Nemets Effect in Deuteron Breakup by Heavy Nuclei

Nemets et al. have suggested that deuteron breakup by heavy nuclei may be appreciably different in closed-shell nuclei from that in neighboring non-closed-shell nuclei because of differences in the diffuseness of the nuclear surface; we refer to this as the Nemets effect. Their experimental evidence, based on measurements at 13.6 MeV in the Ca-Ni region, has been reinvestigated with a more favorable experimental arrangement. It was found that ($d, \mathrm{pn}$) and ($d, \mathrm{np}$) reactions are responsible for a large fraction of the events; they seem to account for most of the differences observed, so there is no good evidence for the effect in this region. However, a study with 17- and 14.5-MeV deuterons on ${\mathrm{Pb}}^{208}$, ${\mathrm{Bi}}^{209}$, ${\mathrm{Au}}^{197}$, and ${\mathrm{Pt}}^{198}$ revealed a strong Nemets effect, with ${\mathrm{Pb}}^{208}$ having a cross section 1.2 to 2.5 times larger than the others. Since a Nemets effect could not be explained if the breakup were by Coulomb forces, nuclear forces must contribute strongly. From this, it is deduced that the usual method for determining the radius at which breakup occurs is invalid. An alternative treatment based on the difference between the neutron and proton nuclear potentials is given, and it is concluded that the breakup occurs at $r\simeq 1.3A^{\frac{1}{3}}$ F.