CH-2 XSM

Solar X-ray Monitor (abbreviated as XSM) is part of the remote X-ray fluorescence spectroscopy experiment on board the Chandrayaan-2 mission to the Moon. XSM assists its companion payload CLASS (Chandrayaan-2 Large Area Soft X-ray Spectrometer) that measures the fluorescence X-ray spectrum from the lunar surface in determining the surface elemental composition of the Moon on a global scale by providing measurement of the incident solar X-ray spectrum.

It observes the Sun as a star and measures the solar X-ray spectrum in the energy range of 1-15 keV with an energy resolution of ~175 eV at 5.9 keV and a time cadence of one second. The broadband soft X-ray spectra with XSM provides diagnostics of the solar corona at various levels of solar activity from quiescent Sun to large solar flares.

Salient Features

State-of-the-art Silicon Drift Detector (SDD) and in-house designed space-qualified readout electronics: Energy resolution and dynamic range
Active temperature control of the detector at -35^oC: Stable performance with time
High time cadence: Full spectrum at 1 second, light curves in three energy bands at 100 ms
Silver coated aluminium collimator with wide FOV and small aperture: Low background and extended dynamic range
Filter wheel mechanism with Be-filter and Fe-55 calibration source and automated filter control with on-board flare detection: Extend the dynamic range and on-board calibration

Specifications

Energy Range	1 – 15 Kev (up to ≈ M5 class) 2 – 15 keV (above ≈ M5 class)
Energy Resolution	~175 eV @ 5.9 Kev
Time Cadence	1 second
Effective area (on-axis)	0.135 mm² @ 1 keV 0.367 mm² @ 5 keV
Field of View	± 40 degree
Filter wheel positions	3: Open, Be-filter, Cal(Fe-55)
Be-filter movement threshold flux	80,000 counts s⁻¹ (≈ M5 class)

Instrument Configuration

The XSM instrument is designed as two separate packages, namely the XSM sensor package and the XSM processing electronics (PE) package. Block diagram below shows the various components in both the packages and the interfaces between them and those with the spacecraft systems.

XSM sensor package

The sensor package houses the detector, front-end electronics, and the filter wheel mechanism. It has a size of 234 x 112 x 133 mm³ and weighs about 650 g and is mounted outside the negative pitch panel of the spacecraft.

XSM processing electronics package

The PE package houses the FPGA based data acquisition system, power electronics, and spacecraft interfaces. Its size is 150 x 127 x 55 mm³ and it weighs about 700 g. This package is mounted on the inner side of the -pitch panel of the spacecraft.

At the heart of the instrument is a Silicon Drift Detector (SDD) housed in the sensor package. X-ray photons incident on the SDD generate a charge cloud proportional to the deposited photon energy, which is collected at the anode of the detector. The front-end electronics, that includes a charge sensitive preamplifier and shaping amplifiers, convert this charge to a voltage signal in the form of a semi-Gaussian pulse. The peak of this pulse is then detected by a peak detector and digitized by a 12-bit analog- to-digital converter (ADC). Histograms of the 10-bit ADC values (ignoring the two least significant bits) are generated at every one second interval and recorded on board. This PHA (Pulse Height Analysis) spectrum at one second is the raw data from the instrument. In level-2/calibrated data generation, the raw PHA spectrum is converted to a Pulse Invariant (PI) channel spectrum of 512 channels of 33 eV width, taking into account the instrument gain, for further scientific analysis.