High energy X-[gamma] ray spectrometer on the Chandrayaan-1 mission to the Moon

The Chandrayaan-1 mission to the Moon scheduled for launch in late 2007 will include a high energy X-ray spectrometer (HEX) for detection of naturally occurring emissions from the lunar surface due to radioactive decay of the^sup 238^U and^sup 232^Th series nuclides in the energy region 20-250 keV. The primary science objective is to study the transport of volatiles on the lunar surface by detection of the 46.5 keV line from radioactive^sup 210^Pb, a decay product of the gaseous^sup 222^Rn, both of which are members of the^sup 238^U decay series. Mapping of U and Th concentration over the lunar surface, particularly in the polar and U-Th rich regions will also be attempted through detection of prominent lines from the U and Th decay series in the above energy range. The low signal strengths of these emissions require a detector with high sensitivity and good energy resolution. Pixelated Cadmium-Zinc-Telluride (CZT) array detectors having these characteristics will be used in this experiment. Here we describe the science considerations that led to this experiment, anticipated flux and background (lunar continuum), the choice of detectors, the proposed payload configuration and plans for its realization[PUBLICATION ABSTRACT]