Vector analyzing power for the proton-6 He elastic scattering at 71 MeV/nucleon has been measured for the first time, with a newly developed polarized proton solid target working at low magnetic field of 0.09 T. The results are found to be incompatible with a t-matrix folding model prediction. Comparisons of the data with g-matrix folding analyses clearly show that the vector analyzing power is sensitive to the nuclear structure model used in the reaction analysis. The α-core distribution in 6 He is suggested to be a possible key to understand the nuclear structure sensitivity. PACS numbers: 24.70.+s, 29.25.Pj Spin observables in scattering experiments have been rich sources of our understanding of nuclear structure, reaction, and interactions. One of the good examples is spin asymmetry in proton-proton and proton-nucleus (p-A) scatterings which is a direct manifestation of spinorbit coupling in the system. The first spin asymmetry measurements carried out by use of a double scattering method [1,2] clearly demonstrated that the spin-orbit coupling in nuclei is an order of magnitude stronger than that due to the relativistic effect [3]. At present the spin-orbit coupling in p-A scattering is quantitatively established through numerous experiments using polarized proton beams for stable targets.It is interesting to use spin asymmetry measurements to study unstable nuclei. Nuclei locating near the neutron drip line occasionally show distinctive structure such as halos or skins. The neutron rich 6 He nucleus is one of the typical nuclides with an extended neutron distribution. Since the extended neutron distribution is prominent at the nuclear surface and the spin-orbit coupling is, in nature, a surface phenomenon, it is stimulating to see how the extended neutron distributions affect the spin asymmetry, i.e., vector analyzing power in proton elastic scattering.In this Letter, we report new results of vector analyzing power for the p-6 He elastic scattering at 71 MeV/nucleon, measured with a newly developed polarized proton target. The results are compared with microscopic folding model calculations.Although cross sections in proton elastic scattering from 6 He have been extensively measured over a wide range of energies [4][5][6][7][8][9], until recently there had been no measurement of vector analyzing power. Since unstable nuclei are produced as secondary beams, we need a polarized proton target, practically in the solid state, for the spin-asymmetry studies. In addition, the solid polarized proton target should work under a low magnetic field of B ∼ 0.1 T for detection of recoiled protons with magnetic rigidity as low as 0.3 Tm. The traditional dynamical nuclear polarization technique [10], demanding a magnetic field higher than a few Tesla, can not be applied therefore. Although this difficulty might be overcome by applying a "spin frozen" operation, efforts to do so have not been successful so far. An alternative approach to overcome the problem is to develop a polarized target based on a new principle which...

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The vector analyzing power has been measured for the elastic scattering of neutron-rich 6 He from polarized protons at 71 MeV/nucleon making use of a newly constructed solid polarized proton target operated in a low magnetic field and at high temperature. Two approaches based on local one-body potentials were applied to investigate the spin-orbit interaction between a proton and a 6 He nucleus. An optical model analysis revealed that the spin-orbit potential for 6 He is characterized by a shallow and long-ranged shape compared with the global systematics of stable nuclei. A semimicroscopic analysis with a α+n+n cluster folding model suggests that the interaction between a proton and the α core is essentially important in describing the p+ 6 He elastic scattering. The data are also compared with fully microscopic analyses using non-local optical potentials based on nucleon-nucleon g-matrices.

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