was measured to be 3-4 mm FWHM irrespective of the position of the crystal within the array, or the interaction location along the length of the crystal. The total light signal and energy resolution was almost independent of the depth of interaction. The measured energy resolution averaged 14% FWHM. The coincidence timing resolution measured using a pair of identical detector modules was 4.5 ns FWHM. These results are consistent with the design goals and the performance required of a compact, high-resolution and high-efficiency PET detector module for small animal and breast imaging applications.

In this paper, investigation of position sensitive avalanche photodiodes (PSAPDs) as optical detectors for reading out segmented scintillation arrays of LSO in high resolution PET modules is reported. PSAPDs with 8/spl times/8 mm/sup 2/ and 14/spl times/14 mm/sup 2/ area have been characterized with single LSO crystals and arrays. Energy resolution of 19% (FWHM) for 511 keV /spl gamma/-rays and coincidence timing resolution of /spl sim/3 ns (FWHM) have been recorded with PSAPD coupled to 1/spl times/1/spl times/20 mm/sup 3/ LSO detectors. Flood histogram studies have been successfully conducted by coupling multi-element element LSO arrays (1 mm pixels, 20 mm tall) to the PSAPDs. Finally, depth of interaction (DOI) resolution of

A gamma ray detector for PET, consisting of an array of mixed lutetium oxyorthosilicate (MLS) scintillator crystals coupled to a position sensitive avalanche photodiode (PSAPD), was evaluated. The scintillator array was constructed from individual MLS crystals with dimensions of 1.5 mm /spl times/ 1.5 mm /spl times/ 15 mm. The assembled 7 /spl times/ 7 array, including intercrystal reflector material, had a pitch of 1.79 mm. The low noise, high gain PSAPD had dimensions of 14 mm /spl times/ 14 mm. Peaks associated with each of the 49 scintillator crystals were readily identifiable in flood histograms, and most of the crystals demonstrated energy resolution in the range of 15% to 20% at 511 keV. Measurements of the timing of the PSAPD in coincidence with a fast-scintillator/PMT detector indicated a timing resolution of approximately 4 ns. The operating characteristics and design attributes, such as compactness and reduced readout channel requirements, of the PSAPD make it attractive for high resolution PET applications.

In this paper, we report on a relatively new cerium-doped scintillator—LaCl 3 for γ-ray spectroscopy. Crystals of this scintillator have been grown using Bridgman method. This material when doped with 10% cerium has high light output (∼50,000 photons/MeV) and fast principal decay time constant (∼20 ns). Furthermore, it shows excellent energy resolution for γ-ray detection. For example, energy resolution as low as 3.2% (FWHM) has been achieved with 662 keV photons ( 137 Cs source) at room temperature. Also, high timing resolution (264 ps—FWHM) has been recorded with LaCl 3 -PMT and BaF 2 -PMT detectors operating in coincidence using 511 keV positron annihilation γ-ray pairs. Details of crystal growth, scintillation properties, and variation of these properties with cerium concentration are also reported.

A TOC scintillator material, lanthanum yttrium oxide ((La,Y)2O3) has been investigated at RMD. The X-ray excited emission shows a broad band that peaks at ∼ 415 nm, originating from the host lattice. Under gamma-ray excitation the emission decays with a fast time constant of ∼ 17 ns, and multiple slower components. The temperature dependence of LYO was studied in the −30°C to +50°C range. The relative light output decreases with the increase in the temperature, whereas, the energy resolution was observed to be fairly constant. The scintillation light yield of the material estimated by a comparison with a BGO single crystal is ∼21,000 photons/MeV. Energy resolution of 4.9% at 662keV (FWHM) was also measured. The composition shows good proportionality in the 60 keV to 1275 keV energy range, with only ∼ 2% deviation from the linear response at 60 keV. The scintillation properties will be further improved by optimization of La concentration and optical quality of the ceramics. The material is not hygroscopic and scale up to large sizes is possible due to cubic structure of Y2O3. Overall, LYO seems to be a very promising material for gamma-ray spectroscopy.

Numerous instruments have been developed for performing gamma-ray imaging and neutron imaging for research, nondestructive testing, medicine and national security. However, none are capable of imaging gamma-rays and neutrons simultaneously while also discriminating gamma-rays from the neutron. This paper will describe recent experimental results obtained using a gamma/neutron camera based on Cs 2 LiYCl 6 :Ce (CLYC) scintillation crystals, which can discriminate gamma-rays from neutrons. The ability to do this while also having good energy resolution provides a powerful capability for detecting and identifying shielded special nuclear materials for security applications. Also discussed are results obtained using a LaBr 3 scintillation crystal.

In this paper we report on the scintillation properties of cerium doped strontium - and barium hafnate. Radioluminescence, pulse height, scintillation decay and timing spectra are presented. Radioluminescence spectra of SrHfO3:Ce3+ and BaHfO3:Ce3+ consist of a broad band due to Ce3+ emission peaking at 410 nm and 400 nm, respectively. The light yield of BaHfO3:Ce3+ and SrHfO3:Ce3+ is approximately 40 000 photons/MeV when compared to a crystal of BGO. The principal decay time constant for SrHfO3:Ce3+ and BaHfO3:Ce3+ is 42 and 25 ns, respectively. A timing resolution of 276 ps (FWHM) was obtained with transparent optical ceramic of SrHfO3:Ce3+.

Lutetium iodide (LuI 3 ) is a new addition to the family of Ce-doped lanthanide trihalide scintillating materials. Crystals of this material show hexagonal structure with density of 5.6 g/cm 3 and have been grown by the Bridgman method. Under X-ray excitation this material exhibits broad, cerium based emission that peaks at 475 and 520 nm. The fastest and major component of scintillation time profile of LuI 3 :Ce emission decays with a 31 ns time constant. The light yield of LuI 3 :Ce for thin samples (∼0.2 mm) was estimated to be ∼50,000 photons/MeV.

New generation of plastic scintillators have been developed at RMD for fast neutron detection technology. These plastics have peak emission wavelength ∼ 440 nm, fast scintillation decay < 10 ns, light output ∼ 13,000 photons/MeV, and excellent Pulse Shape Discrimination (PSD) between gamma rays and neutrons. We have achieved a Figure-of-Merit (FOM) of 2.3 at 1.0 MeVee electron energy threshold for a 2 inch diameter right cylinder sample. At RMD, comparative measurements were made between the plastic scintillator and Eljen liquid scintillator EJ309 both 1 inch diameter × 1 inch length. RMD plastic showed competitive performance. Additionally, in an experiment performed at the University of Kentucky 7 MV Van De Graaff accelerator, RMD plastic scintillator was irradiated with mono-energetic fast neutron beam energies up to 20.8 MeV. The results from this experiment confirm fast neutron spectroscopy capabilities. These results and effects of different electronic systems on the PSD measurements are discussed in this paper.

Scintillator crystal detectors form the basis for many radiation detection devices. Therefore, a search for high light yield single crystal scintillators with improved energy resolution, large volume, and the potential for low cost, is an ongoing process that has increased in recent years due to a large demand in the area of nuclear isotope identification. Alkaline earth halides, elpasolites and rare earth halides are very interesting because many compositions from these crystal families provide efficient Ce3+/ Eu2+ luminescence, good proportionality and good energy resolution. They also have small band-gap leading to higher light yields. Ce3+and Eu2+ are efficient, and the emission wavelengths in the 350-500 nm region matches well with PMTs and a new generation of Siphotodiodes. In this presentation, we will the present progress made in the crystal growth of these compositions, and scintillator properties of large diameter SrI2:Eu2+ single transparent crystals. The crystals were grown successfully using the vertical Bridgeman technique. Crystals with different diameters of 1”, 1.3”, and 1.5” will be discussed. SrI2:Eu was discovered a half century ago, and was recently found to be an outstanding material for gamma ray-spectroscopy with high light yield, very good non-proportionality, and excellent energy resolution. We will also discuss growth and properties of larger Cs2LiYCl6 (CLYC) crystals. Recently, it has been shown that crystals from the elpasolite family, including CLYC, can be successfully employed for a dual gamma ray and neutron detection, which is possible with the help of pulse shape discrimination (PSD). PSD allows for recognition of an incident particle’s nature based on the shape of the corresponding scintillation pulse. CLYC has the potential to minimize the cost and complexity of dual sensing gamma ray and neutron spectrometers. We also address progress in growth of CLYC crystals with large diameters (1” and 2”) that are transparent and crack free.

A new scintillation material CS2LiLaBr6-xCIx:Ce (CLLBC:Ce) belonging to the elpasolite crystal family has been discovered and investigated at RMD. A Cs2LiLaBr4.8Cl1.2: 2% Ce crystal was grown using the melt-based, vertical Bridgman method. The first grown crystal of CLLBC:Ce is 1.5 cm in diameter and a few inches in length. This material has a cubic structure, therefore, potentially can be grown in large sizes. Its scintillation properties have been studied under gamma and neutron irradiation. The presence of 6Li and 35Cl in CLLBC:Ce allows for thermal and fast neutron detection, respectively. CLLBC's response to fast neutrons via 35Cl(n,p)35S reaction was tested at different energies of mono-energetic fast neutron beam, acquired at the University of Massachusetts Lowell 5.5 MV Van de Graaff generator. A fast neutron peak was identified, and its centroid was observed to move linearly with the beam energy. The differences in the decay times of neutrons and gammas allowed pulse shape discrimination to differentiate between the two types of radiation. The material also shows an excellent response to gamma-ray irradiation, with gamma-ray energy resolution of ~3.0 % at 662 keV, and light output of 40,000 photons/MeV. These properties make CLLBC:Ce an excellent candidate for combined gamma-ray and neutron (thermal and fast) spectroscopy.

RadCam™ is a detection system developed by RMD for gamma ray imaging. The system images and tracks radiation contamination and its distribution. The current system is based on a CsI:Na scintillator coupled to a position-sensitive photomultiplier tube (PS-PMT). The image is obtained either using MURA coded aperture or pin-hole masks. Detected signals are decoded to produce radiation distribution that is overlaid on an image from a video camera (real-time). Due to increased focus on neutron detection, we recently started upgrading the system with a new scintillating material capable of simultaneous gamma ray and neutron detection - Cs2LiYCl6 (CLYC). Due to excellent pulse-shape discrimination, CLYC provides dual-mode detection of gamma-rays and neutrons while cleanly separating the signals from these two types of radiation. In addition, CLYC delivers better energy resolution than classical scintillators, such as CsI:Na. In this paper we present our initial results obtained with a system in which the original CsI:Na crystal was replaced with a CLYC scintillator.

Neutron detectors are used in homeland security to improve the identification of special nuclear materials (SNM). The detection of any neutrons above the natural background is a strong indication of SNM presence. Currently, there is a significant dependence on He-3 tubes for neutron detection. These devices are simple and effective detectors; however they have a number of drawbacks. He-3 tubes are pressure vessels and cannot be brought on board an airplane. They also exhibit micro-phonic effects, which are a serious issue for the handheld instrumentation. The last but not least is a diminishing stockpile of the He-3 gas, which stimulated the recent push to create new neutron detection technologies. In the last few years, RMD has developed a Cs2LiYCI6 (CLYC) scintillator that offers (1) efficient thermal neutron detection, which is higher on a per-volume basis than that of He-3; (2) excellent gamma rejection ratio (GRR), better than 1x10-7; and (3) gamma-ray energy resolution as good as 4% at 662 keV.

In this paper we introduce a new scintillator for fast neutron and gamma ray detection, Cs2NaYCI6:Ce (CNYC). The material belongs to the elpasolite crystal family. The fast neutron detection is based on the 35Cl(n,p)35S reaction. The proton produced carries the energy proportional to the excitation energy. We show neutron energy spectra measured under continuous excitation (Cf-2S2, Am/Be) and mono-energetic excitation (from a Van de Graaff generator). The latter show clear peaks in the energy spectra. Gamma detection is also discussed. The 7% (FWHM) energy resolution was measured at 662 keV. The light yield was estimated to be lower than 10,000 photons/MeV. The material shows good proportionality, as most elpasolite scintillators. Therefore improvement in the energy resolution is expected should better light yield be achieved. Decays times and pulse shape discrimination is also discussed. The latter is critical in extracting neutron data.

New scintillators belonging to Cs(Ba,Sr)(Br,I) 3 crystal family have been grown and investigated at RMD. The crystals were grown using the melt-based vertical Bridgman method. The iodide compositions were brighter compared to the bromides. Scintillation properties of these crystals have been studied under gamma irradiation, particularly CsBaI 3 :Eu in more details. CsBaI 3 :Eu has a tetragonal structure, and a density of 4.8 g/cc. The primary investigations of CsBaI 3 :Eu show an excellent energy resolution of 3.8% at 662 keV, high light yield of ∼55,000 photons/MeV, and a primary decay time of 760 ns. For CsBaI 3 :Eu, the emission occurs over a single band (400–500 nm), which is due to d → ƒ transition of Eu2+, with the emission wavelength peaking at 429 nm. The crystal shows high proportionality over a wide range of energies from 14 keV to up to 1.2 MeV.

Recently a number of alkaline earth halide scintillators doped with Eu2+ have been reported on. They are characterized by very good proportionality, high up to 100,000 photons/MeV light yield, and very good energy resolution, as low as 2.8% at 662 keV. Yet, one of the issues facing these materials is radiation trapping. Radiation trapping results from a small Stokes shift that creates considerable overlap between emission and absorption bands. As a result the scintillation light is absorbed and emitted multiple times, leading to a prolongation of the scintillation decay, potential light losses and degradation of energy resolution. Spectroscopic properties of various Eu2+ doped alkaline earth halides are presented. Materials studied include: SrI 2 , BaI 2 , BaBrI and other compounds. It appears that some compositions are less affected by radiation trapping.

A promising composition, Ce doped Cs 2 LiYBr 6 (CLYB) originally introduced by van Loef et al. has been investigated recently as a gamma-ray and neutron detector. It belongs to the elpasolite family and has a cubic structure. A single crystal of this material has been grown using the vertical Bridgman method. Scintillation properties such as energy resolution, emission, light yield, non-proportionality, and decay times are discussed in this paper. It can be used effectively to discriminate between the gamma and neutron events using pulse-shape discrimination (PSD) technique.