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