X-RAY DETECTORS - Two-Dimensional X-Ray Diffraction - page 122

over 5 10 4 . The phosphor screen can also be tailored to suit the different X-ray
energies, so the indirect detection method can offer wider photon energy coverage.
Another important advantage of indirect detection over direct detection is the
phosphor screen and the coupling fiber optic prevents X-ray photons from reaching
the CCD sensor and protects the CCD from X-ray damage.
There are several disadvantages associated with indirect detection methods, in
addition to the previously mentioned zinger noise and spatial distortion caused by
fiber optic taper. The detector may have poorer spatial resolution due to the
scattering in the phosphor screen and scattering of light in the fiber optic taper
or faceplate. A single detected photon could produce a spot of light with a diameter
of approximately 100 mm in the phosphor. Reducing the thickness of the phosphor
layer may improve the resolution. A certain percentage (typically 50-90 percent) of
light is lost in the fiber optic taper due to optical detrapping. To absorb the lost light,
black glass fibers are inserted between the imaging fibers in the fiber optic taper.
However, the absorption is not complete and remaining scattered light in the fiber
optic taper reaches the CCD and results in a loss in spatial resolution. The reduced
sensitivity is another drawback with indirect detection, since visible photons
produced by the X-ray event may be spread over several pixels of the detector,
resulting in signal levels approaching noise levels.
The CCD chip needs to be cooled to reduce the dark current noise. Dark current is
due to thermally produced electron-hole pairs, which accumulate in the potential well
with time. The dark current noise is the dominant noise source for long exposures.
Dark current can be decreased by cooling the detector. Typical cooling temperatures
for CCD detectors used for X-ray diffraction are 40 to 60 C. With every 5-6 C
drop in temperature, the dark current can be reduced by a factor of 2. For example, the
dark current noise of a CCD at 60 C is about one order of magnitude lower than the
noise of a detector cooled only to 40 C.
4.6.4 Microgap Detector
Efforts to improve the local count rate of MWPC led to the introduction of several
detectors categorized as “microgap” detectors, including the GEM [46], the
MSGC [47], the MGC [48], the CAT [49], and the MICROMEGAS [50] detectors.
With reduced cathode-to-anode spacing and increased average electric field
intensity, so as to reduce the ion collection time, microgap detectors can achieve
count rates several orders of magnitude higher than the MWPC. However, the
above microgap detectors suffer a common limitation in that the maximum gain,
spatial resolution, and energy resolution are reduced at high count rates due to
electrical sparks and discharge. A new type of microgap detector with a resistive
anode was recently introduced by Bruker AXS, named the MikroGap detec-
tor [21,22]. MikroGap technology has been used for both one-dimensional and
two-dimensional detectors. It consists of a parallel plate avalanche chamber with a
resistive anode separated from the readout electrode by an insulator. The resistive
anode significantly reduces the number of sparks and the damaging effects of the