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Infrared in the spotlight (2)

15/11/2002

One of the mysterious aspects of detectors is the materials that are used to make them. Not all materials act in the same way when they come into contact with photons and so not all of them are suitable for use in EMIR.

For example, the material used for work in the visible light spectrum (normally silicon) cannot gather infrared light. We need a material sensitive to photons in the infrared range - for EMIR this is an alloy of mercury, cadmium and tellurium (HgCdTe).

We already know (from the last instalment of “Infrared in the spotlight”) that inside the detector, a photoelectric process breaks apart the bonds of atoms so that each photon ejects an electron (this is a slight simplification). These electrons are then ‘captured’ by an electric current (polarisation) and end up in the ‘electron pool’. Actually putting the process to work, however, is far from a simple matter…

Why do infrared instruments take us back to the ice age?

The real challenge is to keep the temperature low enough so that, whilst the electrons can still ‘jump,’ noise (unwanted electrons) is kept out. For work in the visible light range this can be achieved by cooling the atmosphere around the detector to a temperature well below zero, but in the infrared it is more complicated.

We only have to look at the images picked up by infrared night cameras to understand how much energy objects give off. For this reason, not only does EMIR’s detector need to be refrigerated, but the whole instrument has to be immersed in an ‘ice age’ of -196° C.

The cryostat (which contains the thermal insulation material and allows very low temperatures to be maintained), will completely surround the instrument: the sensitivity required of the instrument will determine every aspect of its design.

The cooling process for EMIR will have two stages. First, liquid nitrogen will be introduced until the instrument cools to a temperature around 100 Kelvin (170° below zero). Then helium will be passed through a closed circuit, expanding to create one or more ‘cold spots’. Thus the temperature inside the instrument will be maintained.

EMIR

Emir’s detectors will be located inside its electronics, which will ultimately be adapted for use in the instrument (this process is called ‘characterisation’) by the IAC. The IAC already has experience of this, as it has characterised other instruments such as LIRIS.

These detectors will be pushed to the limit: instead of receiving information via four channels (the norm) they will use 32, eight per band. This is essential as it will speed up the reading process and cut data production time. It will be the only detector of its kind (2048x2048 pixels) in the world and will be better than any other for working in the near infrared (the range used for analysing medium heat objects). It will operate between 0.8 and 2.5 microns and, of all the cutting edge technology that is currently available, it will be the ultimate in infrared light detectors.

This, then is an instrument that will be able to pierce dust and gas clouds, tell us about faraway objects from their infrared radiation and yield up new information on the Universe around us.

All this from 38 square millimeters of mercury, cadmium and tellurium.

Natalia R. Zelman

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