Abstract
Lifetimes of low-lying yrast states in neutron-rich 94,96,98Sr have been measured by Germanium-gated γ−γ fast timing with LaBr3(Ce) detectors using the EXILL&FATIMA spectrometer at the Institut Laue-Langevin. Sr fission products were generated using cold-neutron-induced fission of 235U and stopped almost instantaneously within the thick target. The experimental B(E2) values are compared with results of Monte Carlo shell-model calculations made without truncation on the occupation numbers of the orbits spanned by eight proton and eight neutron orbits and show good agreement. Similarly to the Zr isotopes, the abrupt shape transition in the Sr isotopes near neutron number N=60 is identified as being caused by many-proton excitations to its g9/2 orbit.
Highlights
The experimental B(E2) values are compared with results of Monte Carlo shell-model calculations made without truncation on the occupation numbers of the orbits spanned by eight proton and eight neutron orbits and show good agreement
To the Zr isotopes, the abrupt shape transition in the Sr isotopes near neutron number N = 60 is identified as being caused by many-proton excitations to its g9/2 orbit
It is well known that the ground states of the Sr isotopes with proton number Z = 38 and the Zr isotopes (Z = 40) show an abrupt change from a spherical structure at neutron number N = 58 to a strongly deformed structure at N = 60
Summary
Level energy and B(E2) values were obtained within a single framework in good agreement with experiments, depicting the abrupt shape transition at N = 60 as a consequence of type II shell evolution involving many proton particle-hole excitations to the g9/2 orbit [3,4]. We start with the usual (type I) shell evolution, where some single-particle orbits (for example the proton 1f7/2-1f5/2 splitting) change their energies due to the occupancy of other particular orbits (neutron 1g9/2 orbit, in the present example). The time differences delivered by the TACs of the setup are superimposed independent of the detector-detector combination only by distinguishing between the start and stop detectors This procedure generates two independent fast-timing-array time spectra depending on whether the decay transition of the γfeeder-γdecay cascade provided a stop signal (the delayed time spectrum) or a start signal (antidelayed) as described in more detail in Refs. As can be seen in the LaBr3 projection of Fig. 6(a), the background events are about three times larger than the FEP events at 144 keV
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