A 3-quasiparticle isomer in neutron-rich 183Ta

Excited states in neutron-rich 183Ta have been studied using a two-neutron transfer reaction of 181Ta(18O,16O) . In-beam $ \gamma$ -rays were measured in coincidence with scattered ions detected by a high-resolution $ \Delta$E -E Si telescope for reaction channel selection. Previously known 1-quasiparticle bands are extended to higher spins, and several levels including a T1/2 = 0.9(3) μs 3-quasiparticle isomer are identified.

Information on intrinsic and rotational states of medium to high spins in neutron-rich nuclei has been limited because of the difficulty to access to such nuclei using standard fusion-evaporation reactions. Recent progress, however, in in-beam γ-ray spectroscopic techniques using deep inelastic [1][2][3] and multi-nucleon transfer reactions [4,5] has enable us to study the yrast structure of neutron-rich nuclei. For example, high-spin states in neutron-rich 182 Hf have been successfully identified through a deep inelastic reaction with a 136 Xe beam incident on 180 Hf [1]. Using 18 O-induced multi-nucleon transfer reactions such as ( 18 O, 16 20 Ne), modest spin states of neutron-rich nuclei have been studied [4][5][6][7][8][9]. In the present work, excited states of the neutron-rich 183 Ta have been populated following a two-neutron transfer reaction of 181 Ta( 18 O, 16 O).
The experiment was carried out at the Tokai tandem accelerator facility [10] of Japan Atomic Energy Agency. The 180 MeV 18 O beam was incident on a natural Ta self-supporting foil with a thickness of 3.9 mg/cm 2 , which is thick enough to stop target-like nuclei inside the target material. Outgoing ions were detected by four sets of surface barrier Si ΔE-E detectors with a diameter of 20 mm. These detectors were placed at 28 • with respect to the beam direction. Emitted γ-rays were measured with eight HP-Ge detectors, in coincidence with outgoing ions. Four of these detectors were arranged symmetrically in a plane perpendicular to the beam axis, providing γ-ray anisotropy information for the determination of transia e-mail: shizuma.toshiyuki@jaea.go.jp tion multipole orders [4]. Energy and efficiency calibrations of the Ge detectors were made by using 133 Ba and 152 Eu standard γ-ray sources. The time difference between signals from the Si and Ge detectors was measured by time-to-amplitude converters (TAC). The energy and time information on outgoing ions and γ-rays were recorded event by event on magnetic tapes. A total of 2.3 × 10 8 and 6.8 × 10 7 events for particle-γ and particle-γ-γ coincidences, respectively, were collected. The details of the experimental setup are described in ref. [6].
The E-ΔE plot for outgoing ions is shown in fig. 1  and atomic numbers. The particle energies were calibrated assuming that the most intense peak in the E-ΔE matrix corresponds to elastically scattered events of 18 O ions entering at the center of the each Si detector. In the present data analysis, the narrow gate window on 16 O ions, shown with the enclosed area in fig. 1, is used to eliminate events associated with 181 Ta and 182 Ta nuclei. These can be produced by two-and one-neutron evaporation from 183 Ta when the compound-like 183 Ta is excited above the neutron separation energies. The higher-energy 16 O gate therefore gives clean γ-ray spectra for 183 Ta. The level scheme of 183 Ta deduced from the present experimental data is shown in fig. 2. The ground state is assigned to be the 7/2 + [404] 1-quasiparticle configuration, the same as other Ta isotopes with 175 ≤ A ≤ 185 [11]. Levels at 74, 144, 459, and 573 keV, identified in the β-decay study of 183 Hf [12], are confirmed. While the 144 keV level is known as a member of the 7/2 + [404] band, the 459 and 573 keV levels are based on the 5/2 + [402] configuration. The present data extend the 7/2 + [404] and 5/2 + [402] bands to higher spins. It is noted that the inband transition energies of these bands, which are pseudospin partners [13], are very close to each other. The 74 keV state is based on the 9/2 − [514] configuration, but its rotational band has not been observed. The energy spectrum of γ-rays in coincidence with the 74 keV transition which depopulates the bandhead is shown in fig. 3(a). Rotational band members up to I π = (19/2 − ) are identified in this band. Analysis of the in-band γ-ray branching ratios using the rotational model expressions [14] gives an average g K value of 1.4(4) with Q 0 = 6 eb and g R = 0.3. This is consistent with the expected value of g K = 1.29 for the 9/2 − [514] configuration. Spin assignments for the observed levels are based on γ-ray anisotropy data if available. The γ-ray anisotropy is given as an intensity ratio of γ-rays detected by the Ge detectors placed in and out of the reaction plane. The ratio depends on the transition multipole order, the degree of polarization, and the mixing ratio. In practice, the intensity ratio R[= I γ (in)/I γ (out)] is greater than unity for stretched quadrupole (ΔI = 2) and R ≈ 0.5 for stretched pure dipole transitions [15]. For mixed ΔI = 1 transitions, the ratio varies with the mixing ratio, and e.g., R ≈ 1 for positive small mixing ratios [15] which can be applied for the transitions of 1quasiparticle bands observed in 183 Ta. Note that the sign  906 keV is too low to be a 3-quasiparticle state, and could be a γ vibrational state coupled to the 9/2 − [514] configuration which is implied by the observed inter-band transitions to the 9/2 − [514] band. States formed by γ vibrations on the 9/2 − [514] configuration are known to exist in 177 Lu at 1306 keV [18], 177 Ta at 899 keV [19], and 187 Re at 793 keV [20]. Information on the γ-ray energies, intensities and in-and out-of-plane intensity ratios if obtained is summarized in table 1.
From the analysis of the delayed coincidence spectrum shown in fig. 3(b), a new isomer has been identified above the 1311 keV level. The half-life of this isomer is determined as T 1/2 = 0.9(3) μs from the particle-γ time difference spectrum shown in fig. 5. In the decay curve of the 208 keV γ-ray, a prompt decay component can be seen. This indicates that the 1311 keV level depopulated by the 208 keV transition is not isomeric itself, but it is fed from the isomer. Note that a half-life for the 74 keV state is deduced as T 1/2 = 101 (20) ns (see fig. 5) which agrees with Table 1. Energies Eγ, relative intensities Iγ, and initial level energies E i for the γ-ray transitions observed in 183 Ta. Intensity ratios I γ (in)/Iγ(out) for in-plane to out-of-plane anisotropies are also given.

Eγ
Iγ the adopted value of 107(11) ns [11] within the quoted uncertainties. The γ-ray linking the 1311 keV state and the isomer, labeled by "Δ" in fig. 2, has not been observed.
On the basis of detection efficiency and conversion coefficient consideration, the possible energy Δ of the missing transition is deduced to be less than 50 keV for E1 and 100 keV for M 1 and E2. Consequently, spins and parities of 19/2 ± or 21/2 − are likely for the isomeric state. Most of these assignments, however, can be excluded by the following discussion of the hindrance factors f ν per degree of K forbiddenness for the isomeric transition. The f ν factor is defined as f ν = (T γ 1/2 /T W ) 1/ν where T γ 1/2 and T W are the partial γ-ray half-life and the corresponding Weisskopf single-particle estimate, respectively, and ν is the order of K forbiddenness, defined as ν = ΔK − λ for transitions of multipole order λ. The consideration of the hindrance factor estimated for the isomeric transition can exclude the 19/2 − assignment because of the large hindrance deduced for the isomeric transition which would be M 1 with f ν = 474 assuming Δ = 100 keV and the 21/2 − assignment because of the abnormally strong E2 transition with f ν = 2 assuming Δ = 100 keV. This leads only the 19/2 + assignment with f ν factor of 28 (Δ = 50 keV assumed) which is consistent with values obtained for K forbidden E1 transitions in 175 Ta [21] and 177 Ta [19]. Note that the T W value for the E1 transition was multiplied by 10 3 before calculating f ν . From the consideration of low-lying single-particle states a K π = 19/2 + , 3-quasiparticle configuration of π{9/2 − [514]} ⊗ ν5 − {1/2 − [510]11/2 + [615]} is likely assigned to the isomer. The ν5 − configuration is obtained by a favored spin-spin coupling associated with the empirical Gallagher-Moszkowski interaction [22] and are known in 184 W (E x = 1285 keV) [23] and 186 Os (E x = 1629 keV) [24]. A similar isomer with the same configuration is known at 1682 keV in 187 Re [20].
We thank the staff of the JAEA tandem accelerator facility for providing the 18 O beam.