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High energy particle physics is one of the coolest and most mysterious branches of physics around. This experiment provides a taster on what it is like to work with particles studied in this field, using the highest energy sources for exotic particles around - cosmic rays. This experiment mashes up concepts from relativity, particle physics, and electrodynamics, touching on physics problems both solved and yet to be understood. it is a wonderful way to build up your skills in creating a detailed picture from simple components and information.
Before you get into this experiment, you are going to want to familiarise yourself with the particles of interest - muons. For beginning your journey, it helps to think of these exotic particles as electrons with extra mass tacked on. This is reflected in its decay chain, muons decay into electrons or positrons and neutrinos. It is this property that will enable their measurement, keep that in mind when developing a method for this experiment. Lots of muon experiments exist which describe both their properties in the context of collider physics, and cosmic origins, including how cosmic rays decay to produce muons and the energy expected from their production. The book Introduction to Elementary Particles by David Jeffrey Griffiths and the paper Determining the muon mass using a scintillator-based detector by Neal Woo and John Essick are some good starting points.
Hopefully now you know a bit about the properties of what we are hunting, and you are starting to think about how we can use these properties to our advantage. These next couple of sections will introduce the tools that are provided as a part of this experiment. It should be clear how everything comes together; I will provide some hints to the background knowledge that will be useful in compiling your lab report.
At the experiment station if things have not changed much, you should see a big black box. Now, if everything is turned off, I think you should have a peek. Oh look, it is a big hunk of plastic wrapped in foil! Neat! On closer inspection, you may notice this seemingly ordinary plastic cylinder has an eerie blue glow to it. That is because it has made from scintillating material. What does that mean? It is structured in such a way that it gets excited by ionizing radiation. Charged particles on impact with the scintillator excite electrons in the material, causing fluorescence when the electron relaxes back to its ground state, emitting a photon corresponding to the energy it needs to lose. This is important! Consider the decay chain of a muon, which itself is negatively charged. How many times to you expect the process of a muon passing through a scintillator to cause fluorescence? Furthermore, is there anything about the timing of these emissions that can inform you on the properties of a muon?
We need a way to get usable data out of the flashes of light in the scintillator. Photomultipliers are a perfect fit for the job. When you looked inside the black box you may have noticed a metal cylinder sitting on top of the scintillator. This is the photomultiplier, a Hamamatsu H3178-51. The reason the scintillator is wrapped in foil is to maximise the amount of light that reaches this detector, there's obviously room for improvement here, note the exposed top where light is neither reflected nor passing through the cylinder, and the roughness of the foil.
Photomultiplier tubes work via the photoelectric effect in a vacuum tube producing electrons with energy levels corresponding to the impacting photons, with a series of dynodes exponentially increasing the number of electrons, creating a detectable current. This pulse of current needs to be transformed into a safe form for electronics without losing temporal information. We do this with a Hamamatsu C6438-02 amplifier, which takes the signal from the photomultiplier tube as input and provides a 3.3V CMOS safe output. It works on a bipolar power supply, needing a positive and negative current of 5V as well as grounding. As of writing there are two lab power supplies provided to hook up to the amplifier, one to provide the positive voltage and one to provide the negative. Before you plug everything in draw a circuit diagram to ensure that power is supplied correctly!
This experiment as it exists is interesting, however there is plenty of room to do something different and exciting. Dive in deeper with some of these extra tasks.