COSMIC RAYS
By DR. J. GORDON COOK THE world is better informed about scientific research today than it has ever been before. For this we owe a debt of gratitude to the atomic bomb, which brought home to us with some insistence that the scientists were tracing out the pattern of our lives in their laboratories. It is inevitable and understandable, however, that much of what is written for the layman is about applied science ; for the results of applied science can be related directly to our every-day affairs. Pure science, on the other hand, is more difficult to explain, and much of the more academic research being carried out today receives less attention than is its due.
One such field of under-publicised research is that which deals with the phenomenon of cosmic rays. All over the world physicists are studying the origin and nature of these rays, to try to provide a scientific explanation for one of nature's most intriguing mysteries. And yet we hear little of what is going on. The discovery of cosmic rays belongs to the early days of the present century, when physicists were working out the internal structure of the atom as we recognise it today. Detection of atomic particles like the electron and the proton is made possible by the ability of such moving particles to ionise gases and render them capable of conducting an electric current. A charged atomic particle entering an apparatus in which two electrodes are separated by a gas will clock itself in by causing a momentary surge of electricity to pass through the gas from one terminal to the other. Some thirty years ago, it was found that, in spite of all precautions to shield an ionisation device of this sort from atomic particles—fired off, for example, by radioactive sub- stances—some form of ionising particles would persist in entering the detector. Many experiments were carried out to try and find out where these " wild " particles were coming from, and it was discovered that the number of rays increased with increasing distance from the surface of the earth. Balloons carrying detectnrs brought back records of intense activity in the upper atmosphere. Many workers confirmed the existence of the rays, and it became apparent that they were entering the earth's atmosphere from somewhere out in space. They arrived from all directions and did not have their origin in the sun. The name that science has given them—cosmic rays—is a confession of our ignorance of their origin or cause.
Not only are cosmic rays able to penetrate the atmosphere and reach the surface of the earth, but they have been detected at the bottom of the deepest mines. Such powers of penetration indicate that some types of cosmic. rays at least are different from the normal sub-atomic particles like the proton or electron. For these could penetrate only short distances through the atmosphere before being absorbed. It is now known that the rays detected at ground- level comprise a number of different types of moving particles. These are not the cosmic rays themselves, but are secondary rays produced when the true cosmic rays collide with atoms in the atmosphere. Some of the rays are " soft " and easily absorbed by matter ; others are " hard" and are the rays with the unusual powers of penetration. The soft rays consist of "normal" particles like electrons, the expected results of atomic disruption, and their positively charged counterparts, the positrons. But the nature of the hard rays has only recently been established, and they are now regarded as being a new form of particle called the "meson." The place of this particle in atomic structure is still a subject for conjecture.
In the classical picture of the atom we have a miniature solar system in which there is a relatively large nucleus. Around this a number of small negatively charged planetary electrons are circu- lating in their orbits. The nucleus itself consists of large particles— positively charged protons and neutral neutrons—held together tightly by powerful forces of attraction. The function of the new meson particles discovered in cosmic rays is probably concerned in some way with the holding together of the neutrons and the protons in the nucleus. Mesons may act as a nuclear glue inside the atom ; and the nature of the forces that are involved are those that have given us atomic energy. The mass of the meson has been estimated as between that of the electron and the proton.
The nature of the primary cosmic ray as it enters our atmosphere from space has been the subject of considerable speculation. It is now thought to be a high-speed proton. As it arrives in the atmosphere, the proton collides with an atomic nucleus giving rise to the " soft " particles and the mesons that continue in approxi- mately the same direction as the proton. The energy possessed by cosmic ray mesons is remarkable, and it is this that is making them so valuable as a research tool for the physicist today. For the cosmic ray meson is an atom-splitting bullet that is many times more effective than any high-energy particle produced by artificial means. Some mesons, for example, carry more than five thousand times the energy involved in fission of the uranium nucleus. Much of the cosmic ray research, therefore, is concerned with the effects produced by mesons as they batter at the nuclei of atoms.
Three main centres of cosmic-ray research are active in Britain. In Manchester, Professor P. M. Blackett has a team of scientists working under his direction ; there is a team at Bristol University and a third headed by Dr. Paul George at Birkbeck College in London. A most effective technique for studying the results of cosmic-ray collisions has been worked out by these British physicists, using photographic plates to record the effects of nuclear splitting by the high-speed meson. The meson collides with the nucleus of an atom in the emulsion layer of the photographic plate, and the paths of the particles resulting from the atomic explosion are traced out in the layer. The physicist is thus provided with a photo- graphic record of the results of the atomic fission.
Variation in the intensity of cosmic rays with altitude is taking the scientists to laboratories in unusual places. In London the Birkbeck College team have a cosmic-ray laboratory on the top of the University Senate building—the nearest thing to a skyscraper that London can provide. There is another laboratory on a disused platform in Holborn underground station, where a cosmic-ray detector counts the mesons that are penetrating through three hundred feet of London's clay. Last year this equipment—including a three- ton shield of lead—was carried to the international laboratory in Switzerland. Here, for four months, Dr. George and his team studied cosmic rays at the highest laboratory in the world, 12,000 feet up in the snow on the Jungfrau. Another member of this team has been attached to the Piccard expedition to study cosmic rays in the ocean depths.
In spite of all the research that is going on, we still know com- paratively little about the cosmic rays. It may be, for example, that they have some direct effect upon our daily lives. Experiments are being carried out with mice at present to try to determine whether cosmic rays have any influence upon the development of cancer. Some scientists believe also that cosmic rays may be responsible for mutations which cause hereditary upsets and make the offspring of plants and animals differ from their parents. But so far as any practical applications are concerned, it seems probable that we can look for little from these showers of energy that are presented to us gratis by the universe. For cosmic rays will not generate our electricity for us, nor make atomic bombs ; they cannot run our motor cars, nor will they cure the common cold. But they are none the less important for all that. In the help they are giving our physicists in their exploration of atomic structure, cosmic rays are serving mankind well. They could be a decisive factor in bringing us industrial atomic energy in our time.
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