Study of subnanosecond high-spin isomers in 186, 188, 190Pt by conversion electron spectroscopy

Evidence for shape coexistence in186Pt

“…Level systematics of (+,0) levels in even-mass Pt isotopes [15][16][17]. The gb members are connected by full lines, the members of the oblate (vi13/z) 2 bands by dashed lines and the members of the prolate two-quasiparticle bands by dash-dotted lines energy systematics a similar shape is suggested for the high-spin levels of the (+, 0)0 band in lS6pt" This interpretation is supported by the fact that the half life of the 12 + yrast level in 186pt is T1/2~0.05 ns [5].…”

“…This quantity is related to the centroid shift. No centroid shift has been Observed for the high-energy transitions deexciting these levels whereas a distinct shift was found for those depopulating the 12 + isomer in 188pt which has a half life of T1/2=0.66 ns [5]. The 753.6 keV interband transition has then a partial half life of <0.7 ns, a result being incompatible with the WeiBkopf estimate of 4 ns if M2 character is assumed since enhanced M2 transitions are not known.…”

“…It can be seen that the energies of the gb members increase with mass number displaying a significant change between 186pt and 188Pt, which is interpreted as a transition from predominantly prolate to predominantly oblate shape [1,5,14]. The members of the predominantly oblate i 13/2 two-quasineutron bands in 188, 190, 192pt [1,5,15] connected by dashed lines show a slight decrease of the excitation energy with increasing mass number. The levels belonging to the newly established (+, 0)1 band in 186pt follow very well this energy dependence, indicating that this band also has an il 3/2 two-quasineutron configuration and that the nuclear shape is predominantly oblate,…”

“…Arguments for a gradual change from predominantly prolate shape (+ 30~ 7 > -30~ for Pt nuclei with mass number A < 184 to predominantly oblate shape (-30~ 7> -90 ~ for those with mass number A>188 are (i) that the i13/z quasineutron bands are decoupled in 187, 189, 191,193pt [1] but strongly coupled in 18spt [2,3] and (ii) that the excitation energies of the ground band (gb) levels in the even-mass Pt nuclei change from small values for light Pt isotopes to large values for the heavy ones [4,5]. Evidence for shape coexistence in the Pt nuclei has not been given hitherto but is suggested (/) because a prolate-oblate shape coexistence was found in neighbouring oddmass Au nuclei by the observation of decoupled n h 9/2 and decoupled nh 11/2 bands [6][7][8] and (ii) since lowlying excited 0 + levels were observed in the even-mass nuclei 182-194pt.…”

“…Previous knowledge about high-spin states in 186pt results from studies of Piiparinen et al [1] who established the gb up to U= 14 + and from half life studies of Richter et al [5] who searched for subnanosecond isomers by recoil-shadow conversion-electron spectroscopy. In order to investigate the shape evolution of 186pt at higher rotational frequencies it was necessary (i) to extend the yrast band to higher spins and (ii) to search for side bands.…”