This PDF file contains the front matter associated with SPIE Proceedings Volume 10392, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Eu-doped strontium iodide single crystal growth has reached maturity and prototype SrI₂(Eu)-based gamma ray spectrometers provide detection performance advantages over standard detectors. SrI₂(Eu) offers a high, proportional light yield of >80,000 photons/MeV. Energy resolution of <3% at 662 keV with 1.5” x 1.5” SrI₂(Eu) crystals is routinely achieved, by employing either a small taper at the top of the crystal or a digital readout technique. These methods overcome light-trapping, in which scintillation light is re-absorbed and re-emitted in Eu²⁺-doped crystals. Its excellent energy resolution, lack of intrinsic radioactivity or toxicity, and commercial availability make SrI2(Eu) the ideal scintillator for use in handheld radioisotope identification devices. A 6-lb SrI₂(Eu) radioisotope identifier is described.

Plastic scintillators such as Polyvinyl Toluene (PVT) are used for radiation detection but due to their poor performance they are not widely implemented. In order to circumnavigate this, dopants are added to enhance scintillation by energy transfer otherwise lost through non-radiative processes. In this work, we exploit the effects of energy transfer through the use of short wavelength emission Cadmium Sulfide Quantum Dots (QD) as the transfer stimulant. Scintillation enhancement was observed as Cadmium Sulfide QD with scintillating dyes are embedded in PVT polymer matrix for beta and gamma radiation. Energy transfer was observed between Quantum Dots, scintillating dye, and the host polymer. Different concentrations of QD and 2,5-diphenyloxazole (PPO) dye are investigated to characterize the energy transfer.

There is growing interest in high-energy astrophysics community for the development of sensitive instruments in the hard X-ray energy extending to few hundred keV. This requires position sensitive detector modules with high efficiency in the hard X-ray energy range. Here, we present development of a detector module, which consists of 25 mm x 25 mm CeBr3 scintillation detector, read out by a custom designed two dimensional array of Silicon Photo-Multipliers (SiPM). Readout of common cathode of SiPMs provides the spectral measurement whereas the readout of individual SiPM anodes provides measurement of interaction position in the crystal. Preliminary results for spectral and position measurements with the detector module are presented here.

Pixelated Cadmium Zinc Telluride (CdZnTe) detectors are currently flying on the Nuclear Spectroscopic Telescope ARray (NuSTAR) NASA Astrophysics Small Explorer. While the pixel pitch of the detectors is ≈ 605 μm, we can leverage the detector readout architecture to determine the interaction location of an individual photon to much higher spatial accuracy. The sub-pixel spatial location allows us to finely oversample the point spread function of the optics and reduces imaging artifacts due to pixelation. In this paper we demonstrate how the sub-pixel information is obtained, how the detectors were calibrated, and provide ground verification of the quantum efficiency of our Monte Carlo model of the detector response.

The Italian Institute of Nuclear Physics is currently involved in the development of a prototype for a camera based on Silicon Photomultipliers (SiPMs) for the Cherenkov Telescope Array (CTA), a new generation of telescopes for ground{based gamma{ray astronomy. In recent years, SiPMs have proven to be highly suitable devices for applications where high sensitivity to low{intensity light and fast responses are required. Among their many advantages are their low operational voltage when compared with classical photomultiplier tubes, mechanical robustness, and increased photo{detection efficiency (PDE). Moreover, due to the possibility of operating them during bright moonlight, SiPMs can therefore considerably increase telescope duty cycle.

Here we present a full characterization of a particular type of SiPM produced in Italy by the Fondazione Bruno Kessler, which is suitable for Cherenkov light detection in the Near-Ultraviolet (NUV). This device is a High{Density (HD) NUV SiPM, based on a micro cell of 40 μm × 40 μm and with an area of 6×6 mm², providing low levels of dark noise and high PDE peaking in the NUV band. NUV-HD SiPMs will be arranged in a matrix of 8×8 single units to become part of the focal plane of the Schwarzschild-Couder Telescope prototype for CTA. An update on recent tests of the front-end electronics based on signal sampling with the TARGET-7 chip will be given as well.

The elemental composition and its distribution on planetary surface provide important constraints on the origin and evolution of the planetary body. The nuclear spectrometer consisting of a neutron spectrometer and a gamma-ray spectrometer obtains elemental compositions by remote sensing. Especially, the neutron spectrometer is able to determine the hydrogen concentration, a piece of information that plays an important role in thermal history of the planets. In this work, numerical and experimental studies on the neutron spectrometer for micro-satellite application were conducted. It is found that background count rate of neutron produced from micro-satellite is very small, which enables to obtain successful results in short time observation. The neutron spectrometer combining a lithium-6 glass scintillator with a boron loaded plastic scintillator was used to be able to detect neutrons in different energy ranges. It was experimentally confirmed that the neutron signals from these scintillators were successfully discriminated by the difference of scintillation decay time between two detectors. The measurement of neutron count rates of two scintillators is found to determine hydrogen concentration on the planetary surfaces in the future missions.

The PixFEL collaboration has developed the building blocks for an X-ray imager to be used in applications at FELs. In particular, slim edge pixel detectors with high detection efficiency over a broad energy range, from 1 to 12 keV, have been developed. Moreover, a multichannel readout chip, called PFM2 (PixFEL front-end Matrix 2) and consisting of 32 × 32 cells, has been designed and fabricated in a 65 nm CMOS technology. The pixel pitch is 110 μm, the overall area is around 16 mm2. In the chip, different solutions have been implemented for the readout channel, which includes a charge sensitive amplifier (CSA) with dynamic signal compression, a time-variant shaper and an A-to-D converter with a 10 bit resolution. The CSA can be configured in four different gain modes, so as to comply with photon energies in the 1 to 10 keV range. The paper will describe in detail the channel architecture and present the results from the characterization of PFM2. It will discuss the design of a new version of the chip, called PFM3, suitable for post-processing with peripheral, under-pad through silicon vias (TSVs), which are needed to develop four-side buttable chips and cover large surfaces with minimum inactive area.

Plastic scintillators are widely deployed for ionizing radiation detection, as they can be fabricated in large sizes, for high detection efficiency. However, commercial plastics are limited in use for gamma spectroscopy, since their photopeak is very weak, due to low Z, and they are also limited in use for neutron detection, since proton recoils are indistinguishable from other ionizing radiation absorption events in standard plastics. We are working on scale up and production of transparent plastic scintillators based on polyvinyltoluene (PVT) loaded bismuth metallorganics for gamma spectroscopy. When activated with standard organic fluors, PVT scintillators containing 8 wt% bismuth provide energy resolution of 11% at 662 keV. When Iridium complex fluors are used, we can load plastics up to 20 wt% bismuth, while obtaining energy resolution of 10% at 662 keV. Another formulation, activated with Ir fluors for use as neutron radiography scintillator may be used for high energy neutron radiography. Acknowledgements This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and has been supported by the US DOE National Nuclear Security Administration, Defense Nuclear Nonproliferation Research and Development under Contract No. DE-AC03-76SF00098

Locating a gamma source can be done by a variety of methods, ranging from rotating shields, to inter-detector shadowing, to Compton and Coded aperture imaging. Directional information is usually available in spectra from detector arrays, but the information is often ignored. Compton data from an array can be exploited to yield precise source locations, but that technique is slow. Processing data from occlusion, or mutual detector shadowing, is less precise, but much faster. Instruments currently implementing an occlusion method tend to focus only on azimuthal angles, but with some modification the technique can be extended to estimate elevation angles as well. We present a generalization of the occlusion method for estimating both azimuthal and elevation angles to a source, and calibration of the method involves singular value decomposition. The method works even for ill-chosen detector array geometries, and we show results from several detector systems.

Future HEP experiments at the energy and intensity frontiers require fast inorganic crystal scintillators with excellent radiation hardness to face the challenges of unprecedented event rate and severe radiation environment. This paper reports recent progress in application of very fast inorganic scintillators in future HEP experiments, such as thin layer of LYSO crystals for a shashlik sampling calorimeter and a precision TOF detector proposed for the CMS upgrade at the HL-LHC, undoped CsI crystals for the Mu2e experiment at Fermilab and yttrium doped BaF₂ crystals for the proposed Mu2e-II experiment. Applications for Gigahertz hard X-ray imaging will also be discussed.

There is strong evidence that water-ice is relatively abundant within permanently shadowed lunar surface materials, particularly at the poles. Evidence for water-ice has been observed within the impact plume of the LCROSS mission and is supported by data gathered from the Lunar Exploration Neutron Detector (LEND) and the Lunar Prospector Neutron Spectrometer (LPNS). Albedo neutrons from the Moon are used for detection of hydrogen, where the epi-thermal neutron flux decreases as hydrogen content increases. The origin on the concentration of water within permanently shadowed regions is not completely understood, and the Lunar Polar Hydrogen Mapper (LunaH-Map) mission is designed to provide a high-resolution spatial distribution of the hydrogen content over the southern pole using a highly elliptical, low perilune orbit. The LunaH-Map spacecraft is a 6U cubesat consisting of the Miniature Neutron Spectrometer (Mini-NS). Mini-NS is not collimated, requiring a low altitude to achieve a higher spatial resolution compared to previous missions. To develop a compact neutron detector for epi-thermal neutrons, the Mini-NS comprises of 2-cm thick slabs of CLYC (Cs₂LiYCl₆), which provide a sensitivity similar to a 10-atm, 5.7-cm diameter He-3 tubes, as used in LPNS. The Mini-NS digital processing electronics can discriminate by shape and height to determine signal (albedo neutrons) from background (cosmic rays). The Mini-NS achieves a total active sensing area of 200 cm² and is covered with a cadmium sheet to shield against thermal neutrons. The research and development on the detector modules show a robust design ready for space flight.

Solid-state thermal neutron detectors with improved detection efficiencies and sensitivities are highly sought after for the detection of special nuclear materials (SNM). Due to the inherent nature of low flux of fission neutron emission from a potential SNM as well as the fact that the neutron flux is inversely proportional to the square of the distance from the SNM source, it is challenging in many cases to detect neutron signals at relatively large distances. Consequently, large size high efficiency detectors are desired to improve the detector performance and to provide a reasonable count rate (or sensitivity). We report here the successful synthesis by MOCVD of freestanding 10B enriched hexagonal boron nitride (h-BN) epilayers with a thickness of about 50 microns. The progress towards the realization of vertical photoconductor-like thermal neutron detectors with the detector area increasing from 1 mm × 1 mm to 3 mm × 3 mm while upholding a record high detection efficiency of about 53% among solid-state neutron detectors is described. The effects of resistivity, dark current density, and the carrier mobility-lifetime products on the device performance were monitored and optimized via MOCVD growth parameters. With improved material quality, the noise related dark counts have been significantly reduced. The work laid the groundwork for realizing large area detectors. Our results indicate that h-BN epilayers are highly promising for realizing sensitive solid-state thermal neutron detectors with expected advantages resulting from semiconductor technologies, including compact size, light weight, low operating voltage, and low cost.

We present a preliminary design for a novel neutron detection system that is compact, lightweight, and low power consuming, utilizing the CubeSat platform making it suitable for space-based applications. This is made possible using the scintillating crystal lithium indium diselenide (⁶LiInSe₂), the first crystal to include 6Li in the crystalline structure, and a silicon avalanche photodiode (Si-APD). The schematics of this instrument are presented as well as the response of the instrument to initial testing under alpha, gamma and neutron radiation. A principal aim of this work is to demonstrate the feasibility of such a neutron detection system within a CubeSat platform. The entire end-to-end system presented here is 10 cm x 10 cm x 15 cm, weighs 670 grams and requires 5 V direct current at 3 Watts.

Lithium indium diselenide pressed ceramics.

Intrinsic materials can offer advantages over doped materials for some important applications. The doped material might suffer from non-uniform distribution of the dopant, such as fine-scale striations and larger scale segregation, which might affect the overall device response, especially for large-volume detectors such as those in demand for homeland security applications for gamma spectroscopy. Cs2LiCeCl6 (CLCC), being an intrinsic scintillator, can be grown in large volume to produce large detectors with good performance, provided the crystals are free from unwanted scattering centers. CLCC belongs to the elpasolite family and the structure is cubic, so large-volume ingots can be grown without the strains resulting from anisotropic thermal expansion coefficients. In this presentation, we will discuss extensive material characterization and device response of CLCC for gamma and thermal neutron detector applications.

We present new results from testing a small array of position-sensitive virtual Frisch-grid gamma-ray detectors. Such arrays provide high-detection efficiency, excellent energy and position resolution. They can be used in compact hand-held instruments or in large-area gamma ray imaging cameras. The high granularity position sensing enables these detectors to correct the response non-uniformity caused by crystal defects. This important feature allows one to achieve high detection performance while using standard-grade (unselected) CZT crystals, which is expected to reduce the overall cost of field deployable high-resolution CZT gamma ray detection instruments. Here, we report the results of testing several array prototypes with configurations designed for different applications.

With cadmium zinc telluride's (CZT) success as a gamma and x-ray detector material, there is need for high-quality, monocrystalline CZT in large volumes. Bridgman and gradient freeze growth methods have consistently produced material containing significant amounts of micron-sized, tellurium-rich inclusions, which are detrimental to device performance. These inclusions are believed to arise from a morphological instability of the growth interface driven by constitutional undercooling. Repeatedly rotating the crucible back and forth via the accelerated crucible rotation technique (ACRT) has been shown to reduce the size and number of inclusions. Via numerical techniques, we analyze the impact of two different applied temperature gradients, 10 K/cm and 30 K/cm, on the flow, temperature, tellurium distribution, and undercooling during growth with and without applied ACRT. Under growth without rotation, a higher axial thermal gradient results in stronger thermal-buoyancy driven flows, faster interface growth velocity, greater tellurium segregation, and stronger undercooling. ACRT improves the stability of the growth interfaces for both systems; however, contrary to conventional wisdom, the case of the shallow thermal gradient is predicted to exhibit a more stable growth interface, which may result in fewer inclusions and higher quality material.

Wirebonds, although proven for space application and perceived necessary for hybrid sensors like CdZnTe (CZT) detectors, introduce assembly complexity and undesirable gaps between detector units. Thus, they pose a serious challenge in building a low cost large area detector. We are developing Through-Silicon Vias (TSVs) to make all connections (both power and data) through ASICs, which will eliminate wirebonds and enable simple direct flip-chip bonding between the ASIC and a substrate electronics layer. TSVs also enable a more compact layout of the ASIC, which reduces the inactive area of the detector plane, and thus enables nearly gaplessly tilable detector arrays. We demonstrate the first successful TSV implementation on ASICs used for CZT detectors onboard the Nuclear Spectroscopic Telescope Array (NuSTAR) mission as part of our program to develop large area CZT imagers for wide field coded aperture imaging.

High-resolution position-sensing has been proposed to correct response non-uniformities in Cadmium Zinc Telluride (CZT) gamma ray detectors by virtually subdividing the area into small voxels and equalizing responses from each voxel. 3D pixelated detectors coupled with multichannel readout electronics are the most advanced type of CZT devices offering many options in signal processing and enhancing detector performance. The main hurdle in achieving high sub-pixel position resolution is the relatively low signal induced on the neighboring pixels because of the electrostatic shielding effect caused by the collecting pixel. In addition, to achieve high position sensitivity one should rely on time-correlated transient signals, which means that digitized output signals must be used. Previous results have shown the benefit of using a focused laser beam to study position resolution in 3D pixelated detectors. We present the results of our studies to measure the amplitude of the pixel signals so that these can be used to measure positions of the interaction points. This is done with the processing of digitized correlated time signals measured from several adjacent pixels taking into account rise-time and charge-sharing effects. In these measurements we used a focused pulsed laser to generate a 10-micron beam at one milliwatt (650-nm wavelength) over the detector surface while the collecting pixel was moved in cardinal directions. The results include measurements that present the benefits of combining conventional pixel geometry with digital pulse processing for the best approach in achieving sub-pixel position resolution with different pixel dimensions ranging from 0.5 mm to 1.72 mm.

Gadolinium Garnet transparent ceramics doped with Ce, ((Gd,Y,Ce)3(Ga,Al)5O12), for gamma-ray spectroscopy provide high density, high light yield, high energy resolution , high Z, mechanical robustness, and they are unreactive to air and water. Gadolinium garnet single crystals are costly to grow, due to their high melting points, and suffer from non-uniform light yield, due to Ce segregation. In contrast, transparent polycrystalline ceramic Garnets are never melted, and therefore are less costly to produce and provide the uniform light yield required to achieve high energy resolution with a scintillator. GYGAG(Ce) transparent ceramics offer energy resolution as good as R(662 keV) = 3.5%, in a pixelated detector utilizing Silicon photodiode array readout. We have developed a modular handheld detector based on pixelated GYGAG(Ce) on a photodiode array, that offers directional detection for point source detection as well as gamma spectroscopy. Individual modules can be assembled into detectors ranging from pocket-size to large panels, for a range of applications. Large GYGAG(Ce) transparent ceramics in the 2-5 in3 size range have been fabricated at LLNL. Instrumentation of these ceramics with Silicon photomultipliers (SiPMs) and super bi-alkali PMTs has been explored and energy resolution as good as R(662 keV) = 5% has been obtained. Further improvements with SiPM readout will leverage their high quantum efficiency in the 500-650 nm range where GYGAG(Ce) emits, and implement electronics that minimize the effect of SiPM dark current and capacitance on the pulse height spectra. This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, and has been supported by the US Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded IAA HSHQDC-12-X-00149 under Contract No. DE-AC03-76SF00098. LLNL-ABS-724480.

Alloying of CdZnTe (CZT) with selenium has been found to be very promising and effective in reducing the overall concentration of secondary phases (Te precipitates/inclusions) and sub-grain boundary networks in the crystals. These two types of defects are the main causes for incomplete charge collection, and hence they affect the yield of high-quality CZT, resulting in a very high cost for large-volume, high-quality detector-grade CZT detectors. The addition of selenium was also found to very effective in increasing the compositional homogeneity along the growth direction of the CdZnTeSe (CZTS) ingots grown by the traveling heater method (THM) technique. The compositional homogeneity along the growth direction can enhance the overall yield of detector-grade CZTS, which should therefore be possible to produce at a lower cost compared to CZT. The electrical properties and detector performance of the CZTS crystals will be presented and discussed.

Thin film and nanocrystalline materials of oxides have been very attractive choice as low cost option for γ-ray detection and have shown great promise. Our studies on pure oxide films indicated that thickness and microstructure have pronounced effect on sensitivity. Since the interaction of γ-ray with composites involves all three interaction processes; photoelectric effect, Compton scattering, and pair production, composites containing ionic organics have better chance for enhancing sensitivity. In the composites of ionizing organics oxidation effect of unusual oxides changes much faster and hence increases the sensitivity of radiation. In this study, we have used nickel oxide and titanium oxide in ionic organics to develop composite materials for low energy γ-ray sensing. We prepared composites containing ethylene carbonate and evaluated the effect of commercial Cs-137 radiation source by studying current-voltage relationship at several frequencies. Radiated samples showed higher resistivity compared to as prepared composites.

Thallium bromide is a promising semiconductor for room temperature radiation detection. A primary remaining challenge of this soft material is defects associated with impurities, strain, reactivity, or electromigration. Defect states near mid-gap promote rapid loss of carriers at high densities by Shockley-Read-Hall bimolecular recombination. We describe measurement and 3-D mapping of nonlinear photoconductive response using femtosecond pulse trains at 836 nm (1.48 eV), somewhat above mid-gap of TlBr. A confocal two-photon microscopy setup with photocurrent detection affords spatial resolution perpendicular to the surface. We report analysis of the irradiance dependence of the induced current as a function of depth in the sample. There are three separate contributions to the second-order current response. There is intrinsic two-photon absorption of non-defective TlBr. A finding of this work is that at relatively low irradiance, there is additional significantly enhanced two-photon absorption concentrated within about 1 mm of the surface and attributed to resonant enhancement by real intermediate states near mid-gap. Those same mid-gap states potentially contribute an electron-hole recombination term proportional to the square of carrier density (for intrinsic production as in a radiation detector), and this is in fact confirmed by analysis of the measurements. The spatial distribution of the defects can be displayed and the cross section for bimolecular carrier quenching can be extracted.

Solid-liquid phase transitions in Cd_0.95-xMn_xZn_0.05Te alloys with x = 0.20 and 0.30 were investigated by differential thermal analysis (DTA). The heating/cooling rates were 5 and 10 K/min with a melt dwell time of 10, 30 and 60 minutes. Cd_0.95-xMn_xZn_0.05Te (x=0.20, 0.30) single-crystal ingots were grown by the vertical Bridgman method guided by the DTA results.

Te inclusions (1-20 micron diameter), typical of melt-grown CdTe and Cd(Zn)Te crystals, were observed in the ingots by infrared transmission microscopy. The measured X-ray diffraction patterns showed that all compositions are found to be in a single phase. Using current-voltage (I-V) measurements, the resistivity of the samples from each ingot was estimated to be about 105 Ohm·cm. The optical transmission analysis demonstrated that the band-gap of the investigated ingots increased from 1.77 to 1.88 eV with an increase of the MnTe content from 20 to 30 mol. %.

The elemental composition on planetary surface provides us an essential information to improve geological and geochemical understanding of planets. An active X-ray spectrometer (AXS) was developed and proposed as one of the mission payloads to perform in-situ X-ray fluorescence analysis. The AXS consists of multiple pyroelectric X-ray generators (PXGs) and a silicon drift detector (SDD). Although the PXG is light in weight and low in electric consumption, the limited X-ray intensity and reproducibility hamper obtaining the elemental composition by short time observation. This is attributed mainly to the less known mechanism of X-ray and electron emission by the pyroelectric crystal. In this study, we observed the crystal surface during X-ray emission as a function of the distance between the crystal top and a Cu target. Two types of light emission derived from the electric discharge were observed. The dependence of light emission on the distance was found to be related with the physical mechanism of pyroelectric X-ray emission. The study provides clues to obtain high intensity and reproducible X-rays emission. The experimental results and discussion are presented in this paper.

One electrode configuration frequently used in CZT detectors is the cross-strip pattern. The cathode-to-anode signal ratio (C/A) can be used to estimate the position of photon interaction in the direction perpendicular to the electrode plane. In addition, C/A is used to calibrate for the depth-dependent anode signal. Based on the design configuration of the electrode widths with respect to the CZT thickness, their measured energy spectrum varies due to the small pixel effect and charge trapping. In this work we evaluate the effect of a poor cathode energy resolution on the depth of interaction correction and anode signal recovery.

Modern ultraviolet (UV) cameras, when combined with UV-transmitting lenses/filter arrangements, can be used to detect radiation dose in air. Ionizing radiation excites nitrogen molecules in ambient air, the resulting decay includes weak emission of ultraviolet photons. Previous work has proven this phenomenon is detectable using highly-sensitive electronically cooled cameras traditionally used in astronomy for low-background imaging ^6,7 . While the ability to detect the presence of radiation (i.e. qualitative measurement) has been demonstrated at Sandia National Laboratories, there are several challenges in correlating images to known dose-fields (quantitative measurement). These challenges include: a low signal to background ratio, interferences due to electronic noise and direct radiation interactions with the camera, and a complex source-dependent detection efficiency. Based on measurements of low-level radioactive sources as well as high-level sources at several irradiation facilities at Sandia National Laboratories, researchers are developing deeper understanding of these challenges in an attempt to engineer a system that can be used for quantitatively measuring radiation dose fields remotely. This paper will describe these efforts and share the lessons learned from several experiments.

The near-infrared radioluminescence and dosimetric properties of Yb-doped silica optical fibers, coupled with an optical detector prototype based on an avalanche photo-diode, were studied by irradiating the fibers with clinical beams generated by a Varian Trilogy accelerator. The performances of the system in standard and small field sizes have been also investigated comparing the output factor, percent depth dose and off axis ratio measurements of the prototypal dosimetric system with other commercial sensors.

The results demonstrated that the drawback due to the stem effect in Yb-doped silica optical fibers can be managed in a simple but effective way by optical filtering. These features, together with the accuracy and precision achieved by Ybdoped fibers in relative dose assessments make the device promising for in-vivo dosimetry studies in radiation therapy.