Detectors. What?

Okay, so I've talked briefly about what I'm working on (LUX dark matter direct detection experiment, if you weren't listening), and I've talked at relative length about dark matter. However, I haven't really described the actual detection process. This is, as you can imagine, rather important. Detectors don't just tell you 'yes, I found what you're looking for!', or 'no, nothing, check back later.' Instead, detectors are extremely complicated devices with extremely complicated signals to interpret. Later I'll talk about the latter (signals), here I'll discuss the former (detector itself).

The art of detection (okay, science), is really a complex collection of different steps. Let's take an example that you're probably familiar with (unless you're reading this in braille): vision. You're eyes are your light detectors. They convert photons of light that enter the eye into electrical signals that your brain (DAQ) interprets. Now, with dark matter, you can't just look at it. Mostly because dark matter is dark and so is invisible to light. But it does interact with the nuclear force (bumps into an atom's nucleus). So when you want to detect something, you have to find a way to interact with it. For LUX, we use a big bucket of liquid xenon, which is a massive nucleus that provides a large target for the dark matter to interact with. When dark matter bumps into a xenon atom it creates a flash of light (the light escapes as the interaction energy). Now, light is something that we can detect. However, the flash of light is so small that we can't see it with our eyes. So now what? We need a quantitative way to detect small amounts of light. With experiments our goal is to turn some interaction evidence (in this case, the light) into an electrical signal that we can record with the DAQ computer. This can then be analyzed. But first, to the problem at hand: turning the light, into an electrical signal. There are several ways that we do this in the physics world. The predominant method is to use something called a photomultiplier tube (PMT). This device has a special piece of metal that, when hit with a photon of light, ejects an electron (this is like billiards, the photon has a lot of energy in it, and when it bumps into the electron, it knocks it right out of the metal, known as a photocathode). This electron is accelerated with a strong electric field into another piece of metal (called a dynode), knocking out more electrons, and so on with several stages of dynodes, each ejecting more electrons. By the time this is all over, one photon that converts into a single electron on one end, becomes over a million electrons on the other end! This signal is easily recorded by a computer. In LUX, we have 120 of these PMTs (60 on the top of the bucket, and 60 on the bottom) so that we catch all of the light that comes out when a dark matter particle (WIMP) interacts with a xenon atom. So in the next issue I'll talk about how we interpret and analyze these 120 signals. Eventually I'll also discuss an alternative to PMTs that I have been working on, known as an Avalanche Photodiode (APD). So now at least, you know a bit about what 'detectors' are and how they work (sort of).