Isomers and oblate collectivity at high spin in neutron-rich Pt isotopes

Abstract

Abstract. Isomers and high-spin structures with rotation-aligned oblate configurations have been studied in several Pt isotopes. The 12 + states in the even Pt isotopes from 192−198 Pt are found to be metastable, and have (i 13/2 ) 2 neutron character. The progression of E2 transition probabilities from the 12 + to 10 + states across the Pt isotopic chain implies reduction in collectivity, followed by an abrupt decrease at N=120 ( 198 Pt). This behavior is quite distinct from the gradual decrease of B(E2) values near the respective ground states. A large contribution from aligned angular momentum, to the rotational sequences built on the 12 + states, is visible. This is due to the relatively small crossing frequencies for nucleons in low-Ω orbitals at oblate deformation in comparison to higher values for prolate shapes. As a result, oblate rotation is found to be increasingly favored for higher neutron numbers.

Structure of odd- A Pt isotopes along the line of stability

“…Various multi-dimensional energy and time histograms were created and analyzed to establish the level structure and lifetimes of isomeric states, in addition to the multipolarities of observed transitions. Details about the data analysis may be found elsewhere [2,3].…”

“…These are in addition to the long-lived, 13/2 + spin isomers in odd-A Pt isotopes, arising from the occupation of the i 13/2 orbital by the unpaired neutron. In even-A isotopes, I π =7 − states are known to have isomeric character, with T 1/2 = 1.85(17), 3.45(12), 5.2(2) and 3.4(2) ns in 192,194,196,198 Pt, respectively [2,3,[13][14][15][16][17]. This situation is mirrored in the odd-A Pt isotopes, where the I π = 25/2 − levels are isomers with T 1/2 = 1.07(6), 3.3(4), 5.0(5) and 9.4(5) ns in 191,193,195,197 Pt, respectively [7,11,16].…”

“…In the above Pt isotopes (particularly the heavier ones), these negative-parity states result from a semi-decoupled ν 2 (i 13/2 , f 5/2 or p 3/2 ) 2-quasiparticle (qp) configuration coupled to a 0-qp and 1-qp νi 13/2 one in the even-and odd-A isotopes, respectively. In the lighter isotopes, the 2-qp configuration π 2 (h 11/2 , d 3/2 or s 1/2 ) is also involved [2]. The isomeric character may be attributed primarily to the low energy of the 25/2 − → 21/2 − E2 transition.…”

“…The structure of nuclei in the A≈190-200 region exhibits a complex interplay between collective effects and those due to individual nucleons. These nuclei show evidence of moderate deformation and excited states appear to result from collective rotation [1][2][3]. However, owing to the small number of valence nucleons, collectivity is not as well developed as, for example, in the deformed, mid-shell A≈170-180 region.…”

“…Further, the effect of the unpaired nucleon on deformation can be explored. The loss of collectivity as the Fermi level approaches N=120 determined in the investigation of even-A isotopes [2] can be examined further through a study of the odd-A isotopes 193,195,197 Pt with neutron numbers N=115,117,119, respectively.…”