A 4π γ-ray multidetector system for heavy-ion physics ; population and decay of high spin states in ¹⁵⁷⁻¹⁶¹Yb

Abstract

Construction of a new type of spectrometer for the investigation of nuclear structure at high angular momentum and of the mechanisms of heavy-ion induced reactions is reported. The spectrometer consists of 72 NaI(Tl) detectors packed in a 4π arrangement. With this instrument the γ-ray multiplicity, the total γ-ray energy, the angular correlations of emitted γ-rays, and the time relationships between the γ-rays in a cascade can be obtained for each event. The pulse height spectra of the γ-rays associated with these parameters can also be obtained. The geometric arrangement and the associated electronics and data aquisition system are discussed. The calibrations and data analysis methods for obtaining the entry state populations and for obtaining the associated γ-ray energy spectra are discussed. Unique studies of the role of the angular momentum in heavy-ion fusion reactions and the structure of transitional rare earth nuclei are reported. The statistical model calculations reproduce the main features of the observed entry state populations in heavy-ion fusion reactions. From the observed γ-ray multiplicity distributions the entrance channel orbital angular momentum distribution leading to fusion has been deduced. A new decay mode involving localized dipole radiction at half the accompanying stretched E2 radiation is found in 157-161Yb at high spin. The appearance of this decay mode correlates smoothly with neutron number. In addition a region of noncollective behaviour of 157Yb and 158Yb at intermediate spins is observed. Possible interpretations are presented in terms of the evolution of the nuclear shapes from prolate to aligned-quasiparticle oblate to collective oblate nucleus.