The X-Ray Spectrometer (XRS) experiment was comprised of three
          identical gas proportional counters (GPC) that measure X-rays
          emitted from the surface of Mercury in the energy range from about
          1 to 10 keV. X-rays in this energy range sample the planetary
          surface to depths of a few tens of microns. The GPCs each had a
          10-cm2 active area and used both anti-coincidence wires and pulse
          shape discrimination to minimize background. Balanced filters were
          used to resolve the lower energy X-ray lines from Mg, Al, and Si.
          This technique was used previously on other orbital X-ray
          experiments flown on the Apollo 15, 16 and NEAR-Shoemaker missions.
          One GPC had a thin aluminum filter, which filtered out Si photons,
          one had a thin Mg filter, which filtered out Al and Si photons, and
          the third GPC had no filter. The energy resolution of the gas
          counters was sufficient to resolve higher energy lines from S, Ca,
          Ti, and Fe. A small Si-PIN detector was used as a solar monitor,
          because the Sun is the source of the planetary X-ray fluorescence.