P. Sreekumar

Characterisation of swept-charge devices for the Chandrayaan-1 x-ray spectrometer (C1XS) instrument

The Indian Space Research Organisation (ISRO) Chandrayaan-1 mission is Indias first lunar spacecraft, containing a suite of instruments to carry out high-resolution remote sensing of the Moon at visible, near infrared and X-ray wavelengths. Due for launch in early 2008, the spacecraft will carry out its two year mission in a polar orbit around the Moon at an altitude of 100 km. One of the eleven instruments in the spacecraft payload is the Chandrayaan-1 X-ray Spectrometer (C1XS), a descendant of the successful D-CIXS instrument that flew on the European Space Agency SMART-1 lunar mission launched in 2003. C1XS consists of 24 swept-charge device (SCD) silicon X-ray detectors arranged in 6 modules that will carry out high quality X-ray spectroscopic mapping of the Moon using the technique of X-ray fluorescence. This paper presents an overview of the Chandrayaan-1 mission and specifically the C1XS instrument and describes the development of an SCD test facility, proton irradiation characterisation and screening of candidate SCD devices for the mission.

Radiation Study of Swept-Charge Devices for the Chandrayaan-1 X-ray Spectrometer (C1XS) instrument

The Chandrayaan-1 X-ray Spectrometer (C1XS) will be launched as part of the Indian Space Research Organisation (ISRO) Chandrayaan-1 payload in September 2008, arriving at the Moon within 7 days to begin a two year mission in lunar orbit conducting mineralogical surface mapping over the range of 1 - 10 keV. The detector plane of the instrument consists of twenty four e2v technologies CCD54 swept-charge devices (SCDs). Such devices were first flown in the Demonstration of a Compact Imaging X-ray Spectrometer (D-CIXS) instrument onboard SMART-1 [4, 5]. The detector plane in each case provides a total X-ray collection area of 26.4 cm2. The SCD is capable of providing near Fano-limited spectroscopy at -10°C, and at -20°C, near the Chandrayaan-1 mission average temperature, it achieves a total system noise of 6.2 electrons r.m.s. and a FWHM of 134 eV at Mn-Kα. This paper presents a brief overview of the C1XS mission and a detailed study of the effects of proton irradiation on SCD operational performance.

Modeling charge transport in swept charge devices for X-ray spectroscopy

We present the formulation of an analytical model which simulates charge transport in Swept Charge Devices (SCDs) to understand the nature of the spectral redistribution function (SRF). We attempt to construct the energy-dependent and position dependent SRF by modeling the photon interaction, charge cloud generation and various loss mechanisms viz., recombination, partial charge collection and split events. The model will help in optimizing event selection, maximize event recovery and improve spectral modeling for Chandrayaan-2 (slated for launch in 2014). A proto-type physical model is developed and the algorithm along with its results are discussed in this paper.

Performance of new generation swept charge devices for lunar x-ray spectroscopy on Chandrayaan-2

The Chandrayaan-2 Large Area Soft X-ray spectrometer (CLASS) is due to be launched by the Indian Space Research Organisation (ISRO) in 2014. It will map the elemental composition of the lunar surface, building on the Chandrayaan-1 X-ray spectrometer (C1XS) heritage. CLASS will use an array of e2v technologies CCD236 swept charge devices (SCD) providing an active detector area of approximately 64 cm2, almost three times the active area of C1XS which used the first generation of SCD, the CCD54. The CCD236 is designed as a soft X-ray detector, 0.8 keV to 10 keV, and benefits from improvements in design to allow for increased detector area, a reduction in split X-ray events and improvements to radiation hardness. This paper describes the investigation into the performance requirements of the CCD236, focussing on an optimisation of the energy resolution of a device irradiated to the estimated worse case end of life proton fluence.

The effects of radiation damage on the spectral resolution of the Chandrayaan-1 x-ray spectrometer

The Chandrayaan-1 X-ray Spectrometer (C1XS) was launched onboard the Indian Space Research Organisation (ISRO) Chandrayaan-1 lunar mission in October 2008. The instrument consisted of 24 swept-charge device silicon X-ray detectors providing a total collecting area of ~24 cm2, corresponding to a 14° field of view (FWHM), with the ability to measure X-rays from 0.8 - 10 keV. During the 10 months the spacecraft was located in orbit around the Moon a number of solar flare X-ray events were detected, along with calibration data from X-ray sources housed inside the movable door of the instrument. This paper presents a study of the degradation in spectral resolution of the measured X-ray calibration lines, comparing those recorded mid way through the mission lifetime with ground based calibration data collected prior to the launch of the instrument. The radiation environment the detectors were subjected to is discussed in light of the actual radiation damage effects on the spectral resolution observed in flight.