Shallow and diffuse spin-orbit potential for proton elastic scattering from neutron-rich helium isotopes at 71 MeV/nucleon

Vector analyzing powers for proton elastic scattering from ${}^8$He at 71 MeV/nucleon have been measured using a solid polarized proton target operated in a low magnetic field of 0.1 T. The spin-orbit potential obtained from a phenomenological optical model analysis is found to be significantly shallower and more diffuse than the global systematics of stable nuclei, which is an indication that the spin-orbit potential is modified for scattering involving neutron-rich nuclei. A close similarity between the matter radius and the root-mean-square radius of the spin-orbit potential is also identified.

Figure 1

Differential cross section (upper) and analyzing power (lower) of $p$-${}^{6,8}$He elastic scattering at 71 MeV/nucleon.

Figure 2

Radial dependence of the optical potential between a proton and a ${}^8$He nucleus.

Figure 3

Two-dimensional distribution of the radius and diffuseness parameters of the spin-orbit term of the local (filled circles) and global (dot-dashed: CH89, dashed: KD03) potentials for spin-zero nuclei. Solid and dotted lines indicate $\Delta {\chi }_{A_y}^2=$ 1 and 3, respectively.

Figure 4

The ${\langle r_{ls}^2\rangle }^{1/2}$ and $r_m$ (upper) values and the peak amplitude $V_s/2a_s$ (lower) of the spin-orbit potentials for light spin-zero nuclei. For ${}^8$He, the results of set A are shown. Those with sets B and C are consistent with them within the statistical uncertainties. The symbols for $r_m$ are shifted horizontally by $-$0.5 fm to prevent overlap.