The University of Adelaide Muon Detector

The original University of Adelaide muon detector 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 masters project work in the late 1990s. Abdullrahman's continuing interest later led to some financial support from his home institution for the High-Energy Astrophysics Muon System (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 Field Programmable Gate Arrays (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 Department of Physics at 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 muon detector 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.

Muon Data Files 

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.

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.

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.