H. Gabelmann

Neutron-rich isotopes around the r-process “waiting-point” nuclei 29 79 Cu50 and 30 80 Zn50

Beta-decay half-lives (T1/2) and delayed-neutron emission probabilities (Pn) of very neutron-rich Cu to As nuclei have been measured, among them the new isotopes77Cu48,79Cu50,81Zn51 and84Ga53. With the T1/2 and Pn-values of now four N ∼-50 ”waiting-point” nuclei known, our hypothesis that the r-process has attained a local β-flow equilibrium araound A∼-80 is further strengthened.

Beta-delayed neutron emission from93–100Rb to excited states in the residual Sr isotopes

Beta-delayed neutron emission from the precursors93–100Rb to excited states in the residual nuclei92–99Sr has been measured by means ofγ-ray and neutron spectroscopy. In general, considerable neutron feeding of excited final states is observed. The experimental branching ratios (Pni) are compared to predictions from statistical model calculations. It is demonstrated that the main parameter affecting thePni-values is the shape of theβ-strength function (Sβ). For several Rb isotopes hindrance, respectively enhancement of neutron branches to specific final states is observed giving evidence for the persistence of intermediate structure of the neutron-emitting states. By the example of98Rb it is shown that a few integralβ-delayed neutron properties may be sufficient to derive first estimates on the real shape ofSβ above the neutron binding energy. This result is of importance for exotic nuclei where detailed spectroscopic investigations are precluded.

P n-values of short-lived Sr, Y, Ba and La precursors

Beta-delayed neutron emission probabilities (Pn) of short-lived Sr, Y, Ba and La precursors were measured. As expected from systematics, but in disagreement with recent data from the SOLIS-group, only small Pn -values were obtained.

The beta-decay of95Rb and97Rb

Theβ-decay of 377 ms95Rb and 168ms97Rb has been thoroughly investigated by means ofγ-ray, conversion electron andβ-delayed neutron spectroscopy. More than 97% of theβ-decay to particle-bound states has been placed in level schemes of95Sr and97Sr. High-resolution neutron spectroscopic studies have allowed to extend the knowledge of the excitation spectra up to 9 MeV, respectively 10 MeV. The density of neutron-unbound levels in95Sr and97Sr has been derived from peak stripping analyses of the neutron spectra. Beta-strength functions (Sβ(E)) have been investigated in detail. They indicate that Gamow-Tellerβ-decay of both precursors is dominated by nuclear structure. The discovered pronounced resonances and the variation in the shape ofSβ(E) are compared with predictions from the gross theory ofβ-decay and shell model calculations in the random phase approximation (RPA).

Evidence for excited 0+ states in94Sr

Beta-delayed neutron measurements on95Rb have indicated evidence for two heretofore unknown excited O+ states in94Sr which are not fed by ß -decay from the Jπ = 3− ground state of94Rb.

Structure of the highly deformed nucleus101Sr63 and evidence for identical K=3/2 bands

The low-energy level scheme of theN=63 nucleus101Sr has been obtained from a β-decay study of101Rb. The ν[532]5/2 and ν[411]3/2 orbitals are well established as the ground state and a band head at 271.2keV, respectively. The general properties of the level scheme indicate a quadrupole deformation ofβ ≃ 0.4, confirming the unique feature of saturation of deformation inN ≥ 60 Sr isotopes. The energies of theK=3/2 intraband transitions in the odd-neutron neighbours99Sr61 and101Sr63 are very similar. More generally, the identical bands at low spin in98–101Sr nuclei are correlated with the mass-independent moments of inertia in this region.

Lifetime measurements in99Sr and100Zr

The lifetime of the first excited state in99Sr has been determined by the delayed γ-γ coincidence method. The value of t/2=0.58(9) ns corresponds to a deformation parameter ß=0.38(4). The deduced value confirms the previously proposed ν[411]3/2 Nilsson assignment for the99Sr ground state. In addition, lifetime values for the 2+ (t1/2=0.40(8) ns) and 0+ (t1/2=5.36(23) ns) states in100Zr are obtained, which deviate from older measurements and indicate a larger deformation of ß=0.39(4).

Identical transitions in the strongly deformed 99 Sr and 100 Sr

The decay of the very neutron-rich nucleus

Saturation of deformation atN=60 in the Sr isotopes

{}^{100}\mathrm{Rb}

Beta-delayed neutron spectra for application in reactor technology, nuclear physics and astrophysics

has been studied by \ensuremath{\gamma} spectroscopy of online mass-separated samples. Schemes for \ensuremath{\beta} decay to