Ground calibration of Alpha Particle X-ray Spectrometer (APXS) on-board Chandrayaan-2 Pragyaan rover: An empirical approach N.P.S. Mithun * , Santosh V. Vadawale, M. Shanmugam, Arpit R. Patel, Nishant Singh, Sushil Kumar, Neeraj Kumar Tiwari, Shiv Kumar Goyal, Amit Basu Sarbadhikari, Garima Arora, Yash Srivastava, Hitesh Kumar Adalja, Tinkal Ladiya, Deepak Kumar Painkra, Swaroop B. Banerjee, Vishnubhai R. Patel, Anil Bhardwaj Physical Research Laboratory, Ahmedabad, India ARTICLE INFO Keywords: Abundances X-ray spectroscopy Moon ABSTRACT Alpha Particle X-ray Spectrometer (APXS) was one of the scientific experiments on-board the Pragyaan rover of the Chandrayaan-2 mission to Moon, with the objective to determine the elemental composition of the lunar surface near the landing site in the south polar region of the Moon. APXS estimates the abundances by measuring the characteristic X-ray lines of the constituent elements present in the target surface, when irradiated by the onboard Cm-244 radioactive source. X-ray spectral measurement in the energy range of 0.8–25 keV with an energy resolution of e135 eV at 5.89 keV enables APXS to detect all the major rock-forming elements, such as Na, Mg, Al, Si, Ca, Ti, and Fe. This paper describes the calibration of APXS as an X-ray spectrometer, provides a procedure for quantitative estimation of elemental concentrations based on the measurements of standard geochemical reference materials and provides the detection limits of the major elements. It is shown that, following this procedure, the concentration of the major elements in any sample can be determined with typical accuracy of better than 10% of the actual value. 1. Introduction In-situ measurements of elemental composition have been an integral part of all planetary rover missions. Such measurements complement the mineralogical studies and provide inputs to constrain various models on the origin and evolution of planetary bodies. In-situ measurement of elemental abundance on the local scale also plays a vital role in providing ‘‘ground truth’’ for the remote sensing measurements, which provide global estimates of abundance. One of the important techniques to esti- mate elemental concentration is by measuring intensities of the charac- teristic X-rays emitted by a particular element when excited by any incident radiation. Experiments based on this technique, when con- ducted from an orbiting spacecraft to study atmosphere-less bodies, usually use the solar X-rays as excitation radiation. However, for in-situ experiments, it is essential to have an independent source of excitation radiation, typically in the form of a radioactive source emitting α parti- cles and X-rays. Such experiments have been carried out on the Martian surface by all three generations of rovers, the Sojourner rover of Mars Pathfinder mission (Rieder et al., 1997), the Mars Exploration Rovers - Spirit and Opportunity (Rieder et al., 2003) and the Curiosity rover (Gellert et al., 2009); as well as on the lunar surface by the Yutu rover of the Change-3 mission (Zhang et al., 2015), though in this case, excitation was only with X-rays. The Pragyaan rover of Chandrayaan-2, the second Indian mission to the Moon, carried an experiment named as Alpha Particle X-ray Spec- trometer (APXS) (Shanmugam et al., 2020) in order to determine the elemental composition of the lunar surface surrounding the landing site. The APXS is designed to measure the X-ray spectrum in the energy range of 0.8–25 keV, which covers fluorescence lines of almost all major and minor elements expected to be present in the lunar surface. A key feature of the APXS on Pragyaan rover is that it employs the radioactive source, Cm-244, an ideal choice due to its high specific activity, the moderate half-life of e18:1 years, and for having both α-particles (of energy 5.8 MeV) and X-rays (main energies 14.3 keV and 18.3 keV) as excitation radiation. Thus the X-ray spectrum of the irradiated sample contains a contribution from both, the atoms excited by the incident X-rays - the process known as X-ray Fluorescence (XRF), and the atoms excited by incident alpha particles - the process known as Particle Induced X-ray * Corresponding author. E-mail address: mithun@prl.res.in (N.P.S. Mithun). Contents lists available at ScienceDirect Planetary and Space Science journal homepage: www.elsevier.com/locate/pss https://doi.org/10.1016/j.pss.2020.104923 Received 10 November 2019; Received in revised form 19 February 2020; Accepted 30 March 2020 Available online 16 April 2020 0032-0633/© 2020 Elsevier Ltd. All rights reserved. Planetary and Space Science 187 (2020) 104923