M. Shanmugam

High energy X-γ ray spectrometer on the Chandrayaan-1 mission to the Moon

The Chandrayaan-1 mission to the Moon scheduled for launch in late 2007 will include a high energy X-ray spectrometer (HEX) for detection of naturally occurring emissions from the lunar surface due to radioactive decay of the238U and232Th series nuclides in the energy region 20–250 keV. The primary science objective is to study the transport of volatiles on the lunar surface by detection of the 46.5 keV line from radioactive210Pb, a decay product of the gaseous222Rn, both of which are members of the238U decay series. Mapping of U and Th concentration over the lunar surface, particularly in the polar and U-Th rich regions will also be attempted through detection of prominent lines from the U and Th decay series in the above energy range. The low signal strengths of these emissions require a detector with high sensitivity and good energy resolution. Pixelated Cadmium-Zinc-Telluride (CZT) array detectors having these characteristics will be used in this experiment. Here we describe the science considerations that led to this experiment, anticipated flux and background (lunar continuum), the choice of detectors, the proposed payload configuration and plans for its realization

Measurement of Low Energy Detection Efficiency of a Plastic Scintillator: Implications on the Lower Energy Limit and Sensitivity of a Hard X-Ray Focal Plane Compton Polarimeter

The polarization measurements in X-rays offer a unique opportunity for the study of physical processes under the extreme conditions prevalent at compact X-ray sources, including gravitation, magnetic field, and temperature. Unfortunately, there has been no real progress in observational X-ray polarimetry thus far. Although photoelectron tracking-based X-ray polarimeters provide realistic prospects of polarimetric observations, they are effective in the soft X-rays only. With the advent of hard X-ray optics, it has become possible to design sensitive X-ray polarimeters in hard X-rays based on Compton scattering. An important point that should be carefully considered for the Compton polarimeters is the lower energy threshold of the active scatterer, which typically consists of a plastic scintillator due to its lowest effective atomic number. Therefore, an accurate understanding of the plastic scintillators energy threshold is essential to make a realistic estimate of the energy range and sensitivity of any Compton polarimeter. In this context, we set up an experiment to investigate the plastic scintillators behavior for very low energy deposition events. The experiment involves the detection of Compton scattered photons from a long, thin, plastic scintillator (a similar configuration as the eventual Compton polarimeter) by a high resolution CdTe detector at different scattering angles. We find that it is possible to detect energy deposition well below 1 keV, though with decreasing efficiency. We present detailed semianalytical modeling of our experimental setup and discuss the results in the context of the energy range and sensitivity of the Compton polarimeter involving plastic scintillators.

A new technique for measuring the leakage current in Silicon Drift Detector based X-ray spectrometer—implications for on-board calibration

In this work, we report a new technique of measuring the leakage current in Silicon Drift Detectors (SDD) and propose to use this technique as a tool for on-board estimation of the radiation damage to the SDD employed in space-borne X-ray spectrometers. The leakage current of a silicon based detector varies with the detector operating temperature and increases with the radiation dose encountered by the detector in the space environment. The proposed technique to measure detector leakage current involves measurement of the reset frequency of the reset type charge sensitive pre-amplifier when the feedback capacitor is charged only due to the detector leakage current. Using this technique, the leakage current is measured for large samples of SDDs having two different active areas of 40 mm2 and 109 mm2 with 450 micron thick silicon. These measurements are carried out in the temperature range of -50°C to 20°C. At each step energy resolution is measured for all SDDs using Fe-55 X-ray source and shown that the energy resolution varies systematically with the leakage current irrespective of the difference among the detectors of the same as well as different sizes. Thus by measuring the leakage current on-board, it would be possible to estimate the time dependent performance degradation of the SDD based X-ray spectrometer. This can be particularly useful in case where large numbers of SDD are used.

Space radiation induced displacement damage effects on the performance of the silicon drift detector onboard chandrayaan-2 mission

The space radiation induced displacement damage effects on the performance of the Silicon Drift Detector (SDD) based X-ray spectrometer has been studied using X-ray (Fe-55) and gamma ray (Co-60) radiations. The spectroscopic performance of the SDD based spectrometer degrades due to radiation damage during the transit and in-orbit operations. Silicon detectors are sensitive to displacement damage which is due to the non ionizing energy loss of the incident radiation. The displacement damage increases the leakage current of the SDD and hence the energy resolution. The X-ray irradiation is to quantify the X-ray fluence level up to which the SDD provides stable energy resolution. After X-ray irradiation tests, it is observed that there is no change in the leakage current and the energy resolution for dose up to 64 krad. The gamma ray irradiation test is to quantify the space radiation damage effects on the SDD and shown that the energy resolution degrades from ~160 eV at 5.9 keV to ~210 eV for the detector operating temperature of ~-40°C for the gamma ray dose of ~10 Krad. It is observed that the increase in the leakage current due to displacement damage is ~0.15 pA and its contribution to the degradation in the energy resolution is insignificant. The degradation in the energy resolution is attributed to the radiation damage of the electronic components inside the SDD module and meets the Chandrayaan-2 requirement of <; 250 eV for the mission life of 2 years.

Dependence of leakage current on the performance of Silicon Drift Detector based X-ray spectrometer

We have developed Silicon Drift Detector (SDD) based X-ray spectrometer for the future planetary/space exploration missions. This spectrometer provides the energy resolution of ~150 eV at 5.9 keV for the pulse peaking time of 3 μs and the detector kept at -40°C. The energy resolution of the SDD based X-ray spectrometer depends on the detector leakage current and the electronics noise associated with the signal readout and processing electronics. We have measured energy resolution and leakage current for two sets of SDDs having active area of 40 mm2 and 109 mm2 respectively. It is shown that the leakage current for small area (40 mm2) SDD detector varies from ~0.6 nA at 20°C to ~0.2 pA at -40°C and for large area (109 mm2) SDD detector, the leakage current varies from ~0.9 nA at 20°C to ~1 pA at -50°C. The total measured Equivalent Noise Charge (ENC) of the spectrometer system varies from -34.5 erms at -3°C to 11 erms at -40°C for small area detector and 42 erms at -8°C to 13 erms at -50°C for large area detector.

Laboratory XRF measurements using Alpha Particle X-ray Spectrometer of Chandrayaan-2 rover: Comparison with Geant4 simulation results

Indias 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 X-rays 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.

Characterization of high count rate capability of Solar X-ray Monitor on-board Chandrayaan-2 — the second Indian mission to the Moon

Chandrayaan-2 is the second Indian mission to the Moon, planned to be launched during 2015. One of the scientific experiments onboard Chandrayaan-2 is the remote X-ray Fluorescence spectroscopy to measure abundances of the major rock forming elements of the lunar surface. The experiment involves measuring spectra of fluorescent X-rays from lunar surface which are generated due to the incident solar X-rays. Since the flux of fluorescent X-ray lines critically depend on the flux and spectrum of the incident solar X-rays which are highly variable, it is essential to have simultaneous measurement of X-ray from the Sun in order to have quantitative interpretation of the lunar X-ray fluorescence spectra. The Solar X-ray Monitor (XSM) onboard Chandrayaan-2 orbiter will accurate and real time measurement of the solar X-ray spectrum using state-of-the-art Silicon Drift Detector (SDD). Here we present the overall design of the XSM instrument as well as initial results of the XSM laboratory model tests at different incident count rate. The X-ray spectrometer provides energy resolution of ~200 eV at 5.9 keV for the pulse peaking time of 0.8 μs by cooling the detector to -35°C. We have characterized the engineering model of XSM for high count rate measurement and shown that the energy resolution is stable at ~200 eV for the count rates up to 70K couts/s and the change in peak position is <;0.5%.