Payloads : 5. High Energy X-ray spectrometer (HEX)

The high energy X-rays and gamma rays aboveabout 20 keV are produced as a consequence ofcosmic ray interactions in the lunar surface materialand inherent radioactivity (e.g., K, and Uand Th decay nuclides) present in the Moon.The nuclear interactions of primary and secondarycosmic rays with lunar material are confined tothe upper meter or two of the lunar surfaceand produce gamma rays by de-excitation, spallation,decay of induced radionuclides and by neutroncapture reactions, etc. Thus their flux hasthe signature of the lunar composition. An X-rayspectrometer, sensitive in the 20–250 keV regionhas therefore been included in the Chandrayaan-1payloads. In this region, there are a number ofgamma ray lines due to U and Th decay seriesnuclides like 210Pb (46.5 keV), 228Th (238.6 keV)and also due to neutron capture in rare earth elementslike Gd and Sm which have high neutroncapture cross section. However, because of the highCompton background, the signal to backgroundratio is poor. The background is also producedby the space craft and detector material whichmakes it difficult to determine the peak strengthswith good precision. However, the flux of scatteredgamma rays in this energy region is itself characteristicof the lunar terrain, being high in KREEP,gradually decreasing in basalt, highland and waterbodies and can possibly be used to map the variouslunar terrains.

The flux of gamma rays from radionuclides producedin decay of radon, e.g., 210Pb, depends, notonly on the in situ production in lunar surface butalso on degassing of radon from the lunar interior(Bhandari et al 2004a). Once in the lunaratmosphere, radon decays to 210Pb while it getsdeposited in lunar cold traps (e.g., poles or coolnight side). According to the model of Heymannand Yaniv (1971) radon is expected to pile upand show a peak at the morning and evening terminatorsand for this reason, radon (210Pb) canbe used as a tracer for transport of volatiles onthe lunar surface. Brodzinski and Langford (1975)have summarized the observations on 210Po andradon made at the Apollo landing sites. Consideringplausible diffusivity coefficients of radon in thelunar regolith, the signal due to 210Pb, depositedon the lunar surface as a thin paint, must bemeasurable.

There are many suitable solid state detectors(e.g., CdZnTe) and scintillators (e.g., BGO,CsI) available for the measurement of low energygamma rays. The measurement of excess 210Pbdue to diffusion of 222Rn from the lunar interior(Bhandari et al 2004a), over the amount producedin situ in the lunar surface due to U, requires highspatial resolution and therefore a 10? field of viewcollimator is proposed for a large area gamma raydetector, which will have a spatial resolution of20 km from a nominal altitude of 100 km.