2. Calculation of the characteristic decay times of K-capture isotopes
In the process of electron attachment an electron in the target medium becomes bound to the cosmic ray (Letaw, Silberberg and Tsao, 1984, see also Silberberg et al., 1991). This process is termed radiative attachment if the electron may be considered free and if a photon is emitted to conserve energy and momentum. The cross section for attachment of a free electron is calculated as that for the inverse process of the photoelectric effect using the principle of detailed balancing (Heitler, 1966; Meaker Davisson & Evans, 1952: Raisbeck & Yiou, 1971). The value of this cross section and of its energy dependence around 500 MeV/nuc agree within 10% or less with those taken from the curves of Letaw et al. (Letaw et al. 1985) and with the calculation of Crawford on argon at 400 Mev/nuc (Crawford, 1979). Radiative attachment dominates at cosmic ray energies above 200 MeV/nuc in an interstellar medium of normal cosmic abundances (Raisbeck & Yiou, 1971). For the isotopes listed in Table 1 stripping is neglected and the effective K-capture decay is determined by the attachment rate. The lifetimes are calculated for an IS medium with 90% H + 10% He and with a density n = 0.3 atoms cm-3. In Table 1, we show the calculated lifetimes for nuclear destruction, radiative attachment (and subsequent K-capture) and their combination as a function of charge and at two interstellar energies. Note that the ratio of these lifetimes does not depend on the adopted value of the density of the medium.
Table 1. K-capture isotopes
© European Southern Observatory (ESO) 1998
Online publication: July 27, 1998