Multi - quasiparticle states in 184 W via multi - nucleon transfer

A multi-nucleon transfer reaction using an 850 MeV Xe beam incident on a Pt target was used to populate high-spin states in W, 14 nucleons from the target. A 4-quasiparticle isomer with t1/2 = 188±38 ns has been observed for the first time and its de-excitation through collective and intrinsic structures studied. The results are compared with predictions of blocked BCS Nilsson calculations. Observation of this metastable state completes the tungsten 4-quasiparticle isomer systematics from A = 176 to 186. Mapping out the most yrast 4-quasiparticle isomers in these even-even tungsten isotopes suggests the existence of a highly favoured state in W, within reach of current experimental set-ups. PACS. 21.10.Tg Lifetimes – 23.20.Lv γ transitions and level energies – 27.70.+q 150 ≤ A ≤ 189 High-seniority states in well-deformed neutron-rich A = 180 → 190 nuclei are difficult to study due to the lack of suitable reaction mechanisms. The stable nucleus W lies in this historically difficult-to-reach landscape. Recently, advances have been achieved in this mass-region using deep inelastic reactions, including both inelastic and multi-nucleon–transfer mechanisms, e.g. refs. [1,2]. This enables the study of the interplay between intrinsic and collective states as the nucleus accommodates angular momentum and as a function of neutron number. Previous studies of W [3,4] have observed 2-quasiparticle states up to 7 including a t1/2 = 8.3 μs K = 5− level. In this Letter, new data on seniority-2 and -4 structures are reported, observed as the 4-proton, 10-neutron channel in a deep inelastic reaction; a by-product, well away from the strongest reaction channels. a e-mail: [email protected] An 850 MeV 136 54Xe beam provided by the 88 ′′ cyclotron at Lawrence Berkeley National Laboratory, was incident on a thin, 420 μg cm−2, self-supporting target of 198 78Pt enriched to > 92%. The beam energy was chosen to be ≈ 25% above the Coulomb barrier in order to maximise the population of high-spin states. The beam had a natural micro-pulsing period of 178 ns. Gamma rays were detected using the GAMMASPHERE array of 102 Compton-suppressed Ge detectors. The CHICO gas-filled PPAC ancillary detector [5], was used to measure the angles (both θ and φ) of, and time-of-flight difference between, the 2 heavy-ion recoils. The trigger condition for a good event required 2 CHICO elements and at least 3 Ge detectors to fire. The timing condition was set in software such that the 4th (and subsequent) Ge signals could be delayed by up to 670 ns with respect to the first 3 prompt (within ±45 ns of the detection of the recoils) Ge energies. Isomers in the ns → μs range are observable as the recoiling products stop in the PPACs, at a distance 366 The European Physical Journal A Fig. 1. Partial level scheme for 184 74W110 showing the newly observed 4-quasiparticle isomer at Ex = 3862 keV (this work) together with relevant previously known levels [3,4,6]. The widths of the arrows are proportional to the relative γ-ray intensity (black) and deduced/assumed electron conversion intensity (white) except for the tentative transitions (dashed arrows). Level energies in parentheses indicate unestablished