Gamma-Gamma Energy Correlations and Moment of Inertia in Barium Nuclei

Abstract

A two-dimensional γ-ray coincidence spectrum is recorded using several NaI (T1) detectors during the bombardment of a ¹²⁴Sn target with 118 MeV ¹²C ions. Essentially four residual nuclei are produced, namely ^128,129,139Ba and ¹²⁶Xe. After subtraction of an uncorrelated background from the coincidence matrix, a correlated spectrum is obtained. This spectrum exhibits a valley along the 45° diagonal from about E_γ 0.5 MeV to E_γ 1.35 MeV, reflecting the rotational properties of the residual nuclei. The width of the valley decreases up to a transition energy E_γ 0.8 MeV. This width is directly related to the moment of inertia _band⁽²⁾ = dI/dω and is found to be in good agreement with the deduced values from known ground-band energies, although the nucleus probably decays through many different paths in this energy region. For the regions 800 < E_γ < 1050 keV and 1200 keV < E_γ < 1300 keV, the width of the valley remains nearly constant and the moment of inertia (2_band⁽²⁾/² = 80 and 95 MeV^-1, respectively) is close to the rigid-body value. A "bridge" across the valley at E_γ = 1.12 MeV is interpreted as being due to band crossing. Cranked shell model calculations indicate that the aligned angular momentum at this rotational frequency may result from the occupation of the highest aligned h_9/2 or i_13/2 neutron orbitals.