The ep -->e'p eta reaction at and above the S11(1535) baryon resonance

New cross sections for the reaction e p-->e p eta are reported for total center of mass energy W = 1.5--1.86 GeV and invariant momentum transfer Q2 = 0.25--1.5 (GeV/c)(2). This large kinematic range allows extraction of important new information about response functions, photocouplings, and eta N coupling strengths of baryon resonances. Newly observed structure at W approximately 1.65 GeV is shown to come from interference between S and P waves and can be interpreted with known resonances. Improved values are derived for the photon coupling amplitude for the S11(1535) resonance.     

Suppression of the sawtooth instability in a storage ring by free-electron laser: an example of nonlinear stabilization by noise

The stabilization of nonlinear excitations by noise is a topic of fundamental importance in many physical problems. We discuss a genuine example within the context of storage ring-free electron laser physics, by presenting a model which allows the characterization of the system evolution and the determination of the conditions leading to the suppression of instabilities of sawtooth type. The conclusions of the model are confirmed by a comparison with experimental results on the Super Aco Storage Ring-Free Electron Laser.     

Proton MRI of (13)C distribution by J and chemical shift editing.

The sensitivity of (13)C NMR imaging can be considerably favored by detecting the (1)H nuclei bound to (13)C nuclei via scalar J-interaction (X-filter). However, the J-editing approaches have difficulty in discriminating between compounds with similar J-constant as, for example, different glucose metabolites. In such cases, it is almost impossible to get J-edited images of a single-compound distribution, since the various molecules are distinguishable only via their chemical shift. In a recent application of J-editing to high-resolution spectroscopy, it has been shown that a more efficient chemical selectivity could be obtained by utilizing the larger chemical shift range of (13)C. This has been made by introducing frequency-selective (13)C pulses that allow a great capability of indirect chemical separation. Here a double-resonance imaging approach is proposed, based on both J-editing and (13)C chemical shift editing, which achieves a powerful chemical selectivity and is able to produce full maps of specific chemical compounds. Results are presented on a multicompartments sample containing solutions of glucose and lactic and glutamic acid in water.     

Scintillation properties of K2LaCl5 with Ce doping

In this work we present the scintillation properties of K₂LaCl₅ doped with Ce³⁺ concentrations of 0, 0.1, 1 and 10%. Under X-ray excitation the crystals show the efficient Ce³⁺ (5d–4f) luminescence between 340 and 400 nm. Depending on the Ce concentration, the light yields vary between 24,000 and 50,000 photons/MeV (ph/MeV). Experiments with 662 KeV gamma ray excitation show light yields varying between 13,000 and 30,000 ph/MeV. For Ce luminescence a single exponential decay time of approximately 40 ns is expected. However, for K₂LaCl₅:10%Ce, the decay is not single exponential. As a function of the time t, it can be described by a 1/t^1.63 behaviour for large t. With a crystal of K₂LaCl₅:10%Ce we obtained an energy resolution of 5.1% for 662 KeV gamma ray excitation of ¹³⁷Cs.     

The Second Noether Theorem in the formalism of jet-bundles: Symmetries and degeneration

By means of the jet-bundle formalism, the Second Noether Theorem is formulated for a general first-order Lagrangian field theory with infinitesimal local symmetries. These symmetries are implemented by a linear differential operator acting between the sections of a vector bundle and vector fields on the configuration bundle. The problem of the degeneration of the Lagrangian system is examined from a covariant and an instantaneous (i.e. space+time split) viewpoint. It is shown that in the instantaneous approach the presence of infinitesimal local symmetries leads to degeneration of the theory. Vertical local symmetries are shown to imply degeneration also in the covariant formalism. These results can be extended to higher-order Lagrangians as well.     

Humidity effects on calcium zirconate ceramics in electrochemical hydrogen cells

In the eighties, Iwahara discovered that doped Sr and Ba cerates show significant proton conduction, at high temperature, and in hydrogen- or water-containing atmosphere [1]. Begin ‘90 it was proved that also doped zirconates (Sr, Ba, Ca) join this family of proton conducting perovskites [2]. Within this group, indium doped CaZrO₃ is one of the more interesting materials for applications, being mechanically and chemically particularly stable, and producible in very dense form. Specifically, it was already succesfully incorporated in a hydrogen sensor for liquid aluminium [3]. In principle, this sensor works as a simple electrochemical hydrogen concentration cell described by Nernst. However, as already remarked phenomenologically from the beginning [1], a hydrogen concentration cell using these perovskites shows the theoretically predicted emf, only in the presence of humidity. In this paper we want to go deeper into the reasons for this phenomenon. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995