Energy levels in the nucleus 156Dy through the 159Tb(p, 4nγ)156Dy reaction

doi:10.1016/0375-9474(76)90272-4

Nuclear Physics A

Volume 271, Issue 1, 26 October 1976, Pages 133-161

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Abstract

Using the ¹⁵⁹Tb(p, 4nγ)¹⁵⁶Dy reaction at E_p = 27 to 51 MeV and standard on-line γ-ray spectroscopy methods, the energies and decay properties of members of various rotational bands in the nucleus ¹⁵⁶Dy have been investigated, i.e. the ground-state band up to 14_g⁺, the β-vibrational band up to 14_β⁺,the γ-vibrational band up to 11_γ⁺,and two other bands, one with odd-spin levels up to 11, the other with even-spin levels up to 10. The results are compared with various calculations in the framework of the collective model, and no satisfactory fit is obtained; possible improvements of the model to remove these discrepancies are suggested.

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Rotational band structures in ¹⁵⁶Dy have been populated via the reactions ¹⁴⁸Nd(¹²C, 4n) and ¹²⁴Sn(³⁶S, 4n) at beam energies of 65 MeV and 155 MeV, respectively. High-spin states (I ≳ 40kh) have been observed and a detailed level scheme is presented. Two positive-parity bands have been observed to spins I = 42 and 36. Four negative-parity bands have been identified up to or above spin I = 36. New levels are also observed in octupole and gamma vibrational bands. The band structures are understood in terms of quasiparticle configurations using the cranked-shell model. The second and third i_$132$ neutron alignments, the systematics of the first h_$112$ proton alignments in even-even N = 90 isotones and the observation of a third discontinuity in the lowest energy positive- and negative-parity bands near spin 40 in ¹⁵⁶Dy are discussed.
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Lifetimes of states with spins up to 30ħ have been measured in the nuclei ¹⁵⁶Dy, ¹⁵⁷Dy, and ^ll58Dy using the recoil-distance technique together with inverse reactions of the type $Mg (^{136} Xe, x n)$ . The applied method, which benefited from the high velocities of the fusion residues as well as from improvements of the recoil-distance technique, allowed us to determine lifetimes and feeding times down to 0.1 ps. Below the first backbending the resultant B(E2) values in the ground-state band of ^{156, 158}Dy increase faster with increasing rotational frequency than expected for rigid rotors, reaching values similar to those observed for the well-deformed neutron-rich Dy isotopes. In contrast to this, the E2-transition probabilities between high-spin states are clearly retarded. The retardation gradually evolves from the rotation alignment of nucléons and indicates deformation changes most likely towards a triaxial shape. From the analysis of the side-feeding times of the high-spin yrast states it could be furthermore deduced that the E2 component of the preyrast γ-decay stems from transitions along highly collective bands.

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^†: Work supported by the Institut Interuniversitaire des Sciences Nucléaires, Bruxelles (Belgium).

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Energy levels in the nucleus 156Dy through the 159Tb(p, 4nγ)156Dy reaction

Abstract

Z. Phys.

Phys. Lett.

J. de Phys.

Nucl. Phys.

Nucl. Phys.

Annual Report of the Research Institute of Physics, Stockholm

Nucl. Data Sheets

Phys. Lett.

Phys. Lett.

Z. Phys.

Math. Fys. Med. Dan. Vid. Selsk.

Nucl. Phys.

Nucl. Phys.

Nucl. Phys.

Phys. Reports

Rev. Mod. Phys.

J. Phys.

Phys. Lett.

Phys. Rev. Lett.

Phys. Lett.

Nucl. Phys.

Energy levels in the nucleus ¹⁵⁶Dy through the ¹⁵⁹Tb(p, 4nγ)¹⁵⁶Dy reaction