Australian Cosmic Ray Modulation Research
M. L. Duldig
, PASA, 18 (1), in press.

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Next Section: Recent Instrumentation
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Subsections

• Surface Muon Telescopes
• Underground Muon Telescopes
• The Neutron Monitor Network
• Liaweenee Air Shower Experiment

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Establishing the Australian Network of Observatories

Surface Muon Telescopes

In parallel with the development of the East-West experiment in Hobart the Physics Department at Melbourne University also began research in cosmic rays. The research team included several notable names including David Caro, Fred Jacka, John Prescott and Phil Law. A Geiger Müller four tray telescope and an ionization chamber were developed. This equipment made observations from Melbourne. Three further sets of equipment were constructed for deployment when the newly formed Australian National Antarctic Research Expeditions (ANARE) bases were opened. In the summer of 1947-48 one set of equipment was sent to each of Heard and Macquarie Islands and the final set was operated from the HMAS ``Wyatt Earp''. The testing and operation of the equipment and the ``Wyatt Earp'' voyage is described in Law (2000). The results from the voyage were published the same year as the expedition (Caro et al. 1948). In 1949 the equipment was returned from the islands for overhaul and maintenance but in 1950 the Melbourne group decided to discontinue cosmic ray research, putting its efforts into nuclear physics instead. Phil Law invited the Physics Department at the University of Tasmania to take over the ANARE work and Geoff Fenton was put in charge. Early in 1949 the Hobart group was already building an East-West telescope similar to the one above for deployment to Macquarie Island. This experiment was established on the island in 1950 and operated alongside the Melbourne University experiment. A replacement telescope for the Melbourne University instrument was constructed at Hobart during 1951 and deployed the following summer. It continued operating until 30 March 1959 when fire destroyed the cosmic ray laboratory at Macquarie Island. Perhaps the most significant result from the instrument was the recording of the giant flare-induced Ground Level Enhancement (GLE) observed worldwide in February 1956 (Fenton et al. 1956). The East-West telescope had been returned to Hobart at the end of 1951. A description of the Macquarie Island experiments and operations can be found in K.B. Fenton (2000). N.R. (Nod) Parsons was appointed as the Australian Antarctic Division's officer in Hobart for the cosmic ray program following the changeover in responsibility for the research from Melbourne University to the University of Tasmania. Nod had been involved with the programs at both universities and had ``wintered'' at Macquarie Island with K.B. (Peter) Fenton in 1950. The Mawson station was established in 1954 on the Antarctic Continent and a cosmic ray laboratory was added the following year. This housed a vertical telescope and an inclined telescope that could be set to any desired zenith angle and automatically rotated each hour to the next azimuth of a preset series. Nod Parsons was responsible for the installation and commissioning of the equipment and handed over a fine facility to R.M. (Bob) Jacklyn at the end of the year for the International Geophysical Year. Bob would later take over as head of the Australian Antarctic Division research program. These telescopes and various upgraded replacements continued operating at Mawson until 1972 when a new laboratory was constructed at the station that incorporated underground observations. During August 1953 a vertical telescope was installed at the University campus in Hobart. A new cosmic ray observatory was constructed on the campus in 1975 and a new vertical telescope was run in parallel with the old one for some time. Continuous recording has continued to the present giving almost 50 years of data. Ken McCracken installed a small 60 cm square muon telescope at Lae at the same time as the neutron monitor was installed (see Section 3.3 below). In 1968 a new telescope system was added to the Mawson cosmic ray observatory. This consisted of two units viewing north and south at a zenith angle of 76^o giving an effective atmospheric absorber depth of 40 metres water equivalent (mwe). This experiment would complement observations from the Cambridge underground telescopes (see Section 3.2 below) and results from the observations supported the case for an underground observatory at Mawson. A small vertical telescope was also operated at Macquarie Island in 1969. The new observatory at Mawson was constructed during 1971 and early 1972 as described below. The surface telescopes comprised three north and south high zenith angle crossed systems using coincidences between vertical walls of Geiger Müller counters to view at the same zenith angle as the inclined system in the old observatory. The south pointing telescope viewed across the geographic pole into the opposite temporal hemisphere as well as perpendicular to the local geomagnetic field. The result of such a view was to spread the rigidity dependent responses in time due to geomagnetic deflection (see Section 6.1.1 below) of the incoming particles. The northern view reached equatorial latitudes which, in partnership with the underground system, gave complete southern hemisphere coverage from a single observing site (Jacklyn et al. 1975). These surface counters were replaced by larger area multi-zenith angle proportional counter systems during 1986 and 1987 (Jacklyn & Duldig 1987; Duldig 1990).

Underground Muon Telescopes

One of the most important early developments was the decision by Geoff Fenton to operate underground telescopes in a disused railway tunnel at Cambridge near Hobart. The depth was shallow enough that the counting rate was still sufficiently high for useful studies and, perhaps more fortuitously, not so shallow that changes in atmospheric structure would have complicating effects on the telescope response. This latter feature was not known at the time. At the selected depth studies of both solar and sidereal variations and their energy dependencies could be investigated. The instruments, based on those already put into operation at Mawson by Nod Parsons, commenced observations on 19 July 1957 (Fenton et al. 1961). Planning for further underground telescopes was well underway in the early 1970's. A deep underground system was installed in the Poatina power station in central Tasmania late in 1971. The depth of 357 mwe meant that the observations should be at energies beyond the influence of solar modulation but the count rate was also low and several years of observation would be required to obtain significant results for the sidereal anisotropy (Fenton & Fenton 1972, 1975; Humble et al. 1985; Jacklyn 1986). Also during 1971 construction of the new Mawson surface/underground observatory was commenced (Jacklyn 2000). Bob Jacklyn, who had assumed leadership of the Australian Antarctic Division cosmic ray program, optimized the available telescope viewing directions to take advantage of both the geographic polar location and the position of the Mawson station relative to the magnetic pole. Telescopes were designed and constructed by Attila Vrana to put these plans into place. An 11 m vertical shaft was blasted into the granitic rock and two chambers were excavated at the bottom of the shaft. A surface laboratory was then constructed over the shaft. One underground chamber housed five cosmic ray telescopes. The remaining chamber was used for seismic observations. Three telescopes viewed north at a zenith angle of 24^o. This is aligned to the local magnetic field and the response is thus unaffected by any geomagnetic deflection of the arriving cosmic ray particles. Two smaller telescopes viewed south-west at 40^o zenith angle. After accounting for geomagnetic bending (see Section 6.1.1 below) the view of these telescopes is effectively along the Earth's rotation axis. They are therefore insensitive to the daily rotation of the Earth, viewing a fixed region over the south pole. Changes in their response do not arise from scanning an anisotropy but rather from isotropic variations in the cosmic ray density (i.e. changes in the total number of cosmic rays) near the Earth. Both the north and south-west telescope systems were subsequently upgraded to proportional counter systems in the early 1980's (Jacklyn & Duldig 1983).

The Neutron Monitor Network

During 1955 Ken McCracken began construction of Hobart's first neutron monitor as part of his PhD studies (McCracken 2000). This monitor followed the Chicago design developed by John Simpson (Simpson et al. 1953) that was later adopted as the standard neutron monitor for the International Geophysical Year (IGY). The counters thus became known as IGY counters and installations of this type are described by the number of counters followed by the mnemonic (eg 12 IGY). Because the count rate of neutron monitors increases rapidly with altitude the new instrument was sited at ``The Springs'' on Mt Wellington, 700 m above sea level. At the time this was the highest point on the mountain with good road access and electrical power. The counters employed BF<sub>3</sub> gas enriched in B<sup>10</sup>. Cosmic ray neutrons interact with lead surrounding the counters producing additional neutrons. These neutrons were further thermalized when passing through an inner paraffin moderator so that the cross section for neutron capture by the Boron was optimal. Paraffin also surrounded the lead to act as a partial ``reflector'' redirecting some of the scattered neutrons back toward the counters. The neutron capture by Boron produced an -particle and a Lithium ion which were then detected by the proportional counter as a pulse, amplified and counted.

For an extensive review of neutron monitor design see Hatton (1971). The monitor was installed at ``The Springs'' in July 1956, unfortunately after the giant GLE of February. As part of the IGY several other monitors were also being constructed at this time. One was sent to Mawson and installed in early 1957 and another was installed at Lae, New Guinea in April of the same year (McCracken 2000). IGY counters were added to the network at Brisbane, Casey, Darwin, Wilkes and on the University campus in Hobart (Table 1). The Wilkes monitor was moved to Casey station with the rest of the Australian Antarctic operations in that region in 1969. The Mt Wellington monitor was destroyed by the major bushfires of 1967 around Hobart. An improved monitor design (Carmichael 1964), known as NM-64 or IQSY (International Year of the Quiet Sun) monitors, led to the eventual replacement of most IGY monitors worldwide. Installations of this type are also described by the number of counters followed by the mnemonic (eg 18 NM-64). The Darwin monitor was constructed using the new design in 1967 and the Mt Wellington monitor was similarly rebuilt in 1970. Subsequently, Brisbane, Hobart and Mawson all upgraded to IQSY monitors. For a complete worldwide history of neutron monitor development, installation and use readers should refer to the special issue of Space Science Reviews published recently (Bieber et al. 2000).

Liaweenee Air Shower Experiment

In the early 1980's it was becoming clear that the sidereal daily variation at energies of 10¹²--10¹⁴ eV had an amplitude of about 0.05% as measured by deep underground and small air shower experiments in the northern hemisphere. The only measurements in the southern hemisphere were from the Poatina power station telescopes at the bottom end of this energy window (Fenton & Fenton 1975; Humble et al. 1985; Jacklyn 1986). A new air shower experiment was therefore installed in the central plateau region of Tasmania at Liawenee (Fenton et al. 1981, 1982; Murakami et al. 1984). This experiment showed that the southern hemisphere sidereal response was much smaller than the northern hemisphere at 0.02% (Fenton et al. 1990) which was to have important implications for understanding the structure of sidereal anisotropies (see Section 7 below).

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Next Section: Recent Instrumentation
Title/Abstract Page: Australian Cosmic Ray Modulation
Previous Section: The Early Years
Contents Page: Volume 18, Number 1

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© Copyright Astronomical Society of Australia 1997