Cosmic Radiation

When cosmic radiation was discovered in 1912, it was believed to consist of penetrating, high-energy y rays which were, therefore, called cosmic rays. The radiation is now known to consist mainly of particles having energies in the range of 108—1019 eV. Although much has been learned about cosmic radiation, especially in recent years from data obtained by means of satellites and spacecraft, many mysteries remain. It is known that the sun is a varying source of cosmic radiation at the low end of the cosmic energy range, that galactic sources (sources in the Milky Way, our galaxy) produce radiation having energy from 1010—1019 eV, and that there are sources that produce very low intensity cosmic radiation with energy that may be as high as 3 x 1020 eV.

About 77% of the primaries are protons and about 20% are 4He nuclei. The remainder consist mainly of nuclei of elements heavier than helium (up to Ni, Z = 28) plus a small contribution from electrons, positrons, and photons. There is evidence that these high-energy particles undergo a complex nuclear

''Although x rays such as those used for medical diagnosis are not nuclear in origin, they are equivalent to low-energy y rays emitted by radionuclides and, in some cases, x rays are emitted as a consequence of nuclear processes. X rays contribute to the total dose of ionizing radiation that an individual receives and are, therefore, discussed in this chapter.

reaction (spallation)2 with CorO nuclei in the interstellar medium to produce most of the Be and B and some of the Li in the universe.

Most of the primaries have energies below 1016eV. Above this energy, the flux of primary cosmic particles drops rapidly, and very few of the highest energy primaries reach the earth's surface. Perhaps 0.05% of the primaries reach sea level. Almost all the cosmic radiation to which the earth's inhabitants are exposed reaches the surface of the earth as showers of secondary radiation produced in the earth's atmosphere (below an altitude of about 25 km) by the interaction of high-energy primary cosmic particles with nuclei of nitrogen, oxygen, argon, and the minor constituents of the atmosphere. Neutrons are released in these interactions. Cosmic radiation intensity near the earth's surface varies with magnetic latitude as the earth's magnetic field bends the trajectories of the primaries so that the intensity is highest near the poles. Cosmic radiation intensity increases with altitude and with decreased barometric pressure at a given altitude as a result of decreased absorption of the secondary radiation in the atmosphere. Because of the solar component of cosmic radiation, the intensity at an altitude of about 12 km can increase by as much a factor of 30 or more during a solar flare, which typically lasts one or two days.

Space travelers are exposed to the high-energy cosmic primaries and to high-energy protons and electrons that are trapped in the radiation belts in the earth's magnetosphere.3 During the Apollo missions, the astronauts, who were shielded by the equivalent of about 2 g/cm2 of aluminum, received a dose between 1.6 and 11.4 mGy (0.16 and 1.14 rads).4 In addition to being a health hazard for people traveling in spacecraft or living in space stations, cosmic radiation also creates problems with electronic components of instruments, especially computer chips in spacecraft and satellites. Thus, a high-energy proton can produce + and - charges (electron-hole pairs) by ionization in or near the depletion region of a junction and can cause a change in the logic state of the junction (see Figure 15-13a). Alternatively, a high-energy proton can undergo a nuclear reaction whose products would cause ionization.

The linear energy transfer (LET) for highly charged, high-energy primary cosmic particles can be 10 or more times that of a particles, making it difficult to evaluate Q, the quality factor, needed to calculate the dose equivalent in

2In spallation, the incident particle does not share its kinetic energy with the nucleons and become part of the target nucleus, which then emits a photon or some other reaction product or undergoes fission. Instead, a very high energy incident particle spalls or chips off small fragments (i.e., light nuclei) from the target nucleus.

Also in the environment above the earth's atmosphere are x-rays and y-rays emitted in pulses or "flashes" by sources unknown at the time of writing.

J. E. Angelo, W. Quam, and R. G. Madonna, Radiation protection issues and techniques concerning extended manned space missions in Radiation Protection in Nuclear Energy, Vol. 2, Proceedings Series, International Atomic Energy Agency, Vienna, 1988.

sieverts from the absorbed dose in grays. At sea level, however, Q for the ionizing radiation can be taken as unity.

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