K. Nakanishi

Complete electric dipole response and the neutron skin in 208Pb

A benchmark experiment on (208)Pb shows that polarized proton inelastic scattering at very forward angles including 0° is a powerful tool for high-resolution studies of electric dipole (E1) and spin magnetic dipole (M1) modes in nuclei over a broad excitation energy range to test up-to-date nuclear models. The extracted E1 polarizability leads to a neutron skin thickness r(skin) = 0.156(-0.021)(+0.025) fm in (208)Pb derived within a mean-field model [Phys. Rev. C 81, 051303 (2010)], thereby constraining the symmetry energy and its density dependence relevant to the description of neutron stars.

Pygmy dipole resonance in 208Pb

Scattering of protons of several hundred MeV is a promising new spectroscopic tool for the study of electric dipole strength in nuclei. A case study of 208 Pb shows that, at very forward angles, J π = 1 − states are strongly populated via Coulomb excitation. A separation from nuclear excitation of other modes is achieved by a multipole decomposition analysis of the experimental cross sections based on theoretical angular distributions calculated within the quasiparticle-phonon model. The B(E1) transition strength distribution is extracted for excitation energies up to 9 MeV; that is, in the region of the so-called pygmy dipole resonance (PDR). The Coulomb-nuclear interference shows sensitivity to the underlying structure of the E1 transitions, which allows for the first time an experimental extraction of the electromagnetic transition strength and the energy centroid of the PDR.

High-resolution study of the Be-9(He-3,t) B-9 reaction up to the B-9 triton threshold

A high-energy-resolution {sup 9}Be({sup 3}He,t){sup 9}B charge-exchange reaction was performed around a scattering angle of 0 deg. and at an intermediate incident energy of 140 MeV/nucleon for the study of precise Gamow-Teller (GT) transition strengths. The energy resolution of 30 keV allowed a precise deconvolution of the spectrum and the determination of angular distributions of cross sections, excitation energies, and decay widths. The GT strength of 10 states has been determined for the first time using the GT strength of the analogous {beta} decays of {sup 9}Li and {sup 9}C as standards. The large difference between the GT strengths going to the low-lying T=1/2 and the highly excited T=3/2 states is interpreted as a result of their different spatial structures. The obtained GT strength distribution is compared to the results of a (p,n) experiment with lower resolution performed in the 1980s. In particular, the width of the 16.8 MeV, J{sup {pi}=}(5/2{sup +}) state has been determined for the first time.

High‐Resolution B(GT) studies with (3He,t)reactions

It has been shown that the (3He,t) charge exchange reaction at intermediate energies, used at zero momentum transfer in combination with high‐resolution magnetic spectrometers is a very powerful probe for Gamow‐Teller strengths. Several N = Z nuclei such as 46V [1] or 50Mn [2] have been investigated. Recently ab‐initio nuclear structure calculations have become possible for light nuclei. Thus experimental studies on light nuclei have become of great current interest [3]. We review previous results obtained in 11C with respect to nuclear structure aspects and the Hoyle state, and present some early results of a 9Be(3He,t)9B experiment carried out recently, pointing out the significance of high‐resolution charge‐exchange reactions and the investigation of Gamow‐Teller strengths for the understanding of nuclear structure in light nuclei.

High-resolution study of the Be9(He3,t)B9 reaction up to the B9 triton threshold

A high-energy-resolution {sup 9}Be({sup 3}He,t){sup 9}B charge-exchange reaction was performed around a scattering angle of 0 deg. and at an intermediate incident energy of 140 MeV/nucleon for the study of precise Gamow-Teller (GT) transition strengths. The energy resolution of 30 keV allowed a precise deconvolution of the spectrum and the determination of angular distributions of cross sections, excitation energies, and decay widths. The GT strength of 10 states has been determined for the first time using the GT strength of the analogous {beta} decays of {sup 9}Li and {sup 9}C as standards. The large difference between the GT strengths going to the low-lying T=1/2 and the highly excited T=3/2 states is interpreted as a result of their different spatial structures. The obtained GT strength distribution is compared to the results of a (p,n) experiment with lower resolution performed in the 1980s. In particular, the width of the 16.8 MeV, J{sup {pi}=}(5/2{sup +}) state has been determined for the first time.

ISOSPIN SYMMETRY OF Tz = ±3/2 → ±1/2 GAMOW-TELLER TRANSITIONS IN A = 37 AND A = 41 NUCLEI

Under the assumption of good isospin symmetry, Tz = ±3/2 → ±1/2 Gamow-Teller (GT) transitions in a mass A isobar are analogous, where Tz is the z component of isospin T defined by 1/2(N - Z). We studied the Tz = +3/2 → +1/2 GT transitions by using the 37Cl(3He,t)37Ar and 41K(3He,t)41Ca reactions at Ebeam = 140 MeV/nucleon. The GT transition strengths in 37Ar and 41Ca were obtained up to the excitation energy (Ex) of 14.2 MeV and 10.4 MeV with energy resolutions of 30 and 35 keV, respectively. The obtained GT strengths were compared with those of the Tz = -2/3 → -1/2 mirror transitions, measured by the β-decay of 37Ca and 41Ti, respectively. It was found that the overall distributions of the mirror transitions were similar in both A = 37 and A = 41 systems. In A = 37 the differences seen at lower excitation energies are suggested to be the effect of the tensor interaction in the charge-exchange reaction. The differences seen at higher excitation energies are suggested to be the effect of the Coulomb interaction that can break the mirror symmetry of the strength distributions.

Complete dipole response in 208Pb from high-resolution polarized proton scattering at 0°

The structure of electric and magnetic dipole modes in 208Pb is investigated in a high‐resolution measurement of the (p,p′) reaction under 0°. First results on the E1 strength in the region of the pygmy dipole resonance are reported.

Study of M1 and E1 excitations by high-resolution proton inelastic scattering measurement at forward angles

Experimental technique for measuring proton inelastic scattering with high‐resolution at 295 MeV and at forward angles including zero degrees is described. The method is useful for extracting spin part of the M1 strength via nuclear excitation as well as E1 strength via Coulomb excitation. An excitation energy resolution of 20 keV, good scattering angle resolution, and low background condition have been achieved. The experimental technique was applied for several sd and pf shell nuclei.