Cosmic Rays

views updated

COSMIC RAYS

The idea that there was some type of unknown energetic radiation falling on the Earth from space arose from studies of radioactivity that began in the 1890s. As instruments and understanding of radioactivity improved, the presence of a residual background of radiation that could not be accounted for became more troubling and significant. As early as 1900, C. T. R. Wilson suggested that there could be some form of "cosmic" radiation. However, it was not until 1912 that Victor Hess, in a classic and daring series of manned balloon flights, proved that indeed some form of energetic radiation was penetrating the atmosphere from space.

Initially this cosmic radiation was assumed, by analogy, to be similar to the most penetrating form of radiation produced by radioactivity, gamma radiation. In the 1930s, it was found that the cosmic radiation was influenced by the Earth's magnetic field and hence had to be made up of charged particles, presumed to be electrons. When further studies showed that these particles were positively charged, the assumption was that these particles were positively charged electrons, positrons, examples of which had recently been discovered in the cosmic rays. By 1940 it was becoming apparent that the majority of the incident particles were not electronic but nuclear, presumably protons. It is now known that although most of the particles are indeed protons, there are also energetic nuclei of all the elements in the periodic table present with roughly the same abundance as those found in the solar system. These particles originate from outside the solar system, although, in addition, the sun sometimes produces bursts of copious but relatively low energy particles, solar energetic particles (SEP). The energies of the cosmic ray particles cover an enormous range, from values typically found in radioactive decays, 10⁶ electron volts (eV), to greater than 10²¹ eV, energies that are greatly above any that can be produced in the largest accelerators made by humans.

Few, if any, of the primary particles that enter the top of the earth's atmosphere reach the ground unaffected. Instead they typically undergo nuclear interactions with the nuclei in the atmosphere and produce showers of secondary particles. If the initial energy is high enough, this process may continue for many generations of interactions, resulting in a large burst of secondary particles reaching sea level. While the majority of these secondary particles are electrons, there are also present many of the unstable elementary particles that are produced in high-energy nuclear interactions. The lower-energy particles will lose their energy by ionization in the atmosphere even if they do not interact. As a result, the number of particles in the atmosphere reaches a maximum at an altitude of about 20,000 meters and declines nearer the surface.

Until about 1954, primary and secondary cosmic ray particles were the only source of available high-energy particles that could be used to study the physics of nuclear interactions at energies greater than those typical of radioactive decays. Many of the fundamental processes of the production and existence of elementary particles were first studied by looking at the secondary cosmic ray particles.

Pair Production and the Positron

The first new particle discovered in the secondary cosmic rays was the positively charged electron, the positron, the first example of antimatter proved to exist. In 1932 Carl Anderson used a cloud chamber to look at the tracks produced by cosmic ray particles passing through the chamber. A magnetic field was applied to the chamber so that the sign of the charge of each particle could be determined from the curvature in the field. A lead plate was placed in the chamber so that the direction of motion could be determined from the loss of energy in the lead. Anderson found an example of a particle with the mass of an electron but with a positive charge. This discovery was rapidly confirmed by pictures in cloud chambers of the tracks of pairs of electrons with opposite curvature, examples of the pair production of electrons and positrons by gamma rays. The discovery of these positrons confirmed the theoretical predictions of Paul Dirac (1928) of negative energy states of the electron.

Light Mesons

The presence of unstable particles in the secondary cosmic rays with masses intermediate between that of the proton mass of 1836 electron masses, m_e, and that of the electron, was established as the result of a wide range of experiments. In 1937 particles were observed which had both positive and negative charges, did not lose energy as fast as electrons, but were not as massive as a proton. These particles, originally called "mesotrons," then "mesons," appeared at first to resemble those predicted in 1935 by Hideki Yukawa to explain the forces within the nucleus. The observed mesons were found to have a mass some 200 times that of the electron and to be unstable. However, they had a half-life of 2.1 × 10⁶ second, which was some twenty times longer than predicted, and did not interact strongly with matter, as the Yukawa particle should.

After World War II rapid advances in experimental techniques and in particular the development of sensitive nuclear photographic emulsions by Cecil Powell and his group solved the problem of the discrepancies between the predicted and observed properties of these mesons. Emulsions exposed on mountaintops and on high-altitude balloons showed that there were two types of light meson. Pi-mesons, with a mass of about 273 m_e, were observed coming to rest in the emulsion and decaying into a lighter mu-meson with a mass of about 207 m_e. This mu-meson then also came to rest and decayed into an electron. Now we know that charged pi-mesons decay to mu-mesons and a neutrino. The mu meson then decays into an electron and two more neutrinos. The lifetime of the pi meson is 2.5 × 10^-8 second, much less than that of the mu meson. In addition, negatively charged pi mesons, which are not repelled by the positive nuclei in atoms, tend to be captured and interact with nuclei before they can decay, proving that they are strongly interacting particles of the sort predicted by Yukawa. Mu mesons, on the other hand, do not interact but decay even though they have a much longer lifetime. These pi mesons are now known to exist with positive, negative, and neutral charges, with the neutral pi mesons having very short lifetimes (8.4 × 10^-17 second) and generally decaying into two energetic gamma rays.

Heavier Mesons and Hyperons

Although a light meson was predicted theoretically, the existence of additional mesons heavier than the pi meson was not anticipated. However, as early as 1944 there was a cosmic ray report of the observation of a particle with a mass about 1,000 m_e. Further isolated examples were found over the next few years. Examples were seen of particles that came to rest in emulsion and decayed into three pi-mesons. Other events showed decays, both in flight and at rest, of heavy mesons into various lighter particles. It became clear from observations in cloud chambers and emulsions that there were a number of different modes of decay of so called "K mesons" with masses around 1,000 m_e being produced in high-energy nuclear interactions. By 1954 there had been observations of at least six different modes of decay of charged K mesons and several modes of decay of neutral K mesons.

At the same time there were also observations of unstable particles with masses between that of the proton and deuteron. These "hyperons" were initially regarded as being excited states of the nucleon. They could be charged or neutral, decaying into a proton and a charged or neutral pi-meson.

In every case only a few examples of each type of particle were observed. In many cases there was limited information on the properties of the incident particles and the decay products. Only when artificial accelerators were built with sufficient energy to create these particles and study them under controlled conditions was it possible to begin to understand the underlying physics of nuclei and their forces.

Current Status

The studies using cosmic ray particles only just touched on the problem of these unstable particles. The full complexity of the possible unstable forms of matter that could be created in high-energy nuclear interactions was not unraveled until extensive experiments using high-energy particles from artificial accelerators led to the development of the quark theory of matter. In this theory strongly interacting particles known as hadrons consisted either of baryons (nucleons and hyperons) composed of three quarks or mesons composed of two quarks. Hence, unlike the earlier assumption in the cosmic ray studies, mesons could include particles with masses greater than that of a nucleon. Hyperons also were found to be more than just excited nucleons.

See also:Astrophysics