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 .