Nuclear axial polarizability and the α + α interaction

The 27π-exchange part of the α + α interaction is studied in terms of the response of the α -particle to a static pion field which is related to the static nuclear axial polarizability. We show that the experimental α + α phase shifts for l ? 4 and T_lab ? 50 MeV (that probe the asymptotic tail of the potential) cannot be fitted taking only into account the nucleonic responses of the individual nucleons in the α-particle. This result is interpreted as evidence for an appreciable contribution of the nuclear polarization of the α-particle in the scattering process, nearly twice as large as the nucleonic contribution. The magnitude of this effect, as extracted from the experimental data, is consistent with the theoretical expectations.     

Axial polarizability and weak currents in nuclei

The effect of the isobaric excitations on the weak axial coupling constants in nuclei is studied through P.C.A.C. We first establish the Klein-Gordon equation for the virtual pion field in the nucleus; it takes into account pion rescattering. The influence of isobar excitation is contained in the axial polarizability coefficient which is linked to the p-wave π-N scattering volume. The derivation of this equation stresses its analogies with electromagnetism. We give then a basic relation between the axial current and the pionic field. It incorporates the effects of the isobars in the axial polarizability, which leads naturally to an electromagnetic analog. We show that this relation leads in heavy nuclei to a quenching of the axial coupling constant by the Lorentz-Lorenz factor, which may originate from the short range or the Pauli correlations, depending on the range of the π-N forces. Hence this quenching may have a different origin than the existence of short-range correlations and may arise from a Pauli blocking effect. On the other hand, the pseudoscalar coupling constant is found to be strongly suppressed. In finite nuclei, these basic quenchings can be masked by surface effects, the general features of which are studied with the help of a solvable model. This model is further used to obtain the asymptotic pion field which is linked to the effective pion-nucleus coupling constant and can be determined experimentally through π-nucleus dispersion relations. We find that this quantity is quenched, in agreement with recent experimental data.     

Two-pion production processes, chiral symmetry and NN interaction in the medium

We study the two-pion propagator in the nuclear medium. This quantity appears in the ππ T-matrix and we show that it also enters the QCD scalar susceptibility. The medium effects on this propagator are due to the influence of the individual nucleon response to a scalar field through their pion clouds. This response is appreciably increased by the nuclear environment. It produces an important convergence effect between the scalar and pseudoscalar susceptibilities, reflecting the reshaping of the scalar strength observed in 2π production experiments. While a large modifications of the σ propagator follows, due to its coupling to two pion states, we show that the NN potential remains instead unaffected.     

Polarizability effects in electronic and muonic atoms

Zeitschrift für Physik A Hadrons and nuclei - TheS state polarizability shifts are derived from the virtual forward Compton scattering in the unretarded dipole approximation. In the...     

Solvent delivery in the new application areas of high-performance liquid chromatography

Summary The development of new column packings for biological macromolecules, as well as the techniques for handling micropacked columns, sets new demands on solvent delivery systems for HPLC. The main implications of these developments with respect to pump design are discussed and the evaluation of a solvent delivery system (pump and gradient programmer) optimized for these purposes is presented.Presented at the 14th International Symposium on Chromatography London, September, 1982     

Resident neuroelectrochemical interfacing using carbon nanofiber arrays

Carbon nanofiber electrode architectures are used to provide for long-term, neuroelectroanalytical measurements of the dynamic processes of intercellular communication between excitable cells. Individually addressed, vertically aligned carbon nanofibers are incorporated into multielement electrode arrays upon which excitable cell matrixes of both neuronal-like derived cell lines (rat pheochromocytoma, PC-12) and primary cells (dissociated cells from embryonic rat hippocampus) are cultured over extended periods (days to weeks). Electrode arrays are characterized with respect to their response to easily oxidized neurotransmitters, including dopamine, norepinephrine, and 5-hydroxytyramide. Electroanalysis at discrete electrodes following long-term cell culture demonstrates that this platform remains responsive for the detection of easily oxidized species generated by the cultured cells. Preliminary data also suggests that quantal release of easily oxidized transmitters can be observed at nanofiber electrodes following direct culture and differentiation on the arrays for periods of at least 16 days.     

Binding effects in coherent π0 photoproduction on deuterium

We have calculated the threshold amplitude for π⁰ photoproduction on deuterium including binding corrections and the pion mass difference. Due to large cancellations among the binding corrections our result is remarkably well described by the static approximation. The main corrections come from the P-wave contribution to the single-scattering term.     

An anomaly in the reaction n + p → D + 2γ

We analyze the recently observed doubly radiative n-p capture. It is shown that the experimental result is impossible to explain within the present theoretical framework.