Alan Owens

The X-ray energy response of silicon Part A. Theory

Abstract In this, the first part of a two-part study of the interaction of soft X-rays with silicon, motivated by the calibration requirements of CCD imaging spectrometers in astronomy, we describe a Monte Carlo model of X-ray energy loss whose products are the energy- and temperature-dependences of (i) W , the average energy required to create an electron-hole pair, and (ii) the Fano factor F , W and F have invariably been treated as material constants in previous analyses of Si X-ray detector performance. We show that in fact, at constant detector temperature T , W is an increasing function of X-ray energy for E F is predicted to increase slowly with E . The temperature coefficient d W /d T has a calculated value ∼ 1 × 10 −4 K −1 at a typical CCD operating temperature of 170 K. We discuss the practical implications of these results. Finally, we describe our separate calculations of the near-edge variation of CCD quantum detection efficiency arising from silicon K-shell Extended X-ray Absorption Fine Structure (EXAFS).

The X-ray energy response of silicon (B): Measurements

Abstract In this, the second part of a detailed study of the interaction of soft X-rays with silicon, we summarise the results of a large number of experiments on charge coupled devices (CCDs), carried out both in our laboratory and at the Daresbury Synchrotron Radiation Source (SRS). Measurements of the energy variation of the W parameter and of the Fano factor F are in substantial agreement with the predictions of the model developed in Part (A) of the study [G.W. Fraser et al., Nucl. Instr. and Meth. A 350 (1994) 368]. The consequences of using a Gaussian pulse height distribution model in the experimental determination of F are discussed. Variations in X-ray event morphology (i.e. the frequency distribution of single-, two-, three-pixel events) across the silicon K edge are described. Measurements of CCD quantum detection efficiency Q (counts/photon) showing XAFS (X-ray absorption fine structure) modulation in the vicinity of the Si K edge are compared with calculations based upon new, experimentally-determined linear absorption coefficients for Si, SiO 2 and Si 3 N 4 . Finally, the X-ray photoyield from silicon is described, both experimentally and theoretically.

Modelling the X-ray response of charge coupled devices

Abstract Based on the physics of charge generation, diffusion and collection, we have developed a simple three-dimensional Monte Carlo model to predict the X-ray response of deep depletion front-illuminated CCDs. It is shown that the measurable properties of the device (i.e., energy resolution, quantum efficiency and event morphology) can all be reproduced by a simple treatment of charge creation and diffusion within each of the active layers of the device. The simulation reproduces the distinct spectral signatures of X-ray interactions in the depletion layer and the field-free regions. Pulse height spectra can be reproduced to good accuracy and quantum efficiencies and energy resolutions could be predicted to within a few percent over the energy range 500 eV to ∼ 10 keV. Refinements such as the effect of altering electrode voltages have been incorporated into the model.

Mapping X-ray absorption fine structure in the quantum efficiency of an X-ray charge-coupled device

Recent measurements have shown that high-resolution silicon detectors are beginning to resolve x-ray absorption fine structure (XAFS) produced in the detectors themselves. Unless calibrated, such structure threatens to ‘pollute’ spectroscopic measurements. An experiment has begun map this structure to systematically in an x-ray charge-coupled device (CCD) over the energy range 300–2100 eV. The program is novel in that the synchrotron radiation source has to be operated with a ring current reduced by five orders of magnitude so that individual photons can be recorded by the CCD. Results of the detailed spectroscopic response around the nitrogen, oxygen and silicon K edges are presented. The measured quantum efficiency shows considerable near-edge structure which is modified by the presence of the various insulation layers used in MOS construction, reflecting the chemical rather than elemental nature of the interaction environment. It is pointed out that the measurement of XAFS in x-ray CCDs is potentially a powerful diagnostic tool with which to explore surface structures.

Gamma-ray observations of the Crab Region using a coded-aperture telescope

The region of the Galactic anticenter, including the Crab Nebula, was observed during a balloon flight of the University of New Hampshire Directional Gamma-Ray Telescope employing the coded-aperture imaging technique to image celestial gamma-radiation between 160 keV and 9.3 MeV. The background systematics are treated with a simple and relatively straightforward correction procedure. The results demonstrate that the coded-aperture procedure is a viable approach for imaging not only point sources of radiation, but also extended sources of emission. The results for the Crabs photon spectrum are consistent with a power-law spectrum. Upper limits on the flux levels of line emission at 405 keV and 1050 keV and on the flux from the X-ray binary source A0535 + 26 and diffuse Galactic emission from the anticenter region are derived. 35 references.

Event recognition in X-ray CCDs

Abstract The signatures of photon and charged particle interactions in an X-ray CCD have been investigated and techniques for reducing spectral data developed. It is found that the energy-loss spectrum of X-rays which interact in the depletion layer are well described by simple functions and show little evidence of redistributive processes. However, photons which interact in the “so-called” field-free region introduce multi-pixel, line-like features into the spectral data which are shifted in energy and slightly broadened with respect to depletion region events. The efficiency of this process can be surprisingly high, being comparable to the depletion layer efficiency at 10 keV for the devices considered in this study. For astrophysical applications, these artifacts can have energies and intensities comparable to expected non-solar line and edge features. We have developed procedures to identify, quantify and correct for these effects to arbitrary precision. Simple statistical techniques are described to ascertain when, and if, they need to be applied.

On the relationship between total electron photoyield and X-ray absorption coefficient

Abstract We have tested the hypotheses that the total electron photoyield and X-ray absorption coefficient are simply related and that the drain current method can then be reliably used to derive absorption coefficients. By comparing gold photocurrent measurements taken at the Daresbury Synchrotron Radiation Source (SRS) with experimentally derived linear absorption coefficients in the same energy band, we find that the expected relationship is accurate over the range 2200–3500 eV to a precision of a few percent.

TGRS OBSERVATION OF THE GALACTIC CENTER ANNIHILATION LINE

The Transient Gamma-Ray Spectrometer (TGRS) experiment is a high-resolution germanium detector launched on the WIND satellite on 1994 November 1. Although primarily intended to study gamma-ray bursts and solar flares, TGRS also has the capability of studying slower transients (e.g., X-ray novae) and certain steady sources. We present here results on the narrow 511 keV annihilation line from the general direction of the Galactic center accumulated over the period 1995 January through 1995 October. These results were obtained from the TGRS occultation mode, in which a lead absorber occults the Galactic center region for of each spacecraft rotation, thus chopping the 511 keV signal. The occulted region is a band in the sky of width 16° that passes through the Galactic center. We detect the narrow annihilation line from the Galactic center with flux = (1.64 ± 0.09) × 10-3 photons cm-2 s-1. The data are consistent with a single point source at the Galactic center, but a distributed source of extent up to ~30° cannot be ruled out. No evidence for temporal variability on timescales longer than 1 month was found.

X-ray-induced radiation damage in CsI, Gadox, Y2O2S and Y2O3 thin films

The stability of CsI, CsI(Tl), Gd O S(Tb), Gd O S(Eu), Y O S(Eu) and Y O (Eu) thin lms under bombardment by 9}18 keV X-rays is described. Both external photocurrent and scintillation light yield were measured as functions of accumulated dose at radiation #uxes of 10}10 photons s mm on Beamline 2.2 of the Daresbury Synchrotron Radiation Source (SRS). All of the samples studied showed changes of several percent (both reductions and increases) in photocurrent and scintillation light yield of several percent for accumulated doses of up to 510 photons mm .N o signicant dependence of the lm response on the angle of X-ray incidence was observed for angles up to 453 from the normal. It was found that the accumulated dose is not the only parameter determining the degradation of photoconverter performance; the #ux rate has also to be taken into account. Scanning Electron Microscope studies of the irradiated samples did not reveal any signicant surface modication. 2001 Elsevier Science B.V. All rights reserved.

Gamma-ray observations of Cygnus X-1 and Cygnus X-3 using a coded-aperture telescope

A balloon-borne coded-aperture telescope, measuring gamma-ray photons in the 160 keV to 9.3 MeV range, was used to observe the Cygnus region of the sky on October 1 and 2, 1984. In the 2-9.3-MeV band, evidence is found for a hard spectral component with a mean flux level at the top of the atmosphere of 7.4 + or - 2.5 x 10 to the -7th photons/sq cm per s per keV, inconsistent with the predictions of the inverse Compton models normally used to describe the X-ray emission. Both Cyg X-1 and Cyg X-3 could be observed simultaneously with the telescope. The results are used to establish 1-sigma upper flux limits on the spectral emission from Cyg X-3.