Magnetic properties of smooth terminating dipole bands in 110 , 112 Te

Three strongly coupled sequences have been established in 110,112Te up to high spins. They are interpreted in terms of deformed structures built on proton 1-particle–1-hole excitations that reach termination at I ∼ 40h̄. This is the first observation of smooth terminating dipole structures in this mass region. Lifetime measurements have allowed the extraction of experimental B(M1; I → I −1) and B(E2; I → I −2) reduced transition rates for one of the dipole bands in 110Te. The results support the deformed interpretation.  2006 Elsevier B.V. All rights reserved. PACS: 27.60.+j; 21.10.Re; 23.20.Lv Nuclei close to doubly magic 100Sn generate their spin predominantly through the alignment of single-particle angular momenta of the valence nucleons resulting in complex energy * Corresponding author. E-mail address: [email protected] (E.S. Paul). 1 Present address: Department of Chemistry, Washington University, St. Louis, MO 63130, USA. 2 Present address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA. 3 Present address: Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia. 4 Present address: Schuster Laboratory, The University of Manchester, Brunswick Street, Manchester M13 9PL, United Kingdom. 5 Present address: National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA. 0370-2693/$ – see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physletb.2006.03.020 spectra. The limited number of valence particles is insufficient to break the spherical symmetry of these nuclei and their structure can be interpreted in terms of a shell model. Quadrupole deformation can, however, be induced through 2-particle– 2-hole (2p–2h) proton excitations across the Z = 50 shell gap [1–4], involving holes in the g9/2 high-K proton orbital. Such deformed structures are able to generate spin through collective nuclear rotation leading to the observation of band structures consisting of regular sequences of I = 2 γ rays. In this mass region, 1p–1h proton excitations, involving a hole in the same g9/2 high-K proton orbitals, result in I = 1 bands of intense dipole transitions. Such structures in 50Sn [5–7] and 51Sb [8] isotopes, which have only been observed to modest spins, drop off quickly in M1 strength showing ‘shears’ dynamics (‘magnetic rotation’) related to the perpendicular coupling at low spin of a high-K proton hole to low-K neutron orbitals in a weakly 26 A.O. Evans et al. / Physics Letters B 636 (2006) 25–30 deformed nucleus. Similar dipole bands have also been observed in nuclei with Z < 50, e.g. 110Cd [9]. These structures have been interpreted through comparison with tilted-axis cranking (TAC) calculations [10,11], which predict that the shears mechanism should be active around proton or neutron closed-shell regions, i.e. regions of weak quadrupole deformation. This Letter presents the observation in 110,112 52Te of three strongly coupled ( I = 1) bands up to unusually high spins, two of which have been connected to the low-spin level schemes. These new dipole bands in 110,112Te maintain their M1 strength up to high spin, indicating a significant stability against the shears mechanism over a large spin range. Both experimental B(M1; I → I − 1) and B(E2; I → I − 2) reduced transition rates have been extracted for one of the bands in 110Te; the results show that the new dipole bands represent the first observation of deformed 1p–1h excitations to high spin in this mass region. From comparisons with cranked Nilsson– Strutinsky (CNS) calculations, we conclude that, for each of the bands which have been linked to the low spin states, one signature is observed to the maximum spin value in the respective configuration. Two experiments were carried out at the 88-Inch Cyclotron of the Lawrence Berkeley National Laboratory. In the first, thin-target experiment, high-spin states in A∼ 110 nuclei were populated with the 58Ni(58Ni, xα yp znγ ) fusion-evaporation reaction at 250 MeV. The Gammasphere [12] γ -ray spectrometer, containing 83 HPGe detectors, was used in conjunction with the Microball [13] charged-particle detector and an array of 15 neutron detectors in order to provide clean exit channel selection by determining the number of evaporated particles (x, y, z). Gamma rays selected for 110Te (α2p) and 112Te (4p) were sorted into Radware format [14,15] matrices (γ 2), cubes (γ 3), and hypercubes (γ 4) for subsequent level scheme construction. In the second experiment a target consisting of 1 mg/cm2 of 58Ni on a 208Pb backing of thickness 15 mg/cm2 was bombarded by a 240 MeV 58Ni beam. Gammasphere containing 98 HPGe detectors was used in conjunction with the Microball charged-particle detector. The backed target was used to facilitate the measurement of mean level lifetimes through a Doppler broadened lineshape (DBLS) analysis [16]. From the measured lifetimes of levels within strongly coupled bands, experimental B(M1; I → I − 1) and B(E2; I → I − 2) values can be obtained providing both information on nuclear single-particle (magnetic) and collective (electric) aspects of the bands. Partial level schemes for 110,112Te are presented in Fig. 1 showing the relevant dipole bands up to high spins of ≈ 40h̄; the remarkable feature is that the I = 1 inband transitions are clearly seen up to γ -ray energies of ∼ 900 keV, indicating significant M1 strength. Band 1 represents a strongly coupled band structure in 110Te, with a cascade of 20 M1 transitions extending up to high spin. This structure is linked to the lowspin level structure by two high-energy transitions of I = 1 character (1569 and 1858 keV) and one of I = 2 character (1904 keV). Band 2, including 13 M1 transitions, represents another strongly coupled structure in 110Te, but is populated with Fig. 1. Partial level schemes for 110,112Te, showing the new dipole bands. much lower intensity. As a consequence, band 2 is unlinked to the low-spin 110Te level scheme and hence its excitation energy, along with the spin and parity of its levels, remain unknown. Band 3 in 112Te represents the third example of a strongly coupled structure, including 17 M1 transitions extending to high spin. This band is connected to states at 28+ and 30+ in the 112Te level scheme, as well as the yrast 18+ state. Examples of fitted lineshapes, using the codes of Ref. [17], are shown in Fig. 2 for the 396, 432, and 463 keV transitions of 110Te band 1. Slowing down of the recoiling nuclei in the 58Ni target and 208Pb backing was simulated using Monte Carlo techniques, based on the formalism of Currie [18]. Five thousand recoil histories with a time step of 1 fs were created, containing information on the velocity and direction of the recoils for each time step. The electronic and nuclear components of the slowing-down process were modelled using the stopping A.O. Evans et al. / Physics Letters B 636 (2006) 25–30 27 Fig. 2. Fitted γ -ray lineshapes for the 396, 432, and 463 keV transitions of 110Te band 1 at forward (top) and backward (bottom) angles. Contamination peaks are shown by the dotted lines. Table 1 Experimental B(M1) and B(E2) values extracted for 110Te band 1