Laboratory XRF Measurements Using Alpha Particle X-ray Spectrometer of Chandrayaan-2 Rover: Comparison with Geant4 Simulation Results S. K. Goyal*, M. Shanmugam, S. Vadawale, D. Banerjee, Y. B. Acharya, and S. V. S. Murty Abstract– India’s second mission to the Moon – Chandrayaan – 2, will have a rover for in-situ exploration of lunar surface around the landing site. Alpha Particle X-ray Spectrometer (APXS) is one of the instruments on board Chandrayaan-2 rover, for measuring elemental composition of the lunar surface using state – of – the – art X-ray detector (Silicon Drift Detector - SDD) with higher energy resolution. The objective of the APXS instrument is to analyze several soil / rock samples along the rover traverse for the major elements with the characteristics Xrays in the 1 to 25 keV energy range. We have carried out XRF measurements in the laboratory using Alpha Particle X-ray Spectrometer which provides energy resolution of ~150 eV at 5.9 keV, when the detector is cooled to -35°C. These measurements are carried out using six 55Fe X-ray sources for various target materials at different detector to target heights. We have also carried out detailed Monte-Carlo simulation based on GEANT4 for the APXS for various compositions of the lunar surface. Here we are comparing the GEANT4 simulation results with the experimentally acquired XRF data.
I. INTRODUCTION Particle X-ray Spectrometer is a well Alpha instrument for quantitative elemental analysis
proven of the planetary surfaces through in-situ measurements. Measuring the elemental composition of the planetary bodies, particularly the major surface constituent elements such as Mg, Al, Si, Ca, Ti and Fe is very important as it can constrain various theoretical / phenomenological models of the origin and evaluation of the respective bodies. Several Mars rovers such as Mars Path Finder (MPF), Mars Exploration Rover (MER) [1] and Mars Science Lab (MSL) [2] have carried APXS instruments onboard. So far, APXS instrument has not been flown on the Moon for surface elemental composition studies. Chandrayaan – 2, the second Indian Moon mission having Orbiter, Lander and Rover gives us unique opportunity to plan such an instrument to carry out the in-situ elemental composition measurements. In-situ measurements on the Moon have been initiated in late 1960s through detection of back scattered alpha particles with surveyor Lander program on Moon. Though lack of atmosphere on Moon offers an advantage in terms of the absence of attenuation of alphas and X-rays, the fluffy regolith
Manuscript received November 15, 2013. S. K. Goyal, M. Shanmugam, S. Vadawale, D. Banerjee, Y. B. Acharya and S. V. S. Murty are with Physical Research Laboratory, Ahmedabad, India - 380009 (Ph: +91-79-26314427, e-mail:
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[email protected]). *Corresponding author
and temperature extremes offer new challenges for the operation of APXS on lunar rovers. As compared to the Mars, the environment on Moon is under very high vacuum and also experiences temperature extremes during day / night cycles. The measurement at one spot depends on the distance between the detector and the target surface. The intensity of characteristic lines emitted from the surface is expected to be low as the APXS instrument is mounted under the chassis of Chandrayaan-2 rover at a height of 180 mm from the lunar surface [3]. Hence it is necessary to estimate the integration time required at a fixed height to accumulate the statistically significant spectrum. APXS has two types of excitation sources, PIXE (due to alpha particles) and XRF, which cause variations in the characteristic line intensities with the atomic number Z. Hence in order to understand the variations of the characteristic lines with height and atomic number Z, we have carried out the Monte-Carlo simulation using GEANT4 for APXS lab model at several heights with different type of compositions of the target materials. In this paper, we are comparing the GEANT4 simulated XRF data with the XRF measurements of APXS (using six 55Fe radio-active sources). II. SCIENTIFIC OBJECTIVE The objective of the APXS experiment onboard Chandrayaan – 2 rover is to analyze several soil / rock samples by measuring fluorescence from the lunar surface, along the rover track for the major elements composition. The measurements will be carried out on several selected samples during the rover life time. This is the first time, when such measurements will be carried out on the Moon. III. PRINCIPLE APXS involves the measurement of characteristic X-rays emitted from the lunar surface due to alpha Particle Induced X-ray Emission (PIXE) and X-ray Fluorescence (XRF) processes. The 244Cm has been chosen as the radio-active source which emits both alpha particles with energy 5.8 MeV and X-rays with energies of 14.3 keV and 18.4 keV. PIXE is dominant for low Z elements, while XRF is more prominent for high Z elements, allowing the determination of elements from Na to Br, spanning the energy range of 0.9 keV to 25 keV, for the Kα X-rays. APXS uses six radio-active sources, each with activity of ~5 (± 25%) mCi (total activity ~30 mCi). The arrangement of sources and detector is shown in Fig. 1.
Electronics head
Alpha source SDD
Sample surface Fig. 1. Arrangement of six radioactive sources and detector in APXS.
Each source is an 8 mm circular disc with
