+
and 
- mesons were
produced in the new large cyclotron at Berkeley by bombarding carbon nuclei
with alpha particles. In 1950, the 
0 was discovered as a result of similar collisions.
What are cosmic rays ?
Cosmic rays are sub-atomic particles and gamma-ray photons which
bombard the Earth from outer space. They have a large range of energies,
usually measured in electron-volts [eV], from a few GeV to more
than 100 EeV.
( 1 EeV = 10 18 eV ) The highest energy detected
so far was a proton with 320 EeV of kinetic energy - equivalent to a tennis
ball travelling at over 300 kmph ! The proton must therefore have been
travelling at practically the speed of light - if it had raced a photon
from one end of the Milky Way to the other, the photon would have won,
but by only half a centimetre at the finish line. This makes cosmic rays
the most energetic particles the Universe can produce. Reseachers are interested
in solving the problem of the origin of these highest energy rays, but
their discovery has a long and fascinating history.
The Early Days
In the last decades of the 19th century, there was a general feeling that all the major discoveries in physics had been made and all that remained was to pursue the values of the fundamental constants to extra decimal places. In reality, this could not have been further from the truth as subsequent events were about to prove. One area of research which was to bring surprising results was the electrical conductivity of gases.
Following the invention of the incandescent filament lamp by Joseph Swan in 1878, the cathode-ray tube, developed in the 1880s, became a major tool for studying electrical conduction in gases. It was found that the cathode rays could be deflected by electric and magnetic fields and it was suspected that the "rays" were in fact negatively charged particles, a fact that was established in 1897 by Joseph John Thomson (no relation to the author) at the Cavendish laboratory in Cambridge. Thomson correctly identified the electron as a particle which could be knocked out of atoms in the gas, leading to the production of positively charged ions which, when accelerated by the high voltage, produced so-called positive rays. This line of research would eventually lead to the invention of the mass spectrograph by Aston in 1919 and to the discovery of isotopes - atoms of the same element but which have different nuclear masses.
Meanwhile in Wurtzburg , a German physicist called Wilhelm Roentgen was studying fluorescence in minerals. One day in November 1895, he noticed that a fluorescent screen glowed when at a considerable distance from a cathode ray tube and even when the tube was completely covered by black cardboard. Roentgen had discovered X-rays and in 1902 he was awarded the first Nobel Prize for physics. Following Roentgen's line of research into fluorescence, Henri Becquerel working at the Sorbonne, reported in February 1896 that he had discovered an "emission of rays" from uranium compounds which fogged wrapped photographic plates even when stored in a dark cupboard.
The nature of these rays eluded Becquerel, who incorrectly associated them with X-rays. It was left to his Polish student Maria Slodowska, soon to be married to Pierre Curie, to correctly identify the source of "Becquerel rays" which she renamed radioactivity. Marie and Pierre Curie realised that the radiation was being emitted from the uranium itself and over the next few years, the same property was found in thorium, as well as in the previously unidentified elements they named radium and polonium. They shared the 1903 Nobel Prize with Becquerel for the discovery of radioactive elements. Marie Curie was the first female recipient of the prize and also the first person to win it twice, for her work on radium and its compounds. She died of anaemia caused by overexposure to radiation in 1934.
Back in England, a young New Zealander named Ernest Rutherord was working with Thomson to identify the exact nature of the radiation emitted by uranium. Between 1896 and 1900, sharing a corner of the laboratory with Thomson and his cathode tubes, Rutherford identified three types of radiation. A heavy particle which could be stopped by sheets of paper he named alpha and identified it as the core of a helium atom. A lighter particle, named beta was found to be an energetic electron which could penetrate thin sheets of aluminium. The third type of radiation was found to be not a particle at all but a more powerful form of electromagnetic wave similar to X-rays, called gamma-rays. He was awarded the Nobel Prize for chemistry in 1908. Rutherford went on to discover the nucleus of the atom, in Manchester in 1911, and he produced the first artificial nuclear transmutation in 1919 when he bombarded nitrogen gas with alpha particles to produce oxygen nuclei and protons.
This was a line of research which would lead inevitably to nuclear power and the atomic bomb, but we have left the story of cosmic rays rather far behind.
Cosmic rays discovered
As J J Thomson realised , X-rays and radioactive emanations produce ionization in gases enabling them to conduct an electric current. It is this effect which provides the means of detecting and measuring radiation by the use of electroscopes and electrometers, devices which use the electrostatic repulsion between two gold leaves or metal wires to measure charge. When a source of ionizing radiation is present, the leaves, which are repelled from each other due to the electric charge stored, gradually come together as the charge is neutralized by the presence of conducting ions. No matter how many precautions researchers took to eliminate sources of radiation, it was found that it was impossible to prevent the leakage of charge from these instruments, even when they were surrounded by more than 10 cm of lead. Whatever was responsible must have a remarkable penetrating poower.
Numerous attempts were made to reduce the effect of sources of radiation in the ground. Electroscopes were taken to the tops of tall buildings; in 1910, Father Thomas Wulf took his electroscope 300 metres up the Eiffel Tower and observed a 64% reduction in the leakage rate. He had expected that radiation from the ground would have been absorbed by the atmosphere and he concluded that there must be a contribution from radiation coming down through the atmosphere.
The next step was to go to greater heights and in 1911, Austrian physicist Victor Hess began a series of high-altitude balloon flights which showed that the intensity of ionizing radiation at first decreased with height but, by about 1500 metres, the radiation was definitely more intense than at sea level. After his highest ascent of over 5000 metres in 1912, during which the effect increased several times, Hess was convinced that the explanation was "an extra-terrestrial source of penetrating radiation." Further flights, were conducted by Werner Kolhorster in Berlin and by Robert A. Millikan and Ira S. Bowen in the US. Initially skeptical of their extra- terrestrial nature, by 1926 Millikan had reversed his view and he coined the term cosmic rays.
Cosmic ray detectors
After the accidental discovery of the leakage of charge from electroscopes, it took only a few years before researchers agreed that it was caused by a penetrating radiation, quite different in character from X-rays and radioactivity. It was many more years however before the nature of the cosmic rays (CR) was understood because of the complex changes which occur as they pass through the atmosphere. Millikan showed that the atmosphere acts only as an absorber of, and is not the producer of primary CR. The primary rays approaching the Earth collide with atoms in the stratosphere and produce secondary CR which may in turn have their own collisions. Such secondary CR were responsible for the electroscope leakage. The detection and identification of primary CR had to wait until the development of instruments of greater sophistication than simple electroscopes.
Experiments done by Millikan before 1920 and by Arthur Compton during the 1930s used ionization chambers. The total ionization in a lead-shielded container was monitored in order to record the total flow of CR through the chamber. Variations in CR intensity with altitude and latitude on the Earth were surveyed in this way and the effect of the Earth's magnetic field was deduced.
Around 1911, Hans Geiger, ( who with Ernest Marsden performed the famous alpha scattering experiment that allowed Rutherford to calculate the size of the nucleus of a gold atom) invented a single-particle detector which bears his name. In 1929, Geiger and his student Walther Muller improved on the design. The GM tube consists of a thin wire fixed along the axis of a metal cylinder containing argon gas at low pressure. A high voltage is maintained between the wire and the walls of the cylinder so that, if an ionizing particle enters the tube, it produces a cascade of ions which is detected as a pulse of electric current. By using two GM tubes together it became possible for researchers to determine the direction in which a particle passed through both tubes. This was known as a GM telescope.
The cloud chamber
Developed at about the same time as the Geiger counter, the Wilson cloud chamber was for many years the most widely used particle track detector. Charles Thomson Rees Wilson was a student of J.J.Thomson at Cambridge and one of the discoverers of electroscope leakage. Whilst on holiday in Scotland, Wilson had been entranced with the beauty of the clouds surrounding the meteorological observatory at the top of Ben Nevis. He returned to Cambridge with the intention of making clouds for study in the laboratory. By 1911 he had devised an "expansion chamber" to study condensation under controlled conditions. When Wilson actuated a tightly fitting piston, the pressure in a glass chamber sprayed with water was suddenly reduced. Cloud would form in the chamber if there was sufficient dust to act as nucleation sites for water droplets to condense on. Although Wilson systematically cleaned his tanks to remove all traces of dust, nevertheless he found that cloud could still form inside the chamber. By directing a beam of X-rays into the chamber, Wilson found he could make clouds more easily.
The conclusion that ions were responsible for the formation of clouds led to the discovery that ionizing radiation entering the chamber could leave a track, much as a high-flying jet does in the thin upper atmosphere. A photograph could be taken of these tracks and this could be studied to reveal the mass and energy of the particle causing the ionization. If a magnetic field is applied, the curvature of the path reveals the charge on the particle. In this way, in 1932, Carl Anderson discovered the first evidence for the existence of anti-matter. The curved track of a positron or positively-charged electron, was photographed passing through his new cloud chamber. Positrons are produced as a result of gamma rays colliding with nuclei and their existence had been predicted theoretically by Paul Dirac in 1929.
Anderson shared the 1936 Nobel Prize with Victor Hess for his discovery of cosmic rays. But the story does not end here.
The heavy mob
In 1936 Carl D Anderson and Seth H Neddermeyer, using a cloud chamber in a strong magnetic field at their Pike's Peak laboratory 4300 metres above sea-level, discovered the subatomic particle called the muon in cosmic rays. The positron and the muon were the first of a series of subatomic particles discovered as a result of cosmic ray research and these discoveries gave birth to the science of elementary particle physics. Particle physicists used cosmic rays for their research until the advent of particle accelerators in the 1950s. Even today, the most energetic particles detected in cosmic ray showers exceed the maximum energy which can be produced in an accelerator by more than a hundred million times !
The discovery of the muon, a particle with a mass 200 times greater than that of the electron, gave CR research a new goal, to discover other particles in the host of secondary cosmic rays hurtling down from the stratosphere. In 1935 a Japanese physicist, Hideki Yukawa at Osaka University proposed that the binding force which held protons and neutrons together in the nuclei of atoms was carried by an exchange particle called a pi-meson or pion, with a mass around 200 electron masses. At first it was thought that this was the particle discovered by Anderson, but it was soon realised that it did not exactly fit the bill. Further research was needed.
Photographic Evidence
Before the invention of Geiger counters or cloud chambers, photographic plates were used by Rutherford in his early experiments on alpha particles. When fast charged particles pass through a photographic emulsion, they cause chemical changes similar to light, which show up after development as a trail of silver grains. However, the emulsions used were too thin for long tracks to be seen. An alpha particle travelling at about 0.01c will produce a track consisting of only one grain.Heavy particles with large electric charges leave thick, straight tracks made up of thousands of grains. Lighter particles with smaller charges produce thin, tortuous tracks with well-separated grains.
In November 1935, Captain Albert W. Stevens and Captain Orvil J. Anderson of the U.S. Army carried special Eastman Kodak plates into the stratosphere aboard Explorer II and set the altitude record for balloon flights at 72,395 feet. When the plates were examined, by T.R.Wilkins of the University of Rochester, clear evidence of primary CR tracks was found.
Because high altitude balloons were not easily available
until the late 1940s high mountains were chosen for CR research. Cecil
Powell of the University of Bristol, who had been a student of Rutherford
and Wilson, was joined by Guiseppe Occhialini, who had worked with P.M.S.Blackett
at Manchester, and together they worked with chemists at Kodak and Ilford
to develop more sensitive emulsions. Some of these were taken for exposure
to CR at research stations above 3000 metres on the Pic du Midi and the
Jungfraujoch. In 1947, using the improved emulsions from the Pic du Midi,
Powell and company discovered dark tracks produced by Yukawa's pions as
well as those of their decay products, muons. A year later, +
and - mesons were
produced in the new large cyclotron at Berkeley by bombarding carbon nuclei
with alpha particles. In 1950, the
0 was discovered as a result of similar collisions.
Strange Days
Another discovery was made in 1947 by George Rochester and Clifford Butler of Manchester University while they were examining CR tracks in their cloud chamber. Most of the tracks were produced by particles such as pions and protons which travelled right through the chamber, but some interacted in a 5 cm lead plate across the chamber's centre. Rochester and Butler found two strange tracks which did not correspond to any known types of particle behaviour. The tracks showed abrupt bends which made them look like inverted Vs but the angle between the tracks and the degree of ionisation were too great to be an electron-positron pair. Initially referred to as V-particles it was subsequently established that the tracks were due to a new type of meson called a kaon which had to have a mass over 800 times that of the electron.The K-meson is one of a family of heavy particles called hadrons and soon, as the rush to find other similar particles intensified, it was joined by even heavier and stranger particles known as the lambda, sigma and xi. These particles are all heavier than the proton and neutron and are produced by the interaction of cosmic ray pions and protons in the Earth's atmosphere. Furthermore, these particles are always produced in pairs and subsequently decay into protons and pions. To rationalize this behaviour, in 1953 Murray Gell-Mann proposed a new property of matter called strangeness.
The "nuclear zoo"
During the 1950s, hundreds of research groups set up cloud chambers on mountains and flew emulsions in balloons to heights of over 90,000 feet in attempts to find new heavy particles. On the ground, particle accelerators began to play an increasingly important part. The flood of new observations revealed no initial order or grouping which led J.Robert Oppenheimer to refer to the "nuclear zoo" in a talk he gave in 1956. With better measurements of particle masses and lifetimes, and the application of theory, a semblance of order began to appear and by 1964, Gell-Mann and George Zweig were putting the finishing touches to their quark theory of matter, culminating in another Nobel Prize in 1969.
By 1955, particle accelerators of increasing size had displaced cosmic rays as the source of high-speed particles. More and more particles were being discovered in machines capable of accelerating electrons and protons to almost the speed of light, and as these machines grew in size, more complex theories were proposed to explain their discoveries.But cosmic ray detectors have been evolving as well and today, modern electronics and computers have all but replaced track detection techniques. Among the array of newer devices are scintillation counters, which work on the principle that an ionizing particle produces a brief flash of light when it goes through a clear material such as lucite. Cerenkov detectors respond to the radiation produced when particles travel faster than light does in air or a liquid such as water.
By the year 2001 it is hoped that two large detectors, one in Argentina and one in North America, will be operating to detect ultra-high energy cosmic rays from sources of energy far away in the universe. The project is named after Pierre Auger, who discovered the effect of extensive cosmic ray air showers back in 1938.