Abstract
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 58Ni
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.
Purpose: The aim of the present work is a state-by-state analysis of possible 𝐸1 and 𝑀1 transitions in 58Ni
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.
| Original language | English |
|---|---|
| Article number | 034319 |
| Number of pages | 15 |
| Journal | Physical Review C |
| Volume | 110 |
| DOIs | |
| Publication status | Published - 17 Sept 2024 |