First observation of excited states inPo192

Abstract

␥ rays following the 160 Dy( 36 Ar,4n͒ 192 Po reaction have been identified by employing a high-transmission gas-filled separator in recoil decay tagging measurements. The deduced level scheme reveals a flattening of the energy systematics, when going towards the neutron midshell indicating that the deformed intruder structures have become yrast. ͓S0556-2813͑96͒50212-8͔ PACS number͑s͒: 27.80.ϩw, 23.20.Lv, 21.60.Ev, 25.70.Ϫz The microscopic origin of quadrupole collectivity at lowexcitation energies in neutron midshell nuclei near the Zϭ50 and Zϭ82 shell closures is not well known. In some of these nuclei, deformed intruder states coexist with nearly spherical normal states ͓1͔, while for example in the evenmass Cd nuclei the intruder structures clearly play a role in generating low-lying quadrupole phonon states ͓2͔. These intruder structures are usually associated with two-proton excitations across the main shell gap, but recently the importance of deformation driving high-j neutron orbitals in these states has also been pointed out ͓3-5͔.In this respect, the Po nuclei with two protons outside the Zϭ82 shell closure form an interesting series of isotopes. The ␣and ␤-decay studies by the Leuven group ͓6-9͔ strongly support the view that the abrupt drop of level energies in light even-mass Po isotopes is due to proton 4p-2h intruder configurations while in recent calculations by the Rutgers group ͓4,5͔ this behavior is quite well reproduced by taking into account neutron orbitals, especially the i 13/2 .In order to resolve the ambiguities discussed above, it is crucial to extend the Po energy-level systematics towards the neutron midshell. However, as a consequence of strong fission competition, the fusion-evaporation reaction channels for populating the midshell Po nuclei become very weak and, therefore, special triggering methods are needed.In the present work we describe in-beam ␥-ray studies of 192 Po ͓10͔ by using the recoil decay tagging method ͓11,12͔.A recent determination of the half-life of its ␣-decaying ground state gave a value of 33.2͑14͒ ms ͓9͔. No excited states in 192 Po have been identified before our work. The experiments were performed at the Accelerator Laboratory of the University of Jyväskylä ͑JYFL͒. A beam of 178 MeV 36 Ar ions from the JYFL K130 cyclotron was used to bombard a 70% enriched 500 g/cm 2 thick 160 Dy target. The 192 Po nuclei of interest were produced via the 4n-evaporation channel. Prompt ␥ rays from the target were detected by the DORIS array consisting of 9 TESSA type ͓13͔ Compton suppressed Ge detectors in a truncated dodecahedron frame. The efficiency of the array is about 0.6% at 1.3 MeV.The gas-filled recoil separator RITU ͓14͔ was used to separate fusion-evaporation residues from the dominant fission background. RITU is a charge and velocity focusing device, especially designed for collecting recoiling fusionevaporation residues with high efficiency. Recoils were implanted into an 80ϫ35 mm Si strip detector covering about 70% of the recoil distribution at the fo...

Shape coexistence in the light Po isotopes

“…Only the yrast and yrare states of positive parity and spin J=0,2,4,6,8 and 10 have been included in the gure. While there is substantially more information on the level scheme of the heavier isotopes (A > 200) than shown in the gure [26], the yrast sequence up to spin 8 + summarizes all the levels known in 192 Po [11,12]. The systematics presented also exhaust most of the experimental information on the excitation spectra of 198 194 Po.…”

“…The second excited 0 + states, which were identi ed in the light Po isotopes down to A=196, from + /EC and decay studies [25,28], are nonetheless not reproduced by the model. The alternative i n terpretation is heavily based on the observed low-lying 0 + 2 states, which are discussed in terms of intruder con guration [25,11]. The perturbation in the systematics below A=200 is described qualitatively as being due to the mixing of the associated intruder band with the regular structures.…”

“…The Pb nuclei have been studied in-beam down to N=102 [7,8,9,10], the Po nuclei down to N=108 [11,12] and the Rn nuclei down to N=112 [13,14]. Many of the previous results are very recent and are due to a very intensive activity close to the limits of the current experimental techniques, which concentrates on the shape coexistence phenomenon in the Pb mass region.…”

“…The recent experiments by Helariuta et al [11] and Fotiades et al [12] provided information on excited states in 192 Po, which is the lightest Po isotope studied by spectroscopy, and very close to the last Po isotope known to exist, 190 Po [30]. Although the energies of all the yrast levels continue to decrease, the energy ratios of the type R(E(J+2)/E(J)), with J 2, were found to bestill far from what would beexpected for a rigid rotor [12].…”

“…1 in some detail, since it contains most of the experimental ingredients regarding the pros and cons involved in the two (co-)existing interpretations of the light P o isotopes (see, for example, Refs. [11,12]). …”