The abundance of radioactive nuclei under different physical environments around the solar system

Abstract

Abstract Differential equations describing the propagation of cosmic-ray nuclei from 7Be to 56Fe in a homogeneous interstellar medium (ISM) were solved simultaneously in the time domain, so as to determine the escape path length using the observed B/C ratio as a constraint. The spectra of nuclei thus obtained were then used to obtain the 10Be/9Be ratio as a function of energy. From a comparison with the data, we determined the mean density of nH in the confinement volume of cosmic rays to be 0.2 atom/cc and the corresponding escape life time of 30 My. The effect of the solar system being in a “local bubble” of low density was examined. It was found that if cosmic-ray nuclei have spent considerable amount of time in the local bubble, the mean value of nH in the Galaxy needs to be raised to account for the observation. The ratio of 26Al/27Al and 36Cl/Cl are more sensitive to the propagation in the local hole and our study showed that in order that the data are consistent with each other, the diffusion co-efficient in the local bubble should be > 1029cm2/s. This clearly demonstrates the need to include in the diffusion models the variation of diffusion co-efficient with gas density. We then examined the effect of the local hole on the abundance ratio of 14C/13C and bring out the need to measure 14C to further constrain the diffusion co-efficient in the local hole.