General Meeting 14 July 1999
Hard Rain: Cosmic Rays on Earth
Speaker: Assoc. Professor David N. Jamieson, School of Physics, University of Melbourne
If the title sounds impressive, the equipment and the quality of presentation was even more so: a high-tech Audio Visual projector hooked up to an Ultra thin Notebook computer produced brilliant screen displays, where paragraph headings slid, dropped or bounced into position with a click on the remote control. Illustrated hand-outs complemented the professional approach.
It became obvious very quickly that Professor Jamieson was as familiar with his subject as he was with the equipment. A step by step development led us through the history of cosmic rays, from their first detection in 1912 by Hess on high altitude balloons, their identification and naming in 1925 by Millikan and Cameron, to their analysis of composition, classification into energy levels and the study of environmental effects. His personal research is currently in this last category, radiation damage to semiconductors, and how to protect communication and data equipment against such failures.
Until high altitude rockets became available all cosmic rays detected were of a secondary nature, that is, they are caused by collisions between a primary cosmic ray particle coming from space and a molecule of air in the upper atmosphere. These so-called primary cosmic rays are unbelievably powerful. Their energy spectrum decreases logarithmically in intensity up to a staggering 1020 eV. This is 1012 times the energy of radio-activity, or 108 times that produced by the most powerful Earth-bound particle accelerator. A shower of up to one billion secondary particles can result from a single high energy cosmic ray proton. An effective (albeit somewhat sadistic) demonstration of the energy contained in one of those protons, Professor Jamieson suggested, would be to drop a house brick from a height of two meters onto your toe.
With the help of anecdotes and simple graphic representations, this exposition on a highly technical field somewhat outside the average astronomer’s expertise, became an absorbing adventure into Nature’s mysteries. There was for instance the amusing story of Professor Jamieson taking his equipment on a commercial flight to demonstrate the dramatic rise in radiation with altitude. When he explained to his fellow passengers the meaning of the graphs on his laptop computer, the seats all around him emptied suddenly. Exposure to radiation at cruising height (10,000m) is approximately 50 times that at ground level! And yet, even though the radiation symbol is the most feared danger sign by the general public, in statistical analysis radiation is by far the smallest of the life-threatening risks on Earth. At ground level radiation exposure to cosmic rays is on the average 0.3 milli Sievert per year, mS/Y, which is less than that generated by radioactive decay in the tissue of the human body itself.
The question on everybody’s lips at the end of the session was of course, where does it come from? What natural process can endow these particles with so much energy? Not heat as we know it, not gravity. No, we have to look to magnetic disturbances. Solar flares can produce low energy cosmic rays. Galactic events such as accretion disks of black holes or supernova fronts may be the source for higher acceleration, but for the highest energy detected so far even these are not powerful enough. It needs events of cosmic proportions. Here, where Nature links its smallest particles to the largest cosmic events, we are at the cutting edge of philosophy and technology. We have to measure and understand something that penetrates two meters of lead and may only occur once a year.
Professor Jamieson finished with: We live in an exiting time; there is still so much mystery to be explained. Human knowledge is again working towards a break-through in understanding physics, in magnitude comparable to the great upheavals at the end of last century.