Because, when passing through a grain of dust, a particle traverses < 10-4g.cm-2 of matter (the mean size of grains of dust is = 4 x10-5cm), significantly below its interaction path, the dust grains will not in practice absorb CR and will give only some additional contribution to the fragmentation and production of secondary particles by the gaseous and ionized material in the space. In most cases, however, such a contribution is negligible (some 1% for interstellar space). The rest solids in the space (stars, planets, asteroids, meteorites), whose sizes (in g/cm2) are as a rule much in excess of the nuclear interaction path, will be the CR absorbers and the generators of secondary albedo CR. If the bodies of type i with cross section Si = 4otj (where ri is a radius of body) are spatially distributed
with concentration N = dt (where dt is an average distance between bodies of type i), the mean lifetime T of a CR particle (with velocity v) relative to absorption by all the above mentioned types of celestial bodies will be
In some cases, the importance of CR absorption by the bodies is negligible. According to (Ginzburg and Syrovatsky, M1963), for example, the life-time of a relativistic CR particles in the Galaxy before the particle arrives at any star
is ~ 3 x10 years, the value which is many orders greater than the mean lifetime relative to nuclear interaction in the interstellar medium and CR diffusion from the Galaxy (~ 3x107 years). The absorption, however, should be taken into account in the problems of propagation of galactic and solar CR in vicinities of stars and planets (for example, in interplanetary space) the absorption is always of decisive importance to some modulation models (Dorman, M1963a)
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