Steven E. Kissel

Soft-X-ray CCD imagers for AXAF

We describe the key features and performance data of a 1024/spl times/1026-pixel frame-transfer imager for use as a soft-X-ray detector on the NASA X-ray observatory Advanced X-ray Astrophysics Facility (AXAF). The four-port device features a floating-diffusion output circuit with a responsivity of 20 /spl mu/V/e/sup -/ and noise of about 2 e/sup -/ at a 100-kHz data rate. Techniques for achieving the low sense-node capacitance of 5 fF are described. The CCD is fabricated on high-resistivity p-type silicon for deep depletion and includes narrow potential troughs for transfer inefficiencies of around 10/sup -7/. To achieve good sensitivity at energies below 1 keV, we have developed a back-illumination process that features low recombination losses at the back surface and has produced quantum efficiencies of about 0.7 at 277 eV (carbon K/spl alpha/).

X-ray CCD Calibration for the AXAF CCD Imaging Spectrometer

Acquisition of ground calibration data from the AXAF CCD Imaging Spectrometer, one of two focal plane instruments on NASAs Advanced X-ray Astrophysics Facility, was completed in 1997. Here we summarize results of the detector level calibration effort. Our calibration program has included measurements of CCD response to undispersed synchrotron radiation, measurements of x-ray absorption fine structure, and of sub-pixel structure in the detector. Errors in the energy scale are at the level of a few tenths of one percent, and detection efficiency errors are no large than a few percent. We have also obtained new insights into the mechanisms by which the CCD gate structure and channel stops influence the CCD spectral redistribution function.

Radiation damage in the Chandra x-ray CCDs

Front side illuminated CCDs comprising focal plane of Chandra X-ray telescope have suffered some radiation damage in the beginning of the mission. Measurements of CTI and dark current at different temperatures led us to conclusion that the type of damage is inconsistent with the much studied type of damage created by protons with energies higher than 10 MeV. Intensive ground based investigation showed that irradiation of CCD with low energy protons (about 100 keV) results in the device characteristics very similar to the ones of the flight chips (very low dark current, the shape of the CTI temperature dependence). We were able to reliably determine that only image section of the flight chips was damaged and therefore only fast transfer from image to frame store section was affected. We have developed several techniques in order to determine the parameters of the electron traps introduced into the transfer channel of the irradiated device.

Cyclotron resonance energies at a low X-ray luminosity: A0535+262 observed with Suzaku

The binary X-ray pulsar A0535+262 was observed with the Suzaku X-ray observatory on 2005 September 14 for a net exposure of 22 ks. The source was in the declining phase of a minor outburst, exhibiting 3-50 keV luminosity of ~3.7 × 1035 ergs s-1 at an assumed distance of 2 kpc. In spite of the very low source intensity (about 30 mcrab at 20 keV), its electron cyclotron resonance was detected clearly with the Suzaku Hard X-Ray Detector, in absorption at about 45 keV. The resonance energy is found to be essentially the same as that measured when the source is almost 2 orders of magnitude more luminous. These results are compared with the luminosity-dependent changes in the cyclotron resonance energy, observed from 4U 0115+63 and X0331+53.

Progress in x-ray CCD sensor performance for the Astro-E2 x-ray imaging spectrometer

We have developed X-ray CCD sensors for the Astro-E2 X-ray Imaging Spectrometer. Here we describe the performance benefits obtained from two innovations implemented in the CCD detectors developed for this instrument. First, we discuss the improved radiation tolerance afforded by a novel charge-injection structure. Second, we demonstrate for the first time the potential of a previously-developed chemisorption charging backside treatment process to produce back-illuminated X-ray sensors with excellent soft X-ray spectral resolution as well as improved quantum efficiency. We describe the changes in X-ray event detection algorithms required to obtain this improved performance, and briefly compare the performance of XIS sensors to that of back-illuminated detectors currently operating on-orbit.

Measurement of the subpixel structure of AXAF CCD's

The authors present a method to measure the subpixel structure of a charge-coupled device (CCD), information necessary to accurately determine (<1% uncertainty) the absolute detection efficiency of the device. Their approach uses a thin metal film with periodically spaced holes (small, compared to the pixel size) to localize incident X-rays to a particular region of the pixel. The mesh is rotated to create a small angular misalignment between the grid holes and the CCD pixels, producing a moire effect in the data. The resultant moire pattern is compared to a CCD model, and a best fit minimization technique is used to constrain the parameters that describe the subpixel structure. This technique was developed to measure and calibrate the X-ray CCDs that will comprise one of the two focal plane instruments on-board AXAF, but it is applicable for measuring the structure of any pixelated solid state device.

X-ray CCD response functions, front to back

Abstract For some applications, particularly in X-ray astronomy, one requires accurate knowledge of the charge-coupled device (CCD) spectral redistribution function, with errors of order 1%. We describe some recent advances in response function measurement and modeling. In particular, we show that it is now possible to isolate and separately measure the response contributions of many of the component volumes that comprise the CCD. We describe how we have deployed this capability to determine X-ray photon response functions for front-illuminated detectors, and indicate how these measurements can improve understanding of the response function of the back-illuminated CCD.

CCD soft-X-ray detectors with improved high- and low-energy performance

We describe results from recent efforts to enhance the performance of CCDs to both low- and high-energy soft a rays. For improved low-energy (E<500 eV) sensitivity we show that a low-temperature surface treatment on back-illuminated devices results in generally better performance than that achieved on devices flown on Chandra, which had a more process-intensive high-temperature treatment. For improved high-energy response we describe a design approach for MOS CCDs that allows high substrate biases for deep depletion (>160 /spl mu/m) and thus improved x-ray detection for E>5 keV.

X-ray CCD with charge-injection structure

A frame transfer CCD intended for X-ray detection on-board ASTRO-E2 spacecraft was modified to include an input serial register and a charge injection structure which allows a very uniform injection of charge into the imaging section of the device. A variation of the fill-and-spill method was implemented to inject charge into the CCD. The operation of the structure is described, and results of the measurements are presented. Very small charge packets (a few tens of electrons) can be reproducibly injected with noise as low as 5 electrons rms. The amount of injected charge can be controlled by the external voltage with very high accuracy. We have applied this technique to study various characteristics of the proton irradiated CCD, such as the column-to-column nonuniformity of charge losses and the amplitude dependence of the charge loss. The latter is related to the charge-volume relation in the charge storage site and for the first time we accurately measure this relationship at very low signal levels.

A charge transfer inefficiency correction model for the Chandra Advanced CCD Imaging Spectrometer

Soon after launch, the Advanced CCD Imaging Spectrometer (ACIS), one of the focal plane instruments on the Chandra X-ray Observatory, suffered radiation damage from exposure to soft protons during passages through the Earths radiation belts. The primary effect of the damage was to increase the charge transfer inefficiency (CTI) of the eight front illuminated CCDs by more than two orders of magnitude. The ACIS instrument team is continuing to study the properties of the damage with an emphasis on developing techniques to mitigate CTI and spectral resolution degradation. We will discuss the characteristics of the damage, the detector and the particle background and how they conspire to degrade the instrument performance. We have developed a model for ACIS CTI which can be used to correct each event and regain some of the lost performance. The correction uses a map of the electron trap distribution, a parameterization of the energy dependent charge loss and the fraction of the lost charge re-emitted into the trailing pixel to correct the pixels in the event island. This model has been implemented in the standard Chandra data processing pipeline. Some of the correction algorithm was inspired by the earlier work on ACIS CTI correction by Townsley and collaborators. The details of the CTI model and how each parameter improves performance will be discussed, as well as the limitations and the possibilities for future improvement.