Hold out your hand palm up to the sky for a second.
In that second one or two muons passed through it along with something like a trillion neutrinos. Measuring the neutrinos is a non-trivial task, but muons can be measured by the amateur.
Most of these muons come from cosmic rays that are produced outside the solar system. Cosmic rays are highly energetic particles - mostly protons (about 90%) and alpha particles (about 10%) with a small percentage being the nuclei of heavier atoms. These particles can have amazing amounts of energy - some have been measured with energies on the order of a fastball in professional baseball.
When they encounter the Earth's upper atmosphere reactions occur that produce showers of short lived particles called pions (a good first year physics text or a popular book on particle physics will discuss the basic particles and their interactions). Most of the pions decay into muons and neutrinos which rain down on the Earth's surface.
The muons have a fairly long half life (2.2 microseconds) and are traveling close to the speed of light;. These two properties conspire to make many of them available at sea level, but measurable more at higher altitudes. Sorting all of this out was the birth of particle physics back in the 1920s. It also gives the experimenter a gut level feeling for special relativity.
There are many ways to detect these fast muons. Many of you have seen cloud chambers and geiger counters. These detect charged particles - the problem is many of them aren't cosmic ray produced muons. The trick is to build a telescope.
I've built several simple cosmic ray telescopes over the years. Most of these consist of two devices that create a signal when a muon passes through them. If you stack them and demand that both counters produce a signal, you can work out the direction the muon traveled. Detectors with large areas will collect more muons and increasing the distance between the detectors will look at a smaller area of the sky.
This happens to be an interesting area of study for students and amateurs as it is exotic and relatively simple at the same time. One is dealing with relativistic particles over table top scale distances using a readily available cosmic accelerator. These particles are energetic enough to provide a strong signal in simple apparatus. Finally the flux of these events is high enough to be accessible for the amateur (small detectors are possible), but rare enough (a few times a second at most) that the supporting logic is simple.
If you want to do this on a small budget and learn quite a bit at the same time, I recommend making contact with an experimental particle physics group at a local university. It turns out many physicists are thrilled to share their passion and more than a few are delighted to work with students and amateurs. They also have access to machine shops and materials that are hard to find. Most of these guys can walk you back into their parts bin, pull out a few counters, a HV power supply and a logic crate and demonstrate a telescope in 10 minutes. They can give you important design tips and may have some obsolete materials you can use.
In the near future I'll write about some simple designs that use plastic scintillators coupled with photomultiplier tubes (the stuff most physics departments can help you build), geiger müller based devices and a simple/inexpensive drift tube detector design. You will be able to conduct a variety of interesting experiments with this sort of apparatus and, once you move on, it would be a wonderful gift to a clueful high school physics teacher.
Speaking of high schools. There have been several projects that use arrays of detectors spread over large geographic areas. The idea was to note correlated muon strikes across across large areas. By measuring the areas involved one would be able to calculate the energy of the primary cosmic ray and measuring the energy distribution is real science. A few proposals were made to do these in groups of high schools - one of the largest proposals would involve many high schools throughout Nebraska. Most of these efforts became bogged down for lack of funding (although they weren't terribly expensive). I was involved in the early stages of one proposal ten years ago. I was left with a bitter taste about the status and direction of science education in this country, but that is a story that many of you know.
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