Hard X-ray Detector Research Articles

Development of an experimental platform for the investigation of laser-plasma interaction in conditions relevant to shock ignition regime.

The shock ignition (SI) approach to inertial confinement fusion is a promising scheme for achieving energy production by nuclear fusion. SI relies on using a high intensity laser pulse (≈1016 W/cm2, with a duration of several hundred ps) at the end of the fuel compression stage. However, during laser-plasma interaction (LPI), several parametric instabilities, such as stimulated Raman scattering and two plasmon decay, nonlinearly generate hot electrons (HEs). The whole behavior of HE under SI conditions, including their generation, transport, and final absorption, is still unclear and needs further experimental investigation. This paper focuses on the development of an experimental platform for SI-related experiments, which simultaneously makes use of multiple diagnostics to characterize LPI and HE generation, transport, and energy deposition. Such diagnostics include optical spectrometers, streaked optical shadowgraph, an x-ray pinhole camera, a two-dimensional x-ray imager, a Cu Kα line spectrometer, two hot-electron spectrometers, a hard x-ray (bremsstrahlung) detector, and a streaked optical pyrometer. Diagnostics successfully operated simultaneously in single-shot mode, revealing the features of HEs under SI-relevant conditions.

NuSTAR discovery of the hard X-ray emission and a wide-band X-ray spectrum from the Pictor A western hotspot

Abstract Utilizing Chandra, XMM-Newton, and NuSTAR, a wide-band X-ray spectrum from 0.2 to 20 keV is reported from the western hotspot of Pictor A. In particular, the X-ray emission is significantly detected in the 3 to 20 keV band at 30σ by NuSTAR. This is the first detection of hard X-rays with energies above 10 keV from a jet termination hotspot of active galactic nuclei. The hard X-ray spectrum is well described with a power-law model with a photon index of Γ = 1.8 ± 0.2, and the flux is obtained to be (4.5 ± 0.4) × 10−13 erg s−1 cm−2 in the 3 to 20 keV band. The obtained spectrum is smoothly connected with those soft X-ray spectra observed by Chandra and XMM-Newton. The wide-band spectrum shows a single power-law spectrum with a photon index of Γ = 2.07 ± 0.03, excluding any cut-off/break features. Assuming the X-rays to be synchrotron radiation of the electrons, the energy index of the electrons is estimated as p = 2Γ − 1 = 3.14 ± 0.06 from the wide-band spectrum. Given that the X-ray synchrotron-emitting electrons quickly lose their initial energies via synchrotron radiation, the energy index of electrons at acceleration sites is estimated as pacc = p − 1 = 2.14 ± 0.06. This is consistent with the prediction of the diffusive shock acceleration. Since the spectrum has no cut-off feature up to 20 keV, the maximum electron energy is estimated to be no less than 40 TeV.

Characterization of 128 × 128 MM-PAD-2.1 ASIC: a fast framing hard x-ray detector with high dynamic range

We characterize a new x-ray Mixed-Mode Pixel Array Detector (MM-PAD-2.1) Application Specific Integrated Circuit (ASIC). Using an integrating pixel front-end with dynamic charge removal architecture, the MM-PAD-2.1 ASIC extends the maximum measurable x-ray signal (in 20 keV photon units) to > 107 x-rays/pixel/frame while maintaining a low read noise across the full dynamic range, all while imaging continuously at a frame rate of up to 10 kHz. The in-pixel dynamic charge removal mechanism prevents saturation of the input amplifier and proceeds in parallel with signal integration to achieve deadtime-less measurements with incident x-ray rates of > 1010 x-rays/pixel/s. The ASIC format consists of 128 × 128 square pixels each 150 μm on a side and is designed to be 3-side buttable so large arrays can be effectively tiled. Here we use both laboratory x-ray sources and the Cornell High Energy Synchrotron Source (CHESS) to characterize two single ASIC prototype detectors for both low (single x-ray) and high incident flux detection. In the first detector the ASIC was solder bump-bonded to a 500 μm thick Si sensor for efficient detection of x-rays below 20 keV, whereas the second detector used a 750 μm thick CdTe sensor for x-rays above ∼ 20 keV.

(Mg)ZnO Photoconductive Detector Development for Direct-Conversion Hard X-Ray Detection

Wide-bandgap semiconductor-based X-ray detectors provide a number of advantages. As presented in this letter, (Mg)ZnO photoconductive direct-conversion X-ray detectors with various Mg contents were fabricated and demonstrated. The signal-to-noise ratio (SNR) and response time of the devices under 6 MV hard X-ray irradiation were significantly improved after doping with Mg. The Ag-Mg <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.19</sub> Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.81</sub> O-Ag photoconductive detector with optimal Mg contents exhibited a sensitivity of 3.03 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{C}$ </tex-math></inline-formula> /(mGy <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{\mathrm {air}}\cdot $ </tex-math></inline-formula> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ), a short response time of approximately 0.2 s (rise time) and 0.3 s (decay time), and a large SNR of 278 with a bias of 30 V under hard X-ray irradiation (100 mGy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">air</sub> /s). The results indicate great application prospects for wide-bandgap MgZnO in direct-conversion X-ray detectors.

Ultralow Detection Limit and Robust Hard X-ray Imaging Detector Based on Inch-Sized Lead-Free Perovskite Cs3Bi2Br9 Single Crystals.

Halide perovskites are promising candidates for soft X-ray detection (<80 keV) owing to their high X-ray absorption coefficient, resistivity, and mobility lifetime product. However, the lack of large high-quality single crystals (SCs) renders it challenging to manufacture robust hard X-ray imaging systems (>100 keV) with a low detection limit and stable dark current. Herein, high-quality inch-size two-dimensional (2D) Cs3Bi2Br9 (CBB) single crystals are grown from a melt via the Bridgman method. The crystal quality is enhanced by eliminating inclusions of CsBr-rich phases and restraining the trap-state density, leading to an enhanced resistivity of 1.41 × 1012 Ω cm and a mobility lifetime product of 8.32 × 10-4 cm2 V-1. The Au/CBB/Au single-crystal device exhibits a high sensitivity of 1705 μC Gyair-1 cm-2 in all-inorganic bismuth-based perovskites and an ultralow detection limit of 0.58 nGyair s-1 in all of the bismuth-based perovskites for 120 keV hard X-ray detection. The CBB detector exhibits high work stability with an ultralow dark current drift of 2.8 × 10-10 nA cm-1 s-1 V-1 and long-term air environment reliability under a high electric field of 10 000 V cm-1 owing to the ultrahigh ionic activation energy of the 2D structure. The proposed robust imaging system based on CBB SC is a promising tool for X-ray medical imaging and diagnostics.

Precise Calibration and Application of Hard X-Ray Detector in Energy Range of 10--100 keV

Precise Calibration and Application of Hard X-Ray Detector in Energy Range of 10--100 keV

Development of CuI:Cl-PS composite scintillator

A promising composite scintillator for used in ultrafast X-ray detection was successfully developed by compounding Cl-doped CuI phosphor with polystyrene (PS) (CuI:Cl-PS), and its crystal structure and luminescence characteristics were investigated in detail. The CuI:Cl-PS composite scintillator only exhibited an intense near-band-edge (NBE) luminescence with a decay time of sub-nanosecond, and the slow luminescence of CuI was entirely suppressed under the excitation of ultraviolet light or X-ray. Furthermore, the intensity of the ultrafast NEB emission for the composite scintillator was not basically deteriorated after storage in air at room temperature for six months. It is expected that the CuI:Cl-PS composite scintillator demonstrates a great potential application in ultrafast hard X-ray detection or imaging.

Monte Carlo Modeling and Design of Photon Energy Attenuation Layers for &gt;10× Quantum Yield Enhancement in Si-Based Hard X-ray Detectors

High-energy (&gt;20 keV) X-ray photon detection at high quantum yield, high spatial resolution, and short response time has long been an important area of study in physics. Scintillation is a prevalent method but limited in various ways. Directly detecting high-energy X-ray photons has been a challenge to this day, mainly due to low photon-to-photoelectron conversion efficiencies. Commercially available state-of-the-art Si direct detection products such as the Si charge-coupled device (CCD) are inefficient for &gt;10 keV photons. Here, we present Monte Carlo simulation results and analyses to introduce a highly effective yet simple high-energy X-ray detection concept with significantly enhanced photon-to-electron conversion efficiencies composed of two layers: a top high-Z photon energy attenuation layer (PAL) and a bottom Si detector. We use the principle of photon energy down conversion, where high-energy X-ray photon energies are attenuated down to ≤10 keV via inelastic scattering suitable for efficient photoelectric absorption by Si. Our Monte Carlo simulation results demonstrate that a 10–30× increase in quantum yield can be achieved using PbTe PAL on Si, potentially advancing high-resolution, high-efficiency X-ray detection using PAL-enhanced Si CMOS image sensors.

Development of a hard X-ray detector for measuring continuous spectra generated by runaway electrons in VEST

A hard X-ray measurement system is developed to observe the activities of runaway electrons in versatile experiment spherical torus (VEST) at Seoul National University. The detection system provides information on the generation of runaway electrons by detecting hard X-rays from its bremsstrahlung radiation. To acquire such information, the X-ray spectrum at several tens to hundreds of keV at the maximum is reconstructed using signals from four thin scintillation detectors with different energy sensitivities, and the spectrum is converted to the original X-ray energy. The unfolding process is implemented by an artificial neural network model trained using a measured X-ray spectrum generated by an X-ray tube source at 50 to 320 keV. The system would reveal the generation mechanism of runaway electrons during plasma discharges with various MHD activities in VEST to mitigate and suppress runaway electrons.

Detection of the hard X-ray non-thermal emission from Kepler’s supernova remnant

Abstract We report the first robust detection of the hard X-ray emission in the 15–30 keV band from Kepler’s supernova remnant with the silicon PIN-type semiconductor detector of the hard X-ray detector (HXD-PIN) onboard Suzaku. The detection significance is 7.17σ for the emission from Kepler’s entire X-ray emitting region. The energy spectrum is found to be well reproduced by a single power-law function with a photon index of $3.13^{+1.85+0.69}_{-1.52-0.36}$, where the first and second errors represent $90\%$-statistical and systematic errors, respectively. The X-ray flux is determined to be $2.75_{-0.77-0.82}^{+0.78+0.81}\times 10^{-12}\:$erg s−1 cm−2 in the 15–30 keV band. The wider-band X-ray spectrum in the 3–30 keV band, where the soft X-ray Suzaku/XIS spectrum is combined, shows that the non-thermal component does not have a significant X-ray roll-off structure. We find that the broad-band energy spectrum from the radio band, X-ray data of this work, and TeV upper limits can be reproduced with the one-zone leptonic model with a roll-off energy of νroll = 1.0 × 1017 Hz and magnetic field strength of B &amp;gt; 40 μG. Application of the diagnostic method using indices in the soft and hard X-ray band to the data indicates that the maximum energy of the accelerated electrons in Kepler’s SNR is limited by the age of the remnant. The indication is consistent with the results of the one-zone leptonic modeling.

Hot electrons in a nanowire hard X-ray detector

Nanowire chip-based electrical and optical devices such as biochemical sensors, physical detectors, or light emitters combine outstanding functionality with a small footprint, reducing expensive material and energy consumption. The core functionality of many nanowire-based devices is embedded in their p-n junctions. To fully unleash their potential, such nanowire-based devices require – besides a high performance – stability and reliability. Here, we report on an axial p-n junction GaAs nanowire X-ray detector that enables ultra-high spatial resolution (~200 nm) compared to micron scale conventional ones. In-operando X-ray analytical techniques based on a focused synchrotron X-ray nanobeam allow probing the internal electrical field and observing hot electron effects at the nanoscale. Finally, we study device stability and find a selective hot electron induced oxidization in the n-doped segment of the p-n junction. Our findings demonstrate capabilities and limitations of p-n junction nanowires, providing insight for further improvement and eventual integration into on-chip devices.

Sensitive and Stable 2D Perovskite Single-Crystal X-ray Detectors Enabled by a Supramolecular Anchor.

Perovskite X-ray detectors have been demonstrated to be sensitive to soft X-rays (<80 keV) for potential medical imaging applications. However, developing X-ray detectors that are stable and sensitive to hard X-rays (80 to 120 keV) for practical medical imaging is highly desired. Here, a sensitive 2D fluorophenethylammonium lead iodide ((F-PEA)2 PbI4 ) perovskite single-crystal hard-X-ray detector from low-cost solution processes is reported. Dipole interaction of organic ions promotes the ordering of benzene rings as well as the supramolecular electrostatic interaction between electron-deficient F atoms with neighbor benzene rings. Supramolecular interactions serve as a supramolecular anchor to stabilize and tune the electronic properties of single crystals. The 2D (F-PEA)2 PbI4 perovskite single crystal exhibits an intrinsic property with record bulk resistivity of 1.36 × 1012 Ω cm, which brings a low device noise for hard X-ray detection. Meanwhile, the ion-migration phenomenon is effectively suppressed, even under the large applied bias of 200 V, by blocking the ion migration paths after anchoring. Consequently, the (F-PEA)2 PbI4 single crystal detector yields a sensitivity of 3402 μC Gy-1 air cm-2 to 120 keVp hard X-rays with lowest detectable X-ray dose rate of 23 nGyair s-1 , outperforming the dominating CsI scintillator of commercial digital radiography systems by acquiring clear X-ray images under much lower dose rate. In addition, the detector shows high operation stability under extremely high-flux X-ray irradiation.

The cathode array effects on features and reproducibility of emitted hard x-rays, and anode erosion in a small plasma focus device

Using a small (450 J) Mather-type plasma focus device, this research investigates the effects of cathode rod removal from the cathode array in two steps on hard x-ray emission, both in total and specifically from the plasmoid. The effect on anode rod erosion is also measured. Six hard x-ray detectors were placed around the chamber to measure the emitted hard x-rays. By reducing the number of cathode rods, some features of these hard x-rays such as peak intensity and pulse yield decreased and were deteriorated. Besides, the capability of the device for producing and reproducing the hard x-rays with roughly similar and repeatable characteristics diminished. Moreover, the anode erosion took place during the breakdown phase and by using the detectors around the chamber, similar features were obtained both for the total and the plasmoid hard x-rays, but with different scales. The cathode rod removal could violate the symmetry and uniform formation of the current sheath layer during the breakdown phase and could weaken and change the direction of the Lorentz force vector acting on the current sheath layer during the axial phase. It also causes discharge delay and decreases the axial and radial velocities of the current sheath layer which would result in a weak pinch effect and reduced number of energetic runaway electrons inside the plasma column during the pinch phase. These electrons, by colliding the anode surface, are mainly responsible for producing the great portion of the emitted hard x-ray. On the contrary and based on a different mechanism, the cathode rod removal enhances the number of the runaway electrons during the breakdown phase which increases the anode erosion and enhances the impurities that eventually leads to a weak emission of low intensity hard x-rays.

Fabrication of Bismuth Absorber Arrays for NTD-Ge Hard X-ray Microcalorimeters

The high-spectral-resolution detection of hard X-rays (E > 20 keV) is a challenging and nearly unexplored area in space astrophysics. Traditionally hard X-ray detectors present moderate spectral resolutions, although few tens of eV one could open new frontiers in the study of nuclear processes and high-temperature plasma dynamics in energetic processes. This can be achieved by using cryogenic microcalorimeters. Within a research activity aimed at developing arrays of neutron transmutation-doped germanium (NTD-Ge) microcalorimeters for the high-spectral-resolution detection (about 50 eV@60 keV) of hard X-rays (20 keV 60 μm) bismuth absorber arrays. The adopted technological process and the study of related process parameters are discussed; preliminary results on produced arrays are given.

The ePix10k 2-megapixel hard X-ray detector at LCLS.

The ePix10ka2M (ePix10k) is a new large area detector specifically developed for X-ray free-electron laser (XFEL) applications. The hybrid pixel detector was developed at SLAC to provide a hard X-ray area detector with a high dynamic range, running at the 120 Hz repetition rate of the Linac Coherent Light Source (LCLS). The ePix10k consists of 16 modules, each with 352 × 384 pixels of 100 µm × 100 µm distributed on four ASICs, resulting in a 2.16 megapixel detector, with a 16.5 cm × 16.5 cm active area and ∼80% coverage. The high dynamic range is achieved with three distinct gain settings (low, medium, high) as well as two auto-ranging modes (high-to-low and medium-to-low). Here the three fixed gain modes are evaluated. The resulting dynamic range (from single photon counting to 10000 photons pixel-1 pulse-1 at 8 keV) makes it suitable for a large number of different XFEL experiments. The ePix10k replaces the large CSPAD in operation since 2011. The dimensions of the two detectors are similar, making the upgrade from CSPAD to ePix10k straightforward for most setups, with the ePix10k improving on experimental performance. The SLAC-developed ePix cameras all utilize a similar platform, are tailored to target different experimental conditions and are designed to provide an upgrade path for future high-repetition-rate XFELs. Here the first measurements on this new ePix10k detector are presented and the performance under typical XFEL conditions evaluated during an LCLS X-ray diffuse scattering experiment measuring the 9.5 keV X-ray photons scattered from a thin liquid jet.

Accelerated Electrons Observed Down to

We report the detection of emission from a nonthermal electron distribution in a small solar microflare (GOES class A5.7) observed by the Nuclear Spectroscopic Telescope Array, with supporting observation by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The flaring plasma is well accounted for by a thick-target model of accelerated electrons collisionally thermalizing within the loop, akin to the "coronal thick-target" behavior occasionally observed in larger flares. This is the first positive detection of nonthermal hard X-rays from the Sun using a direct imager (as opposed to indirectly imaging instruments). The accelerated electron distribution has a spectral index of 6.3 ± 0.7, extends down to at least 6.5 keV, and deposits energy at a rate of ~2 × 1027 erg s-1, heating the flare loop to at least 10 MK. The existence of dominant nonthermal emission in X-rays down to <5 keV means that RHESSI emission is almost entirely nonthermal, contrary to what is usually assumed in RHESSI spectroscopy. The ratio of nonthermal to thermal energies is similar to that of large flares, in contrast to what has been found in previous studies of small RHESSI flares. We suggest that a coronal thick target may be a common property of many small microflares based on the average electron energy and collisional mean free path. Future observations of this kind will enable understanding of how flare particle acceleration changes across energy scales, and will aid the push toward the observational regime of nanoflares, which are a possible source of significant coronal heating.

Development of the ZnO:Ga microrods - epoxy composite as a scintillation screen for ultrafast X-ray detection

In order to satisfy the requirements of ultrafast and hard X-ray detection and imaging in the existing and upcoming experimental facilities for high energy physics, the ZnO:Ga microrods - epoxy (ZnO:Ga-EP) composite were successfully synthesized by the hydrothermal and polymerization method. Under X-ray excitation, the ZnO:Ga-EP composite exhibits a narrow and intense near-band-edge emission located at 390 nm. More importantly, the decay time of the near-band-edge emission can reach up to subnanosecond (<1 ns), which is extremely significant for ultrafast X-ray detection and imaging. In addition, there exist an energy transfer from the epoxy host to the ZnO:Ga microrods in the ZnO:Ga-EP composite. In conclusion, this work indicates the ZnO:Ga-EP composite can be used as a potential ultrafast X-ray scintillation screen for applications in ultrafast and hard X-ray detectors.

Microhardness study of Cd1-x ZnxTe1-ySey crystals for X-ray and gamma ray detectors

We performed a detailed Vickers microhardness study of quaternary material Cd1−x ZnxTe1−y Sey (CZTS), which is currently under investigation for applications in the detection of hard X-rays and gamma rays. In this frame, the Vickers microhardness method, resistivity mapping, low-temperature (4.2 K) photoluminescence, and Se energy dispersive X-ray spectroscopy (EDX) composition analysis were employed along the crystal growth axis. The microhardness values measured at 50 g load (HV 0.05), bandgap, and crystal composition along the crystal axis have been correlated. We also performed a comparative study of a set of CdTe, CdZnTe, and CdTeSe samples. We observed a significantly higher microhardness in CZTS crystals when compared to CdZnTe crystals. While in the CdZnTe crystals the microhardness increases by approx. 2.1 HV 0.05 per 1% of Zn + Se content, in CZTS crystals this value is 4.1 HV 0.05. This result indicates a possible additional strengthening of the effect when Zn and Se are mixed in the CdTe lattice.

AGILE search for gamma-ray counterparts of gravitational wave events

AGILE is a space mission of the Italian Space Agency dedicated to X-ray and gamma-ray astrophysics operating since 2007. Since the discovery of gravitational wave (GW) events by the LIGO-Virgo Collaboration (LVC) announced in February 2016, AGILE is playing a very important role in the search for possible X-ray and gamma-ray counterparts. The large fields of view of the gamma-ray and hard X-ray imagers (2.5 sr and 1 sr, respectively) and the current spinning mode allow AGILE to cover at any moment a very large fraction of the sky. We present here an overview of AGILE observations of GW events during the LVC observing periods O1 and O2 which span the time interval September 2015–August 2017. In particular, we focus on the main characteristics of AGILE observations of the events GW150914, GW170104, and GW170817. The latter event is of great relevance being associated with a “kilonova” counterpart: we establish important physical constraints on the possible $$\gamma$$-ray emission from a magnetar-like remnant in the first $$\sim 1000$$ s following the GW event time. We also present further developments and preliminary results obtained in preparation of the O3 observing run (start in April 2019). AGILE is fully operative in the search for GW event counterparts, and continues to observe the sky in a unique way because of its combination of large field-of-view hard X-ray and gamma-ray detectors.

Observation and Investigation of Runaway Electrons in the Start-Up Phase of Taban Tokamak

The plasma current start-up in the Taban tokamak has been achieved via an electron cyclotron resonance (ECR) pre-ionization system. In the present work, the produced runaway electrons (REs) during ECR pre-ionization have been measured and investigated by hard X-ray (HXR) detection under different discharge conditions. The relationship between HXR emission and the loop voltage as well as the filling pressure in the Taban tokamak start-up phase has been reported.