High-spin octupole yrast levels inRn21686

Abstract

The yrast level structure of 216 Rn has been studied using in-beam spectroscopy α-γ -γ coincidence techniques through the 208 Pb( 18 O, 2α2n) reaction in the 91-93 MeV energy range, using the 8π GASP-ISIS spectrometer at Legnaro. The level scheme of 216 Rn resulting from this study shows alternating parity bands only above a certain excitation energy. From this result, the lightest nucleus showing evidence of octupole collectivity at low spins is still 216 Fr, thereby defining the lowest-mass corner for this kind of phenomenon as N 129 and Z 87.

In-beam spectroscopy of215Rn86

“…By "natural" parity states we mean those states having the same parity as the underlying particle configuration. This result is consistent with the measured 216 Rn level scheme [13] which shows interleaved bands of alternating parity only above spin 13…”

“…The close correspondence between the experimentally determined 217 Ra and 215 Rn level schemes, the previously performed work in this region [2,7], and the restricted set of available single-particle states around the Fermi surface strongly suggest for 215 Rn the same positive parity yrast structure as the one previously assigned to 217 Ra. In line with the experimental 216 Rn level scheme [13], no negative-parity low-lying levels have been found here in 215 Rn.…”

“…The addition of the h 9/2 proton does not modify the "quadrupolar-like structure" already partially developed in 215 Rn, but it produces the drastic appearance of the "octupolar structure." Apparently the appearance of low-lying octupole collectivity requires for N = 129 at least five protons (Z = 87, 216 Fr) and for Z = 86 at least five neutrons (N = 131, 217 Rn [20]) outside the closed shells, as shown in this work and by the results on 216 Fr and 216 Rn [13]. In order to illuminate the influence of the neutron number, Fig.…”

“…On the other hand, Sheline et al [7] have analyzed the structure of 215 Rn and 217 Ra from the point of view of the reflection asymmetric model. However, 215 Rn is a transitional nucleus between the isotones with N = 128, 212 Po [14], 214 Rn [15], and 216 Ra [16], whose properties can be described by two neutrons in the g 9/2 subshell with the valence protons coupled to zero angular momentum, and the isotones with N = 130, 216 Rn [13] and 218 Ra [17,18], which show collective characteristics. Some of these collective features can also be found in the N = 129 isotones.…”

“…[11]). This procedure of multipolarity assignment was consistently checked with transitions of known multipolarity belonging to 219,220 Ra [10] and the known transitions in 216 Rn [12,13].…”