The High Resolution Fly's Eye Project.

I am working in the High Energy Astrophysics Group at the University of Adelaide on the High Resolution Fly's Eye Cosmic Ray Detector. We are attempting to detect extremely high energy cosmic rays - particles with energies in excess of 10^18 electron volts (eV). To put this into perspective, and X-ray is 1000 eV and the most energetic particle discovered had in excess of 10^20 eV which corresponds to 50 Joules! That's a single particle with enough energy enough to make a light bulb glow for a second, or about the amount of energy transferred when you drop a heavy physics text book on your foot.

The HiRes project is a collaborative one between the University of Adelaide, the University of New Mexico, The University of Utah and Columbia University. The aim of the project is to obtain as much information as possible on the cosmic rays with energies above 10^18eV. At present our knowledge of these particles is quite incomplete. We definitely know they are coming, but at present there is no known source, or even a theory to explain how they are accelerated to such high energies. We do have a reasonable idea of the environment in which these particles should be created but at present we just haven't detected enough events to know for sure.

The project is both interesting and challenging. The 10^20eV particles only at the rate of 1 per square kilometre per century. HiRes uses the atmospheric fluorescence technique to detect these particles. This allows a large volume of the sky to be observed and hence make detection feasible. Our technique detects the weak fluorescencent signal produced from the particle cascade (air shower) caused by one of these particles interacting in the top of the atmosphere. Such a shower can contain in excess of 10 Billion particles, and the passage of the shower causes the atmospheric nitrogen to fluoresce in the UV. Our detection method consists of using mirrors with sensitive light detectors (Photomultipler Tubes) in their focus. These grid the sky at 1 degree resolution (the gridding is analgous to the composite nature of a Fly's eye, which is where our name comes from). Through the use of several mirrors at two sites we are able to determine the initial cosmic rays energy, type and direction.

A shed containing the mirrors, photomultiplier tubes and electronics used to detect cosmic ray airshowers. A UV filter (to let in only UV light, unlike sunglesses which do the oppposite) has been lowered to reflect the light from the Utah desert where the detector is located.

My PhD Project.

I started my PhD on the project in April 1994. I initially got started on the design and testing of GPS (Global Positioning System) based clocks, and have since gotten involved with the calibration, construction, observation and finally analysis of data from the detector. I submitted my thesis for external examination in May 1998 (my reviewers were Professor R. Morse at the University of Wisconsin and Professor J. Matthews at the University of New Mexico) and it was accepted without changes in June 1998.

We use GPS based clocks to maintain time synchronisation between the two HiRes Sites. The sites are seperated by 12.6km so GPS is an effective and low maintanence way of keeping the sites time synchronised. We operate the receivers in a common view mode so that they track the same satellites and thus solve the same ranging equations. Testing indicates that they maintain a relative timing accuracy of around 10ns (10 billions of a second)! The accuracy that the installation of the GPS clocks provided then allowed me to carry out an in depth calibration of the timing system. Whilst this may sound a little dry it was good solid experimental physics, requiring interfacing, programing and analysis skills to produce a system that gives us data that we can trust.

I have also been involved in observation through three trips to Utah (about 6 months in all). This has allowed me to more fully understand the detector, the technique, and those annoying problems that abound in experimental physics when you try to collect data. Observations are done under clear, dark sky (no moon) conditions, with an observer at each of the two sites. The first 14 mirror site is the primary site whilst the second site consists of 4 mirrors overlooking the same field of view. The second site has been fitted with 52 other mirrors (which I helped construct), which are currently awaiting electronics.

Following my work on the GPS clocks, I became involved with work on reconstructing trajectories for extensive air showers simultaneously viewed by both HiRes detectors. Such stereo observations provides much improved resolution over single site observations. I developed programs to use the both geometrical and timing information to determine air shower trajectories to within 1 degree of their true arrival directions (95% of the time). Knowing the air shower trajectory allows us to estimate more interesting parameters such as energy. Finally I analysed the stereo dataset from the prototype detector, in particular examining the effect of aerosol scattering on parameters such as energy. I then submitted my thesis for external examination on May 6th 1998.

Use the links below to obtain postscript copies of my thesis. I have also made html versions of my abstract and my conclusions and further work chapter.
Chapter 1: Cosmic Rays
Chapter 2: An Introduction to Extensive Air Showers
Chapter 3: The HiRes Prototype Detector
Chapter 4: GPS Clocks and the HiRes Timing System
Chapter 5: EAS Reconstruction
Chapter 6: Analysis of Stereo Observed EAS
Chapter 7: Conclusions and Further Work
Or the whole lot in a 2Meg tarred gzip'ed file