Ponente
Descripción
The neutron-rich rare-earth nuclei that lie mid-way between the proton shell-closures at Z=50,82 are expected to display a maximum of quadrupole deformation close to the double mid-shell N=104, Z=66. In addition, the appearance of deformed shell closures in this region, which depend on the interplay between single-particle and collective degrees of freedom, are important for understanding the $r$-process formation of the rare-earth-element (REE) abundance peak.
Nuclear deformation in these isotonic chains is connected to the appearance of 6+ and 8- K isomers with lifetimes greater than 100~ns. K-isomeric states provide stringent tests of contemporary nuclear models far from stability and allow access to low-lying excited states in such nuclei. Especially, the evolution of the energy of the first 2+ state along an isotopic chain shows a local minimum at N = 98 in Gd and Dy interpreted as the appearance of a deformed sub-shell gap which stabilizes the deformation. With increasing neutron number, E(2+) tend to decrease again towards the mid-shell, developing a minimum at N = 104 in Er, Yb and Dy; but the behaviour for Gd, Sm and Nd, the latter of which has the lowest E(2+) values, is unknown.
Moreover, samarium isotopes undergo $\beta$ decay with half-lives of the order of a few hundred milliseconds, making it possible to study the excited states in their n-rich Eu daughter nuclei, for which information is mostly unknown at present.
In the Sm isotopic chain, $^{164}$Sm is the most exotic nucleus for which an isomer study was performed in the EURICA campaign. Beta-decay half-lives in these isotopes were measured in recent studies in the BRIKEN campaign, while a measurement of half-lives together with discovery of $\beta$-decaying isomers was carried out at ATLAS in recent years.
We propose to measure low-lying excited states in $^{166,168}$Sm and their daughters $^{166,168}$Eu nuclei for the first time, exploiting in-flight fission of a 345~MeV/u $^{238}$U beam. Additionally, we would like to extend the presently-known level scheme in $^{164}$Sm. The newly-available 100~pnA beam intensity, in combination with the increased efficiency of the $\gamma$ detection array compared to previous campaigns, will allow us to reach these exotic species with sufficient statistics to perform gamma spectroscopy.
