A Brief Description of ACIS CCDs

The ACIS focal plane contains ten MIT Lincoln Laboratory CCID17 charge-coupled detectors. The CCID17 ( [Burke et al.1997] is a three-phase, frame transfer imager with 1026 rows of 1024 columns in each of the imaging and framestore areas . The 2 rows nearest the framestore section are ignored by the ACIS digital electronics. The pixels in the imaging area are 24 microns square. Each device is served by four output nodes which are (usually) operated in parallel. The architecture is illustrated in Figure 4.1.

**Figure 4.1:** Layout of the ACIS CCID17 detector, showing framestore architecture and four output nodes. Ten such devices comprise the ACIS focal plane.
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ACIS flight detectors are fabricated of high-resistivity silicon ( $\rho \gt 6500 \Omega$ -cm) to maximize depletion depth and, therefore, high-energy X-ray detection efficiency. The devices are three-side-abuttable to minimize inter-chip gaps in the flight focal plane. Flight CCDs have been fabricated in both front-illuminated and back-illuminated configurations. Other characteristics of the detectors are listed in Table 4.1.

**Table 4.1:** Some ACIS CCD Characteristics (MIT Lincoln Laboratory CCID17)
Parameter	Value
Architecture	3-phase, frame-transfer, 3-side-abuttable
Format	1024 columns by 1026 rows, image array
	2x 512 columns by 1026 rows, framestore
Pixel Size	24 x 24 $\mu$ m, image array
	21(H) x 13.5(V), $\mu$ m, framestore
Output Nodes	4
Readout Rate	100 kpix s^-1 at each of output four nodes
	400 kpix s^-1 total per CCD
Operating Temperature	-120C (nominal)
Readout Noise:
CCD	< 2 e^- RMS, typical
System	2 - 3 e^- RMS
Charge Transfer Inefficiency:
(Low-illumination, 5.9 keV)
FI Devices	$< 3 \times 10^{-6}$ per pixel transfer
BI Devices	$< 1 -3 \times 10^{-5}$ per pixel transfer
Mean Dark Current (-120C)	$< 5 \times 10^{-2} e^{-} s^{-1}$ pixel^-1