in Figure \[fig:propagation\]. The lowered rigidity of [$^3\overline{\text{He}}~$]{}causes a $\sim$50% suppression at low energies relative to the [$\overline{\text{D}}~$]{}modulation factor. It has been shown that at GAPS energies, the Force Field Approximation is within a factor 2 of the minimum and maximum values computed in a full numerical treatment of heliospheric [$\overline{\text{D}}~$]{}transport [@fornengo2013]. Much of the discrepancy between analytic and numerical models should disappear when taking the ratio of modulation between antihelium and antideuterons as the first order rigidity modifications are already captured by the Force-Field Model.

Interaction Cross Sections {#subsec:cross-sections}
--------------------------

In this subsection we discuss [$^3\overline{\text{He}}~$]{}interaction rates with the ISM and compare them to the [$\overline{\text{D}}~$]{}case. $\Gamma_{\mathrm{int}}$ in Eq. (\[eq:transport\]) is given by: $$\begin{aligned}
\label{eq:gamma_int}
\Gamma_{\text{int}}= (n_\text{H} + 4^{2/3} n_{\text{He}})~ v~ \sigma_{\overline{\text{He}},p}\end{aligned}$$ where we have assumed the H and He gas cross-sections are related by a geometrical factor $4^{2/3}.$ For the Galactic Disk’s interstellar hydrogen and helium densities we use $n_{\text{H}}= 1 ~\text{cm}^{-3}$ and $n_{\text{He}} = 0.07 n_{\text{H}} $. $v$ is the antihelium velocity through the ISM, and $\sigma_{\overline{\text{He}},p}$ is interaction cross-section of antihelium with protons.

Direct measurements of the antihelium-proton annihilation and inelastic cross-sections needed in Eq. (\[eq:gamma\_int\]) are not available. Instead, we use the parameterizations in from 