Search

Lifetimes of excited states of the 98;100;102Zr nuclei were measured by using the Generalized Centroid Difference Method. The nuclei of interest were populated via neutron-induced fission of 241Pu and 235U during the EXILL-FATIMA campaign. The obtained lifetimes were used to calculate the B(E2) transition strengths and β deformation parameters which were then compared with the recent theoretical predictions obtained with Monte Carlo Shell Model.

At the Institut Laue-Langevin in Grenoble, germanium-gated γ-γ fast-timing lifetime measurements of nuclear excited states in neutron-rich nuclei have been performed within a prompt γ-ray spectroscopy experimental campaign. We report on results obtained from the cold-neutron induced fission of 235U. The excited secondary fission products were stopped almost instantaneously within the thick target and the γ rays emitted were collected triggerlessly using the EXILL&FATIMA mixed array of HPGe and LaBr3(Ce) detectors. Precise lifetimes could be determined by analysing the γ-γ time difference spectra using the generalized centroid difference method. This picosecondsensitive method provides many advantages and is briefly explained. Still, the major source of systematic errors is related to the contribution of time-correlated Compton background. The EXILL&FATIMA results are discussed with respect to the typical energy-dependent timing behaviour of the background. According to the time response of the background, appropriate methods and a time correction for the sub-nanosecond regime are proposed.

In this work we present the results from two experiments dedicated to search for quadrupolecollective isovector valence-shell excitation, the states with so-called mixed proton-neutron symmetry (MSS), in nuclei around the doubly magic nucleus 208Pb. 212Po was studied in an α-transfer reaction. 204Hg was studied in an inverse kinematics Coulomb excitation reaction on a carbon target. Both experiments provide indications for existence of one-phonon MSSs. Those are the first experimentally identified MSSs in the mass A ≈ 208 region.

In an attempt to describe the excitation spectra and transitions strengths of light even-even Cd-isotopes (A = 98 − 108), large-scale shell-model calculations in the proton (2p1/2, 1g9/2) and the neutron (2d5/2, 1g7/2, 2d3/2, 3s1/2, 1h11/2) model space were performed. Preliminary results are presented and discussed.

At the PF1B cold neutron beam line at the Institut Laue Langevin, the EXILL array consisting of EXOGAM, GASP and ILL-Clover detectors was used to perform (n,γ) measurements at very high coincidence rates. About ten different reactions were measured in autumn 2012 using a highly collimated cold neutron beam. In spring 2013, the EXOGAM array was combined with 16 LaBr3(Ce) scintillators in the EXILL&FATIMA campaign for the measurement of lifetimes using the generalised centroid difference method. We report on the properties of the set-ups and present first results from both campaigns.

A decay spectroscopy experiment was performed within the EURICA campaign at RIKEN in 2012. It aimed at the isomer and particle spectroscopy of excited states and ground states in the mass region below the doubly magic 100Sn. The N = Z nuclei 98In, 96Cd and 94Ag were of particular interest for the present study. Preliminary results on the neutron deficient nuclei 93Ag and 94Ag are presented. In 94Ag a more precise value for the half-life of the ground state’s superallowed Fermi transition was deduced. In addition the energy spectra of the mentioned decay could be reproduced through precise Geant4 simulations of the used active stopper SIMBA. This will enable us to extract Qβ values from the measured data. The decay of 93Ag is discussed based on the observed implantation-decay correlation events.

A novel method for direct electronic “fast-timing” lifetime measurements of nuclear excited states via γ-γ coincidences using an array equipped with N very fast high-resolution LaBr3(Ce) scintillator detectors is presented. The generalized centroid difference method provides two independent “start” and “stop” time spectra obtained without any correction by a superposition of the N(N – 1)/2 calibrated γ-γ time difference spectra of the N detector fast-timing system. The two fast-timing array time spectra correspond to a forward and reverse gating of a specific γ-γ cascade and the centroid difference as the time shift between the centroids of the two time spectra provides a picosecond-sensitive mirror-symmetric observable of the set-up. The energydependent mean prompt response difference between the start and stop events is calibrated and used as a single correction for lifetime determination. These combined fast-timing array mean γ-γ zero-time responses can be determined for 40 keV < Eγ < 1.4 MeV with a precision better than 10 ps using a 152Eu γ-ray source. The new method is described with examples of (n,γ) and (n,f,γ) experiments performed at the intense cold-neutron beam facility PF1B of the Institut Laue-Langevin in Grenoble, France, using 16 LaBr3(Ce) detectors within the EXILL&FATIMA campaign in 2013. The results are discussed with respect to possible systematic errors induced by background contributions.

To investigate the interplay between collective and single particle degrees of freedom in odd nuclei, Recoil distance Doppler-shift and Doppler-shift attenuation lifetime measurements were carried out for levels in 155Dy in coincidence detection of gamma-rays. 26 lifetimes were determined using the Differential decay curve method. Particle plus triaxial rotor model (PTRM) calculations were performed to compare the experimental level scheme and transition strengths with theoretical ones in order to get information on the quadrupole deformation (є,γ) of the bands. As a result, different quadrupole deformations for the one-quasineutron bands at low and medium spins are deduced.

Using the high-resolution Q3D magnetic spectrograph at the Maier-Leibnitz Laboratory (MLL) Tandem accelerator in Munich, we studied 0+ excitations in the mercury isotopes 198Hg, 200Hg, and 202Hg after two-neutron transfer. We only observed 4-6 excited 0+ states per nucleus up to about 3-MeV excitation energy, far fewer than in other experiments of this (p, t) campaign. The results reveal a sharp drop in the number of low-lying 0+ states towards the 208Pb shell closure. We discuss the low-energy 0+ state density as a function of the valence nucleon number Nval. The 0+ excitation energies and the measured (p, t) transfer cross sections indicate a structural change throughout the Hg isotopes, with the most notable result being the peaking in the cross section of the low-lying excited 02+ state in 200Hg.

Studies of even-even nuclei in the A∼100 transitional mass region within the framework of the interacting boson model-1 (IBM-1) have been expanded down to 98Pd nuclei to compare the calculation with new experimental results from measurements obtained at the Institute of Nuclear Physics in Cologne. The low-lying energy levels and the E2 transition rates of 98−100Pd nuclei are investigated and their geometric structures are described in the present work. We have also focused on the new B(E2:21+ → 01+) values of 112,114Pd nuclei to compare with previously calculated values.

Excited states in 106Pd and 106Cd have been studied using the (n,n′γ) reaction. The data include level lifetimes, spins, branching ratios, and multipole mixing ratios, and give a comprehensive view of excitations with spin ≤6ħ. The determined E2 strengths show serious discrepancies with the quadrupole phonon structure expected in these nuclei.

To study how collective nuclear structure evolves towards mid-shell and test next-generation LaBr3(Ce) scintillation detectors, measurements of the lifetimes of 21+ states in 168Hf and 174W were conducted at the Wright Nuclear Structure Laboratory. Preliminary results indicate that the excellent time and energy resolution of LaBr3 detectors make them well suited to fast timing measurements, allowing for improved background subtraction and peak resolution in comparison to BaF2 detectors. Preliminary analysis shows an order of magnitude reduction in the statistical error of the 2+ lifetimes in comparison to literature values for both nuclei. In the case of 174W, a substantial reduction of the observed lifetime hints at the possibility of new physics in the region.

The upcoming GERDA and MAJORANA experiments will use germanium crystals, isotopically enriched in 76Ge to search for the neutrinoless double beta decay (0ν2β). The very long half-life of 0ν2β (> 1025 y for 76Ge) requires an extremely low background level. Neutrons induced by cosmic muons can be captured after thermalization by 74Ge or 76Ge, followed by β− -decay of 75Ge and 77Ge respectively. The prompt γ-cascade after neutron capture and the β-decay contribute to the total backround in these experiments. For a good estimation of the background the poorly known cross-sections for the 74Ge(n,γ)75,75mGe reactions were measured using targets, isotopically depleted in 76Ge. Furthermore the prompt γ-rays in 77Ge were measured with isotopically enriched germanium targets. The results of the latter measurement were combined with the results of a dedicated coincidence measurement to reconstruct the prompt γ-ray decay scheme.