Lifetime measurements in neutron-rich Xe isotopes, S. Ilieva (TU Darmstadt)

par Dr Stoyanka Ilieva (TU Darmstadt)

vendredi 30 oct. 2015, 10:30 → 11:30 Europe/Paris

seminar room, 2nd floor (bat 27)

The nuclear properties in the region around the doubly magic nucleus 132Sn are of special interest. This mass region is important in an astrophysical aspect as "waiting points" of the r-process lie in this region and the nuclear structure there has an impact on the modelling of nucleosynthesis. Theory relies on nuclei near closed shells for predicting other, more exotic systems. Our interest lies in particular on the neutron-rich Xe isotopes (138-144Xe) which exhibit both quadrupole and octupole collective properties. The even neutron-rich xenon isotopes show regular behaviour and the B(E2; 0+->2+) values [1] around N=82 are well reproduced by the systematics unlike the B(E2) value in 136Te. The B(E2) values were measured in a Coulomb excitation experiment. The Coulomb excitation cross section depends on both transition and diagonal matrix elements. The lifetime of an excited state, on the other side is a direct measure for the strength of the transition and allows for a more precise quadrupole moment determination when combined with the Coulomb excitation data. Also of interest are the negative parity states where the increasing induced electric dipole moment is interpreted as a sign of increasing octupole correlations, maybe peaking at an octupole "magic" number N=88 [2]. Nuclear lifetimes also contribute to the understanding of this phenomenon. Within the EXILL&FATIMA campaign at ILL [3], Grenoble, in early 2013, the lifetimes of excited states in the neutron-rich xenon isotopes (138-144Xe), populated in neutron-induced fission of 235U and 241Pu, were measured. The fast timing array (FATIMA) consists of 16 LaBr3 fast scintillators and allows us to measure lifetimes down to 10 ps. The high-resolution EXILL array (EXOGAM@ILL), consisting of 32 HPGe detectors, gives us the possibility to identify the nuclides of interest among the big amout of fission fragments produced. This setup is characterized with high effciency which enables even quadruple (Ge)-(LaBr3) coincidences. Using the generalized centroid difference method [4], lifetimes as short as several picoseconds can be unambiguously determined. In this work we present directly measured lifetimes in odd and even mass neutron-rich xenon isotopes. This was the first measurement of this kind for xenon isotopes, except for 140Xe which was studied before by Lindroth et al. [5]. This work was supported by BMBF under contracts 05P12RDCIA and 05P12RDNUP, by ILL and by HIC for FAIR.

Slides abstract_ilieva.pdf