This thesis is concerned with the measurement of the longitudinal cascade development of large cosmic-ray showers from observations of atmospheric Cerenkov radiation. The purpose of the measurements is to obtain information on the mass composition of the primary cosmic rays and on the gross features of the high-energy hadronic interactions. The characteristics of the Cerenkov light signal which are strongly coupled to the longitudinal shower development are described with reference to computer simulation results. An experiment designed to measure these characteristics was deployed in Dugway, Utah, U.S.A. between October 1977 and March 1980. Measurements were made in showers of primary energy 10(^15) – 10(^18) eV. Existing data analysis techniques have been refined and new procedures developed in order to optimise the reduction of the digital data. A detailed analysis is given of two aspects of the time structure of the Cerenkov light signal in showers of mean primary energy ~ 2 x 10(^17) eV. The measurements are interpreted in terms of the mean depth of electron cascade maximum and the fluctuations occurring between showers. The many results on the cascade development currently available from the Dugway data are summarised. Interpretation of the results is found to be possible in the framework of scaling-based models of the high energy hadronic interactions which incorporate an enhancement of the central-region multiplicity. This allows certain inferences to be made concerning the primary mass composition. Specifically, primaries of energy ~ 10(^16) eV appear to be predominantly heavy nuclei, and the mass composition becomes lighter with increasing primary energy until at least ~ 2 x 10(^17) eV. While no other single experiment has yet been in a position to corroborate these results, broad consistency is found between the Dugway results and the combined results from a number of other experiments.

The search for fluctuations in the Cosmic Microwave Background (CMB) is one of the most important topics in modern cosmology, since their detection would reveal a great many details about the early universe and, in particular, the nature of the primordial density perturbations giving rise to the galaxies and clusters of galaxies seen in the universe today. However, the search for such anisotropies is not trivial- none have been detected as yet - largely as a consequence of the fact that all radiative astrophysical sources can give rise to anisotropic foregrounds which confuse the search for genuine cosmological signals. In this thesis, a detailed study is made of the contribution of such foregrounds to searches for CMB fluctuations. After Chapter 1 summarises the theoretical and observational status of the subject, Chapters 2, 3 and 4 consider the foreground resulting from the Galaxy. It is found that frequencies in excess of 20 GHz are required to reduce the Galactic Synchrotron Radiation to a level where genuine anisotropies might be detected (the synchrotron emission is almost certainly responsible for a previously claimed detection), and that dust emission, especially if very cold dust exists which is unseen and possibly untraced by the dust responsible for the emission seen in the IRAS 100µm passband, can seriously constrain high frequency searches. In Chapter 4 a model is derived for the overall Galactic foreground including a separation of the thermal and non-thermal radio emission in the Galactic plane. Comparison with observations at 10 and 15 GHz suggests that the model is ~ 10%- 20% accurate on average at these frequencies, and maps of the predicted emission at the COBE frequencies 31.5, 53, and 90 GHz are provided. .4. search is made for a frequency window and sky region which minimises the Galactic contribution to the fluctuation measurements: highly sensitive, high resolution searches should be designed with these values in mind. Chapter 5 investigates the anisotropies resulting from discrete extragalactic sources, in particular, the far-infrared emission from dust in spiral galaxies and the integrated radio emission due to radio galaxies, QSOs etc. A frequency window of ~ 30 - 100 GHz is found to minimise both Galactic and extragalactic foreground anisotropies. Chapter 6 considers several cosmological models which could generate the recently observed large-scale structure in the galaxy- distribution: these are constrained by CMB anisotropy limits. It is concluded that genuine cosmological variations in the CMB temperature are now close to detection if the observed galaxy- structures are real. Chapter 7 indicates some future projects which should help in the detection and interpretation of CMB fluctuations.

The discovery of cosmic rays with energies greater than 1014e V has posed a question that has not, as yet, been answered: where are particles accelerated to such high energies? A possible answer to this question came in 1983 with the claim made by Samorski and Stamm of an excess number of cosmic rays from the direction of Cygnus X-3. This result was then confirmed by Lloyd-Evans et al. (1983). The excess was taken to be gamma-rays as the galactic magnetic fields result in charged particles being greatly deflected. These claims led to the birth of Pe V gamma-ray astronomy and the building of numerous instruments designed to search for point sources of Pe V gamma-ray emission. One such instrument was the GREX extensive air shower array built at Haverah Park which began collecting data in March 1986. This thesis describes the GREX array and the methods of analysis used to reconstruct the size and arrival direction of the incident cosmic rays from the detected air showers. The methods used to search for potential point sources are then described. These methods have been applied to data recorded by the GREX array between 6 March 1986 and 18 December 1990. Particular attention has been paid to Cygnus X-3 and 8 other candidate sources. No evidence for steady, periodic or sporadic emission has been found for any of the 9 potential sources. In addition, an all-sky survey has failed to discover any unknown point sources of emission in the Northern sky. Observations made by other groups of Cygnus X-3 and the 8 other candidate sources at 10 12 and 10 15 eV are discussed. Cassiday et al. (1989) claimed to have observed an excess of cosmic rays from Cygnus X-3 with energies greater than Sx10 17 eV. A claimed confirmation of this result was made by the Akeno group (Teshima et al. 1990). A search for emission of 5x10 17 eV cosmic rays from Cygnus X-3 has been made using data from the Haverah Park 12km 2 array and is described in this thesis. The upper limit to the flux from Cygnus X-3 in this search is significantly lower than the claimed flux, even during periods of contemporaneous observations.

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Bibliography: leaves 117-124 / 124 leaves,[31] leaves : ill ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Physics, 1984

Includes bibliographical references / 119 leaves, [67] leaves : ill., maps ; 31 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--Dept. of physics, University of Adelaide, 1983