Chemistry Reference
In-Depth Information
and microgap detectors. Integrating area detectors, also referred to as analogue X-ray
imagers, record X-ray intensity by measuring the analogue electrical signals con-
verted from the incoming X-ray flux. The signal size of each pixel is proportional to
the fluence of incident X-rays. The most commonly used integrating area detectors
include image plates and charge-coupled devices. This section gives a brief descrip-
tion of the working principles and performance of the four most commonly used
detector technologies.
4.6.1 Multiwire Proportional Counter
The multiwire proportional counter, also referred to as a two-dimensional position-
sensitive proportional counter, was developed more than 30 years ago [31-35]. The
MWPC is a gaseous photon-counting detector consisting of an X-ray proportional
chamber, with a two-dimensional multiwire grid, a preamplifier, and decoding
electronics. Figure 4.11 shows a cutaway view of a MWPC detector (Bruker Hi-
Star). The working principle of the MWPC is similar to a gas proportional counter
except with the anode and cathode in a two-dimensional multiwire grid. The grid is
comprised of three parallel planar electrodes made of array of parallel metal wires.
The three planar electrodes consist of an anode layer sandwiched between two layers
of cathodes. One layer of cathode wires is in the horizontal direction (x-cathode) and
another layer in the vertical direction (y-cathode). The chamber is filled with
ionization gas pressurized from one to several atmospheres. The Hi-Star uses a
Xe/methane gas mixture pressurized to approximately 4 atm. The Be-window
attenuation is the only loss of detector efficiency at low count rate. The Be window
of the Hi-Star is 80 percent transparent to 8 keV (Cu-K
a
) radiation and permits
pressurized operation. When an X-ray photon enters the detector it interacts with the
Xenon near the front window, ionizing the gas and creating a small region of 1-2 mm
FIGURE 4.11 Cutaway view of a MWPC detector (Bruker Hi-Star).
