Production of Cosmogenic Radionuclides in the Atmosphere

Abstract

In this chapter, we discuss what happens to a cosmic ray particle after it has successfully travelled through the heliosphere and the Earth's magnetic field and has reached the top of the atmosphere. The fact that it made it so far means that its energy is high (!1 GeV). Depending on the degree of solar activity (Sect. 5.7, also Fig. 10.2.3-1), the majority of the low energy particles fail to get close enough to the Earth to interact with matter even in the polar regions. Each nuclide is characterized by a specific number of protons and neutrons in its nucleus. To produce a cosmogenic nuclide in the atmosphere, it is necessary to change the number of nucleons in an atmospheric atom; for example, 7 Be, 10 Be, and 14 C are almost completely produced from atmospheric nitrogen (14 N) and oxygen (16 O) atoms. Since the binding energy for nucleons is large (>8 MeV), this never occurs as a result of thermal atmospheric collision processes. In the next two sections, we describe how a high-energy cosmic ray particle initiates a chain of nuclear interactions that ultimately result in the nuclei of atmospheric atoms being disrupted, yielding cosmogenic nuclei. The nuclear processes initiated by the cosmic rays nearly always result in nuclei with atomic masses equal to or less than that of the target atmospheric nucleus. The only exception is neutron capture which increases the target mass by one as in the case of 35 Cl(n,g) 36 Cl. Since the main constituents of the atmosphere are nitrogen and oxygen (Table 10.1-1 in Box 10.1.1), the majority of the cosmogenic radionuclides produced have masses below 16. Masses up to 40 are rare because the weight fraction of argon in air is only 1.3%. Heavier cosmogenic radionuclides are very rare because of the very small amounts of Krypton and Xenon in the atmosphere. Neglecting those with short half-lives leaves us with a very limited number of cosmogenic radionuclides produced in the atmosphere. The interaction of a primary cosmic ray particle with the atmosphere consists of various physical processes leading ultimately to the dissipation of the particle's kinetic energy. All these processes have been studied extensively in the last several J. Beer et al., Cosmogenic Radionuclides, Physics of Earth and Space Environments, DOI 10.1007/978-3-642-14651-0_10, # Springer-Verlag Berlin Heidelberg 2012 139 decades using accelerators and are generally well understood. It is this knowledge that is used to calculate the production rates of cosmogenic radionuclides. In Sect. 10.2, the interaction of cosmic rays with the atmosphere and the production of secondary particles (mainly neutrons and protons) are discussed. Many hundreds of interactions occur in each cascade, and we outline the complex physical models used to determine the manner in which the various secondary components develop and attenuate with depth. Then, in Sect. 10.3, we estimate the probability that a secondary particle interacting with oxygen, nitrogen, or argon produces a specific cosmogenic radionuclide. As we will see, the production rate depends on (1) the intensity of the incoming cosmic rays and their modulation by solar activity, and the intensity of the geomagnetic field and (2) the atmospheric depth and the geomag-netic latitude where the production takes place. The calculations show that the galactic cosmic rays are primarily responsible for the production of the cosmogenic nuclides, but that there can be a detectable contribution from solar cosmic rays during limited periods of time. Based on our present knowledge, they do not play a significant role when averaged over 10 years or more. 10.1.1 BOX The Earth's Atmosphere The atmosphere consists of a mixture of gases that form a thin layer around the Earth and act as an interface between the Earth and space. Table 10.1-1 presents some characteristics of seven of the chemical elements present in the atmosphere. It is interesting to note that the presence of 40 Ar is mainly the result of the radioactive decay of terrestrial 40 K, which was produced as part of the nucleosynthesis in a supernova explosion more than five billion years ago. Together with a cloud of interstellar gas and dust, it gave rise to our Sun and the planets. Its long half-life of 2.3 10 9 y means that it is still very common in the Earth's crust. The main characteristics of the atmosphere are that the air gets thinner with increasing altitude and that there is no upper boundary. Figure 10.1-1 shows that both the density and the pressure decrease approximately expo-nentially with height. About half of the total amount of air is below 5.6 km. On account of their different properties, the atmosphere is divided into five regions called the troposphere (lowest), stratosphere, the mesosphere, the