Browsing by Author "Beck, R."

Using the Mainz 48 cm 0 x 64 cm NaI(Tl) detector and the segmented Gottingen recoil detector SENECA in coincidence, Compton scattering by the proton at theta(gamma)(lab) = 136degrees has been measured at MAMI (Mainz) in the energy range from 200 to 470 MeV. The new data confirm the previous observation that there is a systematic discrepancy between MAMI and LEGS (Brookhaven) data leading to different spin polarizabilities gamma(pi) = -38.7 +/- 1.8 and -27.2 +/- 3.1 (x10(-4) fm(4)), respectively.

Differential cross-sections for the reactions d(gamma,pi (o))d and d(gamma,pi (o))pn have been measured at MAMI with the TAPS detector setup in the energy range 140 MeV < E- < 306 MeV. By use of the Glasgow tagging spectrometer an 0.8 MeV energy resolution for photons incident on the target was achieved. The (o) missing energy resolution was sufficient for a reliable separation of coherent and incoherent channels. The data for the break-up channel exhibit very strong final state interaction effects, whereas the observed angular dependence of the inclusive process d(gamma, pi (o))X is in quantitative agreement with predictions for a quasi-free process. The observed absolute d(gamma, pi (o))X cross-sections, on, the other hand, are significantly smaller than predicted by the quasi-free process for E-gamma greater than or similar to 250 MeV. Associating this failure with the pi (o) photoproduction on the neutron would suggest that its cross-section is up to 25% below the presently believed value.

Compton scattering by the proton has been measured over a wide range covering photon energies 250 MeV less than or similar to E-gamma less than or similar to 800 MeV and photon scattering angles 30 degrees less than or similar to theta (lab)(gamma) less than or similar to 150 degrees, using the tagged-photon facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The data are in good agreement with the dispersion theory based on the SAID-SM99K parameterization of photo-meson amplitudes. From the subset of data between 280 and 360 MeV the resonance pion-photoproduction amplitudes were evaluated leading to the multipole E2/M1 ratio EMR(340 MeV) = (-1.6 +/- 0.4(stat+syst) +/- 0.2(model))% From all data below 455 MeV the proton's backward spin polarizability was determined to be gamma pi = (-37.9 +/- 0.6(stat+syst) +/- 3.5(model)) x 10(-4) fm(4). (C) 2001 Elsevier Science B.V. All rights reserved.

Compton scattering by the proton has been measured over a wide range covering photon energies 250 MeV less than or equal to E-gamma less than or similar to 800 MeV and photon scattering angles 30 degrees less than or similar to theta (lab)(gamma) less than or similar to 150 degrees, using the tagged-photon facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The previously existing data base on proton Compton scattering is greatly enlarged by more than 700 new data points. The new data are interpreted in terms of dispersion theory based on the SAID-SM99K parametrization of photo-meson amplitudes. It is found that two-pion exchange in the t-channel is needed for a description of the data in the second resonance region. The data axe well represented if this channel is modeled by a single pole with the mass parameter m(sigma) approximate to 600 MeV. The asymptotic part of the spin-dependent amplitude is found to be well represented by pi (0) exchange in the t-channel. No indications of additional effects were found. Using the mass parameter m(sigma) of the two-pion exchange determined from the second resonance region and using the new global average for the difference of the electric and magnetic polarizabilities of the proton, alpha - beta = (10.5 +/- 0.9(stat+stat) +/- 0.7 model) x 10(-4) fm(3), as obtained from a recent experiment on proton Compton scattering below pion photoproduction threshold, a-backward spin polarizability of gamma (pi) = (-37.1 +/- 0.6(stat+syst) +/- 3.0model) x 10(-4) fm(4) has been determined from data of the first resonance region below 455 MeV. This value is in a good agreement with predictions of dispersion relations and chiral perturbation theory. From a subset of data between 280 and 360 MeV, the resonance pion photoproduction amplitudes were evaluated leading to a E2/M1 multipole ratio of the p --> Delta radiative transition of EMR(340 MeV) = (-1.7 +/- 0.4(stat+syst) +/- 0.2(model))%. It was found that this number is dependent on the parameterization of photo-meson amplitudes. With the MAID2K parameterization, an E2/M1 multipole ratio of EMR(340 MeV) = (-2.0 +/- 0.4(stat+syst) +/- 0.2(model))% is obtained.

The differential cross sections for Compton scattering from the proton have been measured at MAMI with three different detector setups. There is a good agreement among all the results. The theoretical calculation based on dispersion relations shows that there is no drastic change necessary in the parameters of this approach.

The reaction gammap --> pi (+)pi (o)n has been measured at MAMI for photon energies up to 820 MeV. Invariant mass spectra of the particles in the final state (pi (+)n), (pi (o)n), (pi (+)pi (o)) have been determined for several bins of incident photon energy. Differences in pi (+)pi (o) and simultaneously measured pi (o)pi (o) invariant mass distributions are assigned to a rho branch of the D-13(1520) nucleon resonance.

The photoproduction of two neutral pions from the deuteron has been studied for incident photon energies from 200 MeV to 820 MeV with the TAPS spectrometer at MAMI (Mainz Microtron). The total cross-section was determined and used to deduce the cross-section from the neutron. Due to the good statistical quality of the data Dalitz plots for the three particles in the exit channel (pi (0)pi N-0) could be constructed. The invariant mass distributions derived from them are presented in this paper. They indicate that the important reaction mechanism in the second resonance region is a sequential decay pattern involving the population of the Delta (1232)-resonance as an intermediate state.

Winter wheat cultivar Basalt was artificially inoculated with Fusarium culmorum at the end of anthesis and treated with the systemic fungicide tebuconazole (Folicur(R)) a few days before and/or after inoculation. Check plots remained uninoculated and unsprayed. Head infections, yield, yield components and the percentage of Fusarium-infected kernels were determined. Artificial Fusarium inoculation lowered yield significantly by 24.2-45.0%. Any fungicide treatment saved yield, thousand grain weight and kernel numbers per head. Pre-infectional application of tebuconazole was superior to application carried out post-infection. Moreover, the fungicide controlled deoxynivalenol (DON) synthesis in the field to a considerable extent, and enabled good control of Fusarium head blight, glume blotch and the percentage of Fusarium-infected kernels. The levels of Fusarium kernel infection after harvest clearly reflected the DON content of wheat grain.

Coherent photoproduction of eta -mesons from the deuteron has been measured from threshold up to E-gamma approximate to 750 MeV using the photon spectrometer TAPS at the tagged photon facility at the Mainz microtron MAMI. For the first time, differential coherent cross-sections have been deduced from the coincident detection of the eta -meson and the recoil deuteron. A missing-energy analysis was used for the suppression of background events so that a very clean identification of coherent eta -photoproduction was achieved. The resulting cross-sections agree with previous experimental results except for angles around 90 degrees in the gammad c.m. system where they are smaller. They are compared to various model calculations.

Quasi-free photoproduction of eta-mesons from the deuteron has been measured at the tagged photon facility of the Mainz microtron MAMI with the photon spectrometer TAPS for incident photon energies from the production threshold at 630 MeV up to 820 MeV. In a fully exclusive measurement eta-mesons and recoil nucleons were detected in coincidence. At incident photon energies above the production threshold on the free nucleon, where final-state interaction effects are negligible, an almost constant ratio of sigma(n)/sigma(p) = 0.66 +/- 0.10 was found. At lower incident photon energies the ratio rises due to re-scattering effects. The average ratio agrees with the value extracted from a comparison of the inclusive d(gamma,eta)X cross section to the free-proton cross-section via a participant-spectator model for the deuteron cross-section (sigma(n)/sigma(p) = 0.67 +/- 0.07). The angular dependence of the ratio agrees with the expected small deviation of neutron and proton cross-section from isotropy due to the influence of the D-13(1520)-resonance. The energy dependence of the ratio in the excitation energy range of the S-11(1535)-resonance disfavors model predictions which try to explain this resonance as KSigma bound state.

For the first time we checked the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the proton experimentally in the photon energy range from 0.2-2.9 GeV with the tagged photon facilities at MAMI (Mainz) and ELSA (Bonn). New data of the doubly polarized total cross section difference are presented in the energy range from 1.6 to 2.9 GeV. The contribution to the GDH integral from 0.2-2.9 GeV yields [254+/-5(stat)+/-12(syst)] mub with negative contributions in the Regge regime at photon energies above 2.1 GeV. This trend supports the validity of the GDH sum rule.

A direct measurement of the helicity dependence of the total photoabsorption cross section an the proton was carried out at MAMI (Mainz) in the energy range 200 < E- < 800 MeV. The experiment used a 4 detection system, a circularly polarized. tagged photon beam, and a frozen spin target. The contributions to the Gerasimov-Drell-Hearn sum rule and to the forward spin polarizability gamma (0) determined from the data are 226 +/- 5(stat) +/- 12(syst) mub and -187 +/- 8(stat) +/- 10(syst) x 10(-6) fm(4), respectively, for 200 < E- < 800 MeV.

To verify the fundamental Gerasimov-Drell-Hearn (GDH) sum rule for the first time experimentally, we measured the helicity dependent total photoabsorption cross section with circularly polarized real photons and longitudinally polarized nucleons in the photon energy range 0.68-1.82 GeV with the tagged photon facility at ELSA. The experiment was carried out with a 4pi detection system, a circularly polarized tagged photon beam, and a frozen spin polarized proton target. The contribution to the GDH sum rule in this photon energy range is [49.9+/-2.4(stat)+/-2.2(syst)] mub.

The helicity dependence of the total cross-section and the invariant-mass distributions in the (p pi(+/-)) and (pi(+) pi(-)) final states for the gamma p -> p pi(+)pi(-) reaction have been measured for the first time at incident photon energies from 400 to 800MeV. The measurement was performed with the large-acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. Although this channel is found to be predominantly excited by the intermediate production of a Delta pi state, both the measured unpolarized and the helicity-dependent observables are generally not well described by the existing theoretical models.

The helicity dependence of the (gamma) over right arrow(p) over right arrow -+ peta reaction has been measured for the first time at a center-of-mass angle theta(eta) = 70degrees in the photon energy range from 780 MeV to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4pi-detector system, a circularly polarized, tagged photon beam. and a longitudinally polarized frozen-spin target. The helicity 3/2 cross-section is found to be small and the results for helicity 1/2 agree with predictions from the MAID analysis.

The helicity dependence of the (γ) over right arrow(p) over right arrow -->ppi(0) reaction has been measured for the first time in the photon-energy range from 550 to 790 MeV. The experiment, performed at the Mainz microtron MAMI, used a 4pi -detector system, a circularly polarized, tagged photon beam, and a longitudinally polarized frozen-spin target. These data are predominantly sensitive to the D-13(1520) resonance and are used to determine its helicity amplitudes.

The helicity dependence of the single pion photoproduction on the proton has been measured in the energy range from 200 to 450 MeV for the first time. The experiment, performed at the Mainz microtron MAMI, used a 4 pi-detector system, a circularly polarized, tagged photon beam, and a frozen-spin target. The data obtained provide new information for multipole analyses of pion photoproduction and determine the main contributions to the Gerasimov-Drell-Hearn sum rule and the forward spin polarizability gamma(0).

The helicity dependence of the total cross section for the (γ) over right arrow(p) over right arrownpi(+) pi(0) reaction has been measured for the first time at incident photon energies from 400 to 800 MeV The measurement was performed with the large acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. This channel is found to be excited predominantly when the photon and proton have a parallel spin orientation, due to the intermediate production of the D-13 resonance. (C) 2002 Elsevier Science B.V. All rights reserved.

A high-quality double-polarization data set for the helicity dependence of the total and differential cross-sections for both gammap --> Npi channels in the Delta region has been obtained in the framework of the GDH experiment. The experiment, performed at the Mainz microtron MAMI, used a 4pi detection system, a circularly polarized photon beam, and a longitudinally polarized frozen-spin target. These data are included in the database to perform a multipole analysis to determine the properties of the Delta(1232)-resonance. For the resonant Delta(1232) multipoles we find a very good agreement with previous analyses, while the nonresonant ones show significant deviations.

The in-medium properties of the omega meson were investigated at the MAMI-C electron accelerator using the Crystal Ball/TAPS detector system to identify omega mesons through their omega -> pi(0)gamma decay channel. Two solid targets (C, Nb) and a LH2 target as a reference have been used. In the energy range 900 to 1300 MeV the omega lineshapes for the different nuclei are compared and a broadening of the omega signal for the niobium target is observed in comparison to the liquid-hydrogen target. In a second analysis the omega momentum distributions for the solid targets are determined for 50 MeV/c wide bins. For both analyses a comparison to transport calculations shows that the data are consistent with an in-medium modification by collisional broadening, as observed by transparency ratio measurements.