CCDs in the Xray

Figure 7.1 provided hints that CCDs may also be useful detectors for the X-ray region of the spectrum, as the absorption depth within silicon rises shortward of about 1000 A. Figure 7.5 shows us a similar result, only this time we express it as the quantum efficiency of the CCD as a function of photon energy or wavelength. We note that within the X-ray region, backside thinned CCDs are extremely efficient detectors, approaching a quantum efficiency of 100% at times. The X-ray telescopes aboard XMM-Newton and Chandra use CCDs as their detectors (Longair, 1997; Marshall et al., 2004; Sembay et al., 2004).

X-ray detection by CCDs works in a slightly different manner from detection of optical photons. An incident optical photon creates a photoelectron within the silicon lattice, which moves from the valance to the conduction band and is then held there (in a pixel) by an applied potential. The absorption of an X-ray photon by silicon ejects a free, fast moving, photoelectron of energy E — b, where E = hv and b is the binding energy of the electron to the silicon atom, typically 1780 eV. As this highly energetic electron moves through the silicon lattice, it produces a trail of electron-hole (e-h) pairs,

WAVELENGTH, A

- MEASURED

PHOTON ENERGY, eV

Fig. 7.5. Quantum efficiency for a typical thinned, back-side illuminated CCD from the X-ray to the optical spectral regions. From Janesick et al. (1988).

with each requiring an average of 3.65 eV of energy to be produced.1 Each incident X-ray photon collected produces a measureable number of e-h pairs, thus yielding a method by which one can backtrack and obtain the incident photon energy (Longair, 1997). If all the energy of the free electron went into the e-h pair production, the energy of the incident X-ray could be precisely knowable simply by counting the ADUs produced within the CCD pixels. This property leads to an interesting aspect of X-ray imaging in that one can use the number of photoelectrons produced by an incoming photon to tell its incident energy (wavelength), thereby performing imaging and (crude) X-ray spectroscopy simultaneously.

However, a small undetermined amount of the free electron's energy goes into various phononic states of the silicon lattice, thereby causing some uncertainty in the value of the incident photon's energy. The level of this uncertainty, the "Fano" factor,2 is so small that to obtain Fano-noise limited CCD performance, the CCD read noise must be less than about 2 electrons (Janesick et al., 1988; Janesick, 2001). Imaging an 55Fe source (used to measure CTE -Chapter 3), would produce a single spectral line at the 5.9 keV Fe Ka energy level while imaging a real astronomical source would produce a crude X-ray spectrum covering the energy range of the detected photons. This type of X-ray spectroscopy was used to produce very low resolution spectra using the imaging capabilities of the ROSAT X-ray satellite.

The Chandra telescope obtains X-ray images and spectroscopy but, in this case, the spectra are not produced by unfolding the images via monitoring image energy deposition, but through the use of gratings to disperse the X-rays (Marshall et al., 2004) in the same manner as discussed for optical spectroscopy in Chapter 6. The imaging and spectroscopy on Chandra both use MIT/LL CCD detectors. These devices are 1024 x 1024 frame transfer CCDs with 24 micron pixels. The frame transfer nature of the CCDs provides fast readout capabilities and therefore can act as an electronic shutter for X-ray observations. Some of the CCDs are front-side illuminated CCDs but these have suffered a lot of damage from the X-rays incident on their (frontside) gate structures (Grant et al., 2004). The back-side illuminated CCDs fare better in terms of radiation damage as well as having overall better QE at lower energy (Figure 7.6).

Figure 7.7 shows an X-ray spectrum of the star Capella obtained with the Chandra observatory using the high energy transmission grating (HETG).

1 Note this value is about equal to the energy of a typical optical photon, which we already know produces one photoelectron.

2 The term Fano factor is due to U. Fano who, in 1947, formulated a description of the uncertainty in the energy of ion pairs produced in a gas by ionizing radiation.

Quantum Efficiency * filt. trans.

Quantum Efficiency * filt. trans.

Energy [keV]

Fig. 7.6. X-ray QE for the CCDs aboard the Chandra X-ray observatory. These QE curves are those of the CCDs convolved with the X-ray filters used. The QE jumps or "edges" seen are caused by inner electronic shell energies of the elements, such as C, used in the X-ray filters.

Energy [keV]

Fig. 7.6. X-ray QE for the CCDs aboard the Chandra X-ray observatory. These QE curves are those of the CCDs convolved with the X-ray filters used. The QE jumps or "edges" seen are caused by inner electronic shell energies of the elements, such as C, used in the X-ray filters.

The spectrum covers the wavelength range from 6-18 A and shows emission lines (identified in the figure) due to the hot (1 million kelvins or more) stellar corona. XMM-Newton, another X-ray satellite currently in operation, also uses CCDs as detectors. Both of these orbiting X-ray observatories have detailed web pages discussing their telescopes and detectors. A full description is beyond the scope of this volume, but Appendix A contains a number of interesting links to explore.

MEG, m=-1 : HETGS Spectrum, Capella, Obsid 1103

MEG, m=-1 : HETGS Spectrum, Capella, Obsid 1103

Wavelength (A)

Fig. 7.7. X-ray spectrum of the star Capella obtained with CCDs aboard the

Chandra X-ray observatory. The strong emission lines are produced in the hot corona of the star.

Wavelength (A)

Fig. 7.7. X-ray spectrum of the star Capella obtained with CCDs aboard the

Chandra X-ray observatory. The strong emission lines are produced in the hot corona of the star.

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