Understanding the mystery of cosmic rays

Ken Tapping's weekly column on astrophysics from the Herzberg Observatory

In 1896 Henri Becquerel did an experiment that changed the world. He securely wrapped a photographic plate in sheets of thick, black paper, so that no light could get to it, and then placed  a sample containing uranium compounds on top, outside the wrapping, and left it for a few hours. When he developed the photographic plate, he found a dark silhouette of his chemical sample printed on it. He concluded that the uranium sample was giving off rays that passed through the paper and fogged the plate. He had discovered radioactivity. We now know that there are some atoms, usually large ones, which are not stable, and spontaneously, or as the result of being hit by something, break into smaller bits, giving off high-energy waves and particles. It eventually became clear that radioactive elements are distributed throughout the Earth, and the high temperatures in its core are due partly to the heat given off by decaying radioactive material.

In 1912 Victor Hess made a very dangerous high-altitude ascent in a balloon, taking a radiation detector with him. He measured the level of radiation and how it changed as the balloon got higher and higher.  If the radiation was coming from radioactive elements in the ground, radiation levels would fall as the balloon got higher, and further from the radioactive elements. However, what he found was the opposite; as he got higher, the radiation level increased. He concluded that although there is radiation coming from the ground, there is much more coming in from space and being partially blocked by the atmosphere, so as one gets higher the dose rate goes up. We now call this radiation ìcosmic raysî. They are mainly extremely high-energy fragments of atoms.

There followed some very ingenious experiments to find out more. One of them was to make stacks of many layers of photographic film, and to send them up in rockets and balloons. On some lucky occasions a cosmic ray would draw a path down through all the layers of film. Sometimes the cosmic ray particle would hit an atom of the material making up the film cube, and produce a shower of fragments. We have found out that the cosmic ray particles are moving at between 45 and 99.6 per cent of the speed of light. One particle “a fragment of an atom” can have the energy of a baseball moving at almost 200 km/h. The big question is where would particles get accelerated to such high speeds?

Thanks to their meandering route to us through our galaxy, measuring the direction doesn’t tell us where the cosmic rays are coming from. However, we know that stars like the Sun produce lower-energy cosmic rays. Higher-energy ones are accelerated by supernovae  exploding stars, and by black holes. Others gradually gain speed as they are pushed to and fro by the magnetic field of the Milky Way. Here on the Earth’s surface we are largely protected from cosmic rays, but they are a problem we need to solve in order to carry out manned space missions to other planets.

Venus and Jupiter are still conspicuous in the west after sunset, although Jupiter is now getting lower in the sky. Mars is high in the Southeast; Saturn rises about 9 pm. The Moon will be full on the 6th.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory, Penticton.