J. Ajaka

GRAAL: a polarized γ-ray beam at ESRF☆

The GRAAL facility produces a highly polarized gamma-ray beam by Compton scattering of laser photons on the electrons of the European Synchrotron Radiation Facility (ESRF) at Grenoble. Preliminary results have been obtained with the LAGRANGE detector showing its excellent performances.

Beam asymmetry Σ in meson photo-production at GRAAL

Meson photoproduction on the nucleon is a major tool for the investigation of nucleon resonances. In particular eta photoproduction selectively probes certain resonances which are difficult to explore with pions. The low energy behaviour of the eta production is dominated by the S 11 (1535) resonance and the influence of other resonances (like D 13(1520)) is difficult to detect. Polarization observables enhance the sensitivity to the contributions of the less pronounced resonances since they depend mostly on the interference between the dominant E 0+ multipole and the smaller ones (like E 2- + M 2- for the D 13(1520))[l].

Plasmonic Diodes THz Response to Impulse Train and Stochastic Optical Excitations

Terahertz (THz) plasma oscillation in n+nn+ InGaAs vertical diodes is studied by using a numerical approach based on the hydrodynamic (HD) equations. The 1D HD model is coupled to 1D Poisson equation. We simulate the diode response to the optical excitation of plasma waves at room temperature. Our results clearly show the presence of 3D plasma resonances in the THz frequency domain. The selection of appropriately shaped optical excitation pulses helps in the realization of the detected frequency. The investigation is completed by introducing stochastic perturbations according to the Langevin equation.

Vertical diodes response to optical and electrical THz excitations

We use a hydrodynamic model self-consistently coupled to a 1D Poisson solver to simulate the excitation by optical beating as well as by electrical perturbation of plasma waves in n+nn+ InGaAs diodes at room temperature. We calculate the electric field response and the velocity response of the carriers in the middle of the diode regions. Our results show clearly the presence of three-dimensional plasma resonances in the terahertz frequency domain for the two region types (n and n+). The investigation is completed by calculating the local differential mobility.

Experiments with polarized photon beam at GRAAL

Since 1996 the GRAAL experiment is running at the ESRF, using a linearly polarized photon beam produced by backscattering a laser beam on the electron beam in the storage ring. A liquid hydrogen target and a large acceptance detector are used. All sizeable photoproduction reactions are measured and analyzed. We present the results of η meson, one and two pions, and ω photoproduction, which are partly published or still preliminary. Predictions or fits of these data by theoretical models are given.

Beam Asymmetry Σ in Meson Photoproduction at GRAAL

Meson photoproduction on the nucleon is a major tool for the investigation of nucleon resonances. In particular eta photoproduction selectively probes certain resonances which are difficult to explore with pions. The low energy behaviour of the eta production is dominated by the S 11 (1535) resonance and the influence of other resonances (like D 13(1520)) is difficult to detect. Polarization observables enhance the sensitivity to the contributions of the less pronounced resonances since they depend mostly on the interference between the dominant E 0+ multipole and the smaller ones (like E 2- + M 2- for the D 13(1520))[l].

Beam Polarisation Asymmetries in Meson Photoproduction at Graal

Meson photoproduction on the nucleon has been a major source of information on the N and Δ resonances and the transition matrix elements from the nucleon ground state to its excited states.

Isospin splitting of transition amplitudes from photoproduction of eta mesons on deuterium

Measurements of the total and differential cross sections for η-meson photoproduction on 1H, 2D, and 14N liquid targets from threshold to 1.2 GeV, have been taken using the tagged Bremsstrahlung photon beam produced by the electrons extracted from the ELSA storage ring at Bonn. The reaction was identified by detecting the η decay products in the neutral meson spectrometer SPES0-2π, while the recoil baryons (proton, neutron or deuteron) were detected by a variety of large angle scintillator detectors. We present here our experimental results on the Deuterium target, both the comparison of the quasi-free proton and neutron as well as results on the coherent production. Our preliminary results for the counting rates integrated on the S11 excitation show significantly smaller figures for the neutrons than for the protons. This is consistent with an Isoscalar part of the Amplitude roughly ten times smaller than the Isovector one and also explains the very low rate of coherent deuterons observed.

Photoproduction of Eta Mesons on Deuterium from Threshold to 1.2 GeV

Measurements of the total and differential cross sections for η-meson photoproduction on 1H, 2D, and 14N liquid targets from threshold to 1.2 GeV have been taken using the tagged Bremsstrahlung photon beam produced by the electrons extracted from the ELSA storage ring at Bonn. The reaction was identified by detecting the η decay products in the neutral meson spectrometer SPESO-2π, while the recoil baryons (proton, neutron, or deuteron) were detected by a variety of large angle scintillator detectors. We present here our experimental results on the deuterium target, both the comparison of the quasi-free proton and neutron as well as results on the coherent production. Our preliminary results for the counting rates integrated on the S11 excitation show significantly smaller figures for the neutrons than for the protons. This is consistent with an isoscalar part of the amplitude roughly ten times smaller than the isovector one and also explains the very low rate of coherent deuterons observed.