We report on the construction, tests, calibrations and commissioning of an Optical Readout Time Projection Chamber (O-TPC) detector operating with a CO 2 (80%) + N 2 (20%) gas mixture at 100 and 150 Torr. It was designed to measure the cross sections of several key nuclear reactions involved in stellar evolution. In particular, a study of the rate of formation of oxygen and carbon during the process of helium burning will be performed by exposing the chamber gas to intense nearly mono-energetic gamma-ray beams at the High Intensity Gamma Source (HIγS) JINST 5 P12004facility. The O-TPC has a sensitive target-drift volume of 30x30x21 cm 3 . Ionization electrons drift towards a double parallel-grid avalanche multiplier, yielding charge multiplication and light emission. Avalanche-induced photons from N 2 emission are collected, intensified and recorded with a Charge Coupled Device (CCD) camera, providing two-dimensional track images. The event's time projection (third coordinate) and the deposited energy are recorded by photomultipliers and by the TPC charge-signal, respectively. A dedicated VME-based data acquisition system and associated data analysis tools were developed to record and analyze these data.The O-TPC has been tested and calibrated with 3.183 MeV alpha-particles emitted by a 148 Gd source placed within its volume with a measured energy resolution of 3.0%. Tracks of alpha and 12 C particles from the dissociation of 16 O and of three alpha-particles from the dissociation of 12 C have been measured during initial in-beam test experiments performed at the HIγS facility at Duke University. The full detection system and its performance are described and the results of the preliminary in-beam test experiments are reported.

The reactions γp → ηp and γp → η 0 p are measured from their thresholds up to the center-of-mass energy W ¼ 1.96 GeV with the tagged-photon facilities at the Mainz Microtron, MAMI. Differential cross sections are obtained with unprecedented statistical accuracy, providing fine energy binning and full production-angle coverage. A strong cusp is observed in the total cross section for η photoproduction at the energies in the vicinity of the η 0 threshold, W ¼ 1896 MeV (E γ ¼ 1447 MeV). Within the framework of a revised ηMAID isobar model, the cusp, in connection with a steep rise of the η 0 total cross section from its threshold, can only be explained by a strong coupling of the poorly known Nð1895Þ1=2 − state to both ηp and η 0 p. Including the new high-accuracy results in the ηMAID fit to available η and η 0 photoproduction data allows the determination of the Nð1895Þ1=2 − properties.

Measurement of the ω → π 0 e + e − and η → e + e − γ Dalitz decays with the A2 setup at MAMI The Dalitz decays η → e + e − γ and ω → π 0 e + e − have been measured in the γp → ηp and γp → ωp reactions, respectively, with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The value obtained for the slope parameter of the electromagnetic transition form factor of η, Λ −2 η = (1.97 ± 0.11tot ) GeV −2 , is in good agreement with previous measurements of the η → e + e − γ and η → µ + µ − γ decays. The uncertainty obtained in the value of Λ −2 η is lower than in previous results based on the η → e + e − γ decay. The value obtained for the ω slope parameter, Λ −2 ωπ 0 = (1.99 ± 0.21tot) GeV −2 , is somewhat lower than previous measurements based on ω → π 0 µ + µ − , but the results for the ω transition form factor are in better agreement with theoretical calculations, compared to earlier experiments.

The scalar dipole polarizabilities, αE1 and βM1, are fundamental properties related to the internal dynamics of the nucleon. The currently accepted values of the proton polarizabilities were determined by fitting to unpolarized proton Compton scattering cross section data. The measurement of the beam asymmetry Σ3 in a certain kinematical range provides an alternative approach to the extraction of the scalar polarizabilities. At the Mainz Microtron (MAMI) the beam asymmetry was measured for Compton scattering below pion photoproduction threshold for the first time. The results are compared with model calculations and the influence of the experimental data on the extraction of the scalar polarizabilities is determined.