High-spin states populated in deep-inelastic reactions

Abstract

High spin states in the neutron rich 188Os and 190Os nuclei have been populated using the 82Se + 192Os deepinelastic reaction. The level schemes are extended up to spin I <FONT FACE=Symbol>»</FONT>21. The observed new structures are tentatively interpreted as fragments of rotational bands built on multi-quasiparticle configurations.

High-spin states populated in deep-inelastic reactions

S. Mohammadi^{I, II}; Zs. Podolyák^I; G. de Angelis^III; M. Axiotis^III; D. Bazzacco^IV; P.G. Bizzeti^V; F. Brandolini^IV; R. Broda^VI; D. Bucurescu^VII; E. Farnea^III; W. Gelletly^VIII; A. Gadea^III; M. Ionescu-Bujor^VII; A. Iordachescu^VII; Th. Kröll^III; S. Longdown^VIII; S. Lunardi^IV; N. Marginean^III; T. Martinez^III; N.H. Medina^IX; B. Quintana^X; P.H. Regan^VIII; B. Rubio^XI; C.A. Ur^IV; J.-J. Valiente Dobón^VIII; P.M. Walker^VIII; Y.H. Zhang^III

^IDepartment of Physics, University of Surrey, Guildford, GU2 7XH, UK

^IIDepartment of Physics, University of Payam-Noor, Fariman 93914, Iran

^IIILaboratori Nazionali di Legnaro, INFN, Legnaro, Italy

^IVDipartimento di Fisica and INFN, Padova, Italy

^VDipartimento di Fisica and INFN, Firenze, Italy

^VINiewodniczanski Institute of Nuclear Physics, Krakow, Poland

^VIIInsitute of Physics and Nuclear Engineering, Bucharest, Romania

^VIIIDepartment of Physics, University of Surrey, Guildford GU2 7XH, UK

^IXLaboratório Pelletron-IFUSP, São Paulo, SP, Brazil

^XUniversity of Salamanca, Spain

^XIInstituto di Fisica Corpuscular, Valencia, Spain

ABSTRACT

High spin states in the neutron rich ¹⁸⁸Os and ¹⁹⁰Os nuclei have been populated using the ⁸²Se + ¹⁹²Os deepinelastic reaction. The level schemes are extended up to spin I »21. The observed new structures are tentatively interpreted as fragments of rotational bands built on multi-quasiparticle configurations.

1 Introduction

Fusion-evaporation reactions provide the standard mechanism to populate states with high angular momentum. However, by using stable beam-target combinations beta-stable and neutron-rich nuclei cannot be studied. Neutron-rich nuclei with mass A < 150 can be studied in spontaneous and induced fission. Projectile fragmentation has proven to be an efficient method of populating nuclei far from the valley of stability. However, in the case of heavy nuclei this method is still limited to species with isomeric states, needed to enhance the sensitvity of the technique (see for example [1]). Deep-inelastic reactions are the most general reaction mechanism which can be used to study neutron-rich nuclei and are able to populate relatively high-spin states.

We used the ⁸²Se+¹⁹²Os binary reaction to study nuclei both in the vicinity of the ⁸²Se beam and ¹⁹²Os target. The study of nuclei around the N=50 shell closure has given new experimental indications about the persistence of this shell gap down to Z=32 [2]. The structure of nuclei in the A » 180-190 region is characterized by the presence of high-angular momentum intrinsic, i.e. noncollective, states. Such states compete with the collective excitations because the proton and neutron Fermi levels are among orbitals with large angular-momentum projections (W) on the symmetry axis, allowing states with large values of K = SW to be formed [3]. As part of a systematic study of the mass A » 180-190 nuclei, some new results on the stable ^188,190Os nuclei, obtained by using binary reactions, are presented in this paper.

2 Experiment

Excited states in the vicinity of both the ⁸²Se beam and the ¹⁹²Os target nuclei have been populated using heavy-ion multi-nucleon transfer reactions and studied through g-ray spectroscopy in a ''thick target" [4] measurement. The combination of the Tandem-XTU and the superconducting LINAC ALPI accelerators at the Laboratori Nazionali di Legnaro, Italy, was used to accelerate a beam of ⁸²Se ions to an energy of 460 MeV. The ¹⁹²Os target, isotopically enriched to 99.0%, had a thickness of 50 mg/cm² on a 0.2 mm Ta backing, which is sufficient to stop all reaction fragments. High fold g-g coincidences were acquired with the 4p spectrometer GASP [5] consisting of 40 Compton-suppressed, large-volume germanium detectors with an inner BGO ball acting as a multiplicity filter and a total-energy spectrometer. Events were collected on tape during five days of beam time under the conditions that a minimum of three Compton-suppressed Ge detectors and two BGO elements from the multiplicity filter fired in coincidence. With a beam current of 1 particle-nA, the event rate was » 4 kHz and the singles rate in the germanium detectors » 12 kHz. With such a setup g-rays from the deexciting target-like and projectile-like fragments were detected. After gain matching for all the detectors, the coincidence data were sorted into fully symmetrized matrices and cubes for off-line analysis. Since all recoiling fragments were stopped in the target, Doppler broadening prevented the observation of transitions deexciting short-lived states and only the g decay with lifetimes longer than the slowing-down time of the recoiling nuclei ( » 1ps) could be studied. Thus, no Doppler correction was necessary when sorting the data.

3 Results

Two- and three-dimensional g matrices were used to construct the level schemes of ^188,190Os. Typical coincidence spectra are shown in Fig. 1. Partial level schemes of ¹⁸⁸Os and ¹⁹⁰Os, together with the lighter ^184,186Os isotopes [6,7], are presented in Fig. 2.

In ¹⁸⁸Os the levels up to spin 10⁺ of the ground state band are well established [8,9,10] and were confirmed in the present experiment. C.Y. Wu et al. [9] reported two transitions, with energies of 699 keV and 760 keV, populating the yrast 10⁺ state in Coulomb excitation. We did not observe these transitions. Instead, we confirm the existence of the 686 keV transition, previously reported by R.A. Warner et al. [11]. The levels above the 12⁺ state are seen in this experiment for the first time. The properties of the gamma ray transitions belonging to ¹⁸⁸Os are summarized in Table I.

In ¹⁹⁰Os the levels up to spin 10⁺ of the ground state band are known from earlier works [9,10,12]. Here again we did not see the 769 keV and 654 keV g-ray transitions populating the 10⁺ state, as proposed by C.Y. Wu et al. [9]. Levels above the 10⁺ state are seen for the first time in the present work. The properties of the gamma ray transitions belonging to ¹⁹⁰Os are summarized in Table II.

4 Discussion

As first step of the configuration assignment we compare the new structures observed in ^188,190Os with the lighter isotopes (see fig.2), where more experimental data are available. In both ^184,186Os the groundstate band is crossed by a so-called t-band, based on the two-neutron n11/2⁺[615]9/2⁺[624] configuration. The assignments were based on experimentally determined branching ratios, lifetimes of the K^p =10⁺ bandheads, and comparison with blocked BCS calculations [6,7]. The yrast 18⁺ state in ¹⁸⁶Os was interpreted as a K^p =18⁺ four neutron n11/2⁺[615] 9/2⁺ [624] 9/2^- [505] 7/2^- [503] state. Based on the similarities between the yrast structures of the osmium isotopes, we propose that the 14⁺ and 16⁺ states in ¹⁸⁸Os, and 12⁺, 14⁺, 16⁺ states in ¹⁹⁰Os are members of the two-neutron n11/2⁺[615]9/2⁺[624] band. In the same way the states with spin-parity 18⁺, 19⁺, 20⁺, 21⁺ may have the four neutron n11/2⁺[615] 9/2⁺ [624] 9/2^- [505] 7/2^- [503] configuration.

In order to verify whether the assignment based on systematics is reasonable, multi-quasiparticle calculations [13] have been performed using blocked BCS pairing without residual interaction. According to the calculations the K^p =9^-n11/2⁺[615]7/2^-[503] intrinsic state is » 800 keV below the K^p =10⁺n11/2⁺[615]9/2⁺[624] state, both in ¹⁸⁸Os and ¹⁹⁰Os nuclei. However, the lack of observation of low energy dipole transitions favours the K^p =10⁺ assignment. Furthermore, the 9^- bandhead decay is expected to be isomeric, and may be too long lived to be seen in the present analysis,

We couldn't establish the level scheme of ¹⁹²Os up to high spins, although its excited states must have been populated with high cross sections. This might be explained by the existence of a K^p =10^-T_1/2=5.9 s yrast isomer at 2015 keV excitation energy, interpreted as having the n11/2⁺[615]9/2^-[505] configuration. However, a similar yrast isomer with T_1/2=9.9 min at 1705 keV exists also in ¹⁹⁰Os [14]. It seems that in ¹⁹²Os the high spin states decay via the long-lived isomer, whereas in ¹⁹⁰Os the situation is different.

The data analysis is still in progress. Therefore, new experimental results (for example on angular distributions) and theoretical considerations might provide a more complete interpretation.

Acknowledgments

The authors thank the staff of the ALPI linear accelerator, Legnaro, for providing a high quality beam. Zs.P. acknowledges the receipt of an EPSRC Advanced Fellowship Award (GR/A10789/01).

[2] Y.H. Zhang et al., to be published.

Received on 12 September, 2003

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Publication Dates

• Publication in this collection
  26 Oct 2004
• Date of issue
  Sept 2004

History

• Received
  12 Sept 2003