Yrast states in194Os:The prolate-oblate transition region

Abstract

Previously unidentified states in the neutron-rich nucleus 194 Os have been populated following a deep inelastic reaction using a 780 MeV 136 Xe beam on a thick 198 Pt target. ␥-␥ coincidence events were collected using the 8 detector array. The yrast band in 194 Os has been observed up to I ϭ(10 ϩ ), for the first time. This represents the heaviest osmium nucleus where in-beam ␥-ray spectroscopy has been performed to date. The excitation energies of the new levels are compared to the systematics of the lighter even-even osmium nuclei. The evidence for a transition from prolate to oblate-deformed ground states in the heavy osmium nuclei is discussed and total Routhian surface calculations are presented. An alignment analysis together with cranked shell model calculations, suggest that the yrast states have a prolate shape, in contrast to earlier interpretations. Predictions for the neighboring even-even tungsten isotopes are also described.

Weakly deformed oblate structures in76198Os122

“…The potential energy minimisation has been done in the three-dimensional deformation space of β 2 , β 4 and γ. Axially symmetric prolate shapes correspond to γ=0 o , while non-collective o blate shapes correspond to γ=60 o For 198 Os the calculations indicate a very soft system in both γ and β degrees of freedom, with a minimum corresponding to weak oblate deformation with parameters β 2 =0.093, β 4 =-0.028 and γ=60 o (see figure 3.). The same type of calculation performed for lighter osmium isotopes indicates a prolate shape for 190,192,194 Os with decreasing quadrupole deformation for increasing mass, and an oblate shape with β 2 =0.12 for 196 Os [10]. The yrast structures of 190,192,194 Os nuclei were observed up to at least spin-parity 10 + .…”

“…The exact place where this change occurs for the ground state of the osmium isotopes is not clear. Experimental information suggests that up to mass A=194 the osmium isotopes are prolate [10], while the γ degree of freedom is also important [11]. In 196 Os 116 the available experimental information is too scarce to draw a definite conclusion, as only two excited states were observed [7].…”

“…[25]. This type of calculation has been extensively employed and has proved successful in this mass region [10,5]. The potential energy minimisation has been done in the three-dimensional deformation space of β 2 , β 4 and γ. Axially symmetric prolate shapes correspond to γ=0 o , while non-collective o blate shapes correspond to γ=60 o For 198 Os the calculations indicate a very soft system in both γ and β degrees of freedom, with a minimum corresponding to weak oblate deformation with parameters β 2 =0.093, β 4 =-0.028 and γ=60 o (see figure 3.).…”

Shape Evolution in the Neutron-rich Osmium Isotopes: Prompt $\gamma $-ray Spectroscopy of ${}^{196}$Os

“…The heaviest stable isotope, 192 Os, has a prolate J π = 10 − isomer [13,14], but additionally, its γ -vibrational band is the lowest lying in this region with the bandhead at 489 keV [14], a clear signature of γ softness. On the other hand, 194 Os, populated up to J π = (10 + ) via a multinucleon transfer reaction [9], has a level scheme suggestive of prolate deformation, at variance with the interpretation of previous experimental results [15]. In recent years, isomer-decay spectroscopy combined with fragmentation reactions at relativistic energies has enabled the study of, for example, 198 Os; the most neutron-rich osmium isotope for which any spectroscopic information is available.…”

“…However, fully self-consistent beyond-mean-field calculations using the same underlying interaction, which provide spectroscopic information, were still missing in this region and will be discussed in this Rapid Communication. Experimental endeavor has mainly focused on spectroscopy to characterize the shape of the lowest-lying excited states [8][9][10][11][12]. Shape properties can most readily be elucidated by examining changes along isotopic or isotonic chains.…”