Xcalibur II manual

X

TECHNICAL DESCRIPTION scintillator

Taper

CCD cooling electronics

Be

Figure 4.2.1 Schematic of a fibre-optic-coupled CCD detector for x-rays

X-rays enter the vacuum detector enclosure through a Be window. The photons are intercepted by a scintillation screen, which transforms the X-rays to visible light. The light is conducted towards the

CCD detector via a fibre optic taper. The CCD chip integrates the light dose received from the taper.

Its electric signal is then amplified and digitised in a readout circuit.

The central component of the detector is the CCD chip, which is situated in a vacuum enclosure.

The vacuum enclosure isolates the chip thermically and protects it from condensation in the cooled state. The CCD chip requires cooling to reduce its dark current.

The chip is cooled by a Peltier cooler, which dissipates its heat into a Cu heat sink, which is water cooled by the CCD water chiller. The Peltier is powered with 100W to maintain the CCD chip at an operating temperature of –45 to –50ºC. A number of security systems ensure that the Peltier will not overheat the detector due to lack of cooling. The CCD detector power supply is supplied by the

CCD water chiller, which checks the water flow rate and temperature. In the case of loss of cooling water the chiller shuts down the CCD power supply. Additionally there are two heat sensor on the heat sink, which shut down the system, when the heat sink is >40ºC.

The CCD chip has a size of approximately 25 x 25mm arranged in 1024 x 1024 pixels.

In order to obtain an X-ray sensitive area of 62 x 62mm the chip is coupled to fibre-optic taper. The taper is characterised by its reduction ratio. The reduction ratio of the taper is measured in a factory calibration procedure and is typically 2.45-2.5:1. The large end of the taper is covered with scintillation screen. The screen is composed of GdOS

2

:(Tb), which is the most efficient screen material with respect to stopping power and light output. Note that the screen thickness determines the resolution of the detector.

4.2.1 Beryllium Window

The 0.5mm Beryllium window is the entrance for the X-ray photons into the detector unit. The detector unit is held under vacuum to provide thermal isolation for the Peltier cooled CCD chip. The

Be window absorbs incoming X-ray photons in the following way:

Radiation Pure Be

Absorption coeff.

(mm

-1

)

Be with 0.1% contamination (Cu)

Transmission (%) Absorption coeff.

(mm

-1

)

Transmission

(%)

Cu

Mo

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