The structure of low-lying 1− states in 90,94Zr from (α,α′γ) and (p,p′γ) reactions

F C L Crespi ,N Blasi,T Klaus,B Wasilewska,V Werner,A Bracco,M Spieker,A Maj,N Ichige,M Weinert,A Czeszumska,V Derya,S Zhu,S Bassauer,O Wieland,M Krzysiek,F Camera,T Koike,Chris Sullivan ,N Aoi,N Pietralla,Yukio Yamamoto ,A Zilges,Mohsen Harakeh ,E G Lanza ,M Ciemała,L M Donaldson ,Y Fang ,D G Jenkins ,M Liu,Akihisa Inoue ,M P Carpenter ,A Brown ,H.J Ong,S Noji,J Schmitt ,G Steinhilber,D L Balabanski ,G Gey,R.G.T Zegers,D Savran,J Isaak,Hoa Ha ,Tetsuya Yamamoto ,N Kobayashi,Xian-Rong Zhou ,A Tamii,Chihiro Iwamoto ,P Davies ,J J Carroll ,L Morris,E Ideguchi,P Von Neumann-Cosel ,S G Pickstone ,Toshikazu Hashimoto ,M Kumar Raju ,H Fujita

doi:10.1016/j.physletb.2021.136210

Abstract

The low-lying dipole strength in the 90,94Zr nuclei was investigated via (p,p′γ) at 80 MeV and (α,α′γ) at 130 MeV. The experiments, made at RCNP, used the magnetic spectrometer Grand Raiden for the scattered particles and the array CAGRA with HPGe detectors for the γ-decay. For 94Zr these are the first data for both reactions and for 90Zr these are the first data with (p,p′γ) and the first ones at high resolution for (α,α′γ). The comparison of the present results for the two nuclei with existing (γ,γ′) data shows that both nuclear probes produce an excitation pattern different than that of the electromagnetic probes.DWBA calculations were made using form factors deduced from transition densities, based on RPA calculations, characterized by a strong neutron component at the nuclear surface. A combined analysis of the two reactions was performed for the first time to investigate the isoscalar character of the 1− states in 90,94Zr. The (p,p′γ) cross section was calculated using values for the isoscalar electric dipole energy-weighted sum rule (E1 ISEWSR) obtained from the (α,α′γ) data. The isoscalar strength for 90Zr was found to exhaust 20 ± 2.5% of the EWSR in the energy range up to 12 MeV. In case of 94Zr, a strength of 9 ± 1.1% of the EWSR was found in the range up to 8.5 MeV.Although an overall general description was obtained in the studied energy intervals, not all proton cross sections were well reproduced using the isoscalar strength from (α,α′γ). This might suggest mixing of isoscalar and isovector components and that this mixing and the degree of collectivity are not the same for all the 1− states below the particle binding energy.

Highlights

The low-lying dipole strength in the 90,94Zr nuclei was investigated via (p,p γ) at 80 MeV and (α,α γ) at 130 MeV
The red and blue regions correspond to the expected ratios for E2 and E1 transitions, respectively
In panel (a) of Fig. 3, a typical transition density for the pygmy dipole resonance (PDR) is displayed for 90Zr [25], calculated with a SGII interaction [26, 27]. It shows at the surface the characteristic mixing of isoscalar and isovector character, i.e. the proton and neutron contributions are in phase inside the nuclear r2δρ [fm-1]

Summary

Preprint submitted to Physics Letter B

The problem of understanding the features of the lowlying dipole strength at around and below the particle binding energy, commonly denoted as pygmy dipole resonance (PDR), is presently attracting particular interest (see, e.g., [1], [2]) and driving several experimental and theoretical efforts. The spin of the states which form a continuum distribution in the energy region of the PDR can be inferred by looking at the ratio between the number of counts measured at the two angles This ratio is shown by the horizontal bars in Fig. for the two nuclei 90Zr and 94Zr. In the figure, the red and blue regions correspond to the expected ratios for E2 and E1 transitions, respectively. In panel (a) of Fig. 3, a typical transition density for the PDR is displayed for 90Zr [25], calculated with a SGII interaction [26, 27] It shows at the surface the characteristic mixing of isoscalar and isovector character, i.e. the proton and neutron contributions are in phase inside the nuclear r2δρ [fm-1]

Alpha Proton Theo

Findings

Calculation Data