  Alpha Particle X-ray Spectrometer (APXS) is one of the two scientific
experiments on Chandrayaan-2 rover named as Pragyan. The primary scientific
objective of APXS is to determine the elemental composition of the lunar
surface in the surrounding regions of the landing site. This will be achieved
by employing the technique of X-ray fluorescence spectroscopy using in-situ
excitation source Cm-244 emitting both X-rays and alpha particles. These
radiations excite characteristic X-rays of the elements by the processes of
particle induced X-ray emission (PIXE) and X-ray fluorescence (XRF). The
characteristic X-rays are detected by the state-of-the-art X-ray detector known
as Silicon Drift Detector (SDD), which provides high energy resolution as well
as high efficiency in the energy range of 1 to 25 keV. This enables APXS to
detect all major rock forming elements such as, Na, Mg, Al, Si, Ca, Ti and Fe.
The Flight Model (FM) of the APXS payload has been completed and tested for
various instrument parameters. The APXS provides energy resolution of 135 eV at
5.9 keV for the detector operating temperature of about -35 deg C. The design
details and the performance measurement of APXS are presented in this paper.
