Search Results (16 total)

We report the development of different algorithms for the calculation of quality factors of eigenmodes in accelerating cavities, which have resonance frequencies above the cutoff frequency of the beam tubes. The analysis is based on a discretization of such cavity structures by the finite integration technique, and the radiation at the open boundaries is systematically taken into account by different approaches in time and frequency domain. Results indicate that even single cell cavities of the TESLA type show Q values of 10³ and multicell cavities values in excess of 10⁴. Thus these modes may cause considerable beam instabilities. Comparison with the conventional method of analyzing closed cavities and identifying modes with little change in frequency as a function of boundary condition shows qualitative differences. Some modes from the closed cavity model do not exist in the open structure and thus would be misinterpreted as trapped modes when only a closed cavity analysis is employed.

Based on very accurate measurements performed on a superconducting microwave resonator shaped like a desymmetrized three-dimensional Sinai billiard, we investigate for the first time spectral properties of the vectorial Helmholtz, i.e., nonquantum wave equation for a classically totally chaotic and theoretically precisely studied system. We are thereby able to generalize some aspects of quantum chaos and present some results which are consequences of the polarization features of the electromagnetic waves.

The g factor of the first excited, 3/2⁺, 93.6 keV level of ⁹¹Sr was measured using a beam of separated fission products and the time-differential perturbed angular correlation method. The result, g(3/2⁺)=-0.231(11), is discussed in the context of the systematics of g factors of N=53–57 isotones, and found to be in good agreement with the prediction of the core-excitation model.

Experiments designed to provide more information on the spectroscopy of ²¹⁴Bi, and on ²¹⁴Pb(β^-)²¹⁴Bi in particular, were undertaken because of interest in first-forbidden β decay in the lead region. The experiments consisted of γ-γ coincidences and angular correlations, conversion electron measurements, level lifetime determinations, and precision γ-ray energy measurements. The 352-keV level of ²¹⁴Bi was found to be a strong candidate (and the only candidate) for the 0₁^- state. Recent additions to the ²¹⁴Pb decay scheme are confirmed by γ-γ coincidence measurements. A careful evaluation of the ²¹⁴Bi level scheme is made with emphasis on separating experimentally based conclusions from speculations based on systematics and other ‘‘weak’’ arguments. Shell-model calculations of the spectroscopy of ^210,212Bi and ^210,212Pb(β^-)^210,212Bi were performed using a modification of the Kuo-Herling realistic interaction. These calculations and a generalized seniority model provide a basis for an examination of the systematics of the A=210, 212, 214 spectroscopy and β decay. The generalized seniority model is found to be a quite good approximation which provides a quantitative understanding of the ²¹⁴Pb decay rates.

The first postulation of excited states in neutron-rich ⁷⁶Zn is presented. We postulate the levels in ⁷⁶Zn to be populated by high- and low-spin isomers of ⁷⁶Cu with half-lives of 0.57 s and 1.27 s, respectively. Of 12 γ rays attributed to ⁷⁶Zn decay, 11 are placed in a level scheme for ⁷⁶Zn with eight excited states up to 3 MeV. A shell-model calculation has been carried out to reproduce the systematics of the neutron-rich Zn isotopes up to A=76. The model space involves active protons in orbitals between Z=28 and 50 and neutrons in orbitals filling the subshell between N=38 and 50.

A recently developed technique for fast timing measurements using β-γ-γ coincidences with plastic, BaF₂ and Ge detectors, respectively, has been used to measure lifetimes in the range 5–1500 ps in the neutron-rich nuclei ⁹⁷Sr, ⁹⁷Y, and ⁹⁷Zr. The results for ⁹⁷Y are interpreted in terms of quadrupole vibrational excitations. For ⁹⁷Sr the experimental data suggest the coexistence of spherical/vibrational and deformed states and allow the extraction of mixing matrix elements between them. A quadrupole moment of ‖Q₀‖=3.5(4) e b for the deformed states in ⁹⁷Sr has been deduced.

The hyperfine field, H_hf(T), in polycrystalline CuO has been studied by ⁵⁷Fe Mössbauer-source spectroscopy (MS), μ₊ spin rotation μSR, and ¹⁰⁰Rh perturbed angular correlation (PAC) over 4K<T<295K. Antiferromagnetic order is observed below T_N=226(1) K. MS and PAC each exhibit a single dominant site with no K-capture aftereffects while μSR has five components. For μSR and PAC H_hf(T) is well approximated by spin S=1/2 mean-field theory; for MS it is anomalous, indicating a significant difference in spin between Fe and Cu and/or moment coupling for Fe-Cu and Cu-Cu. There is no evidence for long-range dynamic correlations, as observed for La₂CuO₄.

Using perturbed angular correlations, we observed decoration of a well-known ¹¹¹In-vacancy complex in Ni with diffusing H or D atoms. Both partially decorated and saturated configurations are distinguished from the undecorated complex. The saturated configuration is believed to have one hydrogen atom bound in each cavity of vacancy size. Analysis of hydrogen detrapping above 300 K leads to a binding enthalpy ∼0.6 eV, independent of isotopic mass.

Using BaF₂ scintillation counters and a source of ¹⁰¹Fe we have determined the magnetic moment of the 127 keV first excited state of ¹⁰¹Ru, using time differential perturbed angular correlations. Via the known hyperfine field of Ru in Fe we deduce g=-0.138(5) and g=-0.142(5) for annealed and quenched sources, respectively, and obtain a new, accurate value of the half-life, T_1/2=0.655(4) nsec.

Au samples doped with ¹¹¹In impurities and heavily deformed at 77 K were studied by perturbed angular correlations after undergoing isochronal annealing up to 500 K. Between 150 and 500 K we detected formation and dissolution of five defect-probe traps characterized by distinctive quadrupole interaction frequencies. Based on previous identifications of four of the traps and arguments that a previously undetected trap forming at 162 K has vacancy character, our experiments suggest trivacancy, divacancy, and monovacancy recoveries at 162, 180, and 240 K, respectively. From the order of the recoveries, we conclude that all three species are present immediately after deformation. For divacancies we observed thermally activated in situ transformation between two distinguishable trap configurations. The details of our isochronal annealing curves are well simulated via a kinetic rate model which assumes only first-order processes, and the trapping stages are found to explain most but not all features of stored-energy-release measurements.