  We investigate the collision energy dependence of deuteron and antideuteron
emission in the RHIC-BES low- to mid-energy range $\sqrt{s_{NN}} = 4.6-200$ GeV
where the formation rate of antinuclei compared to nuclei is strongly
suppressed. In the coalescence picture, this can be understood as bulk emission
for nuclei in contrast to surface emission for antinuclei. By comparison with
experimental data on the coalescence parameter $B_2$, we are able to extract
the respective source geometries. This interpretation is further supported by
results from the UrQMD transport model, and establishes the following picture:
At low energies, nucleons freeze out over the total fireball volume, while
antinucleons are annihilated inside the nucleon-rich fireball and can only
freeze out on its surface. Towards higher energies, this annihilation effect
becomes irrelevant (due to the decreasing baryochemical potential) and the
system's freeze-out is driven by the mesons. Thus, the nucleon and antinucleon
freeze-out distributions become similar with increasing energy.
