The front MCP is coated with CsI in order to increase the quantum efficiency over that of the bare glass of the MCP. An incident x-ray photon interacts within a channel (pore) of the front MCP producing one more electrons with a probabilty that depends on the photon energy. The initial electron or electrons are accelerated by an applied electric field across the plates and generate secondary electrons; this process ultimately produces and EUV and UV radiation from reaching the sensor surface. The UV/Ion shield is held electrically positive (+100 Volts) with respect to the front surface of the MCP in order to prevent electrons generated in the shield and in the interchannel web from reaching distant channels and degrading the image. a cloud of about 30 million electrons emerging from the rear plate.
The second figure below
illustrates how the centroid of this charge cloud is determined and therefore the
position of the interacting photon. A five-sided plastic scintillator
anti-coincidence shield reduces the detector background due to high energy charged particles.
The fine position (digitized to 6.429 microns) is determined from a "three tap" centroid calculation. The combined MCP/CGCD is capable of < 25 micron (FWHM) resolution over the entire field of the detector. See HRC Position Logic (postscript), by M. Juda for a discussion of how the HRC generates telemetry information on event positions and HRC Degapping Corrections, by M. Juda for a discussion of the removal of image artifacts.
HRC-S' sensor consists of three sets of two-stage (chevron) MCPs arranged as a strip. The MCPs are 100 mm x 27 mm Philips Photonics Corp. radioisotope-free lead oxide glass MCPs. The electronic readout system is essentially the same as used by the HRC-I except that the CGCD is a hybrid consisting of one plane of wires in the cross-dispersion direction and the other plane consists of photo-etched conductors in the dispersion direction.
