Gamma-ray Energy Resolution Research Articles

Performance of the HAWC Observatory and TeV Gamma-Ray Measurements of the Crab Nebula with Improved Extensive Air Shower Reconstruction Algorithms

The High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, located on the side of the Sierra Negra volcano in Mexico, has been fully operational since 2015. The HAWC collaboration has recently significantly improved their extensive air shower reconstruction algorithms, which has notably advanced the observatory performance. The energy resolution for primary gamma rays with energies below 1 TeV was improved by including a noise-suppression algorithm. Corrections have also been made to systematic errors in direction fitting related to the detector and shower plane inclinations, O(0.°1) biases in highly inclined showers, and enhancements to the core reconstruction. The angular resolution for gamma rays approaching the HAWC array from large zenith angles (>37°) has improved by a factor of 4 at the highest energies (>70 TeV) as compared to previous reconstructions. The inclusion of a lateral distribution function fit to the extensive air shower footprint on the array to separate gamma-ray primaries from cosmic-ray ones based on the resulting χ 2 values improved the background rejection performance at all inclinations. At large zenith angles, the improvement in significance is a factor of 4 compared to previous HAWC publications. These enhancements have been verified by observing the Crab Nebula, which is an overhead source for the HAWC Observatory. We show that the sensitivity to Crab-like point sources (E −2.63) with locations overhead to 30° zenith is comparable to or less than 10% of the Crab Nebula’s flux between 2 and 50 TeV. Thanks to these improvements, HAWC can now detect more sources, including the Galactic center.

Compact pixelated scintillator detector investigation for gamma ray detection

Objective. A scintillator is a luminescent material that converts high-energy photons into visible light and is widely used in medical imaging. Different scintillators are applied to gamma imaging in proton therapy and boron neutron capture therapy (BNCT). A pixelated scintillator is suitable for position measurement. The energy resolution, detection efficiency and position measurement of pixelated scintillators coupled to a SiPM were investigated via two kinds of detectors. The goal of this manuscript was to accurately measure the gamma source position through spectrum analysis in selection of energy windows for characteristic gamma lines. Approach. Scintillators were effectively manufactured and encapsulated before testing, especially for the easily deliquescent LaBr3. The compact front-end electronic prototype modules with 2 × 2 array SiPMs in stacked form and 1 × 10 array SiPMs in parallel form were developed for gamma ray energy resolution and efficiency measurements with a radioactive source of 22Na, which had two energy gamma lines at 511 keV and 1274 keV. Energy calibration was used for accurate energy window selection when measuring the position of the gamma source. Main results. Evident inconsistencies were present between different pixels of the same type of scintillator. Thus, an energy calibration method was needed. LaBr3 was the first candidate scintillator for the gamma ray spectrum measurement since it exhibited the best performance with an energy resolution of ∼5%. The recommended size of LaBr3 was 5 × 5 mm2, which had a higher efficiency than the 3 × 3 mm2 size. The gamma count of the multiple mode of the 2 × 2 array was much higher than that of the single mode, while the energy resolution was poorer. Thus, multiple mode was not suitable for gamma ray detection. The 1 × 10 array detector had the potential to measure the gamma ray source position and could be used for proton therapy and BNCT. A small deviation of 0.22 cm was observed in the measurement of the source center position with Energy Window 1 for 511 keV and Energy Window 2 for 1274 keV before the energy calibration. No deviation was observed after energy calibration. Thus, to achieve a higher accuracy position measurement, automatic energy calibration algorithm was coded into data acquisition software. Significance. The characteristic gamma lines produced by particle therapy are abundant and useful for imaging technology. Our developed compact pixelated scintillator detector coupled with SiPMs could measure the gamma spectrum with high resolution. The energy calibration and window selection method could measure the position of the source with high accuracy. Therefore, an advanced imaging device based on the energy spectrum for particle therapy could be potentially attainable.

A hybrid coded-aperture and Compton camera based on cerium-doped Gd3Al2Ga3O12 scintillators coupled with multi-pixel photon counter arrays.

In this study, we propose a hybrid coded-aperture and Compton camera based on cerium-doped Gd3Al2Ga3O12 (GAGG:Ce) scintillator arrays coupled with Multi-Pixel Photon Counter (MPPC) arrays. The sensitive detector of the gamma camera consists of a single GAGG:Ce crystal coupled with a single-chip MPPC unit module. An impedance bridge circuit and a 64-channel data acquisition system were employed to record the code-aperture events and Compton coincidence events. After the calibration of position and energy, the total energy resolution for 662keV gamma-rays from 137Cs was 6.6%. The hybrid camera had the characteristics of mechanical collimation and electronic collimation at the same time. In the code aperture mode, the reconstructed images were obtained by direct deconvolution and maximum likelihood expectation maximization (MLEM) methods. In the Compton imaging mode, the energy-dependent method was applied to order the sequence of Compton scatter events. The simple back-projection algorithm and list-mode MLEM algorithm were adopted for image reconstruction. Practical performances demonstrated that the angular resolutions in two modes were measured as 5.2° and 11.4°, respectively. In addition, the hybrid camera had a desirable imaging capability in a wide energy range (32 keV-2.6MeV) and a wide field of view (∼210° in the horizontal direction). As for the sensitivity, the camera had a commercially available sensitivity level of localizing a 137Cs point source, producing ∼0.026 µSv/h in 5min. Furthermore, the function of distinction for different radiation sources was preliminarily realized.

Active neutron interrogation experiments and simulation verification using the SIngle-scintillator Neutron and Gamma-Ray spectrometer (SINGR) for geosciences

We present a new SIngle-scintillator Neutron and Gamma-Ray spectrometer (SINGR) instrument for use with both passive and active measurement techniques. Here we discuss, the application of SINGR for planetary exploration missions; however, hydrology, nuclear non-proliferation, and resource prospecting are all potential areas where the instrument could be applied. SINGR uses an elpasolite scintillator, Cs2YLiCl6:Ce (CLYC), that has been shown to have high neutron efficiency even at small volumes, with a gamma-ray energy resolution of approximately 4% full-width-at-half-maximum at 662 keV. Active gamma-ray and neutron (GRNS) measurements were performed with SINGR at the NASA Goddard Space Flight Center (GSFC) Goddard Geophysical and Astronomical Observatory (GGAO) outdoor test site using a pulsed neutron generator (PNG) to interrogate geologically relevant materials (basalt and granite monuments). These experimental results, combined with simulations, demonstrate that SINGR is capable of generating neutron die-away curves that can be used to reconstruct the bulk hydrogen abundance and the depth distribution of hydrogen within the monuments. We compare our experimental results with Monte Carlo N-Particle (MCNP) 6.1 transport simulations to constrain the uncertainties in depth and hydrogen abundance from the neutron die-away data generated by SINGR. For future planetary exploration missions, SINGR provides a single detector system for interrogating the shallow subsurface to characterize the presence and abundance of hydrated phases and to provide bulk elemental analysis.

Low-energy excitations and gamma -decay branchings in ^{124}Sn via (p,p\u2019gamma ) at E_p={15},,hbox {MeV}

The dipole response of the proton-magic nucleus {}^{124}hbox {Sn} was previously investigated with electromagnetic and hadronic probes. Different responses were observed revealing the so-called isospin splitting of the Pygmy Dipole Resonance (PDR). Here we present the results of a new study of {}^{124}hbox {Sn} using inelastic proton scattering at low energies to test an additional probe possibly exciting states of the PDR. The response to the new probe as well as the gamma -decay behavior of excited states were studied. The {}^{124}hbox {Sn}(p,p’gamma ) experiment was performed at E_p = {15},,hbox {MeV} using the combined spectroscopy setup SONIC@HORUS at the Tandem accelerator of the University of Cologne. Proton-gamma coincidences were recorded, enabling a state-to-state analysis due to the excellent energy resolution for both particles and gamma rays. J=1 states in the PDR region were populated in the present inelastic proton scattering experiment. Many gamma -decay branching ratios could be determined.

Thermal Characterization of Tl₂LiYCl₆:Ce (TLYC)

Tl$_2$LiYCl$_6$:Ce (TLYC) is a new dual-detection elpasolite scintillator that can detect and distinguish between gamma rays and neutrons using pulse-shape discrimination (PSD). It has a higher density and Z-number than the more mature and well-known elpasolite Cs$_2$LiYCl$_6$:Ce (CLYC), causing it to have a significantly better gamma-ray stopping power. These properties make TLYC an attractive alternative to CLYC for resource-constrained applications where size and weight are important, such as space or national security applications. Such applications may be subjected to a wide range of temperatures, and therefore TLYC's performance was characterized for the first time over a temperature range of -20$^{\circ}$C to +50$^{\circ}$C in 10$^{\circ}$C increments. TLYC's thermal response effects on light-output linearity with energy, gamma-ray photopeak energy resolution, detected neutron energy, pulse shapes, and figure of merit is analyzed and reported. The light output of TLYC was found to be linear with energy over the tested temperature range and was observed to decrease with increasing temperature. The decay time of the scintillation light output was observed to decrease with decreasing temperature at short times, leading to a decreasing PSD figure of merit. The gamma-ray photopeak energy resolution was also observed to degrade with decreasing temperature, due to an asymmetric broadening of the photopeak at low temperatures.

Thallium-based scintillators for high-resolution gamma-ray spectroscopy: Ce[formula omitted]- doped Tl2LaCl5 and Tl2LaBr[formula omitted

In this paper we report on the crystallographic and scintillation properties of Tl2LaCl5:Ce3+ and Tl2LaBr5:Ce3+, two novel thallium-containing high-resolution scintillators for gamma-ray spectroscopy. Crystals of Tl2LaCl5:Ce3+ and Tl2LaBr5:Ce3+ were grown by the Vertical Bridgman method up to 1-inch diameter and 1-inch long. Single crystals of Tl2LaCl5:Ce3+ and Tl2LaBr5:Ce3+ belong to the orthorhombic system with space group 62 and have a density of 5.16 and 5.98 g/cm3, respectively. The scintillators show high light yields of up to 68,000 photons/MeV, excellent gamma-ray energy resolution of ≤ 3% at 662 keV, a fast scintillation decay, and a proportional response over a wide range of energies from 32 keV up to 1275 keV. Density Functional Theory calculations show that the Ce3+ energy levels are inside the bandgap despite the smaller bandgap of Tl2LaCl5 and Tl2LaBr5 compared to K2LaCl5 and K2LaBr5. A systematic Ce3+ concentration study was performed for Tl2LaCl5:Ce3+ and trends observed.

Effects of proton-induced radiation damage on CLYC and CLLBC performance

Cerium-doped Cs$_2$LiYCl$_6$ (CLYC) and Cs$_2$LiLaBr$_x$Cl$_{6-x}$ (CLLBC) are scintillators in the elpasolite family that are attractive options for resource-constrained applications due to their ability to detect both gamma rays and neutrons within a single volume. Space-based detectors are one such application, however, the radiation environment in space can over time damage the crystal structure of the elpasolites, leading to degraded performance. We have exposed 4 samples each of CLYC and CLLBC to 800 MeV protons at the Los Alamos Neutron Science Center. The samples were irradiated with a total number of protons of 1.3$\times$10$^{9}$, 1.3$\times$10$^{10}$, 5.2$\times$10$^{10}$, and 1.3$\times$10$^{11}$, corresponding to estimated doses of 0.14, 1.46, 5.82, and 14.6 kRad, respectively on the CLYC samples and 0.14, 1.38, 5.52, and 13.8 kRad, respectively on the CLLBC samples. We report the impact these radiation doses have on the light output, activation, gamma-ray energy resolution, pulse shapes, and pulse-shape discrimination figure of merit for CLYC and CLLBC.

Preliminary Design of Integrated Digitizer Base for Photomultiplier Tube

In many physical experiments and applications with photomultiplier tubes (PMTs), high-speed pulse digitalization is used extensively. The current pulse from a PMT has a rising time of several to a hundred nanoseconds. The current pulse shape can be used to obtain the pulse energy and discriminate between different particles. The conventional readout system with a PMT uses the discrete or integrated pulse-shaping circuits and analogue-to-digital converter boards which are based on the NIM or VME system to implement the data acquisition function. In many applications, such as security instruments, there are only several PMTs used in the whole system, or in some distributed applications, long-distance analogue cable with high-voltage power cable is not applicable due to the tremendous number of channels and enormous distributed space. In this paper, an integrated digitizer base is designed and interfaced with a Cs2LiYCl6 (CLYC) scintillator with a PMT. The CLYC scintillator has the ability to realize the pulse shape discrimination (PSD) between the neutron and gamma-rays. There is only one Gigabit of unscreened twisted-pair cable needed for over 700 Mbps TCP data throughput and more than 10 W power. The boards of high voltage supply, current sensitive preamplifier, 500 MSPS/12-Bit ADC (Analog-to-Digital Converter), readout module based on ZYNQ SoC (System on Chip), power over Ethernet, and user interface circuits are stacked with board-to-board connectors. The system can achieve 4.6% gamma-ray energy resolution at 662 keV with a 20-{\mu}s pulse integral and PSD figure-of-merit of ~3.0 for the whole energy of neutrons and gamma-rays. This integrated digitizer base can also be interfaced with other detectors with a PMT.

Evaluation of CdZnTeSe as a high-quality gamma-ray spectroscopic material with better compositional homogeneity and reduced defects

X- and gamma-ray detectors have broad applications ranging from medical imaging to security, non-proliferation, high-energy physics and astrophysics. Detectors with high energy resolution, e.g. less than 1.5% resolution at 662 keV at room temperature, are critically important in most uses. The efficacy of adding selenium to the cadmium zinc telluride (CdZnTe) matrix for radiation detector applications has been studied. In this paper, the growth of a new quaternary compound Cd0.9Zn0.1Te0.98Se0.02 by the Traveling Heater Method (THM) is reported. The crystals possess a very high compositional homogeneity with less extended defects, such as secondary phases and sub-grain boundary networks. Virtual Frisch-grid detectors fabricated from as-grown ingots revealed ~0.87–1.5% energy resolution for 662-keV gamma rays. The superior material quality with a very low density of defects and very high compositional homogeneity heightens the likelihood that Cd0.9Zn0.1Te0.98Se0.02 will be the next generation room-temperature detector material.

Characterization and Monte Carlo simulations for a CLYC detector

The CLYC (Cs[Formula: see text]LiYCl[Formula: see text]:Ce) detector is a scintillator detector sensitive to both neutron and gamma radiation and capable of separating the two types of radiation by pulse-shape discrimination. This feature is interesting as pertains to the development of non-destructive assays for the safeguard of nuclear material, nuclear security, and fast-neutron personal dosimetry. A [Formula: see text] CLYC detector highly enriched with [Formula: see text]Li was purchased and tested with analog and digital electronics. In this work, we report on the characterization of the detector in terms of linearity, energy resolution, and full-energy efficiency for gamma rays. This characterization was achieved by measurements with calibrated gamma-ray point-sources with an analog measuring chain, in a well-defined, reproducible geometry. The experimental data were also used to validate a model of the detection system that was developed with the Monte Carlo code MCNP-CP. This work is part of a collaborative agreement between SCK•CEN and JRC-Geel.

Characterization of the new scintillator Cs2LiYCl6:Ce3+

The first domestic inorganic scintillator, Cs2LiYCl6:Ce3+ (CLYC), was grown at Beijing Glass Research Institute using the vertical Bridgman method. In this work, we evaluated the performance of this new CLYC crystal in terms of its gamma-ray energy resolution and pulse shape discrimination (PSD) capability between neutrons and gamma rays. The decay times associated with different scintillation mechanisms were obtained by fitting decay functions to the neutron and gamma-ray waveform structures. We found an energy resolution of ~ 4.5% for 662-keV gamma rays and efficient neutron/gamma PSD with a figure of merit of ~ 2.6. Under gamma-ray excitation, there is an ultrafast scintillation mechanism in CLYC with a decay time of approximately 2 ns, whereas there is no evidence of ultrafast decay under thermal neutron excitation. This work contributes to the promotion of domestic development of CLYC.

Iron distribution of the Moon observed by the Kaguya gamma-ray spectrometer: Geological implications for the South Pole-Aitken basin, the Orientale basin, and the Tycho crater

In this study we describe the distribution of iron on the Moon as obtained by the Kaguya high energy resolution gamma-ray spectrometer (KGRS). We achieved for the first time the identification of iron based on the fast neutron flux obtained by the KGRS. The iron distribution obtained by KGRS is compared to that of the Lunar Prospector Gamma-Ray Spectrometer (LP GRS), showing that the FeO distributions observed by KGRS and LP GRS, in general, are in good agreement. Furthermore, we compare the iron content data obtained by KGRS and LP GRS to spectral reflectance measurements of the Clementine, Kaguya and Chandrayaan-1 spacecraft as well as those inferred from returned samples. We found differences in FeO concentration and distribution in areas of moderate abundance (6–15 wt%) of the South Pole-Aitken basin, Mare Orientale, and around the crater Tycho crater. It implies that high concentrations of FeO at Mare Ingenii in the South Pole-Aitken basin and Mare Orientale are due to the presence of mare basalts, whereas the enriched FeO content in the central depression of the South Pole-Aitken basin and the Tycho crater indicates the presence of mafic materials such as impact melt breccia.

Pulse-shape discrimination and energy quenching of alpha particles in Cs2LiLaBr6:Ce3+

Cs2LiLaBr6:Ce3+(CLLB) is an elpasolite scintillator that offers excellent linearity and gamma-ray energy resolution and sensitivity to thermal neutrons with the ability to perform pulse-shape discrimination (PSD) to distinguish gammas and neutrons. Our investigation of CLLB has indicated the presence of intrinsic radioactive alpha background that we have determined to be from actinium contamination of the lanthanum component. We measured the pulse shapes for gamma, thermal neutron, and alpha events and determined that PSD can be performed to separate the alpha background with a moderate figure of merit of 0.98. We also measured the electron-equivalent-energy of the alpha particles in CLLB and simulated the intrinsic alpha background from 227Ac to determine the quenching factor of the alphas. A linear quenching relationship Lα=Eα×q+L0 was found at alpha particle energies above 5MeV, with a quenching factor q=0.71MeVee/MeV and an offset L0=−1.19MeVee.

Perspectives of the GAMMA-400 space observatory for high-energy gamma rays and cosmic rays measurements

The GAMMA-400 gamma-ray telescope is intended to measure the fluxes of gamma-rays and cosmic-ray electrons and positrons in the energy range from 100 MeV to several TeV. Such measurements concern the following scientific tasks: investigation of point sources of gamma-rays, studies of the energy spectra of Galactic and extragalactic diffuse emission, studies of gamma-ray bursts and gamma-ray emission from the Sun, as well as high precision measurements of spectra of high-energy electrons and positrons. Also the GAMMA- 400 instrument provides the possibility for protons and nuclei measurements up to knee. But the main goal for the GAMMA-400 mission is to perform a sensitive search for signatures of dark matter particles in high-energy gamma-ray emission. To fulfill these measurements the GAMMA-400 gamma-ray telescope possesses unique physical characteristics in comparison with previous and present experiments. The major advantage of the GAMMA-400 instrument is excellent angular and energy resolution for gamma-rays above 10 GeV. The GAMMA-400 experiment will be installed onboard of the Navigator space platform, manufactured by the NPO Lavochkin Association. The expected orbit will be a highly elliptical orbit (with apogee 300.000 km and perigee 500 km) with 7 days orbital period. An important profit of such an orbit is the fact that the full sky coverage will always be available for gamma ray astronomy.

Development of the NPL gamma-ray spectrometer NANA for traceable nuclear decay and structure studies

We present a brief report on the progress towards the construction of the National Nuclear Array (NANA), a gamma-ray coincidence spectrometer for discrete-line nuclear structure and decay measurements. The proposed spectrometer will combine a gamma-ray energy resolution of approximately 3% at 1MeV with sub-nanosecond timing discrimination between successive gamma rays in mutually coincident decay cascades. We also review a number of recent measurements using coincidence fast-timing gamma-ray spectroscopy for nuclear structure studies, which have helped to inform the design criteria for the NANA spectrometer.

Preliminary investigation of scintillator materials properties: SrI2:Eu, CeBr3 and GYGAG:Ce for gamma rays up to 9 MeV

In this work we measured the performance of a 2ʺ×2ʺ cylindrical tapered crystal of SrI2:Eu, a 2ʺ×3ʺ cylindrical sample of CeBr3 and a 2ʺ×0.3ʺ cylindrical sample of GYGAG:Ce. These scintillators are prototypes in volume or material and were provided by the Lawrence Livermore National Laboratory and by the Institut de Physique Nucléaire d׳Orsay. The gamma-ray energy resolution was measured in the energy range of 0.1–9MeV using different sources. Each scintillator was scanned along x, y and z axes, using a 400MBq collimated 137Cs source. Owing to the GYGAG:Ce thickness, it was not possible to obtain the value of the energy resolution at 9MeV and to scan the crystal along the z axis. The 662keV full energy peak position and its FWHM were measured relative to the full energy peaks positions produced by a non-collimated 88Y source. The signals of the detectors were additionally digitized and compared, up to 9MeV, using a 12bit LeCroy 600MHz oscilloscope.

Characteristics of a Frisch collar grid CdZnTe radiation detector grown by low-pressure Bridgman method

A single-polarity charge-sensing method was studied by using a CdZnTe Frisch collar grid detector grown by using a low-pressure Bridgeman furnace at the Korea Atomic Energy Research Institute (KAERI). The Frisch collar grid CdZnTe detector has an active volume of 5 × 5 × 10 mm3, and was fabricated from a single crystal, Teflon tape and copper tape used as a Frisch collar grid. A room-temperature energy resolution of 6% full width at half maximum (FWHM) was obtained for the 662keV peak of Cs-137 without any additional electrical corrections. The detector’s fabrication process is described, and its characteristics are discussed. Finally, the charge transport properties and gamma-ray energy resolution of the fabricated Frisch collar grid detector are compared with those of two other Frisch collar detectors grown by using different crystal growth methods and purchased from eV-products and Redlen technology.

Characterization of new scintillators: SrI2:Eu, CeBr3, GYGAG:Ce and CLYC:Ce

We measured the performance of a 2"×2" tapered cylindrical SrI2:Eu, a 2" × 3" cylindrical CeBr3, a 2" × 0.3" cylindrical GYGAG:Ce. The gamma-ray energy resolution was measured up to 9 MeV. A scan along three axes of each scintillator was performed using a collimated source of 137Cs. The signals of the detectors were also digitized and compared. We tested two 1" × 1" cylindrical CLYC crystals. One was enriched with 6Li at 95% while the other was enriched with 99% of 7Li. The response of the two detectors to gamma rays, thermal and fast neutrons was measured. The PSD performances have been tested with a different types of PMTs.

Development of high-resolution silicon-based particle sensor with integrated amplification

The electronic noise of the front-end preamplifier can potentially limit the energy and position resolutions of radiation sensors, a detrimental effect that can be diminished by the use of on-chip amplification. The implementation of an avalanching structure upon a direct-conversion semiconductor-based radiation detector is modeled and demonstrated using high-resistivity silicon. The avalanche particle sensor configuration is designed analytically, and we validated the design with numerical process and device simulations. Process and device simulation results from the sensors modeled with various geometries, doping profile arrangements, and fabrication conditions are reported. We tested the refined numerical design by fabricating and testing diagnostic sensors. Multiplication junctions, based on a junction termination extension design, were created from micrometer-scale highly-doped layers on silicon wafers using both diffusion and ion implantation techniques. For <100keV energy depositions, our modeling has shown that a gain of ~8 is ideal if one is attempting to optimize the SNR, which is realized in the experimental detectors at an excess bias of 3V. At that voltage, the energy resolution for 81keV gamma-rays can, in principle, be reduced from 2.12% to 0.96% (for a kα=0.2 device), the degree of improvement limited by the leakage current of the devices, and within a factor of 2 of the ultimate Fano limit (0.53%).