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Teaching With the Muon Detector
The Muon Detector responds to significant cosmic ray events resulting from solar processes. These are mainly "Forbush" events in which there is a sharp reduction in cosmic ray intensity followed by a gradual return to earlier levels over a period of a few days. Such events can either be spotted by eye from the muon data or by the use of lists of such events found through the links. Students can search for such effects. Forbush events are more easily recognised if the significant effect of variations in atmospheric pressure is first removed. The figure shows the correlation between count rate and pressure for days 14 Dec to 19 Dec. There is a clear inverse relationship between pressure and the resulting count rate. We use an approximation to this of -0.2% count rate change per millibar for correcting our muon display. Students might check this relationship for themselves and find how it relates to the muon energy spectrum and energy loss in the atmosphere. This dependence is much weaker than the corresponding dependence for cosmic ray showers (about 0.8% mb-1) which is due to the attenuation of the numbers of cascade particles with atmospheric depth. That is close to exponential with an attenuation length of about 200 g cm-2 (about 0.2 of an atmosphere). The process of muon modulation is clearly different. Cosmic rays have a small (solar) daily (diurnal) variation. Students can look for this and find the relationship of the position of its greatest intensity with respect to the position of the sun (it should be roughly 90 degrees from the Solar direction). This can be accomplished by finding the 24 hour component of the Fourier series for the data using the usual Fourier series formula but for the first "harmonic" only. Group members have written a book about cosmic rays, at a popular level, entitled "Cosmic Bullets" by Roger Clay and Bruce Dawson (Australia and U.K.: Allen and Unwin, Sydney, 1997; U.S.A.: Helix Books, Addison Wesley (Frontiers of Science), Reading Mass, 1997). Available from Amazon.com
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