[en] The vibrational first 2₁⁺ and 3₁^- states of the doubly-magic nucleus ¹³²Sn were excited via safe Coulomb excitation (CE) employing the recently commissioned HIE-ISOLDE accelerator at CERN in conjunction with the highly efficient MINIBALL array. The ¹³²Sn ions were accelerated to an energy of 5.5 MeV/nucleon and impinged on a ²⁰⁶Pb target. Dexciting γ rays from the first excited states of the target and the projectile nucleus were recorded in coincidence with scattered particles. The optimized beam energy, the high-energy resolution and good efficiency of the HPGe spectrometer provide a beneficial combination to master the challenging measurement characterized by small CE cross sections and excitation energies above 4 MeV. The reduced transition strengths were determined for 0_g.s.⁺ → 2₁⁺, 0_g.s.⁺ → 3₁^-, and 2₁⁺ → 3₁^- in ¹³²Sn. In the past first preliminary results for the B(E2;0_g.s.⁺ → 2₁⁺) value were obtained with an efficient BaF2 array at ORNL. The results on excited collective states in ¹³²Sn provide crucial information on cross shell configurations that are expected to be dominated by a strong proton contribution. State-of-the-art large-scale shell model calculations and new mean field predictions are compared to the results.

[en] The first 2${}^{+}$ and 3${}^{-<!--\; ???\; -->}$ states of the doubly-magic nucleus ${}^{132}$Sn were populated via safe Coulomb excitation at HIE-ISOLDE, CERN, employing the highly-efficient MINIBALL array. ${}^{132}$Sn ions with a beam energy of 5.49 MeV/nucleon were impinged on a ${}^{206}$Pb target. De-exciting $\mathrm{\gamma <!--\; ??\; -->}$ rays from the low-lying excited states of the target and the projectile were recorded in coincidence with scattered particles. Reduced E1, E2, and E3 transition strengths were determined. A locally enhanced B(E2; 0${}_{g.s.}^{+}\to <!--\; ???\; -->$ 2${}_1^{+}$) strength at doubly-magic ${}^{132}$Sn is found and confronted with the microscopic description of the structure of the respective states within different theoretical approaches. The results provide crucial information on cross-shell configurations which were determined for the first time within state-of-the-art shell-model (LSSM, MCSM) and mean-field calculations (RPA). The presented results of experiment and theory can be considered to be the first direct verification of the sphericity and double-magicity of ${}^{132}$Sn.

[en] A bismuth-germanate (BGO) escape-suppression shield for the high-purity germanium triple-cluster detector of the MINIBALL γ-ray spectrometer was designed and built. Monte Carlo simulations with the simulation code GEANT4 were performed to guide the construction and to determine the detector geometry of the new BGO shield. After the first measurements concerning mechanical properties of the BGO housing and the performance of the photomultiplier tubes at the Institut de Physique Nucléaire, Orsay, the prototype BGO escape-suppression shield was combined with a MINIBALL triple-cluster detector at the Institut für Kernphysik, Cologne. A dedicated electronics and digital data-acquisition system was put into operation in order to determine timing properties of the combined coincidence measurement and to measure values for the energy resolution of the BGO detectors, for the BGO low-energy threshold, and for the crucial peak-to-total ratio (P/T). The measured P/T value for a standard ${}^{60}$Co γ-ray source compares well with expectations and will allow to proceed with the amendment of the MINIBALL triple-cluster detectors with an escape-suppression shield for improved in-beam γ-ray spectroscopy especially at the new HIE-ISOLDE accelerator for radioactive ion beams at CERN.

[en] Reduced transition strengths expressed by B(E2) values are sensitive signatures to describe collective excitations of atomic nuclei and the evolution of shell structures. Along the N = Z line in the 0f1p shell they provide stringent tests of present shell-model interactions. Recently determined B(E2, 2₁⁺ → 0_g.s.⁺) values for self-conjugate nuclei in the 0f_7/2 shell showed an enhanced collective behavior. The soft shell closure at N = Z = 28 is of particular interest as it has been shown that the doubly-closed shell structure is substantially broken. In order to investigate the interplay between single-particle and collective characters of low-lying states, lifetime measurements employing the Doppler-shift attenuation method (DSAM) were performed at the FN tandem accelerator at the IKP, Cologne. Excited states in ₂₂⁴⁴Ti, ₂₄⁴⁸Cr, ₂₆⁵²Fe, and ₂₈⁵⁶Ni were populated with fusion-evaporation reactions. The emitted γ rays were detected by an array of 11 HPGe detectors. First results on lifetimes and corresponding B(E2) values are presented and discussed.

[en] Reduced transition strengths are sensitive signatures to describe collective excitations of atomic nuclei and the evolution of shell structures. They provide stringent tests of present shell-model interactions in the 0f1p shell along the N=Z line. Recently determined B(E2, 2${}_1^{+}$$\to <!--\; ???\; -->$ 0${}_{g.s.}^{+}$) values for ${}^{44}$Ti showed an enhanced collective behavior which was associated with core excitations from the sd-shell. However, there is a lack of information on precise values along the negative parity band in this nucleus. These states arise from a strong interplay between sd- and pf-shell orbitals and allow for refined tests of cross-shell contributions. In order to investigate the single-particle and collective characters of low-lying states, lifetime measurements employing the Recoil-Distance Doppler Shift (RDDS) and the Doppler-Shift Attenuation Method (DSAM) were performed at the FN tandem accelerator at the IKP, Cologne. Excited states in ${}^{44}$Ti were populated with a ${}^{40}$Ca(${}^6$Li,pn)${}^{44}$Ti fusion-evaporation reaction. The emitted $\mathrm{\gamma <!--\; ??\; -->}$ rays were detected by an array of 11 HPGe detectors. First lifetime results will be presented and compared to shell-model calculations including multi particle-hole cross-shell excitations.

[en] Reduced transition strengths expressed with B(E2) values are sensitive signatures to describe collective excitations of atomic nuclei and are indispensable to understand nuclear shell structures. Along the N=Z line in the 1f_7_/_2 shell they provide stringent tests of recent shell-model interactions. So far, B(E2, 2_1"+ → 0_g_._s_."+) values for the self-conjugate "4"4_2_2Ti,"4"8_2_4Cr,"5"2_2_6Fe isotopes are known only with considerable errors. Recoil Distance Doppler-Shift (RDDS) experiments were performed employing the Cologne coincidence plunger device to measure lifetimes with high precision in order to deduce model-independent B(E2) values. Excited states in the nuclei of interest were populated with fusion-evaporation reactions. γ rays were detected by an array of 12 HPGe detectors. Precise B(E2, 2_1"+ → 0_g_._s_."+) values are measured to be 204"+"1"6_-_1_4 e"2fm"4, 262 ± 3 e"2fm"4, and 269 ± 8 e"2fm"4 for "4"4Ti, "4"8Cr, and "5"2Fe, respectively. The results are compared to recent shell-model calculations on the basis of GX1A interaction. While the excitation energies are reproduced quite well, the B(E2) values show considerable differences.

[en] Lifetimes of excited states in the neutron-deficient nucleus ⁶¹Zn were measured employing the Recoil-Distance Doppler-Shift (RDDS) and the Fast-Timing Method at the University of Cologne. The nucleus of interest was populated as an evaporation residue in ⁴⁰Ca(²⁴Mg,n2p)⁶¹Zn and ⁵⁸Ni(α,n)⁶¹Zn reactions at 67 MeV and 19 MeV, respectively. Five lifetimes are measured for the first time, including the 5/2^- → 3/2^- transition depopulating the 124-keV isomer. Short lifetimes from the RDDS analysis are corrected for Doppler-Shift Attentuation (DSA) effects in the stopper foil. Ambiguous results from previous measurements were resolved. The obtained lifetimes are compared to predictions from different sets of shell-model calculations.

[en] Lifetimes of excited states in the neutron-deficient nucleus ⁶¹Zn were measured employing the Recoil-Distance Doppler-Shift (RDDS) and the Fast-Timing Method in fusion-evaporation reactions at the University of Cologne. Five lifetimes were measured for the first time, including the 5/2^- → 3/2^- transition depopulating the 124-keV isomer. Short lifetimes from the RDDS analysis were corrected for Doppler-Shift Attentuation (DSA) effects in the stopper foil. Ambiguous results from previous measurements were resolved. The obtained lifetimes are compared to predictions from different sets of shell-model calculations in the f_p, f_5/2pg_9/2, and multishell fp-g_9/2d_5/2 model spaces. The inclusion of cross-shell excitation into the 1d_5/2 orbital is found to be decisive for the description of collectivity in the first excited positive-parity band.

[en] The high-spin structures and isomers of the N = 81 isotones ¹³⁵Xe and ¹³⁷Ba are investigated after multinucleon-transfer (MNT) and fusion-evaporation reactions. Both nuclei are populated (i) in ¹³⁶Xe+ ²³⁸U and (ii) ¹³⁶Xe+ ²⁰⁸Pb MNT reactions employing the high-resolution Advanced Gamma Tracking Array (AGATA) coupled to the magnetic spectrometer PRISMA (iii) in the ¹³⁶Xe+ ¹⁹⁸Pt MNT reaction employing the gamma-ray array GAMMASPHERE in combination with the gas-detector array CHICO, and (iv) via a ¹¹B + ¹³⁰Te fusion-evaporation reaction with the HORUS gamma-ray array at the University of Cologne. The high-spin level schemes of ¹³⁵Xe and ¹³⁷Ba are considerably extended to higher energies. The 2058-keV (19/2^-) state in ¹³⁵Xe is identified as an isomer, closing a gap in the systematics along the N = 81 isotones. Its half-life is measured to be 9.0(9) ns, corresponding to a reduced transition probability of B(E2,19/2^- → 15/2^-) = 0.52(6) W.u. The experimentally deduced reduced transition probabilities of the isomeric states are compared to shell-model predictions. Latest shell-model calculations reproduce the experimental findings generally well and provide guidance to the interpretation of the new levels. (authors)