Every moment of every day, the Earth is bombarded with particles from outer space. Known as cosmic rays, they have revolutionised our understanding of matter at the subatomic scale.
The discovery of cosmic rays
Radioactivity has fascinated scientists ever since its discovery in 1896. Its ability to ionise air made it easy to detect and led to a surprising discovery: even when no radioactive source was present, detectors revealed the presence of some other radiation that was ionising the air. This radiation even showed up out at sea, with no radioactive rocks in sight. Furthermore, it was very powerful, able to penetrate shielding around the lab apparatus. Another source of unknown rays must exist, and of immense penetrating power – but where?
The first clues came when Theodor Wulf, a physicist and Jesuit priest, ascended the Eiffel Tower and found more radiation up there than he expected. He surmised that these were rays of extraterrestrial origin, and suggested ascending to great heights in balloons as a way of testing the idea. But the spirit of adventure seemed to desert him, and it was left to others, notably the Austrian physicist Victor Hess, to take the risk.
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Hess made 10 ascents in 1911 and 1912 and found that the intensity of the rays increased rapidly above 1000 metres. At an altitude of 5000 metres, their intensity was some five times greater than at sea level. Hess concluded that a powerful radiation originates in outer space and enters the Earth’s atmosphere, diminishing in intensity as it passes through the air.
It is Hess who is traditionally credited with the discovery of cosmic rays, for which he won the Nobel prize in physics in 1936. The evocative name “cosmic rays” was coined by US physicist Robert Millikan in 1925.
Initially Millikan had doubted Hess’s claims, but this changed in the 1920s when Millikan made measurements of his own. He invented an electrometer whose readings were recorded on moving film. This enabled the apparatus to be lofted on unmanned balloons, and extended the measurements to very high altitudes. By 1926 Millikan was convinced of the existence of cosmic rays, even going so far as to claim the discovery for himself.
Anatomy of a ray shower
When a cosmic ray shoots down through the upper atmosphere, its collisions with atoms in the air generate an avalanche of particles (see diagram). Most of the particle shower is absorbed before reaching the Earth’s surface. Whereas each square centimetre of the upper atmosphere is hit by about 20 particles every second, on average, at sea level only a feeble drizzle remains: a mere 1 per minute.
Particles such as pions, and others like them that respond to the strong nuclear force, tend either to be absorbed or to decay into electrons, muons, photons and neutrinos, which penetrate further. The electrically charged particles go on to create their own showers of electrons, positrons and photons. The total energy of the initial cosmic ray is thus shared among ever more constituents, most of which never reach the ground.
One exception are muons, which are essentially heavy electrons. They can punch through the atom-filled atmosphere and reach ground level, even penetrating the soil. If you visit a science exhibition and see a spark chamber recording the passage of cosmic rays, it is most likely muons that are triggering it.
Neutrinos penetrate the most, often passing right through the Earth to fly out the other side. As you read this, neutrinos from cosmic rays hitting the far side of the planet are emerging from beneath your feet and zipping through your body. Yet more are showering down over your head.
In very extreme cases, an incoming cosmic ray particle may have 10 million times the energy of the beams at our most powerful particle accelerator, the Large Hadron Collider at CERN, near Geneva, Switzerland. Such energy can spawn millions of secondary particles and this shower can spread out over several kilometres. Even so, the shower preserves the overall direction of the main thrust. By measuring the relative arrival times of particles at several widely separated locations, it is possible to determine the direction of the primary cosmic ray to within a few degrees.
Particle explosion
In 1927, Dmitry Skobeltsyn of the Leningrad Physicotechnical Institute in the Soviet Union was studying radioactivity using a cloud chamber, a device that makes the invisible visible. Cloud chambers are sealed vessels filled with water vapour on the cusp of condensing. When a charged particle such as an electron zips through, it ionises the vapour and this causes water droplets to form along the trail.
Skobeltsyn was using his cloud chamber with a powerful magnet, which steered the relatively slow-moving electrons in tight circles. He noticed that some trails were nearly straight, though, showing that they had very high momentum far beyond those from any source known at the time. Unknowingly, he had become the first person to directly observe the trails of cosmic rays.
Three years later at the California Institute of Technology in Pasadena, Robert Millikan’s student, Carl Anderson, built a cloud chamber surrounded by very powerful magnets specifically to study cosmic rays. Much to his surprise, he found some trails that curved as if they were electrons with positive electric charge. Anderson had discovered the positron, the antimatter analogue of the electron, which had been predicted earlier by British theoretical physicist .
Positrons are not some peculiar extraterrestrial phenomenon. Patrick Blackett and Giuseppe Occhialini proved this in 1932 when they found that positrons made up roughly half of the particles produced when a cosmic ray struck a metal plate inside a cloud chamber, with the other half electrons.
Anderson and another colleague later went on to discover that some trails penetrated the cloud chamber much further than electrons, and did not create such showers. These were due to a particle that seemed like a heavy version of the electron. This was the discovery of the muon.
The positron and muon were the first of a series of discoveries that showed Earth-bound physics had sampled only a small part of nature’s rich pageant. By the 1950s cosmic rays had revealed yet more new particles that existing theories could not explain.
The pion was discovered in 1947. This, at least, had been predicted, whereas a family of “strange” particles had not. The year marked an explosion of strange-particle finds, with the kaon, lambda, xi and sigma being discovered over the next six years.
In order to understand their nature, physicists built particle accelerators, which in effect simulated the interactions of cosmic rays, but under controlled conditions. So the discovery of cosmic rays led to the modern field of high-energy particle physics.
