A study of the properties of transient changes in the cosmic ray intensity.

Three decades of research have shown that the cosmic ray intensity varies with time. Some of the variations have been shown to be due to atmospheric changes, while others are apparently produced by extra-terrestrial mechanisms. It is now known that most of the variations are small, of the order of a few percent, and that they do not correlate simply with other geophysical phenomena. In order to study them satisfactorily, recorders must be constructed such that the statistical fluctuations in the observations are considerably smaller than the cosmic ray variations themselves. It has become clear that the only worthwhile observational programme is a long term one, lasting for at least a number of years. A number of different types of cosmic ray detector have been developed, among these being the ionization chamber, the geiger counter telescope and the neutron monitor. It has been shown that comparison of neutron monitor data with those from either of the other instruments yields information on the manner in which the cosmic ray spectrum varies with time. Similar information is provided by the comparison of results obtained at high and low geomagnetic latitudes. Furthermore, observatories at different longitudes enable the directional dependence of the variations to be determined. Late in 1953 the University of Tasmania commenced observations of the vertical meson intensity at Hobart using a large geiger counter telescope. During 1954 and early 1955 the candidate, in partial fulfilment of the requirements of the Degree of Bachelor of Science with Honours, investigated and eliminated faults which had previously prevented an ionization chamber from being operated successfully. Thus by the end of 1955, the cosmic ray intensity at Hobart was being measured by two different types of recorder. In the latter half of 1955) I undertook to design and construct a neutron monitor. Boron Trifluoride counters were purchased from a commercial firm, and by June, 1956, a prototype monitor was in operation at Hobart. In order to achieve a higher counting rate, it was decided to site the monitor on the slopes of Mt. Wellington, the mountain at whose foot Hobart is situated. A hut was built and by the middle of July, 1956, the monitor had been transferred from Hobart and was in fully automatic operation.