Directional-polarization correlations using EXILL

Abstract

Eight EXOGAM-type, Ge-clover detectors of the EXILL array, arranged on an octagon plan and acting as Compton polarimeters, were used to measure directional-polarization correlations of γ rays populated in various processes. Measurements of γ radiation following neutron-induced fission of 235U and 241Pu targets; β --decay following fission; (n,γ) reactions on various stable targets and measurements using radioactive sources of 60Co, 133Ba and 152Eu were performed at the PF1B cold-neutron facility of the Institut Laue-Langevin in Grenoble. Digital electronics and a trigger-less acquisition system allowed the collection of signals from Ge detectors down to about 20 keV, providing measurements of linear polarization down to 120 keV. The precise sensitivity calibration was determined for the set of eight EXILL clover polarimeters in a range from 100 keV to 10000 keV. A new formula for calculating the directional-polarization correlations for the upper transition in a γγ cascade is derived from first principles. Directional-polarization correlations were measured for a few dozens of transitions in a number of nuclei, including values for twenty transitions in the 152Sm nucleus measured for the first time, and compared against the calculated values, to illustrate the quality of the technique. The combined analysis of angular and directional-polarization correlations is presented, which helps the determination of spin-parity assignments to excited levels.