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

Abstract

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 spinorbit 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.PACS numbers: 24.10. Ht, 24.70.+s, 25.40.Cm, 25.60.Bx, 29.25.Pj The strong spin-orbit coupling in atomic nuclei plays an important role in nuclear structure and reactions. One good example is the spin-orbit splitting of single-particle levels, which is a key ingredient for the success of the nuclear shell model [1,2]. Spin-orbit coupling is also responsible for many other phenomena such as the dominance of the prolate shape and the emergence of the isomeric intruder state. Moreover, in terms of nuclear reactions, spin-orbit coupling is responsible for the polarization effects in elastic scattering. There has recently been renewed interest in spin-orbit coupling since it is predicted to be modified in neutron-rich nuclei. A number of experimental results suggest a change in the shell structure of neutron-rich nuclei that could be explained by a reduction in the spin-orbit splitting [3][4][5]. However, there has been no experimental study examining how the spin-orbit coupling is modified in nuclear reactions of unstable nuclei.Spin asymmetry in proton-nucleus (p-A) elastic scattering is a prominent manifestation of the spin-orbit coupling in nuclear reactions. The coupling is generally represented by a spin-orbit term in the optical model potential, i.e., the spin-orbit potential. Current understanding of this potential has been based on extensive measurements and analysis of the vector analyzing powers for elastic scattering of polarized protons from various stable nuclei over a wide energy range [6][7][8][9][10][11]. It is now well established that the shape and magnitude of the spin- * Present address:

Spins in exotic nuclei: RI beam experiments with polarized targets

“…• in the center of mass system [54][55][56]. Figure 11 shows the data of the cross section and the vector analyzing power where filled circles, shaded squares, and open circles/squares are from Refs.…”

“…In this method, a microwave at the electron spin resonance frequency is irradiated to the target while an external magnetic field is varied simultaneously to cover the inhomogeneously The first experiments with the target system were to measure the vector analyzing power for the p- 6,8 He scattering at the RIPS facility, RIKEN [54][55][56]. In these experiments, the target was operated in a low magnetic field of about 0.1 T and at temperature of 100 K.…”

“…The triplet-DNP target was used to experimentally measure the vector analyzing power for the p- 6,8 He scattering at RIKEN [54][55][56]. As discussed in Section II A, the vector analyzing power in elastic scattering of non-zero spin particle is a direct manifestation of the spin-orbit coupling in nuclei.…”

“…In the presently used setup, the target material is cooled to ∼100 K by keeping it in cold nitrogen gas atmosphere. The room-temperature operation removes nitrogen gas (∼40 mg/cm 2 ) and films to separate the beam-line vacuum which are serious drawbacks in low-energy experiments [56].…”

Calculation of vector analyzing power in the p+6,8He elastic scattering at intermediate energies

“…The first reliable measurements of the vector analyzing power in elastic р 6 Не and р 8 Не-scattering at 71 MeV/nucleon have been made recently at the RIKEN accelerator [1][2][3]. An appearance of such type of date makes it possible to test different model calculations that take into account the spin-orbit interaction, because the spin-orbit coupling in nuclei manifests itself in the polarization phenomena in nuclear elastic scattering.…”