Cosmic Ray MuonsPHYS 1301 F99Prof. T.E. CoanVersion: 17 Oct ‘99IntroductionYou may find it amusing that, every second, you are bombarded many hundredsof times by an ephemeral elementary particle. The particles pass through you, painlessly,at speeds close to that of light. These ``muons,’’ are similar to the familiar electron that isthe basis of electricity. Both particles have the same electric charge but a muon is about210 times more massive than an electron. Intuitively, you can think of a muon as a heavyelectron, although there are important differences between the two so you shouldn’t thinkof a muon as exactly like a heavy electron. For instance, a muon spontaneously decaysinto electron and a neutrino while an electron doesn’t decay at all. The muons are the end result of a chain of processes. The muons are a portion ofthe decay products of a non-elementary particle composed of a quark and an anti-quark.The muon’s parent particle, a “pion,” lives only a short time, about 26 nanoseconds, andis itself produced by collisions between protons of extraterrestrial origin and protons inthe Earth’s atmosphere. The extraterrestrial protons, called “cosmic rays,” are acceleratedto speeds very close to that of light by a mechanism not yet fully understood and thencollide with essentially stationary atmospheric protons, producing several pions in theprocess. So, the picture we have is, a very fast proton from outside the earth strikes astationary proton in the atmosphere and produces several pions that travel downwardtoward the Earth’s surface. Each of these pions then quickly decays into a muon (and aneutrino). The muons then arrive at the Earth’s surface. Because the original extraterrestrial proton is very fast and much more massivethan either a pion or a muon, both pions and muons move with speeds close to that oflight. You might suspect then that special relativity will be somehow important. You arecorrect. The high speeds of the muons will cause their lifetimes to be lengthened or“dilated.” A muon’s lifetime observed by us on earth is much longer than the lifetime ofsomeone riding along atop the muon. Were this not true, we would observe much fewermuons than we actually do. How do we detect the passage of muons since we have lived a happy life so farwithout even suspecting that we are being pierced every second by hundreds of thesediscreet muons? Your previous laboratory exercise with a simulation of the CLEOexperiment provided you some experience identifying particles by examining theirinteraction with matter. If you recall, you examined how muons, electrons and photonsbehaved in matter and what clues they left about their identity. We will do somethingsimilar here except we won’t use a simulation. We will observe the effects of the passageof muons through matter directly!Actually, when I say we will “directly” observe the muons I am exaggerating abit, but only a bit. When the muons pass through matter, they ionize the atoms andmolecules that comprise the matter and produce free electrons. These electrons are quiteenergetic and can travel some centimeters before they are captured again. It is the motionof these electrons that we will observe. And how will we do this since electrons have noknown size and are therefore a bit difficult to see with our own eyes? We will need to usea particle detector that will respond to the electrons’ motions. This detector is an airtight,transparent box filled with an alcohol vapor. The bottom of the box is chilled to thetemperature of “dry ice” (frozen carbon dioxide) while the top of the box is not chilled atall. The result of this arrangement is that the chilled alcohol vapor will condense aroundthe trail of a charged particle that passes through it. The condensation will look like athin, wispy white thread when light is shined onto the vapor. The condensation traces thepath of the ionized electrons as they travel away from the point of ionization. In essence,the electrons will produce small alcohol cloud chambers. Hence the detector is called a“cloud chamber.”ProcedureVerify that you can actually see the thread-like clouds of condensation called“tracks.” You should stand on the side of the chamber opposite the lamp and your headshould be at about 45 degrees to an imaginary line drawn from the lamp to the chamber.Look down at the bottom of the chamber since the sensitive volume of the chamber is thechilled portion.Q1. Sketch the shapes of some representative tracks.Q2. What is the length, in centimeters, of the longest tracks that you see? Q3. Are most of the tracks horizontal, vertical, or roughly half-and-half?Q4. About how many tracks per second do you see? Use your head to estimate thisnumber. For example, look at only a fraction of the chamber, count for some amount oftime, and then enlarge your answer sensibly to account for the fact that you were onlyexamining a fraction of the chamber.Q5. Introduce the “alpha” source into the chamber. The instructor will show you how.Alpha particles are helium nuclei and are produced by the decay of some of the atoms inthe wire tip. What do these tracks look like? Sketch them. Pay attention to the width andlength of the tracks. How do alpha tracks differ in shape from electron tracks?Q6. Remove the alpha source. Now look for tracks that have shapes like alpha particles.How often do you see them (e.g., 1 per minute, 2 per day, etc.)? The alpha particles comefrom the decay of radon gas atoms in the air and radon itself comes from the decay ofradium atoms that occur naturally in the earth’s soil.Muons from the CosmosPHYS 1301 Fall ‘99Prof. T.E. CoanName: ________________________ Section: