C. M. Hubert Chen

Properties of Pt Schottky Type Contacts on High-Resistivity CdZnTe Detectors

In this paper, we present studies of the I- V characteristics of CdZnTe (CZT) detectors with Pt contacts fabricated from high-resistivity single crystals grown by the high-pressure Bridgman process. We have analyzed the experimental I- V curves using a model that approximates the CZT detector as a system consisting of a reversed Schottky contact, in series with the bulk resistance. Least-square fit to the experimental data yields 0.78- 0.79 eV for the Pt-CZT Schottky barrier height, and <20V for the voltage required to deplete a 2mm thick CZT detector. We demonstrate that, at high bias, the thermionic current over the Schottky barrier, the height of which is reduced due to an interfacial layer between the contact and CZT material, controls the leakage current of the detectors. In many cases, the dark current is not determined by the resistivity of the bulk material, but rather the properties of the contacts; namely, by the interfacial layer between the contact and CZT material.

W/SiC x-ray multilayers optimized for use above 100 keV.

We have developed a new depth-graded multilayer system comprising W and SiC layers, suitable for use as hard x-ray reflective coatings operating in the energy range 100-200 keV. Grazing-incidence x-ray reflectance at E = 8 keV was used to characterize the interface widths, as well as the temporal and thermal stability in both periodic and depth-graded W/SiC structures, whereas synchrotron radiation was used to measure the hard x-ray reflectance of a depth-graded multilayer designed specifically for use in the range E approximately 150-170 keV. We have modeled the hard x-ray reflectance using newly derived optical constants, which we determined from reflectance versus incidence angle measurements also made using synchrotron radiation, in the range E = 120-180 keV. We describe our experimental investigation in detail compare the new W/SiC multilayers with both W/Si and W/B4C films that have been studied previously, and discuss the significance of these results with regard to the eventual development of a hard x-ray nuclear line telescope.

The Effect of Cathode Bias (Field Effect) on the Surface Leakage Current of CdZnTe Detectors

Surface resistivity is an important parameter of multi-electrode CZT detectors such as coplanar-grid, strip, or pixel detectors. Low surface resistivity results in a high leakage current and affects the charge collection efficiency in the areas near contacts. Thus, it is always desirable to have the surface resistivity of the detector as high as possible. In the past the most significant efforts were concentrated to develop passivation techniques for CZT detectors. However, as we found, the field-effect caused by a bias applied on the cathode can significantly reduce the surface resistivity even though the detector surface was carefully passivated. In this paper we illustrate that the field-effect is a common feature of the CZT multi-electrode detectors, and discuss how to take advantage of this effect to improve the surface resistivity of CZT detectors.

Characterization of the HEFT CdZnTe pixel detectors

We have developed large format CdZnTe pixel detectors optimized for astrophysical applications. The detectors, designed for the High Energy Focusing Telescope (HEFT) balloon experiment, each consists of an array of 24x44 pixels, on a 498 μm pitch. Each of the anode segments on a CdZnTe sensor is bonded to a custom, low-noise application-specific integrated circuit (ASIC)optimized to achieve low threshold and good energy resolution. We have studied detectors fabricated by two different bonding methods and corresponding anode plane designs---the first detector has a steering electrode grid, and is bonded to the ASIC with indium bumps; the second detector has no grid but a narrower gap between anode contacts, and is bonded to the ASIC with conductive epoxy bumps and gold stud bumps in series. In this paper, we present results from detailed X-ray testing of the HEFT pixel detectors. This includes measurements of the energy resolution for both single-pixel and split-pixel events, and characterization of the effects of charge trapping, electrode biases and temperature on the spectral performance. Detectors from the two bonding methods are contrasted.

Development of CdZnTe pixel detectors for astrophysical applications

Over the last four years we have been developing imaging Cadmium Zinc Telluride pixel detectors optimized for astrophysical focusing hard X-ray telescopes. This application requires sensors with modest area (approximately 2 cm X 2 cm), relatively small (approximately less than 500 micrometer) pixels and sub-keV energy resolution. For experiments operating in satellite orbits, low energy thresholds of approximately 1 - 2 keV are also desirable. In this paper we describe the desired detector performance characteristics, and report on the status of our development effort. In particular, we present results from a prototype sensor with a custom low- noise VLSI readout designed to achieve excellent spectral resolution and good imaging performance in the 2 - 100 keV band

Constraints on the average magnetic field strength of relic radio sources 0917+75 and 1401−33 from XMM–Newton observations

We observed two relic radio sources, 0917+75 and 1401−33, with the XMM–Newton X-ray observatory. We did not detect any X-ray emission, thermal or non-thermal, in excess of the local background level from either target. This imposes new upper limits on the X-ray flux due to inverse Compton scattering of photons from the cosmic microwave background by relativistic electrons in the relic sources, and new lower limits on the magnetic field strength from the relative strength of the radio and X-ray emission. The combination of radio and X-ray observations provides a measure of the magnetic field independent of equipartition or minimum energy assumptions. Due to increasing sensitivity of radio observations, the known population of cluster relics has been growing; however, studies of non-thermal X-ray emission from relics remain scarce. Our study adds to the small sample of relics studied in X-rays. In both relics, our field strength lower limits are slightly larger than estimates of the equipartition magnetic field.

Optimal contact geometry for CdZnTe pixel detectors

We are developing CdZnTe pixel detectors for use as focal plane sensors in astronomical hard X-ray telescopes. To optimize the spectral response and imaging performance, we are investigating the effect of contact geometry on charge collection. Specifically, we have studied contact designs with orthogonal thin strips placed between pixel contacts. We apply a negative bias on the grid with respect to the pixel potential to force charge to drift toward the contacts. The grid bias is selected to be just sufficient to avoid charge collection on the grid, while increasing the transverse electric field on the surface between contacts. In contrast to focusing electrodes designed to force field lines to terminate on the pixel contact, our approach allows us to overcome the effects of charge loss between the pixels without significant increase of the leakage current, improving the overall energy resolution of the detector. In this paper we describe the performance of a CdZnTe pixel detector containing a grid electrode, bonded to a custom low-noise VLSI readout. We discuss the advantages of this type of detector for high spectral resolution applications.

Hard X-ray Optics: From HEFT to NuSTAR

Focusing optics are now poised to dramatically improve the sensitivity and angular resolution at energies above 10 keV to levels that were previously unachievable by the past generation of background limited collimated and coded-aperture instruments. Active balloon programs (HEFT), possible Explorer-class satellites (NuSTAR - currently under Phase A study), and major X-ray observatories (Con-X HXT) using focusing optics will play a major role in future observations of a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters. These instruments call for low cost, grazing incidence optics coated with depth-graded multilayer films that can be nested to achieve large collecting areas. Our approach to building such instruments is to mount segmented mirror shells with our novel error-compensating, monolithic assembly and alignment (EMAAL) procedure. This process involves constraining the mirror segments to successive layers of graphite rods that are precisely machined to the required conic-approximation Wolter-I geometry. We present results of our continued development of thermally formed glass substrates that have been used to build three HEFT telescopes and are proposed for NuSTAR. We demonstrate how our experience in manufacturing complete HEFT telescopes, as well as our experience developing higher performance prototype optics, will lead to the successful production of telescopes that meet the NuSTAR design goals.

Production and calibration of the first HEFT hard x-ray optics module

Complete hard X-ray optics modules are currently being produced for the High Energy Focusing Telescope (HEFT), a balloon born mission that will observe a wide range of objects including young supernova remnants, active galactic nuclei, and galaxy clusters at energies between 20 and 70 keV. Large collecting areas are achieved by tightly nesting layers of grazing incidence mirrors in a conic approximation Wolter-I design. The segmented layers are made of thermally-formed glass substrates coated with depth-graded multilayer films for enhanced reflectivity. Our novel mounting technique involves constraining these mirror segments to successive layers of precisely machined graphite spacers. We report the production and calibration of the first HEFT optics module.

W/SiC X-ray multilayers optimized for use above 100 keV

We have developed a new depth-graded multilayer system comprising W and SiC layers, suitable for use as hard X-ray reflective coatings operating in the energy range 100 - 200 keV. Grazing incidence X-ray reflectance at E=8 keV was used to characterize the interface widths, as well as the temporal and thermal stability in both periodic and depth-graded W/SiC structures, while synchrotron radiation was used to measure the hard X-ray reflectance of a depth-graded multilayer designed specifically for use in the range E~150 - 170 keV. We have modeled the hard X-ray reflectance using newly-derived optical constants, which we determined from reflectance-vs-incidence angle measurements also made using synchrotron radiation, in the range E=120 - 180 keV. We describe our experimental investigation in detail, compare the new W/SiC multilayers with both W/Si and W/B4C films that have been studied previously, and discuss the significance of these results with regard to the eventual development of a hard X-ray nuclear line telescope.