The performance of superconducting radiofrequency (SRF) cavities used for particle accelerators depends on two characteristic material parameters: field of first flux entry Hentry and pinning strength. The former sets the limit for the maximum achievable accelerating gradient, while the latter determines how efficiently flux can be expelled related to the maximum achievable quality factor. In this paper, a method based on muon spin rotation (µSR) is developed to probe these parameters on samples. It combines measurements from two different spectrometers, one being specifically built for these studies and samples of different geometries. It is found that annealing at 1400 • C virtually eliminates all pinning. Such an annealed substrate is ideally suited to measure Hentry of layered superconductors, which might enable accelerating gradients beyond bulk niobium technology.Recently, to reach high quality factors, a treatment procedure has been established baking cavities at 800 • C and injecting nitrogen gas at the end of this treatment. arXiv:1705.05480v3 [cond-mat.supr-con]

Background: Neutron-rich nuclei around neutron number N = 60 show a dramatic shape transition from spherical ground states to prolate deformation in 98 Sr and heavier nuclei. Purpose: The purpose of this study is to investigate the single-particle structure approaching the shape transitional region. Method: The level structures of neutron-rich 93,94,95 Sr were studied via the 2 H( 94,95,96 Sr, t ) one-neutron stripping reactions at TRIUMF using a beam energy of 5.5 AMeV. γ -rays emitted from excited states and recoiling charged particles were detected by using the TIGRESS and SHARC arrays, respectively. States were identified by gating on the excitation energy and, if possible, the coincident γ radiation. Results: Triton angular distributions for the reactions populating states in ejectile nuclei 93,94,95 Sr were compared with distorted wave Born approximation calculations to assign and revise spin and parity quantum numbers and extract spectroscopic factors. The results were compared with shell-model calculations and the reverse (d, p) reactions and good agreement was obtained. Conclusions: The results for the 2 H( 94 Sr, t ) 93 Sr and 2 H( 95 Sr, t ) 94 Sr reactions are in good agreement with shellmodel calculations. A two-level mixing analysis for the 0 + states in 94 Sr suggest strong mixing of two shapes. For the 2 H( 96 Sr, t ) 95 Sr reaction the agreement with the shell-model is less good. The configuration of the ground state of 96 Sr is already more complex than predicted, and therefore indications for the shape transition can already be observed before N = 60.

In this work we investigate superconducting properties of niobium samples via application of the muon spin rotation/relaxation (SR) technique. We employ for the first time the SR technique to study samples that are cut out from large and small grain 1.5 GHz radio frequency (rf) single cell niobium cavities. The rf test of these cavities was accompanied by full temperature mapping to characterize the rf losses in each of the samples. Results of the SR measurements show that standard cavity surface treatments like mild baking and buffered chemical polishing performed on the studied samples affect their surface pinning strength. We find an interesting correlation between high field rf losses and field dependence of the sample magnetic volume fraction measured via SR. The SR line width observed in zero-field-SR measurements matches the behavior of Nb samples doped with minute amounts of Ta or N impurities. A lower and an upper bound for the upper critical field H c2 of these cutouts is found.

Nb3Sn is currently the most promising material other than niobium for future superconducting radiofrequency cavities. Critical fields above 120 mT in pulsed operation and about 80 mT in CW have been achieved in cavity tests. This is large compared to the lower critical field as derived from the London penetration depth, extracted from low field surface impedance measurements. In this paper direct measurements of the London penetration depth from which the lower critical field and the superheating field are derived are presented. The field of first vortex penetration is measured under DC and RF fields. The combined results confirm that Nb3Sn cavities are indeed operated in a metastable state above the lower critical field but are currently limited to a critical field well below the superheating field.

Background: The region around neutron number N = 60 in the neutron-rich Sr and Zr nuclei is one of the most dramatic examples of a ground state shape transition from (near) spherical below N = 60 to strongly deformed shapes in the heavier isotopes.Purpose: The single-particle structure of 95−97 Sr approaching the ground state shape transition at 98 Sr has been investigated via single-neutron transfer reactions using the (d, p) reaction in inverse kinematics. These reactions selectively populate states with a large overlap of the projectile ground state coupled to a neutron in a single-particle orbital.Method: Radioactive 94,95,96 Sr nuclei with energies of 5.5 AMeV were used to bombard a CD2 target. Recoiling light charged particles and γ rays were detected using a quasi-4π silicon strip detector array and a 12 element Ge array. The excitation energy of states populated was reconstructed employing the missing mass method combined with γ-ray tagging and differential cross sections for final states were extracted.Results: A reaction model analysis of the angular distributions allowed for firm spin assignments to be made for the low-lying 352, 556 and 681 keV excited states in 95 Sr and a constraint has been placed on the spin of the higher-lying 1666 keV state. Angular distributions have been extracted for 10 states populated in the d( 95 Sr, p) 96 Sr reaction, and constraints have been provided for the spins and parities of several final states. Additionally, the 0, 167 and 522 keV states in 97 Sr were populated through the d( 96 Sr, p) reaction. Spectroscopic factors for all three reactions were extracted. Conclusions:Results are compared to shell model calculations in several model spaces and the structure of low-lying states in 94 Sr and 95 Sr is well-described. The spectroscopic strength of the 0 + and 2 + states in 96 Sr is significantly more fragmented than predicted. The spectroscopic factors for the d( 96 Sr, p) 97 Sr reaction suggest that the two lowest lying excited states have significant overlap with the weakly deformed ground state of 96 Sr, but the ground state of 97 Sr has a different structure. * Corresponding author: wimmer@phys.s.u-tokyo.ac.jp arranging the nucleons in certain ways across the valence orbitals, which in turn causes a departure from sphericity [1]. The expense of such re-arrangements is dependent on the size of the energy gaps between single-particle orbitals above the Fermi energy. If the energy spacing is small, the valence nucleons can scatter into valence orbitals which are above the Fermi energy and drive the nucleus into a low-energy deformed configuration. On the other hand, if the energy spacing is large, the valence nucleons are unable to scatter into higher orbitals and this favors spherical shapes. The size of these energy gaps is in turn dependent on the number of valence nucleons, due to the monopole component of the residual