An electron spectrometer consisting of an electron transport system with normal conducting solenoidal coils and a Si(Li)-detector as the energy dispersive element is described. It can be used for in beam spectroscopy of electrons in three different modes. The first one is the usual broad range mode with a low energy cut offofthe transmission performed by a tantalum disk between target and detector. The second one is the lens spectrometer mode. An envelope baffle system permits electron detection in a momentum band A pip = 0.12. To cover a large energy range the magnet current is sweeped. In the third mode-the recoil shadow method -a longitudinal semicylindrical baffle between target and Si(Li)-detector allows spectroscopy of delayed electrons emitted from recoil nuclei in flight. Special features of this method are high transmission, and strong suppression of the prompt 3-electron background. Lifetime measurements based on the detection of conversion electrons are possible by variation of the target position. This was tested with the 152Sm(160, x n) 168 -xyb compound nuclear reaction at a recoil velocity vr = 0.01 c, where half lives between 0.1 ns and 1 ns were determined.
Data on the beam asymmetry Σ in the photoproduction of η mesons off protons are reported for tagged photon energies from 1130 to 1790 MeV (mass range from W = 1748 MeV to W = 2045 MeV). The data cover the full solid angle that allows for a precise moment analysis. For the first time, a strong cusp effect in a polarization observable has been observed that is an effect of a branch-point singularity at the pη threshold [Eγ = 1447 MeV (W = 1896 MeV)]. The latest BnGa partial wave analysis includes the new beam asymmetry data and yields a strong indication for the N (1895) 1 2 − nucleon resonance, demonstrating the importance of including all singularities for a correct determination of partial waves and resonance parameters.
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