Experimental determination of anIπ=2−ground state inCu72

Abstract

This article reports on the ground-state spin and moments measured in 72,74 Cu using collinear laser spectroscopy at the CERN On-Line Isotope Mass Separator (ISOLDE) facility. From the measured hyperfine coefficients, the nuclear observables µ( 72 Cu) = −1.3472(10)µ N , µ( 74 Cu) = −1.068(3)µ N , Q( 72 Cu) = +8(2) efm 2 , Q( 74 Cu) = +26(3) efm 2 , I ( 72 Cu) = 2, and I ( 74 Cu) = 2 have been determined. Through a comparison of the measured magnetic moments with different models, the negative moment reveals a strong πf 5/2 ⊗ νg 9/2 component in the ground-state wave function. Consequently, a negative parity has been assigned to the ground states of 72,74 Cu. Large-scale shell-model calculations illustrate the strong sensitivity of the nuclear moments to configuration mixing and to the effective interaction employed. The neutron-rich nuclei surrounding the Z = 28 and N = 50 shell closures have received a great deal of experimental and theoretical attention in the last decade. This region presents a key proving ground for the latest shell-model interactions, since it offers an attractively simple structure of the excited states in terms of particle-particle or particle-hole couplings. A compelling question in this region is related to the rapid reduction in energy of the 5/2 − state as the νg 9/2 orbital is filled in the Cu isotopes [1,2]. Given that this state remains static at approximately 1 MeV as neutrons fill the fp shell, its abrupt change at N = 40 garnered a great deal of interest and motivated further experimental and theoretical attention [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. A major step in understanding the evolution of nuclear structure in this region was the suggestion to include the monopole term from the tensor force interaction [18,19]. This work predicted a reduction in energy of the 5/2 − state and an inversion with the 3/2 − state in the mid-shell region, which was recently confirmed to occur at N = 46 in the odd-Cu isotopes [20]. Effective shell-model interactions which include this effect have recently been developed in the fpg model space. Two of these interactions start from a 56 Ni core [21,22], while the most recent one also includes excitations of protons from the πf 7/2 orbit across Z = 28 [23]. The odd-odd Cu isotopes are an ideal testing ground for these models, as their properties are extremely sensitive to the proton-neutron interaction.Recent beta-decay studies of 72 Ni [8] have tentatively assigned a spin I = 2 to the ground state (gs) of 72 Cu. Shell-model calculations, based on effective and realistic interactions, could not reproduce such gs spin [8], but placed the 2 − and 2 + states around 400 keV. A gs spin I = 2 is particularly interesting since the spins of 71,73 Cu now have been measured as I = 3/2, and their magnetic moments are compatible with a leading πp 3/2 configuration [20]. However, a [πp 3/2 ⊗ νg 3 9/2 , σ = 1] cannot couple to spin 2, so this configuration cannot be the leading term in the gs wave function of 72 Cu. Alternatively it could be...

NUCLEAR STRUCTURE STUDY WITH CORE EXCITATIONS IN Ni REGION: FOR fpg9/2 SPACE

“…1,2,3,4,5 For the copper isotopes, the important question is related to rapid reduction in the energy of 5/2 − state as the filling of neutrons started in the νg 9/2 orbital . 6,7,8,9,10,11 In the Cu isotopes as N ∼ 40, the state reveal three types of structures -single-particle, collective or coupling of single proton with neighboring even Ni isotopes. In the Zn isotopes with two protons more than the semi-magic Z = 28 nickel nuclei, the spherical N = 40 gap may not be strong enough to stabilize the nuclei in spherical shape when protons are added to the 68 Ni core.…”

Spins and Magnetic Moments ofK49andK51: Establishing thePapuga1, Bissell2, Kreim3 et al. 2013Phys. Rev. Lett.Self Cite

“…In the case of a J = 1/2 to J = 1/2 transition the ratio of the upper-to-lower hyperfine parameter cannot be used to exclude a particular spin (method used e.g. to assign the 72,74 Cu spins [29]), as the relative hyperfine structure separations are unaffected by the nuclear spin in this case. There is, however, an indirect way to determine the 51 K spin, namely by comparing the intensities of the different hyperfine structure components.…”

Coulomb excitation of the 3−isomer in70Cu

“…The B(E2; 6 − → 4 − ) value for 72 Cu and the spin and parity assignment of the low-lying energy levels are deduced from lifetime measurements [27][28][29][30]. However, recent measurements of the ground-state nuclear moment of 72 Cu [6,31] firmly fix the spin and parity of the ground state as I π = 2 − . According to this result, the level scheme of 72 Cu proposed in Refs.…”

“…However, when including the 2d 5/2 orbital, the calculation predicts an enhancement in collectivity in 72 Cu not observed experimentally. In view of the recent spin and parity determination of the 72 Cu ground state [6,31], the experimental result needs revision. The effect of the 2d 5/2 deserves further investigation.…”