Optical model potential analysis of n¯A and nA interactions

Abstract

We use a momentum-dependent optical model potential to analyze the annihilation cross sections of the antineutron $\overline{n}$ on C, Al, Fe, Cu, Ag, Sn, and Pb nuclei for projectile momenta ${p}_{\mathrm{lab}}$ $\ensuremath{\lesssim}500\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/c$. We obtain a good description of annihilation cross section data of Barbina et al. [Nucl. Phys. A 612, 346 (1997)] and of Astrua et al. [Nucl. Phys. A 697, 209 (2002)] which exhibit an interesting dependence of the cross sections on ${p}_{\mathrm{lab}}$ as well as on the target mass number $A$. We also obtain the neutron ($n$) nonelastic reaction cross sections for the same targets. Comparing the $nA$ reaction cross sections ${\ensuremath{\sigma}}_{\mathrm{rec}}^{nA}$ to the $\overline{n}A$ annihilation cross sections ${\ensuremath{\sigma}}_{\mathrm{ann}}^{\overline{n}A}$, we find that ${\ensuremath{\sigma}}_{\mathrm{ann}}^{\overline{n}A}$ is significantly larger than ${\ensuremath{\sigma}}_{\mathrm{rec}}^{nA}$, that is, the ${\ensuremath{\sigma}}_{\mathrm{ann}}^{\overline{n}A}/{\ensuremath{\sigma}}_{\mathrm{rec}}^{nA}$ cross section ratio lies between the values of about 1.5 to 4.0 in the momentum region where comparison is possible. The dependence of the $\overline{n}$ annihilation cross section on the projectile charge is also examined in comparison with the antiproton $\overline{p}$. Here we predict the $\overline{p}A$ annihilation cross section on the simplest assumption that both $\overline{p}A$ and $\overline{n}A$ interactions have the same nuclear part of the optical potential but differ only in the electrostatic Coulomb interaction. Deviation from a such simple model extrapolation in measurements will provide new information on the difference between $\overline{n}A$ and $\overline{p}A$ potentials.