Nuclear Instruments and Methods in Physics Research A 348 (1994) 188-191 North-Holland NUq[.;LI- AH INSTRUMENTS & METHODS IN PHYSICS RESEARCH Seclton A Tabulated responses of X-ray CCDs to optical astronomical sources Alan Owens *, Kieran J. McCarthy Department of Physics and Astronomy, Leicester University, Leicester LE1 7RtI, UK Received 16 March 1994 We calculate the response of both front-illuminated and back-illuminated X-ray CCDs to various astronomical optical inputs. The results are given in tabular form and are useful when calculating the expected optical yields encountered during an observation or in designing optical filters for soft X-ray telescopes. 1. Introduction In recent years, charged coupled devices (CCDs) have emerged as the preferred detectors on all new X-ray astronomy missions [1-4]. This is largely because they offer a combination of superior spectroscopic and spatial resolution [5] coupled with small size and all the inherent advantages of semiconductor technology. However, unlike traditional X-ray detectors, such as proportional counters, the response of deep-depletion CCDs does not conve- niently cut off in the XUV or soft X-ray band, but extends through the optical region down to infrared wavelengths (~ 10000 A). This is illustrated in Fig. 1 in which we plot the broad-band quantum efficiency of a typical front-il- luminated X-ray CCD from the optical to the X-ray band. We can see almost 30% of the total response lies in optical region. Since most astrophysical objects emit many more optical photons than X-rays (by a factor of 10 for M type stars and 106 for A type stars), the signal from the detection plane can easily be swamped by the visible light component. Even worse, for telescopes employing grazing incidence optics, the light from such objects is focused at the same spot as the X-rays, giving rise to excessive dark current. This in turn results in measurable base line shifts, broadening, decreased throughout and spectral distortion. In most cases, filters with high optical to X-ray extinction ratios are required. Unfortunately, highly attenuating filters at visible wavelengths also attenuate soft X-rays and a trade-off between optical contamination and EUV/XUV attenuation must be made. A good understanding of the expected optical yields is therefore essential to this study. In a previous paper [6], we described in detail the rational behind filter design for X-ray telescopes. In this paper, we evaluate the optical yields for a variety of astronomical * Corresponding author. objects. Three types of EEV device were considered in the present analysis; a standard "TV" CCD and two large format devices based on the P88000 series [7] - one being a deep-depletion front-illuminated CCD and the other a deep-depletion rear-illuminated CCD. The thicknesses of the depletion regions are ~ 3, 30 and 30 p~m, respectively. 2. Response to optical sources The linear absorption coefficients of silicon are shown in Fig. 2. From the graph we see that the optical depths for visible photons and keV X-rays are comparable - explain- ing the large sensitivity of X-ray CCDs at optical wave- lengths. Optical photons interact in the first few microns of the depletion region releasing single photoelectrons. The light induced charge causes a shift in baseline over that introduced by readout noise. This, in itself, would not be such a problem were it not that the latter is steady-state while the former is source dependent. For an incident monoenergetic X-ray beam of energy, E, the correspond- ing peak energy is shifted by a factor, ~E/E = (E + wr + OJNe)/(E + ogr), (1) where w is the energy required to liberate an electron-hole pair (3.68 eV in Si), r is the readout noise in units of equivalent number of electrons (typically 3-5 electrons) and N e is the average number of electrons produced by the optical flux. The additional charge resulting from the presence of optical photons also results in a broadening of the detector full-width-at-half-maximum (FWHM) energy resolution. AE = 2.355~/o-g 2 + ~2 + o02, (2) where ~2, o.gZ and 0-2 are the variances (in units of eV 2) of the noise components due to carrier generation statistics, 0168-9002/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-9002(94)00497-U