Tilted Rotation and Backbending in an Odd-Proton Nucleus

Abstract

Tilted-axis rotation, arising from Fermi-aligned configurations, has been observed for the first time to cause backbending in an odd-proton nucleus. In 181 Re, two t-bands are found to be energetically favored relative to the usual rotation-aligned s-bands, presenting an alternative form of cold nuclear rotation. Interactions between the bands are weak, and unambiguous comparisons with tilted-axiscranking calculations can be made. [S0031-9007(97) PACS numbers: 21.10. Re, 21.10.Jx, 23.20.Gq, 27.70. + q Rotational excitations in atomic nuclei have long been a source of information about the underlying nucleonic structure. A dramatic increase in apparent moment of inertia, known as backbending, was first discovered [1] in 160 Dy at an angular momentum of I ഠ 16h, and is now well established as a general feature of nuclear rotation. In well deformed, axially symmetric nuclei, backbending has been understood [2] to be due to the alignment of the angular momentum of a pair of high-j nucleons along the rotation axis, so that the mean angular-momentum component along the symmetry axis (perpendicular to the rotation axis) is ͗K͘ ഠ 0. The aligned structure (s-band) becomes favored in energy at high spin and crosses the nonaligned structure (g band).In the A ഠ 180 mass region, where backbending is due to a pair of i 13͞2 neutrons, it has been proposed [3,4] that a new structure arises due to the Fermi-aligned [4,5] coupling scheme which is active in the middle of the neutron shell (N ഠ 104). In this scheme the nucleon angular-momentum precesses about an axis lying between the rotation and symmetry axes. The projections of the angular momentum onto the symmetry axis, K, and the rotation axis, i, are both localized at a nonzero value. This differs from the strong-coupling scheme in which the nucleon angular-momentum precesses about the nuclear symmetry axis resulting in good K, with ͗i͘ ഠ 0. States formed from a pair of Fermi-aligned nucleons have K ഠ jK 1 6 K 2 j and i ഠ i 1 1 i 2 , resulting in the usual s states, having K ഠ 0, and also t states having large K and a total angular momentum tilted between the rotation and symmetry axes. The s-and t-bands compete for "yrast" status (the lowest energy for a given angular momentum) corresponding to cold nuclear rotation. The nature and extent of this competition is not yet understood, and the possibility of there being large-amplitude high-K components in the yrast bands of nuclei with N ഠ 104 is contrary to the usual interpretation of backbending as intrinsically a low-K phenomenon.Evidence for the role of t-band structures in backbending has been observed in 179,180 W [3,6] and 181,182 Os [7] (with proton numbers Z 74, 76, respectively). However, only in 179 W are the band interactions sufficiently weak to enable a clear signature to be obtained of the high-K band crossing, and in this unique case a chance near degeneracy at the band crossing could be responsible for the special features. Furthermore, rotational models [4] have failed to reproduce the band crossings in 1...

Shape-coexisting rotation in neutron-deficient Hg and Pb nuclei

“…3 A low-K rotation-aligned band (known as s band) can cross with the 0-qp ground-state band (g band) and become yrast. However, when the Fermi surface lies in the middle of the νi 13/2 orbits, bands with high-K configurations (known as t bands) may compete with the g and/or s band [42][43][44]. To illustrate the rotational behavior of each multi-qp configuration as well as its relative energy compared to other components, we can plot rotational bands built on different multi-qp configurations without K mixing in one figure, i.e., a band diagram [18].…”

Heavy Ion Accelerator Facility Department of Nuclear Physics Australian National University

“…The first example in an odd-proton nucleus, lSIRe, has also recently been found in collaborative work with the group from Surrey [13]. A related thrust of the collaborative studies has been the effort to iden- tify, and characterise, high-seniority multi-quasiparticle states.…”

High Spin States Populated via Projectile Fragmentation in Very Neutron-Rich Nuclei Around Mass 180

“…populating K= 35 2 − isomeric state in 175 Hf [3], 179 W [4] and 181 Re[5]. This spin represents the highest value observed so far in projectile fragmentation.…”