Area Detectors

1
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

2
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

3
Rigaku r-axis
4
Image Plate Schematic
5
How IP works
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Small Molecule IP
7
Schematic
8
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

9
Schematic of a ccd
10
Noise

• Dark Currentstored electrons from dark exposure
• Can reduce dark current by cooling the ccd.
• Peltier cooling is frequently used
• Readout Noisefrom moving pixels to counting
  registers
• Slower the movement the lower the noise

11
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.
• Binningfold 4 pixels into 1
• Use large ccd chip 4K

12
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

13
KappaCCD

• Roughly 1K chip
• 12 electrons/photon
• Dark current 0.02electrons/second
• Read out at 200kHznoise about 12electrons
• Pixel size 22.5 x 22.5 micrometers
• Binned 110 x 110
• Readout time about 3 seconds

14
KappaCCD
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Kappaccd