The negative parity bands in¹⁵⁶Gd

Abstract

The high flux reactor of the Institut Laue-Langevin is the world most intense neutron source for research. Using the ultra highresolution crystal spectrometers GAMS installed at the in-pile target position H6/H7 it is possible to measure nuclear state lifetimes using the Gamma Ray Induced Recoil (GRID) technique. In bent crystal mode, the spectrometers allow to perform spectroscopy with a dynamic range of up to six orders magnitude. At a very well collimated external neutron beam it is possible to install a highly efficient germanium detector array to obtain coincidences and angular correlations. The mentioned techniques were used to study the first two negative parity bands in 156 Gd. These bands have been in the focus of interest since they seem to show signatures of a tetrahedral symmetry. A surprisingly high B(E2) value of about 1000 W.u. for the 4 − → 2 − transition was discovered. It indicates that the two first negative parity bands cannot be considered to be signature partners.

Electric Transitions in Hypothetical Tetrahedral/Octahedral Bands

“…The latter comes from the analysis of the negative-parity spectrum in 156 Gd, measured in the ILL laboratory in a series of experiments using the Bragg spectrometry methods, (see, Ref. [2]). …”

“…In this paper, we apply a harmonic-like model, discussed in details in Ref. [1], to reproduce the experimental B(E1) and B(E2) probabilities in 156 Gd, a nucleus supposed to possess low-lying tetrahedral, negative-parity band and for which some data of interest are available from the recent experiments [2,3]. We thus analyze the behaviour of the electric transition probabilities within and between the ground-state and the negative-parity bands in order to find out to which irreducible representation of the tetrahedral group T d (or the octahedral group O) the state of interest could belong.…”

Investigation of negative-parity states in Dy156 : Search for evidence of tetrahedral symmetry

“…Predictions of the possible presence of tetrahedral symmetry [1][2][3][4] at low spins and excitation energy in nuclear spectra have prompted several experimental studies [5][6][7] in the Z ≈ 64 and N ≈ 90 region. These nucleon numbers are considered to be magic with respect to this tetrahedral symmetry.…”

“…In Refs. [3,7], it has been suggested that tetrahedral vibrational bands would have negative parity with K π = 2 − and a transition strength ratio of the inband E2 over the interband E1 decays, B(E2)/B(E1), an order of magnitude greater than for octupole vibrational bands. The current work focuses on interpreting the low-spin, negative-parity states in 156 Dy, as well as on extending the negative-parity, quasiparticle structures.…”