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 fl_ 0.40 [1] and the spherical Cd-Sn region. Evidence for triaxiality of even-even Ru isotopes has been presented in [2][3][4][5], and lifetimes of some of their 2 + states have been measured in fission studies [6][7][8][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/3-decay 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 multiparameter 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-

Analysis of the solar-system r-process abundance pattern with the new ETFSI-Q mass formula

“…They were interpreted as stemming from model deficiencies in neutron separation energies (S n ) and deformation parameters ( 2 ) due to overly strong N=82 (and N=126) shell strength far from stability [5,6,7]. And indeed, first indications for systematic deviations of these quantities from model predictions have recently been observed for very neutron-rich 39 Y to 44 Ru isotopes [8,9].…”

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“…[35,54,56]. Recent experimental information using chemical separation procedures, or ion-guide and projectilefragmentation techniques combined with mass separation can, for example be found in [68][69][70][71][72]. The third region of strong nuclear-structure and astrophysics interest is that around the double-magic nucleus 132 Sn, which we will discussed in subsect.…”

“…Already with the use of Z-selective laser ionization at the General-Purpose Separator (GPS) of CERN/ISOLDE, it has been possible to study decay properties of 14-ms 69 Mn, which is one of the most neutron-rich isotopes with the shortest β-decay half-life beyond the sd-shell. The prospects are good for an extension of such measurements to even heavier isotopes with LIS and the High-Resolution Separator (HRS), eventually including the study of detailed level structures up to 70 Fe.…”