J.P. Didelez

Performance of a BGO calorimeter in a tagged photon beam from 260 to 1150 MeV

Abstract 20 BGO crystals with photomultiplier readout that form one sector of a large solid angle BGO calorimeter, were exposed to an intermediate energy tagged photon beam. Linearity, multiplicity and resolution results are presented and discussed.

PROBING THE LIGHT SPEED ANISOTROPY WITH RESPECT TO THE COSMIC MICROWAVE BACKGROUND RADIATION DIPOLE

We have studied the angular fluctuations in the speed of light with respect to the apex of the dipole of Cosmic Microwave Background (CMB) radiation using the experimental data obtained with GRAAL facility, located at the European Synchrotron Radiation Facility (ESRF) in Grenoble. The measurements were based on the stability of the Compton edge of laser photons scattered on the 6 GeV monochromatic electron beam. The results enable one to obtain a conservative constraint on the anisotropy in the light speed variations Δc(θ)/c<3×10-12, i.e. with higher precision than from previous experiments.

Cross sections and asymmetries for the p(p,π0)pp reaction from threshold to 1 GeV

Abstract Total cross sections, differential cross sections and asymmetries for the reaction p( p ,π 0 )pp from threshold to 1 GeV have been measured. The outgoing π0 was detected by a 4π neutral meson spectrometer. Our data are compared with the corresponding np→NNπ± reactions. The isoscalar partial cross sections are extracted from the relevant total cross sections. They are significant throughout the whole energy range. Below the Δ-region, energy-integrated differential cross sections show a large anisotropy difference between the neutral and charged pion channels, respectively. A partial-wave fit shows that the initial T = 0 state is dominated by the 3D1 wave in this energy region. Finally, predictions of phenomenological models give reasonable good account of the measured observables but cannot reproduce the sign of the asymmetries.

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.

Inner-hole excitations in 89Zr and 91Mo via the (3He,α) reaction at 97 MeV

The nuclei 89Zr and 91Mo have been investigated up to an excitation energy of about 25 MeV using the (3He, alpha ) reaction at 97.3 MeV incident energy. In addition to the well known low-lying levels and analogue states strongly excited groups of levels centered around 4.4 MeV are confirmed to belong to the 1f7/2 neutron inner shell in 89Zr, with a maximum of about 50% of the sum rule strength. A corresponding group, with comparable strength, is found for the first time in 91Mo at nearly the same excitation energy. In addition, and for both nuclei, two much smoother structures are observed lying under and beyond the analogue states. The authors discuss their possible attribution to the 1f7/2 T components respectively. Contributions from 1d inner shells are also considered. In both nuclei, new IAS fragments have been identified.

Meson photoproduction at GRAAL and baryon resonances

Differential cross sections and polarised photon beam asymmetries for eta, pi(0) and pi(1) photoproduction on the proton, produced by the GRAAL Collaboration, are presented. The results cover wide angular and enegy ranges of 40-170degrees and 550-1496 MeV (707-1496 MeV for eta) respectively, and exhibit an unprecedented level of accuracy. The results are compared with predictions of several models.

First observation of T> = 452 inner hole states in 207Pb

Abstract The first four isobaric analog states in 207 Pb, T > = 45 2 , were found around E x = 20 MeV with the reaction 208 Pb(τ, α) 207 Pb at 102 MeV incident energy. The excitation energies, widths and angular distributions were measured. Spectroscopic factors were deduced by DWBA analysis.

ETA PHOTOPRODUCTION ON THE NEUTRON AT GRAAL: MEASUREMENT OF THE DIFFERENTIAL CROSS SECTION

In this contribution, we will present our first preliminary measurement of the differential cross section for the reaction γn→ηn. Comparison of the reactions γp→ηp for free and bound proton (D2 target) will also be discussed.

Measurement of the Sigma beam asymmetry for the omega photoproduction off the proton and the neutron at the GRAAL experiment

We report on new measurements of the beam asymmetry for. photoproduction on protons and neutrons in hydrogen and deuterium targets from the GRAAL Collaboration. The beam asymmetry values are extracted from the reaction threshold (E-gamma = 1.1 GeV in the free nucleon kinematics) up to 1.5 GeV of incoming photon energy. For the first time both the radiative and the three-pion decay channels are simultaneously investigated on the free proton. Results from the two decay channels are in agreement, leading to the worlds most precise measurements of the beam asymmetry for omega photoproduction off free protons. First experimental results on the deuteron allow the extraction of the Sigma beam asymmetry on quasifree nucleons. The beam asymmetry angular distributions obtained for the free and the quasifree kinematics show the same behavior, similar to the findings in pseudoscalar meson photoproduction reactions. First results of the beam asymmetry on the quasifree neutrons are presented, showing different strengths and angular distributions from the results on the proton target.

Measurement of the total photoabsorption cross section on a proton in the energy range 600-1500 MeV at the GRAAL

The total photoabsorption cross section on a free proton was measured at the GRAAL facility in the energy range Eγ = 600−1500 MeV. The large-aperture LAGRANγE detector and a liquid hydrogen target were used in the experiment performed with a back-scattered Compton gamma beam. To improve the accuracy, two alternative methods were employed. First, a subtraction method of using empty-target measurements allowed the cross section σtot to be evaluated directly because of a low level of the electromagnetic background. Second, an algorithm for evaluating σtot on the basis of summing the dominating partial cross sections was developed. Experimental results obtained for σtot by the two methods are compared with existing data.