Current multilayer designs for 10-80 keV hard X-ray telescope missions have focused primarily on the proven
properties of W and Pt based multilayer coatings. Recently a number of new material combinations and coating
capabilities have emerged which allows for more elaborate designs that can further extend the energy band of current
mission designs as well as avoid some of the unwanted absorption edge effects in the effective area near potentially
important line emission energies. These new design possibilities are investigated for current hard X-ray mission designs.
The new material combinations to be considered are recently proven capabilities of enhanced NiV/C coatings and
NiV/SiC coatings in conjuction with the well-established W based coatings.
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.
We have developed a large CdTe pixel detector with dimensions of 23.7 x 13.0 mm2 and a pixel size of 448 x 448 μm2. The detector is based on recent technologies of an uniform CdTe single crystal, a two-dimensional ASIC, and stud bump-bonding to connect pixel electrodes on the CdTe surface to the ASIC. Good spectra are obtained from 1051 pixels out of total 1056 pixels. When we operate the detector at -50°C, the energy resolution is 0.67 keV and 0.99 keV at 14 keV and 60 keV, respectively. Week-long stability of the detector is confirmed at operating temperatures of both -50°C and -20°C. The detector also shows high uniformity: the peak positions for all pixels agree to within 0.82%, and the average of the energy resolution is 1.04 keV at a temperature of -50°C. When we normalized the peak area by the total counts detected by each pixel, a variation of 2.1% is obtained.
The High Energy Focusing Telescope (HEFT) is a balloon-borne, hard x-ray/gamma ray (20-70 keV) astronomical experiment. HEFT's 10 arcminute field of view and 1 arcminute angular resolution place challenging demands on its attitude control system (ACS). A microprocessor-based ACS has been developed to manage target acquisition and sidereal tracking. The ACS consists of a variety of sensors and actuators, with provisions for 2-way ground communication, all controlled by an on-board computer. Ground based pointing performance measurements indicate 1σ jitter of 7" and gyro drift rates of <1" s-1. Jitter is expected to worsen in the flight environment, but star tracker data are expected to reduce drift rates significantly, enabling a predicted 1σ absolute attitude determination of ≥4.7". HEFT is scheduled for flight in Spring 2004.
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.
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.
Over the last six 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, relatively small pixels and sub-keV energy resolution. For experiments operating in satellite orbits, energy thresholds of ~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 1152-channel custom low-noise VLSI readout designed to achieve excellent spectral resolution and good imaging performance in the 5 --
We report on the coating of depth graded W/Si multilayers on the thermally slumped glass substrates for the HEFT flight telescopes. The coatings consists of several hundred bilayers in an optimized graded power law design with stringent requirements on uniformity and interfacial roughness. We present the details of the planar magnetron sputtering facility including the optimization of power, Ar pressure and collimating geometry which allows us to coat the several thousand mirror segments required for each telescope module on a time schedule consistent with the current HEFT balloon project as well as future hard X-ray satellite projects. Results are presented on the uniformity, interfacial roughness, and reflectivity and scatter at hard X-ray energies.
A new generation of hard X-ray telescopes using focusing optics are 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, InFocus), possible Explorer-class satellites, and major X-ray observatories (Constellation-X, XEUS) 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 grazing incidence optics coated with depth-graded multilayer films to achieve large collecting areas. To accomplish the ultimate goals of the more advanced satellite missions such as Constellation-X, lightweight and low-cost substrates with angular resolution well below an arcminute must be developed. Recent experimental results will be presented on the development of improved substrates and precision mounting techniques that yield sub-arcminute performance.
We have studied the design of astronomical multilayer telescopes optimized for performance from 5 to 200 keV. This region of the spectrum contains important nuclear lines that are observable in supernovae and their remnants. The study of these lines can help to differentiate currently competing theories of supernova explosion. Our telescope design will enable us to measure the spectral lines of isotopes such as Ni-56 in Type Ia supernovae and Ti-44 in core-collapse remnants, as well as to observe active galactic nuclei at gamma-ray energies. We considered the performances of multilayers of various material pairs, including W/Si, Pt/C and Ni93V7/Si, as employed in conical-approximation Wolter I optics. We experimented with dividing the energy band of interest into several sections, and optimizing different groups of mirror shells within a single telescope for each smaller energy band. Different material pairs are also used for different energy bands, in order to obtain a higher overall performance. We also consider the significance of the energy bandwidth on the effectiveness of Joensen's parametrization of the multilayer thickness profile, and on the mirror performance within the band.
The High-Resolution Spectroscopic Imaging Mission is designed to be the first instrument to make true images of the hard X-ray/soft gamma-ray (2 - 600 keV) sky. By focusing energetic X-rays and low-energy gamma-rays, HSI will observe the cosmos with an unprecedented combination of sensitivity, spectral resolution, and angular resolving power. HSI is based on an array of multilayer grazing-incidence optics focusing onto high-resolution solid-state germanium pixel detectors with a focal length of 30-50 m. This paper describes the primary scientific objectives, technical approach to the instrumentation, and mission design.
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.
A planar magnetron sputtering facility has been established at the Danish Space Research Institute (DSRI) for the production coating of depth graded multilayers on the thermally slumped glass segments which form the basis for the hard X-ray telescope on the HEFT balloon project. The facility is capable of coating 20-45 mirrors segments in each run. The coatings are optimized W/Si coatings. The paper describes the facility, the results of the calibration and presents data for the X-ray testing of flight mirrors.
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.
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