Thursday, August 21, 2008

The PICASSO dark matter detector, part 1

Some time ago I promised to write something about the PICASSO detector. The following text is based on a talk I gave to some summer students a couple of months ago. If something is not clear, please ask! That's how I learn to explain things.

I assume that you already know something about dark matter, and I will
not go into any details about it this time. (If you have no idea you
can start here
(the less-than-a-minute explanation), or here (explanation for sf writers, with some historical background and a little about detection) or on Wikipedia.)

The most popular hypothesis is that the dark matter comes in the form of weakly interacting massive particles, WIMP, filling our galaxy and passing right through most matter completely unhindered -- just like neutrinos, if you are familiar with them. These WIMPs are what we are trying to discover.

To detect something we need an interaction. When we see things in our everyday life it's through photons interacting in our eyes. To see subatomic particles we need an effect of some kind that can be amplified to human proportions so that we can actually take a look at it. The detectors that are our artificial eyes in the particle world are constructed in different ways to capture the interactions of the particles thay are made to detect.

Since the WIMPs have no electric charge they are completely blind to electromagnetic fields. They interact only through the weak force, which is notoriously short-sighted. This means that an atom has a very small cross section for a WIMP. While an atom looks like a huge cloud to a poor beta particle (an energetic electron), it will look like a tiny dot at the center to a WIMP. Remember that atoms are basically empty, made up mainly of the electric fields of the charged protons
and electrons.

This property can actually be exploited when you try to discover these particles. The question is: how can you tell what particle it was that interacted in your detector? Well, a heavy neutral particle will scatter by hitting an atomic nucleus. In this process it can give a good kick to the nucleus, which will then be thrown out of its place. An atomic nucleus on the run, that's a huge charged particle, which will interact with all of the electrons around it and kick many of them out of their orbits -- it's strongly ionizing, stopping in a short distance. In a suitable detector this will give a very different signal from beta rays or other light particles, which interact directly with the electrons and which lose their energy less efficiently. It's also very different from gamma rays. Dark matter detectors take advantage of this.

Superheated liquids for detection

To understand how we do this with PICASSO we need to take a step back and remember some thermodynamics and the physics of boiling. You probably learned in school that the boiling point is where the vapor pressure needs to be higher than the ambient pressure, so that bubbles that form can continue to grow and not be crushed back to liquid again.

Now, I'm not sure your high school teacher told you (but maybe the professor teaching thermodynamics did?) that there is one more thing that is needed: you need to overcome the little potential wall that comes from the surface tension of the little proto-bubble which also tends to crush it and prevent it from growing, so you need some kind of seed to allow the phase transition to start.

When you boil water on the stove you see bubbles forming on the bottom of your pot, usually in a few places, and rising to the surface. The steam bubbles start forming at little holes or cracks or impurities in the vessel. The water needs some help to start the formation of bubbles when it reaches the boiling point. This is called nucleation.

If you can prevent nucleation from happening, you can heat a liquid much above the boiling point. It will just get hotter and hotter, because there is nothing to start the phase transition. The liquid is then superheated, and if you then introduce something that helps nucleation to happen you will get explosive boiling when the transition go gas spreads to the whole volume.

You can actually superheat water in a microwave oven, as you might have heard. Especially if the water is very pure, and the cup is very smooth -- like a new glass jar. Some people have had terrible accidents, resulting in severe burns on the arms and face. This is why it's a good idea to put the teabag or the instant coffee powder in the cup before heating it, or even leave a spoon in the cup. You can look up "superheated water" on YouTube, and you will se several experiments people have done with this. Be careful!

This effect is the principle behind the bubble chamber detectors that were widely used some decades ago, and it's what we use in PICASSO (and some other detector efforts) to detect recoiling nuclei from WIMP interactions. A recoiling nucleus will interact a lot in a short distance, and deposit the energy it got from the WIMP in the hit in the form of heat. If this little heat spike contains enough energy within short enough distance, it will be the seed needed for a vapor bubble to form and continue to grow.

In PICASSO we don't use superheated water, but instead our liquid is a freon, a fluorocarbon (the kind of molecule used in refrigerators). This is nicer to work with, because it's superheated at room temperature -- no need for extreme temperatures. We have little droplets of this liquid sitting suspended in a gel. You can see all of these little droplets as independent detectors if you like.

When one of these bubbles boil, it will expand suddenly to many times the original size. This will of course induce a pressure wave in the gel, a little sound wave, which can be picked up with piezoelectric sensors.

More to follow...

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