Question: What do you put in the helium dilution refrigerator, or does nothing in-particular go inside it? And how do you use the super-cold temperatures?

Matthew Malek answered on 11 Mar 2014:

We put helium in it, of course!

Okay, that was a joke. You meant what experiments go inside the fridge, not what is the coolant. 😀

I worked with a helium dilution refrigerator when I was searching for dark matter. Although we know that over 85% of the matter in the universe is this mysterious dark matter, we don’t actually know what it is. Thus, when searching for it, we need to make “guesses” about its properties, based on theory.

The type of dark matter that I was searching for is something called a Weakly Interacting Massive Particle (or WIMP). WIMPs, if they exist, almost never interact with matter. They interact more than neutrinos, but that’s not saying much. When they do interact, they bump the nucleus of an atom and give it a little bit of energy, heating it up a tiny amount.

Our dark matter detectors are crystals, where each nucleus is a potential target for a dark matter interaction. However, since the heating from a dark matter collision is so very miniscule, we have to make our detectors extremely cold — just 0.005 degrees above absolute zero, the coldest possible temperature! When they are that cold, our thermometers can detect the tiny temperature rise if dark matter bumps a nucleus in the crystals. Much warmer than that, even one degree above absolute zero, and we would never be able to see the difference!

Pretty cool, huh? (Sorry — awful pun!)