Cosmic Ray Muon Detector

The University of Adelaide operates a one square metre muon detector which records the number of detected cosmic ray muons every 15 minutes. The detector was designed to provide data for undergraduate teaching purposes including student project work.

Cosmic ray muons make up something like half of the natural sea-level radiation background. They are produced high in our atmosphere from the interactions of primary cosmic ray particles with atmospheric gas nuclei. The muons then lose energy as they pass through the atmosphere to reach us. Some will lose so much energy that they fail to reach us and, as a result, there is a dependence of the muon rate on the atmospheric pressure.

The primary cosmic rays reach the Earth after travelling through the solar wind. Not all of them are able to make that journey, especially when there are strong solar outbursts. As a result, the rate of detection of muons depends on the "solar weather" and, at times of solar flare activity, there may be significant changes to the muon rate known as "Forbush decreases". These are naturally more common at times of maximum solar activity which follow an eleven year cycle. Solar activity is currently building towards the next maximum, expected in the year 2000.

The muon detector is located in the Physics Department of the University of Adelaide with about 300 g cm^-2 of building material above it. Our atmosphere has a depth of about 1000 g cm^-2 so, assuming that muons lose energy by ionisation at a rate of about 2 MeV(g cm^-2)^-1, the threshold energy (at production) for the muons we detect is a rather high 2.6 GeV. To get lower energies, neutron monitors are used since neutrons do not suffer ionisation energy loss in passing through our atmosphere. The Earth's magnetic field prevents low energy charged cosmic rays from reaching the atmosphere. There is a rigidity threshold for all place on the Earth due to this. For Adelaide it is about 3 GV. By coincidence then, for protons, this is about the same value as the threshold for muons to reach the detector. A map of rigidities over the Earth can be found through the Chicago  link.

The data show a strong pressure dependence but deviations from that relate to solar effects.

Data

Below is a plot of the data from the last 30 days. The yellow line indicates atmospheric pressure, the blue line represents the count recorded every 15 minutes and divided by 100, and the green line is the count, corrected for atmospheric pressure assuming a relationship of -0.2% count rate for each millibar increase in pressure.

Other Details

Teaching with the Muon Detector.

Information on Cosmic Ray Related Undergraduate Experiments [140KB, pdf].

Details and images of the Muon Detector

Data Collected since July 2003.

Some other related links.

Physics > Research > Astrophysics > Muon Monitor

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A muon detector, with the light-tight box on the left and the computer and high-voltage power supply on the right. Click on the image to see a larger version [1152x864, 351KB, JPEG].