High Energy Focusing Telescope Research Articles

Polarimetry Study With a CdZnTe Focal Plane Detector

Polarimetry has been recognized as a very important observational parameter for high energy astrophysics (>; 100 keV). For this reason the capability to perform accurate polarimetric measurement of high energy cosmic sources should be included in future space mission proposals. This idea has become very appealing in the perspective of new high energy focusing telescope implementing Laue lenses. In order to optimize the focal plane design of such mission as a Compton scattering gamma-ray polarimeter, a CdZnTe detector prototype has been tested at the ESRF (European Synchrotron Radiation Facility) under a ~ 100% polarized gamma-ray beam. The prototype detector tested is a 5 mm thick CdZnTe array with the anode segmented into 16 × 16 pixels of 2.5×2.5 mm2. One of the most important factors that can affect significantly the polarimetric measurements is the direction of the incoming photons from the polarized radiation source with respect to the focal plane of the polarimeter. We have studied and analyzed the polarimetric response of the CdZnTe prototype as a function of the inclination of a polarized beam with respect to the optical axis of the detector from 0° to 10° tilt angle.

Development of the HEFT and NuSTAR focusing telescopes

Hard X-ray/soft gamma-ray astrophysics is on the verge of a major advance with the practical realization of technologies capable of efficiently focusing X-rays above 10 keV. Hard X-ray focusing telescopes can achieve orders of magnitude improvements in sensitivity compared to the instruments based on coded apertures and collimated detectors that have traditionally been employed in this energy band. Compact focal planes enable high-performance detectors with good spectral resolution to be employed in efficient, low-background configurations. We have developed multilayer coated grazing incidence optics and solid state Cadmium Zinc Telluride focal plane systems for the High Energy Focusing Telescope (HEFT) balloon-borne experiment, and for the Nuclear Spectroscopic Telescope Array (NuSTAR) Small Explorer satellite. In this paper we describe the technologies, telescope designs, and performance of both experiments.

Characterization of a large-format, fine-pitch CdZnTe pixel detector for the HEFT balloon-Borne experiment

We have developed a large-format CdZnTe pixel detector with custom, low-noise ASIC readout, for astrophysical applications. In particular, this detector is targeted for use in the High-Energy Focusing Telescope (HEFT), a balloon-borne experiment with focusing optics for 20-70 keV. The detector is a 24/spl times/44 pixel array of 498-/spl mu/m pitch. As a focal plane detector, uniformity from pixel to pixel is very desirable. In this paper, we present the characterization of some detector properties for the 1056 pixels on the HEFT detector. These properties include electronic noise, leakage current, spectral resolution, and count rate.

Numerical modeling of charge sharing in CdZnTe pixel detectors

In this paper, we describe charge collection in CdZnTe pixel detectors, with emphasis on events split between multiple pixels (charge-sharing events). We describe the design of the CdZnTe pixel sensors, being developed for the balloon-borne High-Energy Focusing Telescope (HEFT), and discuss investigations of charge sharing between pixels. We have developed a numerical model that emulates the physical processes of charge transport within the CdZnTe crystal. We discuss this numerical model in detail here. With this model, we are able to reproduce the general features of charge-sharing events. We have found that the amount of charge loss is very sensitive to the surface /spl mu//spl tau/ (the product of charge mobility and trapping time), and we present estimates of (/spl mu//spl tau/) /sub surface/ derived from our model. Further work will focus on more detailed analysis of diffusion in order to gain a complete understanding of these charge-sharing events in CdZnTe pixel detectors.

Development of high spectral resolution CdZnTe pixel detectors for astronomical hard X-ray telescopes

Cadmium zinc telluride detectors (CdZnTe) offer the possibility of achieving excellent spectral and spatial resolution for compact hard X-ray sensors operated without cryogenics. These characteristics make these detectors ideal for several astrophysical applications, including focal plane sensors for multilayer hard X-ray telescopes, and in the image plane of coded-aperture imagers. We are developing a CdZnTe pixel detector and low-noise VLSI readout for use at the focal plane of the balloon-borne High Energy Focusing Telescope (HEFT). In particular, we have concentrated on achieving excellent spectral resolution in a device with 600 μm pixels. Here we report results from the testing of our fully functional prototype hybrid detector. The detector (an 8×7×2 mm 3 single crystal of CdZnTe with an 8×8 pattern of platinum contacts on one side and a monolithic contact on the other) is indium bump-bonded to our 64-channel readout chip, which contains a charge-sensitive preamplifier, shaping amplifier, and combination peak stretcher/discriminator for each pixel. In this paper, we present results taken at room temperature, demonstrating a low-energy threshold near 5 keV and electronic noise of 1.5 keV FWHM at 60 keV. Subsequent testing of this detector at −10°C, reported in Cook et al. [Proc. SPIE 3769 (1999) 92], achieved 670 eV FWHM resolution at 60 keV, with electronic noise of 340 eV.

Fabrication, performance, and figure metrology of epoxy-replicated aluminum foils for hard x-ray focusing multilayer-coated segmented conical optics

We fabricated x-ray mirrors for hard x-ray (*10 keV) tele- scopes using multilayer coatings and an improved epoxy-replicated alu- minum foil (ERAF) nonvacuum technology. The ERAF optics have ;1 arcmin axial figure half-power diameter (HPD) and passed environmen- tal testing. Reflectivity measurements at 8 keV on ERAFs with and with- out multilayer coatings show a 4.4 to 4.8 A root mean square (rms) microroughness for correlation lengths <15 mm. To reduce the dominant contribution of mirror assembly and large-scale distortion in the overall telescope HPD, we designed a figure metrology system and a new mounting technique. We describe a cylindrical metrology system built for fast axial and roundness figure measurement of hard x-ray conical op- tics. These developments lower cost and improve the optics perfor- mance of the HEFT (high-energy focusing telescope) and Constellation-X missions. © 2000 Society of Photo-Optical Instrumentation Engi- neers. (S0091-3286(00)01611-1)

Measured reflectance of graded multilayer mirrors designed for astronomical hard X-ray telescopes

Future astronomical X-ray telescopes, including the balloon-borne High-Energy Focusing Telescope (HEFT) and the Constellation-X Hard X-ray Telescope (Con-X HXT) plan to incorporate depth-graded multilayer coatings in order to extend sensitivity into the hard X-ray ( 10≲E≲80 keV ) band. In this paper, we present measurements of the reflectance in the 18–170 keV energy range of a cylindrical prototype nested optic taken at the European Synchrotron Radiation Facility (ESRF). The mirror segments, mounted in a single bounce stack, are coated with depth-graded W/Si multilayers optimized for broadband performance up to 69.5 keV (WK-edge). These designs are ideal for both the HEFT and Con-X HXT applications. We compare the measurements to model calculations to demonstrate that the reflectivity can be well described by the intended power law distribution of the bilayer thicknesses, and that the coatings are uniform at the 5% level over the mirror surface. Finally, we apply the measurements to predict effective areas achievable for HEFT and Con-X HXT using these W/Si designs.