area detectors like film can detect position like scintillation detector can measure intensity with...

Area Detectors

• Like film can detect position

• Like scintillation detector can measure intensity

• With modern computers can assign hkl from unaligned crystal

• Must convert pixel intensity to intensity by some sort of integration

Image Plates

• Use a phosphor which emits light proportional to x-ray exposure

• Very large dynamic range

• Very low background

• Large input area

• Not a “real time” device

Rigaku r-axis

Image Plate Schematic

How IP works

Small Molecule IP

Schematic

CCD Detector

• The ccd is a standard camera chip

• Need a phosphor to convert x-rays to light

• The ccd pixel is a well that converts photons to electrons and stores them.

• The wells are read out by shifting to special readout wells and using an a-to-d converter

Schematic of a ccd

Noise

• Dark Current—stored electrons from dark exposure

• Can reduce dark current by cooling the ccd.

• Peltier cooling is frequently used

• Readout Noise—from moving pixels to counting registers

• Slower the movement the lower the noise

Size

• More most 1K ccd chips the pixels are too small.

• Use glass taper to expand the face.

• Taper introduces a distortion

• Pixels still to small.

• Binning—fold 4 pixels into 1

• Use large ccd chip 4K

Phosphors

• Thickness is important

• Too thin and x-rays penetrate

• Too thick and emitted photons are re-absorbed

• For Mo radiation phosphor is only about 80% efficient

KappaCCD

Roughly 1K chip12 electrons/photonDark current 0.02electrons/secondRead out at 200kHz—noise about

12electronsPixel size 22.5 x 22.5 micrometersBinned 110 x 110Readout time about 3 seconds

KappaCCD

Kappaccd