|
CHESS
X-ray Detectors
Below is an alphabetical list of the x-ray
detectors at CHESS. X-ray foil filters and several radioactive sources are
available for detector energy calibration.
CCD two-dimensional detectors:
The present detector (1024x1024 50 µm pixels) is supported by the MacCHESS
facility, and is used for the rapid collection of macromolecular
crystallography data.
Ion chambers
Helium/Nitrogen/Argon/Air flow chambers:
Used for general purpose, non-destructive, photon beam intensity monitoring.
We have 8 cm and 30 cm long flow Ion chambers, for which He, N2, and Ar
gasses are available at each station. The graph below shows the efficiency
of the various gasses as a function of energy.
Sealed Xenon and Krypton tubes:
Used for high-energy photon detection. The tubes have beryllium windows. See
the graph below for the efficiency of these tubes as a function of energy.
Efficiency of a 15 cm. long gas ionization chamber as a function of
energy, for different gases at normal pressure (taken from the "X-ray Data
Booklet", Technical Information Department, Lawrence Berkeley Lab., April
1986). Note that 100% efficiency corresponds to complete absorption of the
photon beam by the 15 cm chamber.
Photodiodes
Linear photodiode array (PDA)
A 1024 pixel (25 micron sized pixels) photodiode linear array (Reticon chip)
and a 16-bit ADC are available for special uses. The PDA is read out at a
rate of 5 microseconds per pixel, has a minimum exposure time of 5
microseconds, and a time between exposures of 10 milliseconds. This detector
has a maximum count rate of 1010 photons per second. Contact Ken Finkelstein
for more information.
Scintillation detectors
NaI detectors:
These are general purpose single photon counting detectors with energy
resolution sufficient to distinguish between energy harmonics (typically
DE/E ~ 40%). Useful for count rates up to 50000 per second.
Plastic scintillators:
Used for high count rates, up to several hundred kiloHertz, but tend to be
have more noise than NaI detectors.
Solid-state detectors
Si(Li) or Intrinsic Ge detectors:
Solid-state detectors have very good energy resolution. They are often used
in conjunction with a histogramming multi-channel analyzer to analyze the
energy spectrum of fluorescence or scattered photons. The silicon detector
is most efficienct for photon energies less than 15 keV, and intrinsic
germanium is most efficient for high energy photons (see graph above). The
table on the opposite page gives an inventory of the detectors we have, and
lists their energy resolution. Since these detector must be kept at LN2
temperatures at all times, constant maintenance (refilling every 6-8 hours)
and monitoring is required. The crystals can be easily damaged by rough
handling, and the units are expensive to repair.
Detection efficiency as a function of
energy for semiconductor crystals (taken from the "X-ray Data Booklet",
Technical Information Department, Lawrence Berkeley Lab., April 1986). The
thickness of the detector crystal limits the detection of high-energy
photons; absorption by air, beryllium windows, and the dead layer of silicon
crystals limits efficiency at low energies.
Storage-phosphor image plates:
Mostly used by MacCHESS facility for collection of macromolecular
crystallography data. Two FUJI BAS-2000 image plate scanners are on-line at
all times. Contact
Bill Miller for
more information.
X-ray film and burn paper:
Two darkrooms are available for developing x-ray paper. The burn paper has
somewhat low spatial resolution, but is useful for aligning equipment in
x-ray beams.
Last Update:
2006-09-21
|
