Cold enough to see clearly

Posted on 06. Oct, 2011 by in Academic Departments, Annual Report, Engineering Physics, Issues, Mechanical Engineering, Research

Astrophysics instruments that measure very faint, distant sources of light need to be very cold to be sensitive enough to detect individual photons. These detectors work by measuring the change in temperature that occurs when a single photon hits the detector and deposits energy. Because this temperature rise is extremely tiny, only a very cold semiconductor is sensitive enough to detect it.

Some of the world’s most sensitive X-ray and infrared detectors operate at temperatures below 1 degree Kelvin.

Although space is very cold, the detectors used in orbiting telescopes must be cooled artificially if they are to reach temperatures below about 40 Kelvin, because thermal radiation from the earth and sun warms them.

That’s where Mechanical Engineering Assistant Professor Franklin Miller comes in. Miller, who got his start working on projects like the James Webb space telescope at NASA’s Goddard Space Flight Center, works in the field of sub-Kelvin cryogenics to cool sensitive instruments to temperatures colder than 1 degree Kelvin.

In space, astrophysics detectors  operate at temperatures of about 50 thousandths of a Kelvin, or 50 millikelvin. But Miller is striving to make them even colder—on the order of 20 millikelvin. Although that difference might seem small, only about a hundredth of a degree Fahrenheit, it’s still two and a half times colder than previously achieved in space. Reaching such temperatures in space would pave the way for a new generation of even more sensitive instruments, says Miller.

His project involves adapting dilution refrigeration, which uses an endothermic reaction between superfluid helium-4 and helium-3. Mixing the two isotopes causes them to cool and has been used in laboratories since the 1970s to reach temperatures below 20 millikelvin.

Currently, standard dilution refrigeration won’t work in microgravity, but Miller is currently testing two improvements: a more compact pump with no moving parts, and a means of separating the two isotopes after the mixing that relies on surface tension, not gravity. He expects to have a working prototype around 2013. “If we can get the detectors to be more sensitive we can understand more about how the universe works and may answer some important questions about why the universe does not appear to be working as everyone thought it should,” he says. UW crest

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