Xcalibur II manual

TECHNICAL DESCRIPTION

4.2.7 Dark Current and MPP Mode

The silicon base of the CCD chip generates a so-called dark current. The dark current is material property of the silicon and it is due to spontaneous generation of electron-hole pairs in the silicon lattice due to thermal excitation. Cooling the CCD chip reduces the dark current. At the typical operating temperatures of –45 to –50ºC the dark current is <0.2 electrons/pix.s. The low dark current is archived by a special mode of operation of the CCD chip: The MPP mode. MPP stands for multi-pinned-phase and describes the fact, that each pixel has a special boron implant under a part of the active area, which pins the electrons under it without having to use the actual gates under tension used for the readout transfer.

4.2.8 Radiation Damage

CCD chips (as all unprotected Silicon devices) are sensitive to radiation damage. The glass taper, which is in front of the CCD absorbs all incoming radiation and thus protects the CCD from radiation damage.

4.2.9 Full Well Depth and 18-bit Digitisation

The full well depth is the number of electrons that a CCD pixel can hold. The typical value of the full well depth for a MPP mode operated CCD is 300-400k electrons for a single unbinned pixel. In order to take full advantage of the full well offered by the CCD chip the readout electronics have to match the resolution of the CCD chip: 16 bit readout (65k) is insufficient for this task. 18bit (262k) readout gives the best technically available match.

4.2.10 Anti-blooming

Scientific CCDs use 100% of their surface for image detection. If a very bright object is shines onto the CCD the full well of the chip may be exceeded and charges may spill (“bloom”) into neighbouring pixels. There are optical video CCD designs, which sacrifice a small part of the sensitive area to implement an anti-blooming gate, which takes off overflowing charges. For integrative flux measurement it is better to spill the signal (the integral stays correct) rather than loosing the charges in the anti-blooming gate (integral lost).

4.2.11 Vacuum

The CCD chip and the fibre optic taper are confined in a vacuum enclosure to isolate them thermally and to prevent condensation on the cold detector parts. Static vacuum degrades with time due to degassing of the components in the vacuum enclosure. The system is designed to maintain a sufficient vacuum level in the enclosure for at least 6 months. A bad vacuum can be recognized by the fact that the detector cannot be cooled to the set operating temperature of –45ºC (This can be monitored using the software program ODBench accessible through the plugin menu of

CrysAlis). Loss of vacuum is also indicated by a blinking green light (red light off) on the top of the

CCD detector.

Pumping the vacuum enclosure is a regular service task. A normal rotary vacuum pump is sufficient for this task (<0.04mbar).

4.2.12 Fast Shutter

The CCD X-ray detector is an integrative detector. The precision of the intensity measurement critically depends on the dose released by the X-ray shutter. The normal electro-magnetic shutters are too imprecise for this task. Specially designed fast and reproducible shutters have to be used.

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