As a part of NASA's Discovery Programme, the Applied Physics Laboratory of Johns Hopkins University developed the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft to orbit the planet Mercury.
The X-ray Spectrometer (XRS) aboard the spacecraft uses three Gas Proportional Counter detectors and a silicon PIN diode to measure the surface elemental composition of the planet Mercury, using X-ray fluorescence excited by Sun's radiation.
Oxford Instruments Analytical has delivered the XRS proportional counters and the silicon PIN diode for the mission which was launched on August 3 2004.
"The spacecraft will fly past Earth once, Venus twice and Mercury three times starting a year long orbital study of the innermost planet in March 2011."
Each XRS proportional counter consists of two concentric detectors: surrounded by a graphite walled inner cylindrical detector, there are a number of interconnected veto wires. This cage of veto anodes makes an anticoincidence shield around the center detector. The shield is required to reduce the spectral background induced by the intense charged particle radiation prevalent in Mercury orbit.