References - Bogoliubov Laboratory of Theoretical Physics - JINR

3.3 Effect of valence neutrons on spin-orbit potential
Here, we discuss the effect of valence neutrons on the spin-orbit potential from the systematics
in helium isotopes. For this purpose, an α + xn cluster folding calculation [5]
could be a useful method, since the contribution of an α core and valence neutrons can be
separately evaluated. It has been expected from the calculation that the contribution of
valence neutrons to the spin-orbit potential is one-order of magnitude smaller than that
of an α core. Consequently, the spin-orbit potentials of 6,8 He are expected to be related
to the spatial distribution of the α core in a nucleus, i.e., the LS radius should increase
with the diffuseness of the α core distribution, whereas the LS amplitude should decrease.
The LS radius and LS amplitude of helium
isotopes are plotted in Fig. 5 as a
function of the proton orbital radius, which
represents the diffuseness of the α core distribution.
Although there are large experimental
uncertainties, almost linear relations
can be seen. From this we can derive
a picture that the valence neutrons affect
the spin-orbit potential rather strongly
through the p − α interaction than via the
direct p − n interaction. The recoil motion
Figure 5: LS radius and LS amplitude of helium
isotopes as functions of proton radius.
of the α core induced by valence neutrons can be important in the explanation of the
large difference between spin-orbit potentials in 4 He and neutron-rich helium isotopes.
4 Summary
The vector analyzing power of the proton elastic scattering from 6 He and 8 He have been
measured at 71 MeV/A. We found that the spin-orbit potentials in 6 He and 8 He are
significantly shallower than those in stable nuclei. This shallowness can be naturally
explained by the large diffuseness of the neutron-rich helium isotopes. The dominance of
p − α core interaction in the p− 6,8 He spin-orbit potentials is also indicated.
References
[1] T. Wakui et al., Nucl. Instr. and Meth. 550 (2005) 521.
[2] T. Uesaka et al., Nucl. Instr. and Meth. 526 (2004) 186.
[3] A. Korsheninnikov et al., Nuclear Physics A 617 (1997) 45.
[4] R. Henneck et al., Nuclear Physics A 571 (1994) 541.
[5] Y. Iseri et al., Genshikaku Kenkyu 49 No.4 (2005) 53.
[6] H. Sakaguchi et al., Physical Review C 26 (1982) 944.
[7] B.A. Watson et al., Physical Review 182 (1969) 977.
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