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HEAMS Muon Detector

Image of Muon Dectector

A Muon Detector, with the light-tight box on the left and the computer and high-voltage power supply on the right.

The University of Adelaide operates a one-square-metre muon detector on the Adelaide Campus, which records the number of detected cosmic ray muons every 15 minutes.

NOTE: the campus muon detector is temporarily out of commission; it is hoped that the system will be back online within some weeks (as of 11/1/17). The Buckland Park system remains operational, and is collecting data.

The detector was originally designed to provide data for undergraduate teaching purposes including student project work. There is a larger system 40 km north of Adelaide at Buckland Park. This is made up of eight one-square-metre scintillator muon detectors, with four detectors being in the form of a square above the other four detectors.

Using the two vertically-displaced detector planes, and by taking coincidences between vertically-placed detectors, or diagonally spaced detectors, nine directional 'beams' can be made and we measure the muon rate for each of those. In addition, we record the total count rates, the second-by-second total rate, and the rates of small cosmic ray showers which trigger more than two detectors 'in coincidence'.

The Adelaide and Buckland Park systems make up HEAMS (High-Energy Astrophysics Muon System).

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    The original Adelaide system was intended for teaching undergraduate students about heliospheric processes, including CME's and associated Forbush decreases. The Buckland Park system was then built out of scintillators and electronics previously used for the Buckland Park air shower array. Its construction and commissioning was originally done by Roger Clay and Neville Wild, with Abdullrahman Maghrabi working on it for his Master's project work in the late 1990's. Abdullrahman's continuing interest later led to some financial support from his home institution for the HEAMS upgrade.

    The original Adelaide system showed some progressive drifting to lower sensitivity over a period of about a decade. This was pointed out to us by colleagues at Moscow, and reported by them at the Rio International Cosmic Ray Conference. This drift was believed to be due to a progressive deterioration of one of the four scintillator slabs in use within one of the two Adelaide detectors. The interest of our international colleagues encouraged us to upgrade to HEAMS with the use of an FPGA to add many more monitored channels at Buckland Park. It is hoped that these data will complement scaler data from the Pierre Auger Observatory, which is located in Argentina at the same latitude and about 206 degrees shifted in longitude.

    The Adelaide detector is below three concrete floors in a building, which means that it has a slightly higher energy threshold than the Buckland Park system for vertically arriving muons. The diagonal beams for Buckland Park arrive at zenith angles of 60 degrees or more, and they have even higher thresholds due to the extra atmosphere which they must pass through.

    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 reducing from the most recent maximum in 2013.

    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.

    The data  shows a strong pressure dependence but, when a correction is made for that, variations relate to solar (heliosphere) effects.

    Recent Data

    Below is a plot of the data from the last 30 days. The lines represents the count recorded every 15 minutes for the Adelaide and Buckland Park components of HEAMS, corrected for atmospheric pressure assuming a linear relationship of about -0.2% count rate for each millibar increase in pressure.

Cosmic Rays are high energy particles which reach us from outer space.

Most of the cosmic rays which reach the vicinity of the Earth are atomic nuclei although there are some high energy electrons. All types of nuclei from Hydrogen through Uranium seem to be present in cosmic rays although their distribution is not quite the same as for matter as a whole in the Universe. For instance, there is more Lithium, Beryllium and Boron than in the "Universal Abundance".

The highest energy cosmic rays have energies of many joules (see the Cosmic-ray Research page   ). The lowest energy ones we can detect at Earth have energies of about 1 GeV because lower energy particles cannot reach us through the outflowing solar wind. This also means that as the solar wind changes over a solar 11 year cycle, or as the sun exhibits short term activity, the measured number of low energy cosmic rays is useful as a tool for studying the "weather" in the solar system.

The cosmic rays with energies below about a million GeV are probably accelerated within our galaxy in supernova remnants or at the Galactic Centre. They probably take 1-10 million years to reach us. At much higher energies, the cosmic rays probably mainly come from unknown extragalactic sources.

The muon detector consists of one square metre of plastic scintillator viewed by a photomultiplier tube. The muon produces a flash of light in the scintillator and the photomultiplier tube (a sophisticated photo-electric cell) produces an electronic signal which is proportional in amplitude to the amount of light. The output signal from the tube goes to a piece of electronic circuitry (a discriminator) which senses whether the signal is the right size to be due to a muon passing through the detector. If this is so, the muon is counted. The total count for each 15 minute interval (about 100,000) is recorded in a data file and also displayed using LabView as an hourly average.

Also recorded is the atmospheric pressure (displayed) and the laboratory (inside) temperature.

The level at which the discriminator is set is determined by finding the "single particle peak" using a multichannel analyser and setting the level of the discriminator in the trough below that signal level. Since almost all muons produce a signal close to the single particle peak, this ensures efficient data collection.

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. 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.

The Buckland Park Extensive Air Shower Array was located approximately 40 kms north of Adelaide.

It began operation in the early 1970s, overseen by Dr. Roger Clay. It consisted of approximately 40 scintillation particle detectors which detect cosmic ray showers with energies above 1014 eV. It also was used to search for U.H.E. gamma-rays from the active galaxy Centaurus A and the galactic centre.

A second array, the South East array, was operated at Buckland Park from 1994-98. This array was built for student use and to search for any possible gamma-ray emission from Centaurus A, such as had been detected by the original Buckland Park array.

High-Energy Astrophysics
Please direct any enquiries to:

School of Physical Sciences
The University of Adelaide
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AUSTRALIA

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physicalsciences@adelaide.edu.au