Electron-Capture Isotopes could Constrain Cosmic-Ray Propagation Models

TL;DR

This study investigates electron-capture isotopes within a spiral-arm cosmic-ray propagation model, revealing their potential to constrain diffusion parameters but highlighting current measurement uncertainties as a limiting factor.

Contribution

It applies the spiral-arms model to EC isotopes, showing their dependence on halo size and spiral arm passage time, and discusses their potential to constrain cosmic-ray propagation parameters.

Findings

EC isotope ratios depend on halo size and spiral arm passage time.

Current measurement uncertainties prevent meaningful constraints.

Inconsistencies remain in isotope ratio predictions within the model.

Abstract

Electron capture (EC) isotopes are known to provide constraints on the low energy behavior of cosmic rays (CRs), such as re-acceleration. Here we study the EC isotopes within the framework of the dynamic spiral-arms CR propagation model in which most of the CR sources reside in the galactic spiral arms. The model was previously used to explain the B/C and sub-Fe/Fe ratios \citep{BoverC,Iron}. We show that the known inconsistency between the $^{49}$Ti/$^{49}$V and $^{51}$V/$^{51}$Cr ratios remains also in the spiral-arms model. On the other hand, unlike the general wisdom in which the isotope ratios depend primarily on reacceleration, we find here that the ratio also depends on the halo size ($z_\mathrm{h}$) and in spiral-arms models also on the time since the last spiral arm passage ($\tau_\mathrm{arm}$). Namely, EC isotopes can in principle provide interesting constraints on the diffusion geometry. However, with the present uncertainties in the lab measurements of both the electron attachment rate and the fragmentation cross-sections, no meaningful constraint can be placed.