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Parallax Reduction in a MWPC
Structural studies of samples in a number of disciplines, such as molecular biology, solid state physics and material science, are frequently made with X-ray and neutron scattering, especially with the monochromatic, high fluxes available at synchrotron sources (X-rays) and reactors/spallation sources (neutrons). Typically, these studies involve the measurement of diffraction patterns with position sensitive detectors. In specific biological studies, polymer research and, particularly, sub-second time resolved studies, gas-based proportional detectors play a vital role. These devices are most conveniently fabricated in a planar geometry. They have an absorbing region of typically a centimeter, which has limited their application in many studies because of parallax broadening of the position response at finite angles. Spherical drift chambers and multi-step chambers have been studied to address this problem, but fabrication difficulties appear to have resulted in few applications.
Parallax broadening in a conventional gas detector arises from the flat (parallel plate) field in the absorption region where electrons drift perpendicularly to the window-this is shown in the lower half of the detector in fig.1. To eliminate parallax, electrons should move along field lines starting from a focal point which coincides with the position of the sample under study. Such an ideal, parallax free, drift field could be produced by applying radially changing potentials to the two "flat" boundaries of the region, the front cathode and the window. In practice, this is only possible on the window, which is made with concentric annular electrodes that are biased with progressively decreasing negative potential from the center of the window to the edge. The resulting field distribution is shown in the upper half of the detector in fig.1. The potential on each annulus, derived to minimize parallax with the constraint of an equipotential at the cathode plane, depends upon gas depth, X-ray absorption depth and sample to detector distance. The full paper will present a detailed description of this electrostatic calculation.
To investigate the new technique, the standard beryllium window on a two-dimensional proportional detector (used in structural biology studies at Brookhavens NSLS) was replaced by a mylar window with 60 concentric annuli, whose widths and applied potentials were calculated for 5.4 keV X-rays, a 12.7 mm depth of Xe/10%CO2, and a focal point of 18cm, yielding an effective angular coverage of almost ±15°. A 25 micron X-ray beam was incident on the detector, which could be rotated about a focal point 18cm from the detection plane. Figure 2 shows the position response of the detector to the beam over an angular range of zero to 12°, in 2° intervals. The transformation of the significantly parallax-broadened peaks, especially at the edge of the detector, to relatively narrow peaks, is clear. The FWHM of the central peaks is 110µm, while at 12° incident angle the FWHM is 600µm with a conventional flat field (uniform potential window), and only 180µm for the "spherical field" (annuli at optimized potentials).
The design of a low energy X-ray window requires special care, since the window must have uniform transmission over its entire area. This design aspect is much easier for position sensitive neutron detectors, which we are also investigating, because a hundred of microns, or so, of plastic or conducting metal will not significantly change the window transmission. This new technique offers the prospect of improving the position resolution of all gas-filled imaging devices, with little alteration to the physical structure of the detector, permitting the recording of accurate diffraction data over significantly larger angles than has previously been possible.
Last Modified: Wednesday, 06-Feb-2013 22:33:56 EST