THE WARM NUCLEAR ROTATION IN 174W:CONSERVATION OF THE K QUANTUM NUMBERIN THE ORDER TO CHAOS REGION

The transition between order and chaos is a fascinating subject which is currently investigated in different research fields from solid state to nuclear physics. At very low excitation energy, the nucleus behaves as an ordered system with well-defined quantum numbers and associated selection rules. As the internal energy increases, there is a gradual loss of selection rules and quantum numbers lose their meaning. The extreme regime is the chaotic region of the compound nucleus, where only energy, spin and parity survive. A useful probe to study the evolution of the selection rules with temperature is the K-quantum number. By studying the γ-decay flow in coincidence with low-K and high-K states, information on the order-to-chaos transition can be obtained. An interesting physics case is 174W which is characterized by two high-K bands (K=8 and K=12) built on isomeric states, with lifetimes longer than 120 ns. This nucleus has been populated by the fusion-evaporation reaction of 50Ti (at 217 MeV) on 128Te. The experiment was performed at Legnaro National Laboratories, using the AGATA Demonstrator, coupled to the HELENA array. To probe the gradual weakening of the selection rules on the K-quantum number with temperature, γ-γ matrices selecting low-K and high-K structures have been analysed by statistical fluctuation techniques, allowing to estimate the number of low-K and high-K bands and their correlations. The interpretation of the data has been based on a Monte Carlo simulation code that reproduces the γ-decay flux of 174W, using as input microscopic Cranked Shell Model calculations at finite temperature. The study of the warm rotation in 174W has evidenced the importance of introducing a quenching factor in the E1 decay between low-K and high-K bands, which gives reduced hindrance values fν similar to the ones extracted from standard discrete spectroscopy studies of high-K isomers. For the first time, it is demonstrated that K-mixing is a general mechanism which well explains the hindrance to the γ-decay between low- and high-K states for both regular excited bands and isomeric states, as a function of temperature.