Norbert Pietralla

Lifetimes of excited states of the nucleus Zr 96 were studied with the Doppler-shift attenuation method following the Zr 94 ( O 18 , O 16 ) Zr 96 two-neutron transfer reaction. The obtained B ( E 2 ; 2 2 + → 0 2 + ) and B ( E 2 ; 4 1 + → 2 2 + ) transition strengths, amounting to 26 ( 2 ) and 39 ( 7 ) W . u . , respectively, firmly attribute the 2 2 + and 4 1 + states to the collective intruder band built on top of the 0 2 + state. The nature of this structure is discussed against predictions of the collective model using the Bohr Hamiltonian, the interacting boson model with configuration mixing, and the quasiparticle vacua shell model. The collective, interband B ( E 2 ; 4 1 + → 2 1 + ) = 11.2 ( 8 ) W . u . transition strength, refuting the previously assumed purity of both structures, is reproduced in the calculations. The quasiparticle vacua shell model predicts, in agreement with a simple two-state model based on experimental E 2 transition strengths, considerable mixing between the normal and intruder 2 + states.

The production cross sections of neutron-rich nuclides near the N = 126 neutron shell closure have been measured for the elements ranging from tungsten ( Z = 74 ) to gold ( Z = 79 ) in the fragmentation of a 1000 MeV / u Pb 208 beam on a Be 9 target. Several cross sections, namely those for W 196 − 199 , Re 199 − 201 , and Os 202 − 203 , were determined for the first time with this projectile-target combination. The results are compared with the predictions of several nuclear reaction models and semiempirical parametrizations; special attention is devoted to multiple proton-removal channels and the formation of nuclides with neutron number N = 127 .

The lifetimes of the 2 1 + and 4 1 + states of Pt 200 were measured by applying the recoil-distance Doppler-shift method. Excited states were populated in the Pt 198 ( O 18 , O 16 ) Pt 200 two-neutron transfer reaction at the 9-MV tandem accelerator at the IFIN-HH in Măgurele, Romania. The resulting B ( E 2 ) values of the 2 1 + → 0 1 + and 4 1 + → 2 1 + transitions as well as the B 4 / 2 ratio of 2.10(31) indicate an evolution of the nuclear structure toward sphericity when approaching the neutron shell closure at N = 126 . The B ( E 2 ; 2 1 + → 0 1 + ) values of Pt and Hg are compared to values of Te, Xe, and Ba where similar structural effects are exhibited.

Even-even Yb and W isotopes around the N=104 mid-shell region are known to present deformation. The ground-band states of these nuclei are well described by the SU(3) symmetry, indicating a rigid-rotor character. Excited states of even-even isotopes in the region, namely 176Yb and 182W, were populated through Coulomb excitation and fusion-evaporation reactions, in an experiment performed at IFIN-HH in Măgurele, Romania. The gamma-spectroscopy of these nuclei will be presented in this contribution. Additionally, gamma spectroscopy results for the even-odd 177Yb nucleus, populated by the 1n-transfer reaction mechanism, will be presented. Contradictory to the collective description of the ground-band excitations in 176Yb and 182W isotopes, the excited states of this nucleus can be described by single-particle excitations. The gamma-spectroscopy analysis of this data contributes to the evaluation of the available data in this mass region, while ongoing work is expected to provide further insights for the nuclear structure of these rare-earth isotopes.

The Yb isotopes near the N=104 mid-shell region show a transition from spherical to well-deformed, rigid-rotor-like, structures. The 178Yb isotope (the most neutron-rich Yb isotope with measured excited states) shows signs of high deformation. A γ-spectroscopy experiment was conducted in the 10 MV FN Tandem accelerator of the Institut für Kernphysik at the University of Cologne to study 178Yb and the deformation evolution in the isotopic chain. The 176Yb(18O,16O)178Yb 2-neutron-transfer reaction was used to populate excited states in the isotope of interest. In this proceedings contribution the study of the excitation function of this 2-neutron-transfer reaction, which was measured across a range of beam energies, both below and above the Coulomb barrier, will be presented. Additionally, the study of the 176Yb(18O,4n)190Pt fusion-evaporation reaction occurring will also be discussed.

Magnetic dipole, M1, strengths were studied in semimagic ^{50}Ti up to the neutron-separation threshold by combining data from (d,p) one-neutron transfer, (γ,γ^{'}) real-photon scattering, (e,e^{'}) inelastic scattering at extreme backward angles, and (p,p^{'}) at E_{p}=210 MeV and extreme forward angles. The combination of all probes provided unique access to the neutron spin-flip contribution and the possibility to evaluate its role in generating the spin-flip M1 strengths. The small contribution of the neutron (1f_{7/2})^{-1}(1f_{5/2})^{+1} spin-flip transitions, which were probed with the (d,p) reaction, to the overall strength in ^{50}Ti questions the standard picture for the microscopic origin of spin-flip strength in the fp shell. For ^{50}Ti, this Letter shows that J^{π}=1^{+} states with larger neutron (1f_{7/2})^{-1}(1f_{5/2})^{+1} spectroscopic factors do not correspond to the ones with the largest B(M1;0_{1}^{+}→1_{i}^{+}) strengths.

Bulk niobium is the standard material for superconducting RF cavities but requires operation at 2 K due to its critical temperature (T_c) of 9.3 K, leading to high cooling costs. Nb₃Sn, with a higher T_c (∼18.2 K), allows operation at 4.2 K, reducing cryogenic demands and improving efficiency. Although Nb₃Sn theoretically offers a higher quality factor (Q), achieving this in practice remains difficult. Co-sputtering Nb₃Sn films on copper cavities may improve Q, thermal stability, and delay quenching at high accelerating fields. In this study, Nb₃Sn films were co-sputtered on different substrates to investigate the T_c and lower critical field (H_c1). The films were deposited in a controlled atmosphere and analyzed using X-ray diffraction, resistivity measurements and superconducting quantum interference device magnetometry. The results confirm the successful formation of the Nb₃Sn phase with well-defined superconducting transition. The measured T_c value for films grown on r-cut sapphire reached 17.8 K, close to the bulk Nb₃Sn value. On the technically relevant substrate copper, a T_c value of 15 K was obtained. Notably, the H_c1 value was found to be not only higher than that for bulk Nb₃Sn but also higher than that of pure Nb. A high value of H_c1 is critical for operating high Q cavities at accelerating fields above 20 MV/m.

The lifetime of the 2 1 + state of Pb 210 was measured in the Pb 208 ( O 18 , O 16 ) Pb 210 two-neutron transfer reaction by γ -ray spectroscopy employing the recoil-distance Doppler-shift method. The extracted absolute B ( E 2 ) ↓ value of 119 − 8 + 9 e 2 fm 4 is consistent with previously reported measurements, but with significantly improved precision. The available experimental data for the 2 1 + − 4 1 + − 6 1 + − 8 1 + multiplet are compared with shell-model calculations based on the well-established Kuo-Herling interaction. The new B ( E 2 ) ↓ value agrees well with the shell-model prediction, providing evidence that the properties of the 2 1 + and 8 1 + states of Pb 210 can be consistently described together within the nuclear shell-model framework.