First allowed bandcrossing in neutron deficient nucleus 141Tb

Abstract

The neutron deficient 141 Tb nucleus has been studied with the 92 Mo ( 54 Fe, αp) reaction at 240-MeV incident energy and the multidetector array GASP. For the yrast πh 11/2 decoupled band, excited states up to 6.7 MeV and spin up to 47/2 − have been observed. This band presents an upbend at rotational frequency of ω = 0.38MeV due to the alignment of h 11/2 protons. The results are discussed in terms of the Cranking model.Neutron-deficient odd-proton nuclei in the A = 140 region display many interesting collective properties. The odd-proton nuclei in this region have moderate prolate quadrupole deformations (β 2 ≈0.2 ) at low spin and are soft with respect to changes in γ deformation, which leads to a rich variety of band structures. The πh 11/2 orbital plays an important role in the structure of these nuclei. The low-Ω members of this orbital are close to the Fermi surface, giving rise to decoupled yrast bands at prolate deformations. Pairs of πh 11/2 quasiparticles align at modest rotational frequencies stabilizing prolate configurations (γ = 0 o ). The alignment of νh 11/2 quasiparticles from the upper part of this shell exert a driving force towards collectively rotating oblate nuclear shapes (γ = −60 o , according to the Lund convention[1]) , inducing shape changes in these nuclei.In order to investigate neutron-deficient nuclei in the A=140 mass region we have performed an experiment with the 54 Fe+ 92 Mo reaction at 240-MeV incident energy. The target used was an approximately 1.0 mg/cm 2 thick enriched 92 Mo foil. The incident beam was obtained with the tandem XTU accelerator of Legnaro National Laboratories, Legnaro, Italy. The multidetector array GASP [2], composed of 40 Compton-suppressed high-efficiency HPGe detectors and the 80-element BGO inner ball, was used for obtaining gamma-ray double and triple coincidence spectra. The multi-telescope light-charged-particle detector array (ISIS) [3], consisting of 40 Si surface-barrier ∆E-E telescopes, enabled the selection of the multiplicities of the evaporated charged particles in coincidence with the observed γ-rays. In order to permit the identification of the various masses produced in the reaction, the recoil mass spectrometer CAMEL [4] was coupled to GASP. The coupled GASP-CAMEL system has an intrinsic mass resolution better than 1/300 and the transmission efficiency in this experiment was about 1%. Events have been collected on tape when at least two Compton-suppressed HPGe detectors and two BGO detectors fired in coincidence. A total of 1.3x10 9 Compton-suppressed events was collected. The data have been Doppler corrected and sorted into charged particle-γ-γ, mass-γ-γ and γ-γ-γ cubes. Various nuclei (Z=63 to 66; N=75 to 78) were produced in this experiment, the 3p channel leading to 143 Tb was the most strongly populated nucleus and the results on the high-spin structures observed were described in Ref. [5], where a more detailed description of the experiment and the data analysis may be found. Other nuclei studied in this experiment were 140 Tb [6...