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

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

We have developed a handheld $\gamma $ -ray spectrometer based on 1024 pixels, $2.8\,\,\text {mm} \times 2.8\,\,\text {mm} \times 6$ mm in size, of gadolinium–yttrium–gallium–aluminum garnet (GYGAG) ((Gd,Y,Ce)3(Ga,Al)5O12) ceramic scintillator (total of 48-cm3 detector volume) coupled to silicon photodiode (SiPD) arrays. The SiPD arrays and readout ASIC, originally developed for medical imaging applications, have been adapted for portability in a lightweight box with heatsink and thermoelectric cooling. Custom readout firmware for $\gamma $ -ray spectroscopy has been implemented, and a system user interface was developed that runs on an Android tablet. We have optimized the processing of the GYGAG(Ce), the pixel optical coupling, and electronics readout parameters to obtain single pixel energy resolution as good as $R (662\,\,\text {keV}) = 3.1$ % full-width at half-maximum and full device resolution with singles events from all pixels summed of 4.5%. When Compton-summed events are included, full-energy peak efficiency increases by $\sim 2\times $ , and $R (662\,\,\text {keV}) = 4.7$ % is obtained for the full device. The pixelated architecture is leveraged to locate point sources of radiological materials using Compton imaging and active masking techniques. Directional detection of a 1 mCi Cs-137 source at 10 m can be made to ±10° in ~2 s.

The desire to build nuclear clocks with an unprecedented accuracy of 1 part in $$10^{19}$$ is currently driving the interest in accurate measurements of the first excited nuclear state in $$^{229}$$ Th. We have used superconducting tunnel junction (STJ) radiation detectors to search for the associated decay of $$^{229m}$$ Th and measure its energy accurately. In this experiment, recoiling $$^{229m}$$ Th ions from the alpha decay of $$^{233}$$ U are embedded in the STJ detectors, and their subsequent decay into the ground state is expected to produce a signal at an energy of $$7.8\pm 0.5$$ eV. This approach is particularly promising, because all decay products are captured inside the STJ so that the measured energy does not depend on the decay mode or on chemical effects. Although this approach works well to characterize the decay of metastable $$^{235m}$$ U, no signal from $$^{229m}$$ Th has been observed. This negative result can be explained by recent measurements of the $$^{229m}$$ Th half-life of only $$7\pm 1\,\upmu \mathrm{s}$$ . We discuss our experiments and the modifications required to measure the energy of metastable $$^{229m}$$ Th with an accuracy of order ± 10 meV.

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

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

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

We have used superconducting high-resolution radiation detectors to measure the energy level of metastable $^{235m}\mathrm{U}$ as 76.737 \ifmmode\pm\else\textpm\fi{} 0.018 eV. The $^{235m}\mathrm{U}$ isomer is created from the \ensuremath{\alpha} decay of $^{239}\mathrm{Pu}$ and embedded directly into the detector. When the $^{235m}\mathrm{U}$ subsequently decays, the energy is fully contained within the detector and is independent of the decay mode or the chemical state of the uranium. The detector is calibrated using an energy comb from a pulsed UV laser. A comparable measurement of the metastable $^{229m}\mathrm{Th}$ nucleus would enable a laser search for the exact transition energy in $^{229}\mathrm{Th}\ensuremath{-}^{229m}\mathrm{Th}$ as a step towards developing the first ever nuclear (baryonic) clock.

This paper is the fifth in a series of articles on the basic physics of light yield nonproportionality in scintillators. Here, we compare and contrast the nonproportionality as registered by gamma rays and high-energy electrons. As has been noted in the past, these two types of data have different curve shapes (for plots of the light yield against electron or gamma energy). Herein, we show how the experimental gamma nonproportionality curve can be calculated from the electron response by accounting for the distribution of high energy electrons created by the gamma photon via the photoelectric interaction. Similarly, we measure and model the gamma-induced resolution as a function of energy and compare this data to predictions from our model. The utility of the model is explored using data acquired with the scintillators ${\rm SrI}_{2}$ (Eu), GYGAG(Ce) and CsI(Na).

Transparent plastic scintillators based on polyvinyltoluene (PVT) have been fabricated with high loading of bismuth carboxylates for gamma spectroscopy, and with lithium carboxylates for neutron detection. When activated with a combination of standard fluors, 2,5-diphenyloxazole (PPO) and tetraphenylbutadiene (TPB), gamma light yields with 15 wt% bismuth tripivalate of 5000 Ph/MeV are measured. A PVT plastic formulation including 30 wt% lithium pivalate and 30 wt% PPO offers both pulse shape discrimination, and a neutron capture peak at ~400 keVee. In another configuration, a bismuth-loaded PVT plastic is coated with ZnS(6Li) paint, permitting simultaneous gamma and neutron detection via pulse shape discrimination with a figure-of-merit of 3.8, while offering gamma spectroscopy with energy resolution of R(662 keV)=15%.

Thallium bromide (TlBr) crystals subjected to hydrochloric acid (HCl) chemical treatments have been shown to advantageously affect device performance and longevity in TlBr-based room temperature radiation detectors, yet the exact mechanisms of the improvements remain poorly understood. Here, we investigate the influence of several HCl chemical treatments on device-grade TlBr and describe the changes in the composition and electronic structure of the surface. Composition analysis and depth profiles obtained from secondary ion mass spectrometry (SIMS) identify the extent to which each HCl etch condition affects the detector surface region and forms of a graded TlBr/TlBrCl surface heterojunction. Using a combination of X-ray photoemission spectroscopy (XPS) and hybrid density functional calculations, we are able to determine the valence band offsets, band gaps, and conduction band offsets as a function of Cl content over the entire composition range of . This study establishes a strong correlation between device process conditions, surface chemistry, and electronic structure with the goal of further optimizing the long-term stability and radiation response of TlBr-based detectors.

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.

We are working on scale up of several new plastic scintillator compositions. One composition uses Iridium complex fluors to obtain up to 3x higher light yields and high α/β ratios, for improved particle (alpha, neutron, fission product) detection and imaging. Other compositions include high loading of Bismuth for gamma spectroscopy and with Lithium for neutron detection. Plastics containing 21 wt% elemental Bismuth and an Ir fluor provide strong photopeaks and R(662 keV) ~9%. When activated with standard organic fluors, 10 in3 plastic scintillators containing 8 wt% Bismuth provide a strong photopeak with R(662 keV)~19%. A 5 in3 plastic formulation including 1 wt% Lithium-6 provides a neutron capture peak at 350 keVee, with 11% resolution for the capture peak.

New transparent ceramic scintillators offer advantages for applications in gamma spectroscopy and X-ray imaging. For gamma spectroscopy, excellent light yield, material uniformity, light yield proportionality, mechanical and environmental ruggedness can be achieved in polycrystalline ceramic oxide garnets. We have fabricated 5 in3 Ce-doped Gd garnet transparent ceramics. GYGAG(Ce) garnet transparent ceramics offer $^{\mathbf {\rho \, =\, 5.8g/cm^{3},\,}}$, $z_{eff}=48$, principal decay of 2 O 3 structure structure. The 12” x 12” GLO imaging plates outperform scintillator glass for MeV radiography, due to higher light yield (55,000 Ph/MeV) and better stopping, while providing spatial resolution of >8 lp/mm for MeV X-rays.

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

Breakthrough energy resolution, R(662keV)

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

We report on the development of two new mechanically rugged, high light yield transparent ceramic scintillators: (1) Ce-doped Gd-garnet for gamma spectroscopy, and (2) Eu-doped Gd-Lu-bixbyite for radiography. GYGAG(Ce) garnet transparent ceramics offer ρ = 5.8g/cm3, Zeff = 48, principal decay of 8 lp/mm.

Degradation of room temperature operation of TlBr radiation detectors with time is thought to be due to electromigration of Tl and Br vacancies within the crystal as well as the metal contacts migrating into the TlBr crystal itself due to electrochemical reactions at the metal/TlBr interface. Scanning Auger electron spectroscopy (AES) in combination with sputter depth profiling was used to investigate the metal contact surface/interfacial structure on TlBr devices. Device-grade TlBr was polished and subjected to a 32% HCl etch to remove surface damage and create a TlBr1-xClx surface layer prior to metal contact deposition. Auger compositional depth profiling results reveal non-equilibrium interfacial diffusion after device operation in both air and N2 at ambient temperature. These results improve our understanding of contact/device degradation versus operating environment for further enhancing radiation detector performance.

TlBr radiation detector operation degrades with time at room temperature and is thought to be due to electromigration of Tl and Br vacancies within the crystal as well as the metal contacts migrating into the TlBr crystal itself due to electrochemical reactions at the metal/TlBr interface. X-ray photoemission spectroscopy (XPS) was used to investigate the metal contact surface/interfacial structure on TlBr devices. Device-grade TlBr was polished and subjected to a 32% HCl etch to remove surface damage prior to Mo or Pt contact deposition. High-resolution photoemission measurements on the Tl 4f, Br 3d, Cl 2p, Mo 3d and Pt 4f core lines were used to evaluate surface chemistry and non-equilibrium interfacial diffusion. Results indicate that anion substitution at the TlBr surface due to the HCl etch forms TlBr 1-x Cl x with consequent formation of a shallow heterojunction. In addition, a reduction of Tl 1+ to Tl 0 is observed at the metal contacts after device operation in both air and N2 at ambient temperature. Understanding contact/device degradation versus operating environment is useful for improving radiation detector performance.

Development of the Europium-doped Strontium Iodide scintillator, SrI2(Eu2+), has progressed significantly in recent years. SrI2(Eu2+) has excellent material properties for gamma ray spectroscopy: high light yield (

Breakthrough energy resolution, R(662keV) < 4%, has been achieved with an oxide scintillator, Cerium-doped Gadolinium Yttrium Gallium Aluminum Garnet, or GYGAG(Ce). Transparent ceramic GYGAG(Ce), has a peak emission wavelength of 550 nm that is better matched to Silicon photodetectors than to standard PMTs. We are therefore developing a spectrometer based on pixelated GYGAG(Ce) on a Silicon photodiode array that can provide R(662 keV) = 3.6%. In comparison, with large 1-2 in3 size GYGAG(Ce) ceramics we obtain R(662 keV) = 4.6% with PMT readout. We find that ceramic GYGAG(Ce) of a given stoichiometric chemical composition can exhibit very different scintillation properties, depending on sintering conditions and post-anneal treatments. Among the characteristics of transparent ceramic garnet scintillators that can be controlled by fabrication conditions are: scintillation decay components and their amplitudes, intensity and duration of afterglow, thermoluminescence glow curve peak positions and amplitudes, integrated light yield, light yield non-proportionality - as measured in the Scintillator Light Yield Non-Proportionality Characterization Instrument (SLYNCI), and energy resolution for gamma spectroscopy. Garnet samples exhibiting a significant fraction of Cerium dopant in the tetravalent valence also exhibit: faster overall scintillation decay, very low afterglow, high light yield, but poor light yield proportionality and degraded energy resolution.

Device-grade TlBr was subjected to various chemical treatments used in room temperature radiation detector fabrication to determine the resulting surface composition and electronic structure. Samples of as polished TlBr were treated separately with 2%Br:MeOH, 10%HF, 10%HCl and 96%SOCl 2 solutions. High-resolution photoemission measurements on the valence band electronic structure and Tl 4f, Br 3d, Cl 2p and S 2p core lines were used to evaluate surface chemistry. Results suggest anion substitution at the surface with subsequent shallow heterojunction formation. Surface chemistry and valence band electronic structure were further correlated with the goal of optimizing the long-term stability and radiation response. Keywords: Thallium bromide, photoelectron spectroscopy, radiation detection 1. INTRODUCTION The development of room temperature radiation detectors requires new materials with large band gaps and high atomic number species. Thallium bromide (TlBr) meets these requirements having a wide band gap (2.68 eV), high Z (

The precise measurement of time has been a goal of physicists for centuries. With every new increase in our ability to measure time we have discovered new phenomena. The most advanced clocks available to us currently are atomic clocks that use electronic transitions to track the passage of time. In this proposal, I put forward the framework for the first nuclear clock estimated to be 1000 to 10000 times more precise than the current atomic clocks. This research will explore in detail the atomic nuclear interactions and help perfect and refine current atomic-nuclear interaction models. The realization of a {sup 229}Th nuclear clock will allow tests of cosmology by measuring the change of the fine structure constant as a function of time. The results of these experiments could dramatically alter our view of the universe, its past and future evolution. Precision clocks - with fundamental physics applications - require a long-lived quantum transition (two-level system) that is immune to external perturbations. Nuclear transitions would be better suited than atomic transitions for these applications except that nuclear transitions are typically much higher in energy and therefore cannot be accessed with table-top lasers. There is, however, one promising nuclear transition: the doublet between the ground and first excited states of the {sup 229}Th nucleus discovered by Helmer and Reich. This doublet has an energy splitting of 7.6 {+-} 0.5 eV, a spin difference of 1 h-bar, and an excited state half-life that could be as long as hours. A precision clock based on the {sup 229}Th nuclear doublet has been proposed by Peik et al. Their design is similar to the ion clock research being conducted at NIST in Boulder, CO. However, the NIST researchers use atomic transitions for their frequency standards. In the {sup 229}Th nuclear doublet transition is the frequency standard while atomic transitions are used to cool the ions and for probing the state of the {sup 229}Th nucleus. Recently, Campbell et al. have trapped and cooled {sup 232}Th{sup 3+} at Georgia Institute of Technology. This is a large step forward in the realization of a nuclear clock. The Georgia Tech group is already a collaborator on this project and we are in discussions with the NIST Boulder group about collaboration. In order to determine the suitability of the {sup 229}Th nuclear doublet for a precision clock, the half-life of the excited-state needs to be measured. Current estimates of the half-life vary from 10 {micro}s to 1000 hours. The longer the half-life, the narrower the natural linewidth of the state and the more desirable the transition is for potential applications. In this proposal, I outline the necessary research to be conducted to determine the half-life and exact wavelength of the nuclear doublet transition in {sup 229}Th. This research will lead to a deeper understanding of atomic-nuclear interactions important for our knowledge of high energy density science. It will provide a spectroscopy measurement of the lowest known nuclear transition ever and open the doorway for the development of a nuclear clock with unprecedented precision.