A.D. Holland

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.

The X-ray polarisation sensitivity of CCDs

Abstract The regular array of pixels in a silicon Charge Coupled Device (CCD) may be used to measure photoelectron emission directions following the absorption of high energy X-rays ( E > 15 keV). CCDs offer, therefore, the possibility of combining X-ray imaging and spectroscopy with measurement of the linear polarisation of the incident beam. We describe a simple model of electron transport in CCDs which leads to an estimate of the energy-dependent modulation factor M ( E ) — the parameter determining polarisation sensitivity — for arbitrary pixel geometries. The predictions of the model are in good agreement with published measurements. The sensitivity of an optimised CCD “pixel polarimeter” for cosmic X-ray astronomy is assessed.

Modelling the electro-thermal response of superconducting transition-edge X-ray sensors

Abstract A two-temperature model for simulating the electrothermal response of superconducting transition-edge sensor (TES) X-ray detectors is presented. The model considers that the absorber and the superconducting layer (SL) of the detector are at different temperatures. The time constants within the absorber and the SL are assumed to be much smaller than all remaining time constants of the TES. The time dependences of the current through the SL, the temperature of the absorber, and the temperature of the SL after the absorption of single photons of different energy are found by numerically solving a set of nonlinear differential equations. In order to verify the model, a calculated current transient is compared with a previously measured pulse. Exploring the parameter space defined by appropriate ratios of electro-thermal time constants, we derive constraints for the asymptotically stable, oscillation-free operation of a TES.

Development of a distributed read-out imaging TES X-ray microcalorimeter

We report on the development of a linear absorber detector for one-dimensional imaging spectroscopy, read-out by two Transition Edge Sensors (TESs). The TESs, based on a single layer of iridium, demonstrate stable and controllable superconducting-to-normal transitions in the region of 130 mK. Results from Monte Carlo simulations are presented indicating that the device configuration is capable of detecting photon positions to better than 200 μm, thereby meeting the resolution specification for missions such as XEUS of ∼250 μm.

The Hard X-Ray Telescope : Instrument Concept and Detectors

The Hard X-ray Telescope (HXT) is a new instrument concept for the 1–60 keV band. Based on X-ray focusing using microchannel plate technology, HXT aims to combine arcminute imaging with X-ray spectroscopy and polarimetry. We describe the instrument design, its potential sensitivity and its focal plane concept - a hybrid small-pixel Si CCD / GaAs detector array. The former element provides polarimetric sensitivity and high spectral resolution (FWHM < 200 eV @ 10 keV) at the lower end of the HXT bandpass. The latter element produces good efficiency (20% @ 60 keV) and medium energy resolution (1 keV @ 60 keV) at the higher energies.