[en] Understanding the energy resolution terms for LYSO based calorimeters with APD readout at low energy (< 500 MeV) is relevant both for the completion of the KLOE-2 experiment, at DAΦNE, and for the design of the Mu2e calorimeter. In this work, we present a dedicated comparison between experimental data, taken in 2011 at the MAMI tagged photon beam facility with a crystal matrix prototype, and a full Geant-4 simulation of this detector. The crystal prototype matrix consisted of 9 2×2 × 15 cm³ LYSO crystals read-out by 10×10 mm² Hamamatsu avalanche photodiodes (APD) surrounded by 8 PbWO₄ crystals read-out by Bialkali photomultipliers for outer leakage recovery granting a total transverse coverage of 3 Rm. An energy resolution of ∼5.4% at 100 MeV has been achieved. A fit to the energy dependence of the resolution provides the following parametrization: σ_E/E = 2.1%/E^1/4 ⊕ 3.6%, where the sum is in quadrature. The noise term is completely negligible as expected by the reduced level of the electronic noise achieved. The constant term is still leakage dominated.

[en] We present the first detailed measurement of incoherent photoproduction of neutral pions to a discrete state of a residual nucleus. The ¹²C(γ,π⁰)¹²C_4.4MeV* reaction has been studied with the Glasgow photon tagger at MAMI employing a new technique which uses the large solid angle Crystal Ball detector both as a π⁰ spectrometer and to detect decay photons from the excited residual nucleus. The technique has potential applications to a broad range of future nuclear measurements with the Crystal Ball and similar detector systems elsewhere. Such data are sensitive to the propagation of the Δ in the nuclear medium and will give the first information on matter transition form factors from measurements with an electromagnetic probe. The incoherent cross sections are compared to two theoretical predictions including a Δ-hole model

[en] Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz in the three isospin channels γ-vectorp→π⁺π⁰n, γ-vectorp→π⁰π⁰p, and γ-vectorp→π⁺π^-p. The circularly polarized photons, produced from bremsstrahlung of longitudinally polarized electrons, were tagged with the Glasgow magnetic spectrometer. Charged pions and the decay photons of π⁰ mesons were detected in a 4π electromagnetic calorimeter which combined the Crystal Ball detector with the TAPS detector. The precisely measured asymmetries are very sensitive to details of the production processes and are thus key observables in the modeling of the reaction dynamics.