[en] Using the Mainz 48 cm null-set x64 cm NaI(Tl) detector and the segmented Goettingen recoil detector SENECA in coincidence, Compton scattering by the proton at θ_γ^lab=136 deg. 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 γ_π=-38.7±1.8 and -27.2±3.1 (x10^-4 fm⁴), respectively

[en] Measurements of the ⁴He(γ,π⁺n)³H and ⁴He(γ,π⁺p)nnn reactions in quasifree π production kinematic regimes have been performed using unpolarized and linearly polarized tagged photons in conjunction with large solid angle π and n detectors at the MAMI-B accelerator, Mainz. Differential cross sections are presented for photon energies spanning the Δ(1232) excitation region. These data combined with previously presented results for the ¹²C(γ,π⁺n)¹¹B reaction provide an extensive data set for future investigations of Δ-medium effects in nuclei. The ⁴He(γ,π⁺n)³H cross sections are significantly smaller than theoretical predictions based on the free proton amplitude for Δ excitation with a crude correction for final state interactions, and exhibit different trends, which could possibly be a consequence of Δ-medium effects. From a comparison with published ¹²C(γ,π⁺p)¹¹Be data, it is concluded that the ⁴He(γ,π⁺p)nnn reaction mechanisms are probably two-step processes, although the possibility that part or all of the cross section arises from interactions with preexisting Δ⁺⁺ particles cannot be ruled out

[en] 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

[en] Differential cross-sections for quasi-free π⁰ photoproduction from the proton and neutron bound in the deuteron have been measured for E_γ=200-400 MeV at θγ^lab=136.2 using the Glasgow photon tagger at MAMI, the Mainz 48 cm diameter x 64 cm NaI(Tl) photon detector and the Goettingen SENECA recoil detector. For the proton measurements made with both liquid-deuterium and liquid-hydrogen targets allow direct comparison of ''free'' π⁰ photoproduction cross-sections as extracted from the bound-proton data with experimental free cross-sections which are found to be in reasonable agreement below 320 MeV. At higher energies the ''free'' cross-sections extracted from quasi-free data are significantly smaller than the experimental free cross-sections and theoretical predictions based on multipole analysis. For the first time, ''free'' neutron cross-section have been extracted in the Δ-region. They are also in agreement with the predictions from multipole analysis up to 320 MeV and significantly smaller at higher photon energies. (orig.)

[en] Differential cross-sections for quasi-free Compton scattering from the proton and neutron bound in the deuteron have been measured using the Glasgow/Mainz photon tagging spectrometer at the Mainz MAMI accelerator together with the Mainz 48cm diameter x 64cm NaI(Tl) photon detector and the Goettingen SENECA recoil detector. The data cover photon energies ranging from 200MeV to 400MeV at θ^LAB_γ=136.2 . Liquid deuterium and hydrogen targets allowed direct comparison of free and quasi-free scattering from the proton. The neutron detection efficiency of the SENECA detector was measured via the reaction p(γ,π⁺n). The ''free'' proton Compton scattering cross-sections extracted from the bound proton data are in reasonable agreement with those for the free proton which gives confidence in the method to extract the differential cross-section for free scattering from quasi-free data. Differential cross-sections on the free neutron have been extracted and the difference of the electromagnetic polarizabilities of the neutron has been determined to be α_n-β_n=9.8±3.6(stat)^+2.1_-1.1(syst)±2.2(model) in units of 10^-4fm³. In combination with the polarizability sum α_n+β_n=15.2±0.5 deduced from photoabsorption data, the neutron electric and magnetic polarizabilities, α_n=12.5±1.8(stat)^+₁.1_-0.6(syst)±1.1(model) and β_n=2.7±1.8(stat)^+0.6_-1.1(syst)±1.1(model) are obtained. The backward spin polarizability of the neutron was determined to be γ⁽ⁿ⁾_π=(58.6±4.0) x 10^-4fm⁴. (orig.)