SCI-TECH & AGRI

Magnesium, aluminium, silicon detected on moon

K.S. RAJGOPAL

The mission’s X-ray spectrometer has proved more sensitive than expected



C1XS uses an advanced version of conventional CCD sensors, called swept charge devices

C1XS swept charge devices operate well at around -15 degrees C



The Chandrayaan-1 X-ray Spectrometer (C1XS) aboard the Chandrayaan-1 spacecraft, now orbiting the moon, began sending back data indicating the presence of magnesium, aluminium and silicon from December 12th during a period of weak solar flaring.

C1XS works by looking at X-rays from the sun which have been absorbed by atoms in the lunar soil, then re-emitted in such a way as to reveal the make-up of the surface.

Illumination dependent

The spectrometer is sensitive to magnesium, aluminium and silicon X-rays. When the solar illumination is bright, for example during a big solar flare, it may also be able to make measurements of other elements such as iron, titanium and calcium.

The calibration data reduction, instrument response modelling and daily operational elements and performance monitoring of C1XS are carried out by Mrs. Shyama Narendranath of ISRO Satellite Centre, some of which form core elements of her PhD thesis work.

“The significance of the detection is that the C1XS has exceeded expectations as to its sensitivity and has proven by its performance that it is the most sensitive X-ray spectrometer of its kind in history,” said Ms. Narendranath, to this correspondent. The device was designed and built at UK’s Science and Technology Facilities Council (STFC)’s Space Science and Technology Department in the Rutherford Appleton Laboratory, UK. It was developed in conjunction with the Indian Space Research Organisation.

It employs new technology to make a compact, lightweight, sensitive instrument. C1XS uses an advanced version of conventional CCD (charge coupled device) sensors such as one might find in a digital camera, called swept charge devices.

The baseline design consists of 24 nadir pointing Swept Charge Device (SCD) detectors, which provide high detection efficiency in the 1 to 7 keV range, which contains the X-ray fluorescence lines of interest.

The processing electronics associated with C1XS converts the swept out charge to a voltage pulse and further digitizes it. This signal is proportional to the energy of the X-ray photon that is incident on the SCD. Higher energy photons will have a voltage pulse of higher amplitude.

Solar X-rays incident on the lunar surface trigger the phenomenon called X-ray fluorescence and Moon emits X-rays. Essentially each atom of Mg, Al, Si etc on the Moon produces these X-ray photons.

Photon energy

C1XS SCDs measure these. So C1XS knows the energy of the X-ray photon. Now, each element produces an X-ray photon of unique energy. For example, Mg produces photons at energy 1.25 keV. So in the C1XS SCDs, when 1.25 keV lines are seen (a lot of them will create a peak in the data) it indicates the presence of Mg.

Why do conventional CCDs take up much more power than swept charged devices? And why do they have to be cooled for X-ray purposes, taking up even more power? Mrs. Narendranath replied: “A conventional X-ray CCD requires more power mainly because it has to be cooled up to -60 to -80 deg C. Whereas C1XS SCDs operate well at around -15 deg C. This is because, unlike X-ray CCDs, the charge is read out very quickly and continuously so that the background noise in the system is low. Generally we cool the detectors because the noise in the system is higher at higher temperatures.

“This would mean that the ‘peaks’ I mentioned earlier would be very wide and cannot be extracted from the noise.

On the C1XR you have 24 swept charged devices. Does each swept charged device have a number of pixels or just one pixel? If many pixels, how many?

“Each SCD has 1725 electrodes on it and conventionally in a three phase device, 3 electrodes where clocking is done, can be called a pixel. But this would normally mean that we can identify where the charge came from, on the detector.

“In SCDs, since the charge is swept across the diagonal, we cannot say where on the SCD it came from. So it is equivalent to a single pixel detector. Sweeping it diagonally makes the read out faster. Conventionally CCDs are read as rows and columns,”noted Ms. Narendranath.



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