Hard X-ray Applications Research Articles

Study of gain, noise, and collection efficiency of GaAs SAM-APDs single pixel

III-V-compound semiconductors offer many advantages over silicon-based technologies traditionally used in solid-state photodetectors, especially in hard X-ray applications that require high detection efficiency and short response times. Amongst them, gallium arsenide (GaAs) has very promising characteristics in terms of X-ray absorption and high carrier velocity. Furthermore, implementing charge-multiplication mechanisms within the sensor may become of critical importance in cases where the photogenerated signal needs an intrinsic amplification before being acquired by the front-end electronics. This work reports on the experimental characterization by means of lasers and synchrotron radiation of gain, noise, and charge collection efficiencies of GaAs avalanche photodiodes (APDs), realized by molecular beam epitaxy (MBE), featuring separate absorption and multiplication regions (SAM) and different absorption region thicknesses. These devices have been fabricated to investigate the role of the thickness of the absorption region and of possible traps or defects at the metal-semiconductor interfaces in the collection efficiency in order to lay the groundwork for the future development of thicker GaAs devices for detection of hard X-rays.

Achieving High-Resolution Hard X-ray Microscopy using Monolithic 2D Multilayer Laue Lenses

Abstract: This article introduces the 2D multilayer Laue lens (MLL) nanofocusing optics recently developed for high-resolution hard X-ray microscopy. The new optics utilized a micro-electro-mechanical-system (MEMS)-based template to accommodate two linear MLL optics in a pre-aligned configuration. Angular misalignment between the two lenses was controlled in tens of millidegrees, and the lateral position error was on a micrometer scale. Using the developed 2D MLLs, an astigmatism-free point focus of approximately 14 nm by 13 nm in horizontal and vertical directions, respectively, at 13.6 keV photon energy was obtained. The success of 2D MLL optics with an approaching 10 nm resolution is a significant step forward for the development of high-resolution hard X-ray microscopy and applications of MLL optics in the hard X-ray community.

Synchrotron Radiation Study of Gain, Noise, and Collection Efficiency of GaAs SAM-APDs with Staircase Structure.

In hard X-ray applications that require high detection efficiency and short response times, such as synchrotron radiation-based Mössbauer absorption spectroscopy and time-resolved fluorescence or photon beam position monitoring, III–V-compound semiconductors, and dedicated alloys offer some advantages over the Si-based technologies traditionally used in solid-state photodetectors. Amongst them, gallium arsenide (GaAs) is one of the most valuable materials thanks to its unique characteristics. At the same time, implementing charge-multiplication mechanisms within the sensor may become of critical importance in cases where the photogenerated signal needs an intrinsic amplification before being acquired by the front-end electronics, such as in the case of a very weak photon flux or when single-photon detection is required. Some GaAs-based avalanche photodiodes (APDs) were grown by a molecular beam epitaxy to fulfill these needs; by means of band gap engineering, we realised devices with separate absorption and multiplication region(s) (SAM), the latter featuring a so-called staircase structure to reduce the multiplication noise. This work reports on the experimental characterisations of gain, noise, and charge collection efficiencies of three series of GaAs APDs featuring different thicknesses of the absorption regions. These devices have been developed to investigate the role of such thicknesses and the presence of traps or defects at the metal–semiconductor interfaces responsible for charge loss, in order to lay the groundwork for the future development of very thick GaAs devices (thicker than 100 m) for hard X-rays. Several measurements were carried out on such devices with both lasers and synchrotron light sources, inducing photon absorption with X-ray microbeams at variable and controlled depths. In this way, we verified both the role of the thickness of the absorption region in the collection efficiency and the possibility of using the APDs without reaching the punch-through voltage, thus preventing the noise induced by charge multiplication in the absorption region. These devices, with thicknesses suitable for soft X-ray detection, have also shown good characteristics in terms of internal amplification and reduction of multiplication noise, in line with numerical simulations.

Transition-Edge Sensor Optimization for Hard X-ray Applications

Arrays of superconducting transition-edge sensor (TES) microcalorimeters are attractive for some X-ray experiments where the combination of higher collection efficiency than crystal spectrometers and much better energy resolution than semiconductor detectors is enabling. For hard X-ray (>2 keV) applications, a TES is often integrated with an extra absorber to extend its dynamic range, quantum efficiency and collecting area. For >100 keV applications, millimeter size absorbers have been manually attached to TES pixels after fabrication. At lower energies, the absorber size needs to be much smaller, because a large heat capacity degrades energy resolution. In that case, the absorber is directly electroplated or evaporated on the TES during the fabrication process. In this work, we present TES microcalorimeters optimized for the intermediate hard X-ray energy region (20 keV ~ 100 keV) that have thick absorbers directly deposited during the TES fabrication process. This microcalorimeter is designed to measure high-resolution muonic atom transition X-ray spectra as a method to study quantum electrodynamics (QED) effects. The absorber is comprised of evaporated gold and electroplated bismuth layers and is designed to optimize dynamic range, collecting efficiency, and energy resolution in the 20 keV to 45 keV energy range. A prototype with pure gold absorbers has been fabricated and characterized, in order to obtain a first experimental reference in the detector development process.

Depth-graded Mo/Si multilayer coatings for hard x-rays.

This manuscript presents the first systematic study of non-periodic, broadband Mo/Si multilayer coatings with and without B4C interface barrier layers for hard x-ray applications with large field of view. The photon energy of operation in this work is 17.4 keV, the Mo Kα emission line. The coatings involve layers with varying thicknesses in the nanometer scale and the behavior at the layer interfaces plays a crucial role in their performance. Reflectivity measurements and modeling at 8.05 keV and 17.4 keV, Transmission Electron Microscopy (TEM), as well as thin film stress measurements, are employed to examine and optimize the reflective performance of these coatings and the physics of their constituent layers and interfaces. Mo/Si with B4C barrier layers on the Mo-on-Si interface is shown to produce the highest reflectivity among all design configurations considered in this work.

Multilayer on-chip stacked Fresnel zone plates: Hard x-ray fabrication and soft x-ray simulations

Fresnel zone plates are widely used as x-ray nanofocusing optics. To achieve high spatial resolution combined with good focusing efficiency, high aspect ratio nanolithography is required, and one way to achieve that is through multiple e-beam lithography writing steps to achieve on-chip stacking. A two-step writing process producing 50 nm finest zone width at a zone thickness of 1.14 μm for possible hard x-ray applications is shown here. The authors also consider in simulations the case of soft x-ray focusing where the zone thickness might exceed the depth of focus. In this case, the authors compare on-chip stacking with, and without, adjustment of zone positions and show that the offset zones lead to improved focusing efficiency. The simulations were carried out using a multislice propagation method employing Hankel transforms.

Comparison of three types of XPAD3.2/CdTe single chip hybrids for hard X-ray applications in material science and biomedical imaging

The CHiPSpeCT consortium aims at building a large multi-modules CdTe based photon counting detector for hard X-ray applications. For this purpose, we tested nine XPAD3.2 single chip hybrids in various configurations (i.e. Ohmic vs. Schottky contacts or electrons vs. holes collection mode) in order to select the most performing and best suited configuration for our experimental requirements. Measurements have been done using both X-ray synchrotron beams and 241Am source. Preliminary results on the image quality, calibration, stability, homogeneity and linearity of the different types of detectors are presented.

Nanofabrication of tungsten zone plates with integrated platinum central stop for hard X-ray applications

We present a nanofabrication process for producing tungsten zone plates used in hard X-ray applications including a method of integrating a high-energy absorbing central stop with the optic. Tungsten zone plates are structured with electron-beam lithography and subsequent reactive ion etching. The central stop originates from a platinum wire. It is cut to dimension by focused ion beam etching, and afterwards attached to the zone plate center using ion beam induced deposition of platinum. A zone plate with integrated central stop will simplify alignment in hard X-ray scanning microscope arrangements where the 0th order light must be eliminated. The focusing performance of the zone plate device was investigated by scanning coherent diffraction imaging (ptychography) at 8keV photon energy. We could demonstrate a diffraction-limited focus size of 53nm diameter full-width-at-half-maximum. Tungsten zone plates with integrated central stops show promising results for use in hard X-ray microscopes at high-brightness facilities.

Platinum zone plates for hard X-ray applications

We describe the fabrication and evaluation of platinum zone plates for 5–12 kV X-ray imaging and focusing. These nano-scale circular periodic structures are fabricated by filling an e-beam generated mold with Pt in an electroplating process. The plating recipe is described. The resulting zone plates, having outer zone widths of 100 and 50 nm, show good uniformity and high aspect ratio. Their diffraction efficiencies are 50–70% of the theoretical, as measured at the European Synchrotron Radiation Facility. Platinum shows promise to become an attractive alternative to present hard X-ray zone plate materials due to its nano-structuring properties and the potential for zone-plate operation at higher temperatures.

Design, hard x-ray source characterization and applications of a plasma focus tailored for flash hard x-ray imaging

A low-energy (2.5 kJ and 30 kV) plasma focus tailored to perform as a hard x-ray radiation source for flash radiography, is presented. The device is operated in a mixture of deuterium and argon to enhance its performance. A study of the breakdown properties of the discharge chamber is presented. The hard x-ray emission was experimentally characterized by means of both: (i) the spatial location of the radiation source inside the discharge chamber and (ii) an effective energy analysis based on systematic measurements of the attenuation of the hard x-ray output through different metallic pieces. Samples of cadmium, copper and nickel with thicknesses in the range 1–6 mm were employed. From single shot images of the set of plates, the effective energy of the radiation was estimated to be around 83 keV. Numerical calculations, based on validated codes, are presented to give an interpretation of the experimental results. Experimental demonstration of the suitability for hard x-ray imaging applications is also presented.

Development of chemical-mechanical polished high-resolution zone plates

State-of-the-art zone plates for soft and hard x rays are commonly fabricated in nickel or gold by electroplating. The most critical fabrication step is the controlled filling of the plating mold, which directly affects the performance of the diffractive optics. One problem is that the electroplating rate depends on the actual zone width resulting in an inhomogeneous height profile across the optics. Another problem is the measurement of the actual zone height during the electroplating process to fill exactly the plating mold. In practice, underplating the mold results in a low diffraction efficiency of the zone plate. Overplating yields in unemployable optics. In this article, the authors apply a chemical-mechanical polishing (CMP) process to overcome the described problems. In the new processing step, the zone plate is planarized after overplating. The authors demonstrate for the first time that nickel zone plates with an outermost zone width down to 25 nm can be polished by applying a CMP process. This new step leads to a much better reproducibility in zone plate fabrication and their performance. In addition, to overcome the technical limit of the current aspect ratios of zone plates the authors propose to superimpose polished zone plate layers on top of each other. The authors assume that the introduced CMP process paves the way toward the development of future volume zone plates with ultrahigh aspect ratios for soft and hard x-ray applications.

A high-energy resolution 4 cm-wide double-sided silicon strip detector

Double-sided silicon strip detectors (DSSD) with an energy resolution of 1–2 keV full-width at half-maximum (FWHM) are attractive devices for future hard X-ray and soft γ-ray applications. For example, they are well suited as scatterer detectors for semiconductor Compton telescopes working in the sub-MeV to MeV band, as well as imaging spectrometers in the hard X-ray band. In this paper, the performance of newly developed 4-cm-wide DSSDs is presented.This DSSD has an active area of 38.4 mm times 38.4 mm, with a thickness of 300 μm. The stip pitch is 400 μm. The detector shows an average energy resolution of 1.5 keV (FWHM) for 59.5 keV γ-rays, operated at −20 °C with a bias of 100 V. A 22 keV hard X-ray image is also obtained with 400 μm resolution.

Design of X-ray super-mirrors using simulated annealing algorithm

An adaptive local search-based simulated annealing (ALSA) algorithm is proposed for the design of the wide band-pass multilayer optical elements operating in the hard and soft X-ray wavelength ranges. At present the SA algorithm has been kept as simple as possible and its performance is being studied before a series of modifications are made to tailor the SA algorithm to optimize super-mirror for particular applications. The algorithm has also been developed with two specific areas in mind: a W/C broad angle super-mirror for hard X-ray applications and a Mo/Si wide band super-mirror operating at normal incidence in the EUV spectral region. The design results show that the ALSA algorithm method has the flexibility to design a wide range of multilayer structures and in comparison to other techniques has good results although computationally more intensive.

Characterization of a CZT focal plane small prototype for hard X-ray telescope

The promise of good energy and spatial resolution coupled with high efficiency and room temperature operation has fuelled a large international effort to develop cadmium zinc telluride (CZT) for hard X-ray applications. We are involved on the development of a hard X-ray telescope based on multilayer optics and focal plane detector operative in the 10-80 keV energy range. This telescope requires a high efficiency focal plane providing both fine spatial resolution and spectroscopy with a compact and robust design. This paper reports preliminary results on the characterization both in spectroscopic and spatial response of two small pixellated CZT detectors (10times10times1 mm3 and 10times10times2 mm3 single crystals) with 0.45 mm pixel size. We present the results obtained using both standard commercial read-out electronics Readout Electronics for Nuclear Applications (RENA) and innovative low noise and low power dissipation ASICs developed within the collaboration

P.1.060 The role of serotonin and the prefrontal cortex in cognitive flexibility

Double-sided silicon strip detectors (DSSD) with an energy resolution of 1–2 keV full-width at half-maximum (FWHM) are attractive devices for future hard X-ray and soft γ-ray applications. For example, they are well suited as scatterer detectors for semiconductor Compton telescopes working in the sub-MeV to MeV band, as well as imaging spectrometers in the hard X-ray band. In this paper, the performance of newly developed 4-cm-wide DSSDs is presented.This DSSD has an active area of 38.4 mm times 38.4 mm, with a thickness of 300 μm. The stip pitch is 400 μm. The detector shows an average energy resolution of 1.5 keV (FWHM) for 59.5 keV γ-rays, operated at −20 °C with a bias of 100 V. A 22 keV hard X-ray image is also obtained with 400 μm resolution.

Superbend upgrade on the Advanced Light Source

The Advanced Light Source (ALS) is a third generation synchrotron light source at Lawrence Berkeley National Laboratory (LBNL). There was an increasing demand for additional high brightness hard X-ray beamlines in the 7–40 keV range, so in August 2001, three 1.3 T normal conducting bending magnets were removed from the storage ring and replaced with 5 T superconducting magnets (Superbends). The radiation produced by these Superbends is an order of magnitude higher in photon brightness and flux at 12 keV, making them excellent sources of hard X-rays for protein crystallography and other hard X-ray applications. The Superbends did not compromise the performance of the facility in the VUV and soft X-ray regions of the spectrum. The Superbends will eventually feed 12 new beam lines, greatly enhancing the facility's capability and capacity in the hard X-ray region. The Superbend project is the biggest upgrade since the ALS storage ring was commissioned in 1993. In this paper we present an overview of the Superbend project, its challenges and the resulting impact on the ALS.

Development of zone plates with a blazed profile for hard x-ray applications

A linear and a circular zone plate with a blazed zone profile (ZPBP) have been fabricated and characterized using synchrotron x rays. The ZPBPs have significantly improved performances in terms of focusing efficiency and the background near the focus compared to those of a zone plate with a square profile, of which the transmission function can be characterized by a binary square wave. In many respects and practical cases, an x-ray ZPBP may be used in a way analogous to an optical lens in the visible light region. In this article, the experimental characterization of the ZPBPs is presented and some special applications are discussed.

Broad-band hard X-ray reflectors

Interest in optics for hard X-ray broad-band application is growing. In this paper, we compare the hard X-ray (20–100 keV) reflectivity obtained with an energy-dispersive reflectometer, of a standard commercial gold thin-film with that of a 600 bilayer WSi X-ray supermirror. The reflectivity of the multilayer is found to agree extraordinarily well with theory (assuming an interface roughness of 4.5 Å), while the agreement for the gold film is less. The overall performance of the supermirror is superior to that of gold, extending the band of reflection at least a factor of 2.8 beyond that of the gold. Various other design options are discussed, and we conclude that continued interest in the X-ray supermirror for broad-band hard X-ray applications is warranted.

Performance study of polycapillary optics for hard x rays

In order to investigate the feasibility of using Kumakhov capillary x-ray optics for high energy x-ray applications, measurements have been performed on the behavior of capillary optics from 10 to 80 keV. Transmission efficiencies of straight polycapillary fibers of different types have been measured as a function of source location and x-ray energy. The measurements are analyzed using a geometrical optics simulation program, which includes roughness and waviness effects. Despite the low critical angle for total external reflection at high energies, capillary x-ray optics appear promising for many hard x-ray applications. Transmission measurements at high energies have also proven to be a very sensitive tool in capillary quality analysis.

Optimization of multilayer reflectivity and bandpass for soft to hard x-ray applications [0.1–200 keV

In the last decades, the major motivation for manufacturing multilayer mirrors has been in soft x-ray applications, particularly for astronomy, microlithography, and polarimetry. The advent of high energy synchrotron storage rings has provided a new significant impetus emphasizing high energy applications and especially when flux, rather than resolution, is desired. In this paper we present the reflection properties of the most promising multilayer material combinations for the energy range from 0.1 keV up to 200 keV. Previous calculations by Rosenbluth were limited to a maximum of 2 keV and to multilayers composed of pure elements and operating under normal incidence. As alloys might be essential for a smooth growth and/or for stability under high heat load, our screening consisted of a list of up to 300 solids having a melting point above 100°C and that could be deposited in a sputtering process. A full computer search calculates 45000 multilayer combinations for each angle (or multilayer d-spacing) and energy of operation, the only necessary input variables. Other manufacture-related parameters can be specified to give a more realistic picture of the performance. As the number of layers is often limited, a nonperiodic design could minimize absorption effects.