High-resolution X-ray Spectroscopy Research Articles

VEGA: A low-power front-end ASIC for large area multi-linear X-ray silicon drift detectors: Design and experimental characterization

We present the design and the first experimental characterization of VEGA, an Application Specific Integrated Circuit (ASIC) designed to read out large area monolithic linear Silicon Drift Detectors (SDD’s). VEGA consists of an analog and a digital/mixed-signal section to accomplish all the functionalities and specifications required for high resolution X-ray spectroscopy in the energy range between 500eV and 50keV. The analog section includes a charge sensitive preamplifier, a shaper with 3-bit digitally selectable shaping times from 1.6µs to 6.6µs and a peak stretcher/sample-and-hold stage. The digital/mixed-signal section includes an amplitude discriminator with coarse and fine threshold level setting, a peak discriminator and a logic circuit to fulfill pile-up rejection, signal sampling, trigger generation, channel reset and the preamplifier and discriminators disabling functionalities. A Serial Peripherical Interface (SPI) is integrated in VEGA for loading and storing all configuration parameters in an internal register within few microseconds. The VEGA ASIC has been designed and manufactured in 0.35µm CMOS mixed-signal technology in single and 32 channel versions with dimensions of 200µm×500µm per channel. A minimum intrinsic Equivalent Noise Charge (ENC) of 12 electrons r.m.s. at 3.6µs peaking time and room temperature is measured and the linearity error is between −0.9% and +0.6% in the whole input energy range. The total power consumption is 481µW and 420µW per channel for the single and 32 channels version, respectively. A comparison with other ASICs for X-ray SDD’s shows that VEGA has a suitable low noise and offers high functionality as ADC-ready signal processing but at a power consumption that is a factor of four lower than other similar existing ASICs.

High spatial resolution spectroscopy of G292.0+1.8 with Chandra

We present high spatial resolution X-ray spectroscopy of supernova remnant G292.0+1.8 made with Chandra observations. The X-ray emitting region of this remnant was divided into 25×25 pixels with a scale of 20″ ×20″ each. Spectra of 328 pixels were created and fitted with an absorbed one component non-equilibrium ionization model. With the spectral analysis results, we obtained maps of absorbing column density, temperature, ionization age and abundances for O, Ne, Mg, Si, S and Fe. The abundances of O, Ne and Mg show tight correlations between each other in the range of about two orders of magnitude, suggesting that they are all from explosive C/Ne burning. Meanwhile, the abundances of Si and S are also well correlated, indicating that they are the ashes of explosive O-burning or incomplete Si-burning. The Fe emission lines are not prominent in the whole remnant, and their abundance is significantly reduced, indicating that the reverse shock may not have propagated to the Fe-rich ejecta. Based on relative abundances of O, Ne, Mg, Si and Fe to Si, we suggest a progenitor mass of 25 – 30 M⊙ for this remnant.

Integrated reflectivity measurements of hydrogen phthalate crystals for high-resolution soft x-ray spectroscopy

The integrated x-ray reflectivity of Potassium Hydrogen Phthalate (KAP) and Rubidium Hydrogen Phthalate (RAP) crystals is studied at a photon energy of (1740±14) eV using a double-crystal setup. The absolute measured reflectivities are in < 5% agreement with the values predicted by the dynamic diffraction theory for perfect crystals when absorption is included. Within 4% experimental error margins, specimen that were exposed to ambient conditions over many years show identical reflectivity as specimen that were cleaved just before the measurement. No differences are observed between cleaving off a 10 μm surface layer and splitting the entire crystal bulk of 2 mm thickness. We conclude that at 1.7 keV photon energy the penetration depth of ∼ 1 μm is large compared to a potentially deteriorated surface layer of a few 10 nm.

High Energy Resolution X-ray Spectroscopy at SSRL and LCLS: Instruments and Applications

High-resolution hard x-ray spectroscopies (XES, HERFD, RIXS, XRS) are now well-established characterization tools for providing insights of material's electronic and geometric structure. The high brilliance synchrotron radiation beamlines have made feasible the routine study of the electronic structure and ligand environment of metal coordination compounds and active centers in metalloproteins, electrochemical process under in-situ conditions, as well as studies on catalytic systems under ambient conditions. Moreover, the recent availability of Linac Coherent Light Source (LCLS), provides some unique opportunities for the study of ultrafast electronic structure dynamics in various phenomena such as electron transfer processes, transient molecular states, molecular dissociation, etc. At SLAC National Accelerator Laboratory we have developed recently a set of high-resolution x-ray spectroscopic capabilities based on various multicrystal spectrometers. At SSRL we have built three multicrystal Johann spectrometers enabling XES/RIXS/HERDF techniques as well as X-ray Raman Spectroscopy. For LCLS, we have developed an energy dispersive multicrystal von Hamos spectrometer that records simultaneously the overall emission spectrum, enabling shot-by-shot time-resolved studies. Representative examples of application will be shown and discussed from the ongoing spectroscopy programs of SSRL and LCLS.

High-resolution X-ray spectroscopy with multi-element HPGe- and Si(Li)-detectors and CUBE preamplifiers

Future synchrotron experiments will strongly require detectors suited for high-energy photons ( > 20 keV) which have a good position resolution combined with an excellent energy resolution (far below 1 keV [FWHM] depending on the element size and capacitance). SEMIKON has already built several fine-structured Si(Li)- and HPGe-detectors [1], but up to now they were read-out by conventional preamplifiers placed far away from the position elements. Therefore the achieved energy resolution ( ∼ 1.6 keV [FWHM] @ 60 keV) was not suitable for synchrotron applications. To achieve a better energy resolution CUBE preamplifiers (developed by XGLab s.r.l.) were used for the read-out of the detector elements. The CUBE preamplifier was originally developed to work in combination with detectors which have a very low capacitance (e.g. Silicon Drift Detectors). With such detectors it was already shown that a very good energy resolution can be achieved even at high count rates [2,3]. With this work, we will show that CUBE can have a good performance also with detectors having a higher capacitance. We will present the very first results of measurements which were performed with structured planar HPGe- and Si(Li)-detectors (multi-element detectors) in combination with the CUBE ASICs.

Plasma-assisted synthesis and high-resolution characterization of anisotropic elemental and bimetallic core-shell magnetic nanoparticles.

Magnetically anisotropic as well as magnetic core–shell nanoparticles (CS-NPs) with controllable properties are highly desirable in a broad range of applications. With this background, a setup for the synthesis of heterostructured magnetic core–shell nanoparticles, which relies on (optionally pulsed) DC plasma gas condensation has been developed. We demonstrate the synthesis of elemental nickel nanoparticles with highly tunable sizes and shapes and Ni@Cu CS-NPs with an average shell thickness of 10 nm as determined with scanning electron microscopy, high-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements. An analytical model that relies on classical kinetic gas theory is used to describe the deposition of Cu shell atoms on top of existing Ni cores. Its predictive power and possible implications for the growth of heterostructured NP in gas condensation processes are discussed.

HIGH RESOLUTION X-RAY SPECTROSCOPY OF THE LOCAL HOT GAS ALONG THE 3C 273 SIGHTLINE

X-ray observations of highly ionized metal absorption lines at z=0 provide critical information of the hot gas distribution in and around the Milky Way. We present a study of more than ten-year Chandra and XMM-Newton observations of 3C 273, one of the brightest extragalactic X-ray sources. Compared with previous work, We obtain much tighter constraints of the physical properties of the X-ray absorber. We also find a large, non-thermal velocity at ~ 100 - 150 km/s is the main reason for the higher line equivalent width when compared with other sightlines. Using joint analysis with X-ray emission and ultraviolet observations, we derive a size of 5 - 15 kpc and a temperature of (1.5-1.8) 10^6 K for the X-ray absorber. The 3C 273 sightline passes through a number of Galactic structures, including the radio Loop I, IV, the North Polar Spur, and the neighborhood of the newly discovered "Fermi bubbles". We argue that the X-ray absorber is unlikely associated with the nearby radio Loop I and IV; however, the non-thermal velocity can be naturally explained as the result of the expansion of the "Fermi bubbles". Our data implies an shock-expansion velocity of 200 - 300 km/s. Our study indicates a likely complex environment for the production of the Galactic X-ray absorbers along different sightlines, and highlights the significance of probing galactic feedback with high resolution X-ray spectroscopy.

Enhanced adhesive strength between shape memory polymer nanocomposite and titanium alloy

Abstract Due to their unique shape memory capability, shape memory materials have been studied in numerous areas that require shape reconfiguration. This capability enables the adaptive wing structural concept. The structure prototype required enhanced interfacial strength between the shape memory polymer composite and structural metal alloys to enable the use of the shape memory polymer composite in this application. The interfacial adhesion properties between the shape memory polymer composite and a titanium alloy were systematically investigated. Surface modification of the titanium alloy with silane coupling agents improved the adhesion strength. The shape memory polymer was toughened using amphiphilic poly(n-butylene oxide)-b-poly(ethylene oxide) and with carbon nanotubes. The toughened shape memory polymer composite showed enhanced adhesion strength compared to the non-toughened system. Toughened shape memory polymer with both 5 wt% poly(n-butylene oxide)-b-poly(ethylene oxide) and 2 wt% carbon nanotube against surface-modified titanium alloy showed up to 113.5% increase in the adhesion strength, compared to the control. To investigate the adhesion mechanism, the adhesive fracture surface was investigated by tracking the locus of failure using high-resolution electron microscopy and energy dispersive X-ray spectroscopy.

A low-power CMOS ASIC for X-ray Silicon Drift Detectors low-noise pulse processing

We present an Application Specific Integrated Circuit (ASIC), named VEGA-1, designed and manufactured for low-power analog pulse processing of signals from Silicon Drift Detectors (SDDs). The VEGA-1 ASIC consists of an analog and a digital/mixed-signal section to achieve all the functionalities and specifications required for high-resolution X-ray spectroscopy in the energy range from 500 eV to 60 keV with low power consumption. The VEGA-1 ASIC has been designed and manufactured in 0.35-μm CMOS mixed-signal technology in single and 32-channel version with dimensions of 200 μm × 500 μm per channel. A minimum intrinsic ENC of 12 electrons r.m.s. at 3.6 μs shaping time and room temperature is measured for the ASIC without detector. The VEGA-1 has been tested with Q10-SDD designed in Trieste and fabricated at FBK, with an active area of 10 mm2 and a thickness of 450 μm. The aforementioned detector has an anode current of about 180 pA at +22°C. A minimum Equivalent Noise Charge (ENC) of 16 electrons r.m.s. at 3.0 μs shaping time and −30°C has been demonstrated with a total measured power consumption of 482 μW.

Development of GEM gas detectors for X-ray crystal spectrometry

Two Triple Gas Electron Multiplier (Triple-GEM) detectors were developed for high-resolution X-ray spectroscopy measurements for tokamak plasma to serve as plasma evolution monitoring in soft X-ray region (SXR). They provide energy resolved fast dynamic plasma radiation imaging in the SXR with 0.1 kHz frequency. Detectors were designed and constructed for continuous data-flow precise energy and position measurement of plasma radiation emitted by metal impurities, W46+ and Ni26+ ions, at 2.4 keV and 7.8 keV photon energies, respectively. High counting rate capability of the detecting units has been achieved with good position resolution. This article presents results of the laboratory and tokamak experiments together with the system performance under irradiation by photon flux from the plasma core.

Impact of Epi-Layer Quality on Reliability of GaN/AlGaN/GaN Heterostructure Field-Effect Transistors on Si Substrate

This paper reports the impact of epi-layer quality on the short-term reliability of GaN/AlGaN/GaN heterostructure field-effect transistors fabricated on Si substrates. At an early stage of 50 V high-temperature operating life tests, they exhibited a significant lowering of the forward breakdown voltage. The mean time to failure (MTTF) determined by this degradation mode was exponentially increased by decreasing full-width at half-maximum (FWHM) values of X-ray rocking curves. In small FWHM samples, extrapolated MTTF values exceeded 1×107 h at Tch=150°C. Alternatively, another degradation mode such as drain current degradation will dominate the long-term reliability at this temperature range. High-resolution transmission electron microscope and energy dispersive X-ray spectroscopy analyses revealed thinning or partial vanishment of GaN cap layer in a degraded sample. Gate current-voltage characteristics calculation based on the self-consistent Schrodinger-Poisson analysis suggested lowering of the effective barrier height by decreasing cap layer thickness is responsible for the Schottky degradation. When considering these experimental and theoretical findings, degradation mechanisms assuming diffusion of interfacial materials along dislocations are discussed.

A HIGH RESOLUTION VIEW OF THE WARM ABSORBER IN THE QUASAR MR 2251-178

High resolution X-ray spectroscopy of the warm absorber in the nearby quasar, MR2251-178 (z = 0.06398) is presented. The observations were carried out in 2011 using the Chandra High Energy Transmission Grating and the XMM-Newton Reflection Grating Spectrometer, with net exposure times of approximately 400 ks each. A multitude of absorption lines from C to Fe are detected, revealing at least 3 warm absorbing components ranging in ionization parameter from log(\xi/erg cm s^-1) = 1-3 and with outflow velocities < 500 km/s. The lowest ionization absorber appears to vary between the Chandra and XMM-Newton observations, which implies a radial distance of between 9-17 pc from the black hole. Several broad soft X-ray emission lines are strongly detected, most notably from He-like Oxygen, with FWHM velocity widths of up to 10000 km/s, consistent with an origin from Broad Line Region (BLR) clouds. In addition to the warm absorber, gas partially covering the line of sight to the quasar appears to be present, of typical column density N_H = 10^23 cm^-2. We suggest that the partial covering absorber may arise from the same BLR clouds responsible for the broad soft X-ray emission lines. Finally the presence of a highly ionised outflow in the iron K band from both 2002 and 2011 Chandra HETG observations appears to be confirmed, which has an outflow velocity of -15600 \pm 2400 km/s. However a partial covering origin for the iron K absorption cannot be excluded, resulting from low ionization material with little or no outflow velocity.

Charge exchange measurements with an x-ray calorimeter at an electron beam ion trap

We present K-shell x-ray spectra of highly ionized Mg acquired with the EBIT calorimeter spectrometer at a resolution of 4.5 eV in charge exchange recombination experiments using the LLNL EBIT-I electron beam ion trap. We measured the Doppler width of Mg11+ Lyα in the same experiments using a high resolution crystal spectrometer, giving an estimate of the ion temperature. We find hardness ratios for Mg11+ ranging from 0.6 to 1.6, depending on the neutral gas target. In most of the experiments, the ion temperature was ∼ 10–15 eV amu−1, indicating that the variations in hardness ratio are intrinsic to the choice of neutral target gas, and are not simply a consequence of variations in the collision velocity resulting from evaporative cooling of the trapped ions. The spectral variations show that high resolution x-ray spectroscopy is highly diagnostic of charge exchange reactions, but requires well-developed theory to interpret.

Investigations on the role of Ni-catalyst for the VLS growth of quasi-aligned GaN nanowires by chemical vapor deposition

We report the fabrication of quasi-aligned GaN nanowires (NWs) on Si(111) substrate by chemical vapor deposition using Ni as a catalyst. The structural and composition analysis of Ni-catalyst on the apex of the GaN NWs were investigated using high-resolution transmission electron microscopy and energy dispersive X-ray spectroscopy. The catalyst contains only Ni–Ga alloy with the composition of ~66:33 having Ni3Ga phase. GaN NWs exhibit wurtzite structure without any amorphous sheath layer or cubic inclusions, where the catalyst contains the mixture of both the crystalline and amorphous phases. During growth, the nitrogen radicals are expected to migrate on the surface of the catalytic droplet to incorporate into the growing lattice via the solid–liquid interface due to poor solubility of N in Ni, which forbids the alloying of nitrogen with Ni. Temperature-dependent photoluminescence reveals that the intensity of donor-bound exciton (D0X) peak consistently increases with a blue shift as the temperature decreases. The D0X peak is centered at 3.467 eV for 10 K. Interestingly, yellow band, the characteristic nature of defects-induced luminescence, is absent in Ni-catalytic-assisted growth of GaN NWs.

Development of Ta-Based Superconducting Tunnel Junction X-Ray Detector Arrays

We are developing new Ta-based superconducting tunnel junction (STJ) X-ray detectors for high-resolution soft X-ray spectroscopy at synchrotrons. STJ detectors combine the high-energy resolution of cryogenic detectors with the high count rate capabilities of athermal devices and the high efficiencies of solid state detectors, which increases the sensitivity for material analysis by fluorescence-detected X-ray absorption spectroscopy. Our STJ detectors are fabricated using thick, high-Z Ta absorber films that enhance quantum efficiency and spectral purity, and extend operational range to several keV compared with earlier Nb-based STJs. They offer an energy resolution of ~ 5 to 10 eV FWHM for soft X-rays up to ~1 keV, and count rates of several 1000 counts/s per detector pixel. For increased solid angle coverage, we have fabricated 36- and 112-pixel Ta-based STJ detector arrays with total areas of 1.4 and 4.5 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. The 208 × 208 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> pixels have an energy resolution between 6.8 and 7.6 eV FWHM at 525 eV with a low-energy shoulder, and their responsivity is uniform to within 2% across the array. Here we discuss the performance of the array in the context of synchrotron science.

Development of Hard X-Ray and Gamma-Ray Spectrometer Using Superconducting Transition Edge Sensor

Superconducting transition edge sensors (TES) are used for high-resolution X-ray spectroscopy. In our group, we have developed Ir/Au-TESs. We have recently started to develop a high-resolution X-ray and gamma-ray spectrometer using our Ir/Au-TES. In this study, we plan to accelerate the response time of the detector system, thus we have designed a new Sn-absorber coupled TES detector system by improving the thermal contact between the TES and the absorber. A superconducting tin absorber is coupled to an Ir/Au superconducting film with a gold (Au) bump post, instead of an epoxy post, which is used conventionally. The pulse height signal, which we obtained from the Au bump post TES, is more than twice as large as that from the epoxy post system, and the slow component of the decay time constant improved to 8.0 ms from 22.0 ms. The energy resolution of the 59.5 keV photoelectron peak is 280 eV FWHM, which is currently dominated by the mechanical vibration of our pulse-tube refrigerator.

Environmental conditions in near-wall plasmas generated by impact of energetic particle fluxes

Directional flows of energetic ions produced by laser-exploded foils were used to investigate transient phenomena accompanying the plasma interaction with surfaces of solid targets (walls). In experiments carried out on the iodine laser system PALS, the formation of energetic plasma jets from burn-through foils of Al and Ta was optimized using the three-frame interferometry and applied to a design of alternate experimental configurations. The interaction of the directional plasma flows with secondary targets was studied via X-ray imaging, optical and high-resolution X-ray spectroscopy. The environmental conditions in near-wall plasmas created at surfaces of plasma-exposed solids, in particular the velocity distribution of impinging and back-scattered ions, were determined via analysis of the observed spatially-resolved spectra of Al Lyα and Heα groups. The validity of the ion velocity gradients derived from the Doppler effect induced shifts and splitting of the spectral lines was supported by theoretical modeling based on a combination of hydrodynamic, atomic and collisional-radiative codes.

Measurement and simulations of hollow atom X-ray spectra of solid-density relativistic plasma created by high-contrast PW optical laser pulses

K-shell spectra of solid Al excited by petawatt picosecond laser pulses have been investigated at the Vulcan PW facility. Laser pulses of ultrahigh contrast with an energy of 160 J on the target allow studies of interactions between the laser field and solid state matter at 1020 W/cm2. Intense X-ray emission of KK hollow atoms (atoms without n = 1 electrons) from thin aluminum foils is observed from optical laser plasma for the first time. Specifically for 1.5 μm thin foil targets the hollow atom yield dominates the resonance line emission. It is suggested that the hollow atoms are predominantly excited by the impact of X-ray photons generated by radiation friction to fast electron currents in solid-density plasma due to Thomson scattering and bremsstrahlung in the transverse plasma fields. Numerical simulations of Al hollow atom spectra using the ATOMIC code confirm that the impact of keV photons dominates the atom ionization. Our estimates demonstrate that solid-density plasma generated by relativistic optical laser pulses provide the source of a polychromatic keV range X-ray field of 1018 W/cm2 intensity, and allows the study of excited matter in the radiation-dominated regime. High-resolution X-ray spectroscopy of hollow atom radiation is found to be a powerful tool to study the properties of high-energy density plasma created by intense X-ray radiation.

The case for massive, evolving winds in black hole X-ray binaries

In the last decade, high-resolution X-ray spectroscopy has revolutionized our understanding of the role of accretion disk winds in black hole X-ray binaries. Here I present a brief review of the state of wind studies in black hole X-ray binaries, focusing on recent arguments that disk winds are not only extremely massive, but also highly variable. I show how new and archival observations at high timing and spectral resolution continue to highlight the intricate links between the inner accretion flow, relativistic jets, and accretion disk winds. Finally, I discuss methods to infer the driving mechanisms of observed disk winds and their implications for connections between mass accretion and ejection processes.

Metal–Organic Framework Luminescence in the Yellow Gap by Codoping of the Homoleptic Imidazolate ∞3[Ba(Im)2] with Divalent Europium

The rare case of a metal-triggered broad-band yellow emitter among inorganic-organic hybrid materials was achieved by in situ codoping of the novel imidazolate metal-organic framework ∞(3)[Ba(Im)2] with divalent europium. The emission maximum of this dense framework is in the center of the yellow gap of primary light-emitting diode phosphors. Up to 20% Eu2+ can be added to replace Ba2+ as connectivity centers without causing observable phase segregation. High-resolution energy-dispersive X-ray spectroscopy showed that incorporation of even 30% Eu2+ is possible on an atomic level, with 2-10% Eu2+ giving the peak quantum efficiency (QE = 0.32). The yellow emission can be triggered by two processes: direct excitation of Eu2+ and an antenna effect of the imidazolate linkers. The emission is fully europium-centered, involving 5d → 4f transitions, and depends on the imidazolate surroundings of the metal ions. The framework can be obtained by a solvent-free in situ approach starting from barium metal, europium metal, and a melt of imidazole in a redox reaction. Better homogeneity for the distribution of the luminescence centers was achieved by utilizing the hydrides BaH2 and EuH2 instead of the metals.