Illuminating rays

Chemistry & Industry, 15 October 2001

A thin cosmic rain: particles from outer space
Michael W Friedlander
London: Harvard University Press
Pp1+241, 20.50, ISBN 0 674 00288 1

The discovery that laid the groundwork for the fundamental science of particle physics was made in 1912, in an instrument-laden basket carried aloft by a hot air balloon. Austrian physicist Victor Hess floated to 5,250 metres to study the source of mysterious radiation that was foxing researchers in this radical new science.

Monitoring his electroscopes as he ascended, Hess discovered that the intensity of ionising radiation decreased up to around 1,500 metres. But by the time he reached his maximum altitude, the radiation had again increased to several times the intensity at ground level. Hess concluded that there must be 'an extra-terrestrial source of penetrating radiation', a phenomenon later given the name 'cosmic rays' by US physicist Robert Millikan.

That discovery opened up a whole science based on the properties of the most basic units of matter. Its fundamental nature was recognised by a disproportionate number of Nobel prizes, awarded to such well-known names as Murray Gell-Mann, Arthur Compton and Subramanyan Chandrasekhar, as well as the pioneer Hess.

The label cosmic rays actually includes a variety of different phenomena, including the gamut of atomic nuclei, electrons, neutrinos and hard gamma radiation, all emitted from sources ranging from our sun to supernovae in the furthest stretches of the galaxy.

What they have in common is that they constantly rain onto the Earth with measurable results. While neutrinos can pass straight through the planet, as John Updike put it, 'like dustmaids down a drafty hall', more energetic particles collide with atoms in the atmosphere, sparking a cascade of particles such as mesons that do not exist in everyday matter.

Individual cosmic ray particles can have energies of up to 10(20) eV, equivalent to a fast-bowled cricket ball. Because these energies far exceeded anything that could be produced in laboratory particle accelerators until the 1950s, they were for some 20 years the only means by which high-energy nuclear collisions and fundamental particles could be studied.

Particles discovered through cosmic ray research include an exotic array of pions, kaons and hyperons, eventually ordered into the Standard Model of particle physics. Study also revealed the origins of many of the basic elements found on Earth and exploited by chemists.

Observations of the relatively high abundance of lithium, beryllium and boron - the so-called L nuclei - in cosmic rays helped lead to the theory of nucleosynthesis, one of the greatest triumphs of astrophysics. Inside stars, hydrogen is fused to create helium and so on through the elements. The L nuclei are created but just as rapidly consumed during the process. Their existence anywhere outside stellar interiors is due to the collision and fission of heavier particles, thrown violently out of an exploding star.

Archaeologists and historians also have reason to be grateful for the existence of cosmic rays. The technique of carbon dating measures the proportion of carbon-14 in some ancient artifact or remain. Carbon-14 is created in the atmospheric cascade of cosmic rays, when neutrons produced in collisions are absorbed by nitrogen nuclei.

But cosmic rays also have their dark side. A third of your daily dose of radiation, half of the natural dose, comes from cosmic rays. Because their intensity is greater at higher altitudes, a transatlantic flight will give you a cosmic ray dose equivalent to your average annual intake, while crew members receive an annual dose equal to the recommended maximum for industrial radiation workers. Around 800 cases of cancer could be attributed to cosmic rays in the US alone every year.

The health effects of cosmic rays will come under more scrutiny as the manned space programme develops. Several Apollo astronauts reported seeing bright flashes of light during their passage to the moon, even with their eyes closed, an effect that can be attributed to high energy rays hitting the retina or producing Cerenkov radiation within the fluid of the eyeball. Their plastic helmets were also pitted by heavy particles. Equally worrying, a cosmic ray passing through a silicon chip can scramble the data contained therein, introducing almost untraceable errors in computer systems.

While particle accelerators, from the Berkeley Bevatron to the proposed Relativistic Heavy Ion Collider, have superceded cosmic ray observations as the basic laboratory of particle physics, research into the thin cosmic rain continues. An advanced cosmic ray experiment called ACCESS is planned for the new international space station, while mammoth installations such as the Japanese Superkamiokande study the still-mysterious neutrinos.

And Hess would doubtless be glad to learn that balloons still play a major part in cosmic ray research. But now, rather than the gentleman scientist gently ascending with his electroscopes, balloons swell to a volume of a million cubic metres as they carry a ton of special photographic plates through altitudes of 39,000 metres for days on end.

Michael Friedlander, a professor of physics at Washington University in St Louis, US, provides an excellent introduction to the history and science of cosmic rays in this updated version of his 1989 book 'Cosmic rays'. Friedlander is enthusiastic about his subject and writes lucidly, making this book a treat for the general reader.

He also allows himself a little speculation, taking the story of cosmic rays from distant supernovae to the very essence of our being. We have been receiving a small steady dose of cosmic radiation throughout our history - what effect could this have had on our genetic development? Cosmic rays may well have a hand in the random mutations that feed evolution and make us what we are.