Astronomical X-ray Telescopes Research Articles

Improved noise performance from the next-generation buried-channel p-MOSFET SiSeROs

The Single electron Sensitive Read Out (SiSeRO) is a novel on-chip charge detector output stage for charge-coupled device (CCD) image sensors. Developed at MIT Lincoln Laboratory, this technology uses a p-MOSFET transistor with a depleted internal gate beneath the transistor channel. The transistor source-drain current is modulated by the transfer of charge into the internal gate. At Stanford, we have developed a readout module based on the drain current of the on-chip transistor to characterize the device. In our earlier work, we characterized a number of first prototype SiSeROs with the MOSFET transistor channels at the surface layer. An equivalent noise charge (ENC) of around 15 electrons root mean square (RMS) was obtained. In this work, we examine the first buried-channel SiSeRO. We have achieved substantially improved noise performance of around 4.5 electrons root mean square (RMS) and a full width half maximum (FWHM) energy resolution of 132 eV at 5.9 keV, for a readout speed of 625 kpixel/s. We also discuss how digital filtering techniques can be used to further improve the SiSeRO noise performance. Additional measurements and device simulations will be essential to further mature the SiSeRO technology. This new device class presents an exciting new technology for the next-generation astronomical X-ray telescopes requiring fast, low-noise, radiation-hard megapixel imagers with moderate spectroscopic resolution.

Optical design of type-1 x-ray telescopes and their application to STAR-X.

An astronomical x-ray telescope's capability rests on the quality of its optics, which in turn rests on its point spread function (PSF), field of view (FOV), and photon-collecting area. The design and implementation of telescope optics must optimize these three parameters in the context of mathematical prescription, optical fabrication, engineering, and resources such as mass and cost constraints. In this paper, after reviewing important features of grazing incidence optics and the many different mathematical prescriptions in the literature, we quantitatively compare the advantages and disadvantages of these prescriptions, using detailed ray trace, to optimize the PSF and FOV for a given set of requirements. Then, we apply this approach to optimizing the designs for a proposed future x-ray telescope, Survey and Time-Domain Astrophysical Research eXplorer (STAR-X), a NASA Medium-Class (MIDEX) mission by optimizing the combination of PSF and FOV.

Shaped silicon wafers obtained by hot plastic deformation: performance evaluation for future astronomical x-ray telescopes

In order to develop lightweight and high angular resolution x-ray mirrors, we have investigated hot plastic deformation of 4 in. silicon (111) wafers. A sample wafer was deformed using hemispherical dies with a curvature radius of 1000 mm. The measured radius of the deformed wafer was 1030 mm, suggesting that further conditioning is indispensable for better shaping. For the first time to our knowledge, x-ray reflection on a deformed wafer was detected at Al K(alpha) 1.49 keV. An estimated surface roughness of <1 nm from the x-ray reflection profile was comparable to that of a bare silicon wafer without deformation. Hence, no significant degradation of the microroughness was seen.

Collimated Magnetron Sputter Deposition for Mirror Coatings

At the Danish National Space Center (DNSC), a planar magnetron sputtering chamber has been established as a research and production coating facility for curved X‐ray mirrors for hard X‐ray optics for astronomical X‐ray telescopes. In the following, we present experimental evidence that a collimation of the sputtered particles is an efficient way to suppress the interfacial roughness of the produced multilayer. We present two different types of collimation optimized for the production of low roughness curved mirrors and flat mirrors, respectively.

X-ray polarimetry with an active-matrix pixel proportional counter

We report the first results from an X-ray polarimeter with a micropattern gas proportional counter using an amorphous silicon active matrix readout. With 100% polarized X-rays at 4.5 keV, we obtain a modulation factor of 0.33±0.03, confirming previous reports of the high polarization sensitivity of a finely segmented pixel proportional counter. The detector described here has a geometry suitable for the focal plane of an astronomical X-ray telescope. Amorphous silicon readout technology will enable additional extensions and improvements.

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.

Imaging and quantitative analysis of tritium-labelled cells in lymphocyte proliferation assays using microchannel plate detectors originally developed for X-ray astronomy

Microchannel plate detectors have been used in many astronomical X-ray telescopes. Recently we have begun to use similar detectors to image electron emission from radiolabelled biological assays. Here we show how a microchannel plate (MCP) detector can be used to image tritium uptake in T lymphocyte proliferation assays. Quantitative analysis using the MCP detector has the same sensitivity and speed as conventional liquid scintillation counter (LSC) analysis whilst obviating the need for scintillation fluid. In addition the system permits the imaging of whole plate harvests from a range of plate sizes. Here we present data obtained with 96-well plates and Terasaki plates.

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.

Development of thermally formed glass optics for astronomical hard X-ray telescopes

The next major observational advance in hard X-ray/soft gamma-ray astrophysics will come with the implementation of telescopes capable of focusing 10-200 keV radiation. Focusing allows high signal-to-noise imaging and spectroscopic observations of many sources in this band for the first time. The recent development of depth-graded multilayer coatings has made the design of telescopes for this bandpass practical, however the ability to manufacture inexpensive substrates with appropriate surface quality and figure to achieve sub-arcminute performance has remained an elusive goal. In this paper, we report on new, thermally-formed glass micro-sheet optics capable of meeting the requirements of the next-generation of astronomical hard X-ray telescopes.

Optimization of graded multilayer designs for astronomical x-ray telescopes

We developed a systematic method for optimizing the design of depth-graded multilayers for astronomical hard-x-ray and soft-gamma-ray telescopes based on the instrument's bandpass and the field of view. We apply these methods to the design of the conical-approximation Wolter I optics employed by the balloon-borne High Energy Focusing Telescope, using W/Si as the multilayer materials. In addition, we present optimized performance calculations of mirrors, using other material pairs that are capable of extending performance to photon energies above the W K-absorption edge (69.5 keV), including Pt/C, Ni/C, Cu/Si, and Mo/Si.

A mercuric iodide detector system for X-ray astronomy: I. Design considerations and predictions of background and sensitivity

This is the first of two papers describing an extremely sensitive astronomical hard X-ray telescope design based on a mercuric iodide (HgI 2) detector shielded by a bismuth germanate (Bi 4Ge 3O 12) scintillator. In a version suitable for a Spacelab mission, this shielded detector is located at the focus of a pyrolytic graphite or lithium fluoride concentrator (the Hard X-ray Diffraction Telescope, or HXDT). The background counting rate of the mercuric iodide detector at balloon altitudes is calculated using a Monte Carlo simulation of the detector assembly in the radiation environment at 40 km above sea level. In paper II of this series, these predictions are compared to actual balloon flight measurements of a prototype detector assembly.