Cryogenic solution to providing continuous operation for THz cameras

Terahertz (THz) wavelengths lie between infra-red light and radio waves, and - like X-rays - can be used to see through opaque objects. This is very useful for astronomy, as it enables us to see through dust in space which is completely opaque in normal photographs and indeed at most other wavelengths. Much work has therefore been carried out by astronomers to develop increasingly sensitive THz detectors. Unlike X-rays, all objects "shine" at these wavelengths, so it is possible to take pictures without using a THz source, unlike an X-ray image where X-rays have to be shone through (or at) the object. There are many applications for THz imaging, including atmospheric science, research into fusion energy, security scanning and even art conservation. One example is to detect metal under clothing to search for concealed weapons.

A difficulty in detecting THz waves is that sensitive and fast detectors need to be cooled to low temperatures (-272.8 C, which is only 0.35 C above absolute zero, the lowest temperature which can exist). Detectors at these temperatures are more than a thousand times more sensitive than detectors which can operate at room temperature. We are currently developing a THz camera which can react quickly enough to take video images, but only if cooled sufficiently.

It is now possible to build a "fridge" which cools to these temperatures, and like a domestic fridge only requires electricity to run (traditionally, it was necessary to keep supplying very cold liquid helium, making such systems difficult to operate). The cooler which provides the final cooling (from 3 C above absolute zero) works by evaporating a rare element (helium-3). When it has all evaporated, cooling must stop for a while as the gas is turned back into a liquid. The longer that continuous operation is required, the more gas is needed. Helium-3 has recently become much more expensive (from around $200 to $3000 a litre), making such coolers very expensive.

We are proposing to develop a new type of cooler which provides continuous cooling. This will be achieved by having two cooling stages, one of which is operating while the other one is being reset. Besides the obvious advantage of not having to pause operations every 10 hours for a few hours while the fridge is reset (not ideal for, say, a security scanner in an airport), it reduces the quantity of expensive helium-3 required, since neither of the two fridges needs to have a particularly long operating time. While ideal for sensitive arrays of detectors such as our THz camera, such coolers will also have many other uses in cooling THz detectors, detectors for other wavelengths, and in other applications.