Mark A. Skinner

Measuring the soft X-ray quantum efficiency of charge-coupled devices using continuum synchrotron radiation

Abstract We have measured the soft X-ray (250–4500 eV) quantum efficiency of a charge-coupled device using continuum synchrotron radiation. The spectrum of the synchrotron radiation can be calculated accurately, so the observed pulse height spectrum is a direct measure of the detector efficiency over the synchrotron bandpass. A key function in modeling the efficiency is the energy dependence of the fraction of detected single pixel events. We find that a model including the effects of charge drift and diffusion plus two different depletion depths (corresponding to the collection and barrier phases under the gates) is required. With this model the absolute QE can be determined to within ∼5% everywhere over the 500 to 4500 eV bandpass.

Preliminary results from the diffuse x-ray spectrometer

The Diffuse X-Ray Spectrometer (DXS) experiment was flown as an attached Shuttle payload in January 1993 aboard the STS-54 mission of the Space Shuttle Endeavor. DXS consists of two large-area Bragg crystal X-ray spectrometers that cover the 44 - 83 angstroms wavelength range, and are designed to measure the spectrum of the low energy (0.15 < E < 0.28 keV) diffuse X-ray background with roughly 10 eV energy resolution and 15 degree(s) angular resolution. These diffuse X-rays are thought to be generated by a very hot (106 K) component of the interstellar medium that occupies a large fraction of the interstellar volume near the Sun. Astrophysical plasmas near 106 K are rich in emission lines, and the relative strengths of these lines, besides providing information about the physical conditions of the emitting gas, also provide information about its composition, history and heating mechanisms. We present preliminary spectra of the soft X-ray background in the energy range 0.15 < E < 0.28 keV. Spectra were obtained from along a great circle that lies 0 degree(s) - 10 degree(s) north of the galactic plane between galactic longitudes 150 degree(s) and 300 degree(s). The spectra show emission lines, the first direct evidence that the soft X-ray background arises in hot interstellar gas. The spectra seen along the great circle in different resolution elements are different from one another. We fitted a range of models to these spectra, and present preliminary results of these fits.

Diffuse x-ray spectrometer experiment

The Diffuse X-Ray Spectrometer (DXS) experiment is scheduled to be flown as an attached Shuttle payload in December 1992. As of July 1992, it has completed pre-flight testing at Goddard Space Flight Center and being prepared for shipment to Kennedy Space Center for launch. DXS is designed to measure the spectrum of the low energy (0. 15 — 0.28 keY) diffuse x-ray background with — 10 eV energy resolution and 15° spatial resolution. During its 5-day Shuttle mission, DXS is to measure the spectrum of ten 15° x 15° regions lying along a single 150°-long great circle arc on the sky. DXS has two large area Bragg x-ray spectrometers to cover the wavelength range 44 —84 A using lead stearate Bragg crystals. The spectrometers are of a novel design and have a very large area—solid-angle product, so as to permit measurement of the wavelength spectrum of the cosmic low-energy diffuse x-ray background with good spectral resolution. The bulk of these x-rays are almost certainly from a very hot (T 106 K) component of the interstellar medium that occupies a large fraction of the interstellar volume near the Sun. Astrophysical plasmas near 1O K are rich in emission lines, and the relative strengths of these lines, besides providing information about the physical conditions of the emitting gas, also provide information about its composition, history and heating mechanisms. Each DXS detector consists of a curved panel of Bragg crystals mounted above a position-sensitive proportional counter. The spectrum is dispersed across the counter and all portions of the spectrum are measured at the same time. This eliminates the serious problem in conventional Bragg spectrometers of false spectral features being introduced by time-varying background. On the other hand, while all wavelengths are measured at the same time, the various wavelengths come from different directions in the sky. The spectrometers are therefore rocked back and forth about an axis perpendicular to the dispersed direction to obtain complete spectral coverage along an arc of the sky. This paper describes the DXS instrument concept and design and presents calculations of the anticipated data. It also provides a brief description of the DXS Shuttle payload and its operations

Soft x-ray spectroscopy using charge-coupled devices with thin poly gates and floating gate output amplifiers

We present data from a charge-coupled device (CCD), collaboratively designed by PSU/JPL/Loral, which incorporates several novel features that make it well suited for soft X-ray spectroscopy. It is a three-phase, front-side illuminated device with 1024x1024 pixels. Each pixel is 18 microns by 18 microns.The device has four output amplifiers: two conventional floating diffusion amplifiers (FDAs) and two floating gate amplifiers (FGAs). The FGA non-destructively samples the output charge, allowing the charge in each pixel to be measured multiple times. The readnoise of a given pixel is reduced as the square root of the number of readouts, allowing one to reduce the amplifier noise of these devices to well below the 1/f knee. We have been able to achieve sub-electron readnoise performance with the floating gate amplifier (0.9 e+-) rms with 16 reads per pixel). Using the FGA, the measured energy resolution at 5.9 keV is 120 eV (FWHM). The CCD also has a thin poly gate structure to maximize soft X-ray quantum efficiency. Two-thirds of the active area of the chip is covered only by an insulating layer (1000 angstrom) and a thin poly silicon electrode (400 angstrom). This design enhances the soft X-ray quantum efficiency, but retains the excellent charge transfer efficiency and soft X-ray charge collection efficiency of front-side illuminated devices. The measured energy resolution at 277 eV is 38 eV (FWHM) with a measured quantum efficiency of 15%. We also show that this device performs well below 100 eV, as demonstrated by the detection of Al L fluorescence at 72 eV with a measured FWHM of 16 eV.

CUBIC: x-ray CCD spectroscopy of the diffuse x-ray background

The cosmic unresolved background instrument using CCDs (CUBIC) is currently scheduled for launch on the Argentine/US SAC-B satellite late this year. This instrument is designed to perform moderate resolution nondispersive x-ray spectroscopy of the diffuse x-ray background over the band 0.2 - 10.0 keV using state-of-the-art photon-counting CCDs. The instrument is optimized for spectroscopy of diffuse emission with a field of view of 5 degrees by 5 degrees below 1 keV and 10 degrees by 10 degrees above 3 keV. Observations will typically last 1 - 3 days, and will obtain high quality CCD spectra of the diffuse background, nearby superbubbles and supernova remnants, and the brightest x-ray point sources. This paper gives an overview of the instrument design and CCD detectors.

CUBIC: preflight calibration status

CUBIC, the cosmic unresolved x ray background instrument using CCDs, is instrumented to make moderate resolution x-ray spectral measurements of diffuse targets at spatial scales of a few degrees. While the energy range is nominally 200 eV - 10 keV, the CCDs have been designed to maximize the soft x ray performance by using novel structures. A two part aperture increases the area-solid angle product above 1 keV to maximize sensitivity to the cosmic component of the diffuse x ray background. Here we report preliminary results of our preflight laboratory calibrations performed at Penn State of the dark current, readnoise, nonlinearity, charge transfer efficiency, energy resolution, and quantum efficiency of the two flight CCDs. We also discuss calibration of the detector field of view and the preliminary area-solid angle product of the instrument.

CUBIC: filters and the space environment

CUBIC, the Cosmic Unresolved x-ray Background Instrument using CCDs, is instrumented to make non-dispersive spectral observations, with moderate energy resolution over the energy range 200 eV - 10 keV. The CUBIC filter set is designed to attenuate optical and UV photons, with special emphasis on blocking geocoronal lines. To still allow useful soft x-ray transmission, a single thin (1200 angstroms) Al/Ti filter has been designed as the Optical/UV blocking filter. Since the detectors are mechanically collimated, the CUBIC filters are susceptible to pin hole punctures from micrometeoroids. We review some of the LDEF micrometeoroid measurements, and find that while there is good probability that micrometeoroids will transit our filter, the holes will be small enough and few enough in number to have no significant impact on our stray light counting rate.

CUBIC: preflight calibration results and initial operations

The cosmic unresolved background instrument using CCDs (CUBIC) was scheduled for launch on the Argentine/U.S. SAC-B satellite in October 1996. This instrument is designed to perform moderate resolution nondispersive x-ray spectroscopy of the diffuse x-ray background over the band 0.2 - 10.0 keV using state-of-the-art photon-counting CCDs. The instrument is optimized for spectroscopy of diffuse emission with a field of view approximately 5 degrees multiplied by 5 degrees below 1 keV and 10 degrees multiplied by 10 degrees above 3 keV. Here we discuss the present state of analysis of our preflight calibration data and present preliminary operational plans.

CUBIC: a nondispersive diffuse x-ray background spectrometer

The Cosmic Unresolved X-ray Background InsLrumenl using CCDs (CUBIC ) is designed to obtain spectral observations of the Diffuse X-ray Background (DXRB) with moderate spectral resolution (E/E 10—60) over the energy range 0.2 — 10 keV using mechanically collimated CCDs. It will be launched on the NASA/Argentine minisat SA C-B in December 1994. At this time, it is the only planned satellite payload devoted to the study of the spectrum of the DXRB. Observations will consist of 1—2 day pointed exposures of each target direction, resulting in a series of high quality spectra. Over the anticipated 3 year lifetime of the satellite, CUBIC will be able to study up to 50% of the sky with 5° x 5° spatial resolution for the subkilovolt Galactic diffuse background, and with 1O x 1O spatial resolution for the extragalactic diffuse background above 2 keV. CUBIC will obtain high quality non-dispersive spectra of soft X-ray emission from the interstellar medium, supernova remnants, and some bright sources, and will make a sensitive search for line emission or other features in the extragalactic cosmic X-ray background from 2 — 10 keY