Adam Keay

Performance of the XMM EPIC MOS CCD Detectors

Measurements made during the selection and evaluation of flight CCD detectors for the XMM EPIC MOS cameras have demonstrated near Fano limited resolution at x-ray energies above approximately 3keV. At lower energies some devices exhibit a fractional charge loss which is believed to be due to recombination at the epitaxy/oxide interface. This has been modeled through a Monte-Carlo simulation by assuming that the pinning implant in the etched electrode structure can cause electrons to flow to the front surface, rather than to the buried channel. In spite of this charge loss, spectral response may be characterized using a double Gaussian with residuals of < 5 percent. Quantum efficiency has been measured using a lithium drifted silicon reference detector and these measurements combined with analytical and Monte Carlo simulation, event size ratios and cosmic ray detection, all give a value for the effective depletion depth of 30 to 35 micrometers .

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.

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.

Imaging X-ray fluorescence spectroscopy using microchannel plate relay optics

A novel imaging low-energy x-ray fluorescence spectrometer with no moving parts, based on a microchannel plate relay optic and an open electrode charge-coupled device with good sub-keV quantum efficiency, is described. Results from a proof of principle experiment using the Rutherford Appleton Laboratory picosecond pulsed laser plasma x-ray source are described and the performance limits of the spectrometer explored. Copyright

Exploring macroscopic surface structures on X-ray CCDs using silicon absorption edge quantum efficiency measurements

Abstract We have begun an experiment to systematically record the quantum efficiency variations of a silicon CCD across the K edges of its constituent materials using the Synchrotron Radiation Source (SRS) at the Daresbury Laboratory. The ultimate goal of the program is to provide a detailed calibration database of instrumental X-ray Absorption Fine structure (XAFS) in CCDs intended for X-ray astronomical space missions. Because X-ray CCDs are single photon counting devices, the maximum flux that can be accommodated while still retaining full spectral resolution is typically a few hundred events cm −2 s −1 , a requirement clearly incompatible with normal synchrotron usage. Thus, in order to carry out our experiments, the SRS had to be operated in a new low current mode. We describe how the experiment was carried out and present a preliminary analysis of data obtained on March 9, 1994. The data show considerable structure in the quantum efficiency above the Si K edge at 1839 eV and, in fact, the response of a silicon CCD in this region is dominated by the electrode and passivation structures.

Escape peak ratios in silicon X-ray charge coupled devices (CCDs)

Abstract The intensity of the escape peak from the CCDs developed for the Joint European X-ray Telescope (JET-X) has been investigated over the energy range 2–10 keV. Both measured and calculated escape peak ratios (i.e., the ratio of counts in the escape peak to the sum of the counts in the escape and main peaks) are found to be in excellent agreement for all event sizes (i.e., single pixel events, 1 and 2 pixel events, etc.). Using a Monte Carlo simulation the escape peak ratio has been investigated as a function of pixel size and depletion depth. For completeness, we list the energy dependent parameterised forms for five CCDs used in three major astronomy missions.

Next-generation detectors for x-ray astronomy

The next generation of X-ray astronomy instruments will require position sensitive detectors in the form of charge coupled devices (CCDs) for X-ray spectroscopy and imaging that will have the ability to probe the X-ray universe with a greater efficiency. This will require the development of CCDs with structures that will improve on the quantum efficiency of the current state of the art over a broader spectral range in addition to reducing spectral features, which may affect spectral resolution and signal to background levels. These devices will also have to be designed to withstand the harsh radiation environments associated with orbits that extend beyond the Earth’s magnetosphere. The next generation X-ray telescopes will incorporate larger X-ray optics that will allow deeper observations of the X-ray universe and sensors will have to compensate for this by an increased readout speed. This study will aim to describe some of the results obtained from test CCD structures that may fit many of the requirements described above.

Investigating chemical structure in a silicon CCD using Extended X-Ray Absorption Fine Structure

Abstract Using the Extended X-ray Absorption Fine Structure (EXAFS) signal extracted from quantum efficiency ( Q ( E )) measurements, the chemical structure of overlying, in-active layers in a silicon Charge-Coupled-Device (CCD) has been investigated. The Q ( E ) of the CCD was measured across the Si K-shell absorption threshold (1839 eV) at the Daresbury Synchrotron Radiation Source (SRS), using the source in low beam current mode. The absorption in the inactive layers was then deconvolved from the Q ( E ) and analysed. The analysis shows that CCD Q ( E ) measurements contain enough inactive layer information to determine the structure of the chemicals found there. A Fourier analysis shows that both crystalline silicon (Si-c) and amorphous silicon dioxide (SiO2-a) are present in the inactive layers. Inter-atomic distances of 1.5 A for the SiO bond, 2.3 A for the first SiSi and 3.7 A for the second SiSi bond are deduced. These distances agree well with data on bond lengths in the current literature.

Advanced MOS CCDs for high-throughput x-ray astronomy missions

We present the latest design concepts of CCDs for the next generation of X-ray astronomical applications together with test results of new detector developments for these applications. In particular we consider ways of overcoming the fundamental limitations of these detectors, namely area coverage and low readout speeds. The manufacture of a high yielding, highly efficient CCD with a well understood response at all energies remains a high priority and we discuss our program to achieve this goal. Amongst other features: increased deep depletion and state of the art noise performance will be examined.

Improving the high-energy sensitivity of x-ray CCDs

We present a method of improving the quantum efficiency of MOS CCDs. We show that a change of operation can increase the depletion depth of standard bulk devices. The conclusions point to a point to a potential increase in the depletion depth of 300 percent by the use of optimized materials and operating conditions.