Measurement of the Sign of the Spectroscopic Quadrupole Moment for the21+State inSe70: No Evidence for Oblate Shape

Abstract

Using a method whereby molecular and atomic ions are independently selected, an isobarically pure beam of 70 Se ions was postaccelerated to an energy of 206 MeV using REX-ISOLDE. Coulomb-excitation yields for states in the beam and target nuclei were deduced by recording deexcitation rays in the highly segmented MINIBALL -ray spectrometer in coincidence with scattered particles in a silicon detector. At these energies, the Coulomb-excitation yield for the first 2 state is expected to be strongly sensitive to the sign of the spectroscopic quadrupole moment through the nuclear reorientation effect. Experimental evidence is presented here for a prolate shape for the first 2 state in 70 Se, reopening the question over whether there are, as reported earlier, deformed oblate shapes near to the ground state in the light selenium isotopes. DOI: 10.1103/PhysRevLett.98.072501 PACS numbers: 21.10.Ky, 25.70.De, 27.50.+e A remarkable property of the nucleus is its ability to assume different configurations giving energy minima corresponding to different shapes of the mean field. It is often claimed that the presence of low-lying excited 0 states in even-even nuclei is strong evidence for such shape coexistence, since it is difficult to account for the presence of such states otherwise [1,2]. Arguably the most striking example of this is the case of 186 Pb [3], where both the first and second excited states have a spin parity of 0 . These states were interpreted as the bandheads of oblate and prolate rotational configurations, in close competition with the spherical ground state expected for this singly magic (Z 82) nuclear system. The region close to the N Z line from 56 Ni to 80 Zr is believed to be one of rapidly changing nuclear shape. At the upper end of this region, strongly prolate deformed shapes are found: 80 Zr is suggested to have 2 0:4 on the basis of the high moment of inertia [4], and a strongly prolate ground state, consistent with 2 0:4, has recently been established from the Gamow-Teller strength distribution in 76 Sr [5]. For the midshell nuclei near N Z 34 and 36, large shell gaps exist for protons and neutrons at both prolate and oblate shape, which would favor shape coexistence near the ground state. In the light krypton isotopes, 72-78 Kr, a low-lying excited 0 state is observed. There is good experimental evidence that this state has an oblate intruder configuration, and it most likely becomes the ground state in 72 Kr [6,7]. An analysis of the mixing of this state with the ground state, through consideration of their perturbed energies and the excited state lifetime, suggests an oblate ground state for 72 Kr [6], while the recently measured BE2; 0 ! 2 value in this nucleus, PRL 98,

Collectivity atN=50:Ge82andGade1, Baugher2, Bazin3 et al. 2010Phys. Rev. CSelf Cite619810View full textAdd to dashboardCite

“…Their properties are driven by shape coexistence and rapid shape changes all the way from the N = Z line into the A ≈ 70 mass region [1][2][3][4][5][6][7][8][9][10][11][12][13]. On the other side of the nuclear chart, the most neutron-rich selenium and germanium isotopes accessible for experiments are around the magic neutron number N = 50.…”

Recent advances in nuclear physics through on-line isotope separation

“…Incorporating information on the lifetime of the first excited state in 70 Se from the literature into the analysis led to the conclusion that the first excited state in 70 Se (ref. 33) was associated with a prolate shape. A subsequent re-measurement of excited state lifetimes for 70 Se using a stable beam led to a revised conclusion-namely that the associated shape was oblate 34 .…”

Competing single-particle and collective behavior in71Se

“…[2][3][4][5][6][7]), and several groups have proposed oblate ground-state configurations [4,6,8] that may persist to higher spin [4]. Other researchers have challenged the notion of oblate-deformed ground states [9,10], however, leaving this issue as an open question in these nuclei. Regardless of the interpretation of the ground-state configurations, most N ≈ Z Se nuclei are expected to be good candidates for oblate deformation at low spin, but then quickly transition to prolate-or triaxial-deformed shapes with increasing spin.…”