Question: What equipment do you use to detect the particles/s

Susan Cartwright answered on 5 May 2020: last edited 5 May 2020 9:50 am

There are lots of ways to detect particles. I will tell you a bit about the experiment I work on, which is called T2K.

T2K consists of two detectors 295 km apart, because we want to see what happens to neutrinos when they travel over long distances. (Note that we never detect the neutrinos themselves, but only the particles that are produced when they interact with the material of our detector – which unfortunately they very rarely do. Most of the neutrinos that go through our detector we never actually see!)

The far detector, Super-Kamiokande, is the simplest one: it is simply a very large tank of water (VERY large – 50000 tons of water). When light travels through water, it is slowed down by the refractive index of the water, so light travels through water at about 3/4 times its speed in space (i.e. about 225000 km/s instead of 300000 km/s). But the particles are not slowed down, so a particle travelling through water can be going faster than the speed of liight in water (though not faster than the speed of light in a vacuum). The effect of this is very like the effect of an aircraft flying faster than the speed of sound in air – you get a “sonic boom”. Only in this case, since it’s light and not sound, it is an “optical boom” – the particle (if it has electric charge) emits a cone of blue light called Cherenkov radiation (after a Czech physicist called Pavel Cherenkov). We have lined the edge of the tank with very sensitive light detectors called photomultiplier tubes. These detect the blue light and allow us to know which direction the cone was pointing in (gives direction of particle) and how much light there was (gives energy).

The near detector is more complicated, and consists of several different types of detector. Much of it is made of a special material called plastic scintillator. When a charged particle passes through matter, it will knock electrons off atoms and/or disturb the material’s molecular structure. Scintillators react to this disturbance by emitting a short burst of light (the colour depends on the scintillator). Again, this is detected by sensitive light detectors – not photomultipliers this time, but a sort of solid-state device called an avalanche photodiode. We divide the scintillator into thin strips, and by noting which strips were hit in what order we can reconstruct the track of the particle through the detector. Another kind of detector that we use is filled with gas. When the particle travels trhough the gas, it knocks electrons off. They are negatively charged, and we make one side of the detector positively charged, so they drift towards it. When they hit that side, we amplify the tiny charge of the electron into something we can detect. The place they hit tells us two coordinates, and the time they took to get to the side tells us a third coordinate, so we can reconstruct the particle’s path in three dimensions. This detector is in a magnetic field, and the magnetic field bends the path of the particle by an amount inversely proportional to the momentum, so we measure the amount of bending and that gives us the momentum of the particle.

T2K is quite a simple detector by modern standards. ATLAS, which some of my colleagues work on, is much more complicated and has some different kinds of detector that we don’t have in T2K. But I hope this gives you some idea.

You can find out more by exploring the “accelerators and detectors” section of The Particle Adventure, https://particleadventure.org/