Ultrahigh-Resolution γ-Ray Spectroscopy of 156Gd: A Test of Tetrahedral Symmetry

Tetrahedral symmetry in strongly interacting systems would establish a new class of quantum effects at subatomic scale. Excited states in Gd-156 that could carry the information about the tetrahedral symmetry were populated in the Gd-155(N, gamma)Gd-156 reaction and studied using the GAMS4/5 Bragg spectrometers at the Institut Laue-Langevin. We have identified the 5(1)(-) -> 3(1)(-) transition of 131.983(12) keV in Gd-156 and determined its intensity to be 1.9(3)x10(-6) per neutron capture. The lifetime tau = 220(-30)(+180) fs of the 5(1)(-) state in Gd-156 has been measured using the GRID technique. The resulting B(E2) = 293(-134)(+61)Weisskopf unit rate of the 131.983 keV transition provides the intrinsic quadrupole moment of the 5(1)(-) state in Gd-156 to be Q(0) = 7.1(-1.6)(+0.7) b. This large value, comparable to the quadrupole moment of the ground state in Gd-156, gives strong evidence against tetrahedral symmetry in the lowest odd-spin, negative-parity band of Gd-156.