Single-particle behavior atN=126: Isomeric decays in neutron-richPt204

Abstract

The four proton-hole nucleus, 204 Pt, was populated in the fragmentation of an E/A = 1 GeV 208 Pb beam. The yrast structure of 204 Pt has been observed up to angular momentum I = 10 by detecting delayed γ-ray transitions originating from metastable states. These long-lived excited states have been identified to have spin-parities of I π = (10 + ), (7 − ) and (5 − ) and half-lives of T 1/2 = 146(14) ns, 55(3) µs and 5.5(7) µs, respectively. The structure of the magic N = 126 204 Pt nucleus is discussed and understood in terms of the spherical shell model. The data suggests a revision of the two-body interaction for N = 126, Z < 82, which determines the evolution of nuclear structure towards the r-process waiting point nuclei.PACS numbers: 29.30. Kv, 23.20.Lv The evolution of the properties of atomic nuclei with respect to neutron and proton numbers is a key question of nuclear physics. The study of unstable, neutron-rich nuclei represents one of the foremost pursuits of modern nuclear physics. Over the coming decade new radioactive ion beam facilities are being built with the main objectives being to probe neutron-rich nuclei. Within recent years surprising phenomena have been observed in neutron-rich nuclei such as neutron skins, halos and dramatic changes in the ordering and spacing of energy levels [1].While the stability of the N = 82 shell gap is an active topic of research [2,3], an open question is whether or not there is a quenching of the N = 126 shell gap as protons are removed from doubly magic 208 Pb. The proton dripline has been experimentally reached up to heavy elements [4], our present knowledge of the neutron dripline is limited to light species. The part of the nuclear chart with the least information on neutron-rich nuclei is the 76 Os to 82 Pb region, with experimental knowledge on only a few isotopes. This mass region is however an ideal testing ground of nuclear theories. With the removal of just a few protons and neutrons the landscape evolves from spherical to elongated prolate through disk shaped oblate and triaxial forms [5]. Consequently the information gained on neutron-rich, N = 126 nuclei is essential for the understanding of nuclear structure in heavy nuclei. From a longer-term perspective, experi-

Nuclear astrophysics with radioactive beams

“…These new results on the level scheme of 130 Cd provided no evidence for a reduction of the N = 82 shell gap in the vicinity of 132 Sn contrary to what was implied by the results of a previous experiment [420]. In a similar measurement at GSI, excited states in the magic nucleus 204 Pt (neutron number N = 126) were populated in the fragmentation of 208 Pb at 1 GeV/nucleon beam energy [421]. Medium spin isomers with half-lives between 150 ns and 55 µs and the level structures fed by their decays were identified.…”

Discovery of the calcium, indium, tin, and platinum isotopes

“…Also, a half-life of 49 min had been assigned to 193 Pt [135]. Isomeric decays in neutron-rich 204 Pt'' by Steer et al [142]. A 1 GeV/A 208 Pb beam from the SIS-18 accelerator at GSI impinged on a 9 Be target and the projectile fragments were selected and identified in flight by the fragment separator FRS.…”

Isomeric decay spectroscopy of theBi217isotope

“…For example, the fragmentation of a uranium beam was used to produce 212 Pb and 211 Bi and measure their isomeric decay [1]. Similarly, the adjacent elements beyond N = 126 and below Z = 82, such as thallium and mercury, have been studied [2,3]. For the elements beyond N = 126 but well above Z = 82 the situation is very different, as they can be populated with comparatively large cross sections with spallation or fragmentation reactions on uranium.…”