TY - JOUR
T1 - Electric and magnetic dipole strength in Ni 58 from forward-angle proton scattering
AU - Brandherm, I.
AU - Von Neumann-Cosel, P.
AU - Mancino, R.
AU - Martínez-Pinedo, G.
AU - Matsubara, H.
AU - Ponomarev, V. Yu
AU - Richter, A.
AU - Scheck, M.
AU - Tamii, A.
N1 - Publisher Copyright:
© 2024 American Physical Society.
PY - 2024/9
Y1 - 2024/9
N2 - Background: Electric and magnetic dipole strengths in nuclei at excitation energies well below the giant resonance region are of interest for a variety of nuclear structure problems including a possible electric dipole toroidal mode or the quenching of spin-isospin flip modes. Purpose: The aim of the present work is a state-by-state analysis of possible E1 and M1 transitions in Ni58 with a high-resolution (p,p′) experiment at 295 MeV and very forward angles including 0∘ and a comparison to results from studies of the dipole strength with the (γ,γ′) and (e,e′) reactions. Methods: The E1 and M1 cross sections of individual peaks in the spectra are deduced with a multipole decomposition analysis (MDA). They are converted to reduced E1 and spin M1 transition strengths using the virtual photon method of relativistic Coulomb excitation and the unit cross-section method, respectively. The experimental M1 strength distribution is compared to large-scale shell-model calculations with the effective GXPF1A and KB3G interactions. Results: In total, 11 E1 and 26 M1 transitions could be uniquely identified in the excitation energy region 6-13 MeV. In addition, 22 dipole transitions with preference for either E1 or M1 multipolarity and 57 transitions with uncertain multipolarity were found. Despite the high level density good agreement is obtained for the deduced excitation energies of J=1 states in the three types of experiments indicating that the same states are excited. The B(E1) and B(M1) strengths deduced in the (γ,γ′) experiments are systematically smaller than in the present work because of the lack of information on branching ratios to lower-lying excited states and the competition of particle emission. Fair agreement with the B(M1) strengths extracted from the (e,e′) data is obtained after removal of E1 transitions uniquely assigned in the present work belonging to a low-energy toroidal mode with unusual properties mimicking M1 excitations in electron scattering. The shell-model calculations provide a good description of the isospin splitting and the running sum of the M1 strength. A quenching factor 0.74 for the spin-isospin part of the M1 operator is needed to attain quantitative agreement with the data. Conclusions: High-resolution forward-angle inelastic proton scattering experiments at beam energies of about 300 MeV are a highly selective tool for an extraction of resolved E1 and M1 strength distributions in medium-mass nuclei. Fair agreement with results from electron scattering experiments is obtained indicating a dominance of spin contributions to the M1 strength. Shell-model calculations are in good agreement with gross properties of the M1 strength distribution when a quenching factor for the spin-isospin part comparable to the one needed for a description of Gamow-Teller (GT) strength is included.
AB - Background: Electric and magnetic dipole strengths in nuclei at excitation energies well below the giant resonance region are of interest for a variety of nuclear structure problems including a possible electric dipole toroidal mode or the quenching of spin-isospin flip modes. Purpose: The aim of the present work is a state-by-state analysis of possible E1 and M1 transitions in Ni58 with a high-resolution (p,p′) experiment at 295 MeV and very forward angles including 0∘ and a comparison to results from studies of the dipole strength with the (γ,γ′) and (e,e′) reactions. Methods: The E1 and M1 cross sections of individual peaks in the spectra are deduced with a multipole decomposition analysis (MDA). They are converted to reduced E1 and spin M1 transition strengths using the virtual photon method of relativistic Coulomb excitation and the unit cross-section method, respectively. The experimental M1 strength distribution is compared to large-scale shell-model calculations with the effective GXPF1A and KB3G interactions. Results: In total, 11 E1 and 26 M1 transitions could be uniquely identified in the excitation energy region 6-13 MeV. In addition, 22 dipole transitions with preference for either E1 or M1 multipolarity and 57 transitions with uncertain multipolarity were found. Despite the high level density good agreement is obtained for the deduced excitation energies of J=1 states in the three types of experiments indicating that the same states are excited. The B(E1) and B(M1) strengths deduced in the (γ,γ′) experiments are systematically smaller than in the present work because of the lack of information on branching ratios to lower-lying excited states and the competition of particle emission. Fair agreement with the B(M1) strengths extracted from the (e,e′) data is obtained after removal of E1 transitions uniquely assigned in the present work belonging to a low-energy toroidal mode with unusual properties mimicking M1 excitations in electron scattering. The shell-model calculations provide a good description of the isospin splitting and the running sum of the M1 strength. A quenching factor 0.74 for the spin-isospin part of the M1 operator is needed to attain quantitative agreement with the data. Conclusions: High-resolution forward-angle inelastic proton scattering experiments at beam energies of about 300 MeV are a highly selective tool for an extraction of resolved E1 and M1 strength distributions in medium-mass nuclei. Fair agreement with results from electron scattering experiments is obtained indicating a dominance of spin contributions to the M1 strength. Shell-model calculations are in good agreement with gross properties of the M1 strength distribution when a quenching factor for the spin-isospin part comparable to the one needed for a description of Gamow-Teller (GT) strength is included.
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U2 - 10.1103/PhysRevC.110.034319
DO - 10.1103/PhysRevC.110.034319
M3 - Article
AN - SCOPUS:85204433285
SN - 2469-9985
VL - 110
JO - Physical Review C
JF - Physical Review C
IS - 3
M1 - 034319
ER -