[$^3\overline{\text{He}}~$]{}injection spectrum from Sec. \[sec:production\].

In Figure \[fig:propagation\] we show the ratio $P_{\mathrm{num}}^{\mathrm{\overline{He}}}/P_{\mathrm{num}}^{\mathrm{\overline{D}}}$ for the MIN/MED/MAX propagation models and two values of the interaction rate, $\Gamma_{\mathrm{int}}$. As we will discuss in § \[subsec:cross-sections\], uncertainty in the antihelium cross-section with interstellar gas can lead to a $\sim$25% enhancement or suppression of the antihelium flux relative to that of antideuterons. Of mild importance is the higher nuclear binding energy of [$^3\overline{\text{He}}~$]{}compared to the very weakly bound [$\overline{\text{D}}~$]{}case. While this can more efficiently deplete the higher energy population where the non-annihilating inelastic cross-section dominates, the low energies of interest here are not significantly enhanced by tertiary contributions which are ignored in our treatment.

In-fact, the two-zone diffusion model neglects all diffusion in momentum space, the most important of which may be a proper treatment of interstellar re-acceleration. Several of these schemes, including diffusive re-acceleration, have been applied to the propagation of elements in more sophisticated numerical codes. While these attempts have been successful in reproducing otherwise anomalous peaks in the secondary to primary ratios of heavy elements such as B/C, they encounter problems for light elements. In particular, diffusive re-acceleration results in a spectral bump near 2 GeV/n for p and He which 