Level lifetimes in neutron-rich Ru isotopes

Abstract

The neutron-rich Ru isotopes are situated between the strongly deformed Sr-Zr region with axial deformation of f l _ 0.40 [1] and the spherical Cd-Sn region. Evidence for triaxiality of even-even Ru isotopes has been presented in [2-5], and lifetimes of some of their 2 + states have been measured in fission studies [6-9]. It seems, however, that in these experiments the nearly identical energies of the 2 + ---* 0 + transitions in l~ and 11~ could not be separated. Therefore, high-resolution measurements are required in order to perceive the evolution of deformation with increasing neutron number. So far only few measurements by the delayed-coincidence method [10] have been performed on chemically or massseparated isotopes. Therefore, a systematic study of level lifetimes in neutron-rich Ru nuclei has been undertaken at the Mainz TRIGA reactor. Fission products were obtained from thermal-neutron induced fission of 249Cf. Technetium isotopes were chemically separated within 2 s by the on-line SISAK method [11], allowing the study of levels and lifetimes of their/3decay daughters. Primarily, the short-lived activities of 1~ to 111Tc were measured, whereas long-lived decay products were kept at a low level due to the continuous flow of the liquid phase (containing the technetium activity) passing the detectors. Apart from Tc, also small contaminations of short-lived Ag isotopes were observed in the 7-spectra. The detector setup consisted of a thin plastic scintillator for fi-particles and two Ge-detectors for 7-rays, one being a planar diode with good energy resolution and timing properties (7 ns at 100 keV) and the other a medium-size coaxial detector. Coincidence events from all detector pairs were recorded with the GOOSY mult iparameter system. The delayed coincidences between the fl-scintillator and the planar 7-detector were used for the measurement of the shortest lifetimes. With this setup, a systematical accuracy of the order of 0.05ns could be obtained for the strongest "/-peaks. The 7-7 coincidences were used to check for contaminations in the 7-gates from the planar detector applied for the fl-