Antiproton stopping power in hydrogen below 120 keV and the Barkas effect

The simultaneous measurement of the spatial coordinates and times of p\ifmmode\bar\else\textasciimacron\fi{}s annihilating at rest in a ${\mathrm{H}}_{2}$ target at very low density \ensuremath{\rho} (\ensuremath{\rho}/${\mathrm{\ensuremath{\rho}}}_{0}$${10}^{\mathrm{\ensuremath{-}}2}$, ${\mathrm{\ensuremath{\rho}}}_{0}$ being the STP density) gives the possibility of evaluating the behavior of the p\ifmmode\bar\else\textasciimacron\fi{} stopping power in ${\mathrm{H}}_{2}$ at low energies (below 120 keV). It is different from that of protons (the Barkas effect). Moreover, it is shown that a rise at low-energy values (\ensuremath{\lesssim}1 keV) is needed to agree with experimental data.