The boron-to-carbon abundance ratio and Galactic propagation of cosmic radiation

TL;DR

This study measures the boron-to-carbon ratio in cosmic rays at high energies to understand Galactic propagation, suggesting a soft source spectrum and energy-dependent path length, using balloon data and propagation models.

Contribution

First measurement of the B/C ratio up to 2 TeV per amu, providing new constraints on cosmic-ray propagation and source spectra.

Findings

B/C ratio decreases with energy up to 2 TeV per amu

Source spectrum is a soft power law with index ~2.37

Propagation path length follows a power law in energy with exponent ~0.53

Abstract

In two long-duration balloon flights in 2003 and 2006, the TRACER cosmic-ray detector has measured the energy spectra and the absolute intensities of the cosmic-ray nuclei from boron (Z = 5) to iron (Z = 26) up to very high energies. In particular, the second flight has led to results on the energy spectrum of the secondary boron nuclei, and on the boron abundance relative to that of the heavier primary parent nuclei, commonly quantified as the "B/C abundance ratio". The energy dependence of this ratio, now available up to about 2 TeV per amu, provides a measure for the energy dependence of cosmic-ray propagation through the Galaxy, and for the shape of the cosmic-ray source energy spectrum. We use a Leaky-Box approximation of cosmic-ray propagation to obtain constraints on the relevant parameters on the basis of the results of TRACER and of other measurements. This analysis suggests that the source energy spectrum is a relatively soft power law in energy E^{-\alpha}, with spectral exponent \alpha = 2.37 \pm 0.12, and that the propagation path length \Lambda(E) is described by a power law in energy with exponent \delta = 0.53 \pm 0.06, but may assume a constant residual value \Lambda_0 at high energy. The value of \Lambda_0 is not well constrained but should be less than about 0.8 g cm^{-2}. Finally, we compare the data with numerical solutions of a diffusive reacceleration model, which also indicates a soft source spectrum.