  We report the first measurement of low-energy proton-capture cross sections
of $^{124}$Xe in a heavy ion storage ring. $^{124}$Xe$^{54+}$ ions of five
different beam energies between 5.5 AMeV and 8 AMeV were stored to collide with
a windowless hydrogen target. The $^{125}$Cs reaction products were directly
detected. The interaction energies are located on the high energy tail of the
Gamow window for hot, explosive scenarios such as supernovae and X-ray
binaries. The results serve as an important test of predicted astrophysical
reaction rates in this mass range. Good agreement in the prediction of the
astrophysically important proton width at low energy is found, with only a 30%
difference between measurement and theory. Larger deviations are found above
the neutron emission threshold, where also neutron- and ${\gamma}$-widths
significantly impact the cross sections. The newly established experimental
method is a very powerful tool to investigate nuclear reactions on rare ion
beams at low center-of-mass energies.
