Physics in Canada / La Physique au Canada - 2012 (68.1)

The 100th Anniversary of Cosmic Ray Research : A Canadian Perspective

David Hanna
McGill University

This year is the centenary of the “discovery” of cosmic rays, charged particles that bombard the Earth from all directions and with a great range of energies. I use the quotation marks since cosmic rays were not so much discovered as figured out, and then only partially. As explained in the article “Cosmic Rays B a 100-year Mystery” in this issue, physicists had noticed anomalous effects in their instruments right from the beginning of the systematic study of radiation at the turn of the last century. It was in 1912 that a big step in understanding this phenomenom was made when Viktor Hess took an electroscope to high altitude during a balloon flight. This is accepted as the date when cosmic ray research really began.

During the last century, cosmic rays were studied as a phenomenon in their own right; their energy spectrum and composition, possible origins, etc, but cosmic rays were also valued as tools for other sciences. Early particle physicists used them for production and study of unstable particles before the advent of high-energy proton synchrotrons. Similarly, nuclear physicists used cosmic-ray protons for measuring basic cross-sections relevant to the nuclear power program. Examples of such work are detailed in the companion articles “Early Muon-Physics Measurements with Cosmic Rays” and “Pioneering Canadian Cosmic-Ray Experiment at Echo Lake Motivated by the Chalk River Nuclear Energy Program”. Finally, cosmic rays have been used as probes of other astrophysical phenomena. An example, discussed in the article “Canada’s Neutron Monitoring Program”, is the effect that solar activity has on the flux of lower energy cosmic rays arriving at the Earth. The solar wind decelerates cosmic rays and blocks the lowest energy ones. This, and the deflection of particles by the Earth’s magnetic field gives rise to latitude-dependent time-varying effects that were studied in detail with a world-wide network of neutron detectors designed in Canada.

Basic research into and with cosmic rays goes on but, as with any mature science, progress slows and more commitment of time, talent, and treasure is needed to go forward. The field has evolved from something within reach of small teams with limited resources to a pursuit carried out by multinational collaborations using apparatus costing tens of millions of dollars and years of planning and execution.

Research may not be as widespread as before but cosmic rays continue to influence physics in particular, and everyday life in general. For example, the raison d’être for underground facilities, like SNOLab, to study rare physics processes is to shelter from the constant rain of muons from cosmic-ray air showers and it is even necessary for some applications to use extra shielding made from lead found in ancient shipwrecks so that it is free of cosmogenic radioactivity. Likewise, detectors fabricated far from the experiment are transported to the lab over land and sea rather than by air to avoid being activated by the particles from space. In a similar vein, airline personnel, especially pregnant women, are monitored for radiation exposure and modern satellites are designed to avoid failure due to cosmic ray interactions in their electronics. The single biggest technical challenge for a manned mission to Mars is radiation protection for the crew.

It’s not all negative. Cosmic-ray muons are a boon for calibrating detectors in the lab and can be used in a variety of undergraduate lab exercises, including showing the effects of relativistic time dilation. Many outreach programs for high-school students make use of simple muon detectors.

Cosmic rays, as ubiquitous as starlight and carrying a similar amount of total energy, are invisible to the naked eye and unknown to the majority of the population. They remain, however, objects of fascination to a dedicated cohort of specialists. One hundred years after the anomalous radiation was first linked to the cosmos, much progress has been made on the known unknowns. It is time to find and explore the rest.

David Hanna, McGill University

Guest Editor, Physics in Canada

hanna [at] physics [dot] mcgill [dot] ca

Comments of readers on this editorial are more than welcome.