The total photoabsorption cross sections for 1H, 2H, 3He, and 4H have been measured (for the first time for 3He) with high precision on the MAMI-B accelerator (Mainz, Germany) with a tagged photon beam and the large acceptance detector DAPHNE. The dynamics of baryon resonances along the periodic table is analyzed, and they are compared with the sum of partial reactions on proton and deuteron. Preparation and the first stage of polarization measurements of photoabsorption cross sections on proton in the Δ resonance region are reported.

In recent years, the physics of electromagnetic interactions has made considerable advances in studying giant multipole resonances, which are highly excited collective states of nuclei. After the advent of new-generation continuous-beam accelerators, (e, e′x) coincidence experiments were performed, and new precision data on giant multipole resonances were obtained from these experiments. A number of examples of cross sections for decay accompanied by proton and alpha-particle emission in p-, sd-, and pf-shell nuclei of mass number in the range 16 ⩽ A ⩽ 64 are given. The relative contributions of these reactions are compared, their general and special features are revealed, and evaluations for semidirect and statistical decays are presented.

The total cross section for the $\ensuremath{\gamma}n\ensuremath{\rightarrow}p{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0}$ reaction has been measured over the photon energy range 450--800 MeV at the 855 MeV MAMI Microtron in Mainz with the large acceptance detector DAPHNE and using a deuterium target. As expected, this reaction has a very similar cross section to the $\ensuremath{\gamma}p\ensuremath{\rightarrow}n{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{0}$ channel and its amplitude is strongly underestimated by the existing double pion photoproduction models.

Differential cross sections for Compton scattering by the proton have been measured in the energy interval between 200 and 500 MeV at scattering angles of θcms = 75° and θcms = 90° using the CATS, the CATS/TRAJAN, and the COPP setups with the Glasgow Tagger at MAMI (Mainz). The data are compared with predictions from dispersion theory using photo-meson amplitudes from the recent VPI solution SM95. The experiment and the theoretical procedure are described in detail. It is found that the experiment and predictions are in agreement as far as the energy dependence of the differential cross sections in the Δ-range is concerned. However, there is evidence that a scaling down of the resonance part of the M1+32 photo-meson amplitude by (2.8 ± 0.9)% is required in comparison with the VPI analysis. The deduced value of the M1+32-photoproduction amplitude at the resonance energy of 320 MeV is: |M1+32| = (39.6 ± 0.4) × 10−3 mπ+−1.

The total cross section for the {gamma}n{r_arrow}p{pi}{sup {minus}}{pi}{sup 0} reaction has been measured over the photon energy range 450{endash}800 MeV at the 855 MeV MAMI Microtron in Mainz with the large acceptance detector DAPHNE and using a deuterium target. As expected, this reaction has a very similar cross section to the {gamma}p{r_arrow}n{pi}{sup +}{pi}{sup 0} channel and its amplitude is strongly underestimated by the existing double pion photoproduction models. {copyright} {ital 1997} {ital The American Physical Society}

The total cross section for the gamma n-->p pi(-)pi(0) reaction has been measured over the photon energy range 450-800 MeV at the 855 MeV MAMI Microtron in Mainz with the large acceptance detector DAPHNE and using a deuterium target. As expected, this reaction has a very similar cross section to the gamma p-->n pi(+)pi(0) channel and its amplitude is strongly underestimated by the existing double pion photoproduction models.

The differential cross section for the scattering of photons by the proton was measured in the energy range Eγ = 250−500 MeV at the scattering angle θcms = 90° ± 3°. The tagged photon beam of the MAMI accelerator was used and both the scattered photon and the recoil proton were detected. The data essentially are consistent with dispersion theories, and remove the discrepancy that has existed above 290 MeV between theory and earlier published experimental results.

Using the tagged-photon beam of MAMI (Mainz) differential cross-sections for Compton scattering by the proton through θcms = 75° have been measured for the Δ-resonance region from 200 to 410 MeV. The data put a new constraint on the M1+32 photoproduction amplitude in the resonance, |M1+32| = (39.6 ± 0.4) · 10−3mπ+−1 at 320 MeV, which is (2.8 ± 0.9) % less than given by the recent analysis SM95 of the Virginia group.

Using the Mainz NaI(Tl)BaF2 photon spectrometer CATS (Compton And Two photon Spectrometer) together with the prototype of a universal spectrometer for recoil protons TRAJAN (TRAjectory ANalyser) we have studied different methods to disentangle Compton scattering by the proton from the large π0 background through the energy range of the Δ resonance. CATS is a modular 48 cm ⊘ × 64 cm NaI(Tl) detector with an energy resolution of 1.5% combined with a 2π array of 61 BaF2 detectors. TRAJAN consists of a hodoscope with two planes, a wire chamber with horizontal and vertical wires and a grid of horizontal and vertical plastic scintillator strips, followed by a wall of nine NaI(Tl) detectors. By this arrangement the proton energy was obtained via a time-of-flight analysis and the recoil angle of the protons via a trajectory reconstruction. Additionally, a ΔE-E analysis of the detected particles allows us to suppress the electromagnetic background. By reconstructing the proton trajectory the segment of the scattering target can be determined where the Compton scattering process or the π0 production process had taken place. In this way it is possible to correct the event distributions for the smearing-out caused by the finite target length and thus to improve on the separation of the two types of events.

The differential cross section for the scattering of photons by the proton was measured in the energy range E(gamma) = 250-500 MeV at the scattering angle theta(cms) = 90 degrees +/- 3 degrees. The tagged photon beam of the MAMI accelerator was used and both the scattered photon and the recoil proton were detected. The data essentially are consistent with dispersion theories, and remove the discrepancy that has existed above 290 MeV between theory and earlier published experimental results.

The elastic and inelastic photon scattering from 12C was investigated in the energy range of the Δ-resonance (Eγ = 200–500 MeV) at a scattering angle of 40°. The experiment was performed with the tagged photon facility at the cw electron accelerator MAMI (Ee− = 855 MeV). By using a large NaI(Tl) detector having an energy resolution of 1.5%, the elastic scattering could be separated from the inelastic process to the first excited state (Ex = 4.44 MeV of 12C. The differential cross section for the coherent scattering is compared with a simple static ansatz, that uses the known charge form factor, and with a recent Δ-hole calculation.