Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Shallow and diﬀuse spin-orbit potential for proton elastic scatteringfrom neutron-rich helium isotopes at 71 MeV/nucleon†
S. Sakaguchi,∗1,∗2 T. Uesaka,∗1 N. Aoi,∗3 Y. Ichikawa,∗1 K. Itoh,∗4 M. Itoh,∗5 T. Kawabata,∗6 T. Kawahara,∗1Y. Kondo,∗7 H. Kuboki,∗1 T. Nakamura,∗7 T. Nakao,∗4 Y. Nakayama,∗7 H. Sakai,∗1 Y. Sasamoto,∗4K. Sekiguchi,∗8 T. Shimamura,∗7 Y. Shimizu,∗1 and T. Wakui∗58Strong spin-orbit coupling in atomic nuclei plays animportant role in nuclear structure and reactions. Its
manifestation in neutron-rich nuclei has attracted extensive interest, since a number of experimental results
suggest a change in the shell structure that could be
explained by a reduction in the spin-orbit splitting. On
the other hand, there has been no experimental study
on how the spin-orbit coupling is modiﬁed in nuclear
reactions. Spin asymmetry in proton–nucleus scattering is a prominent manifestation of the spin-orbit
coupling in nuclear reactions. The spin-orbit term in
the optical model potential is generally expressed by aderivative of the density distribution1–3) . It would beinteresting to probe the nature of the spin-orbit potential for a nucleus with a very diﬀuse surface.
In order to investigate the eﬀect of the exotic density distribution on the spin-orbit potential, we measured the vector analyzing powers for proton elastic
scattering from 6 He and 8 He at 71 MeV/nucleon at
RIPS beamline at RI Beam Factory using the solid
polarized proton target specially constructed for theRI-beam experiment 4) . To determine the spin-orbit
potentials, we performed a phenomenological opticalmodel analysis using the ECIS79 code. For the function of the potential, we used a standard Woods-Saxon
form factor with a Thomas-type spin-orbit term. We
search for a parameter set that reproduces both thedσ/dΩ and Ay data. Details of the ﬁtting procedureand obtained parameters can be found in Refs.5,6) .
The characteristics of the spin-orbit potential is discussed in terms
radius of the potential√∫ of the r.m.s.
∫2 1/22⟨rls ⟩r Vls (r)dr/ Vls (r)dr and the ampli=
tude of rVls (r) at the peak position. Here, r is thedistance from the center-of-mass of 6,8 He and Vls (r) is
the spin-orbit potential. Figure 1(a) shows the mass2 1/2
number dependence of the ⟨rls⟩
values for the spinzero nuclei. The closed circles show the potentials
locally obtained for each nucleus. The dashed anddot-dashed curves represent the global optical poten2 1/2
tials7,8) . We can see that the ⟨rls⟩
values of 6 He and†
∗1∗2
∗3
∗4
∗5
∗6∗7∗8He are remarkably larger than the systematics. Moreover, it is interesting to ﬁnd a close similarity between
2 1/2
the behavior of ⟨rls⟩
and the matter radius rm , plotted as the open squares in Fig. 1(a). This indicatesthe particular sensitivity of the spin-orbit interaction
to the nuclear surface structure.
Figure 1(b) displays the amplitude of rVls (r) at thepeak position. The peak amplitudes for 6 He and 8 He
are considerably smaller than the standard values of
3.5–5.5 MeV fm. From these results, it is concluded
that the spin-orbit potentials between a proton andneutron-rich 6 He and 8 He nuclei are considerably shallower and more diﬀuse than the global systematics ofnuclei along the stability line. This is considered to
be a consequence of the diﬀuse density distribution ofthese neutron-rich isotopes.

Fig. 1. See text for details. The symbols for rm are shiftedvertically by −0.5 fm to prevent overlap.

References
1) C. B. Dover et al.: Nucl. Phys. A 190, 373 (1972).
2) R. R. Scheerbaum: Nucl. Phys. A 257, 77 (1976).
3) E. Bauge et al.: Phys. Rev. C 58, 1118 (1998).
4) T. Wakui: Proc. of PST2005 (World Scientific, 2007).
5) S. Sakaguchi et al.: Phys. Rev. C 84, 024604 (2011).
6) S. Sakaguchi et al.: Phys. Rev. C 87, 021601(R)(2013).
7) R. L. Varner et al.: Phys. Rep. 201, 58 (1991).
8) A. J. Koning et al.: Nucl. Phys. A 713, 231 (2003).

Condensed from the article in Phys. Rev. C 87, 021601(R)(2013)RIKEN Nishina Center
Department of Physics, Kyushu University
RCNP, Osaka University
CNS, University of TokyoCYRIC, Tohoku University
Department of Physics, Kyoto University
Department of Physics, Tokyo Institute of Technology
Department of Physics, Tohoku University- 15 -