Spectator Detection for the Measurement of Proton-Neutron Interactions at ANKE

A telescope of three silicon detectors has been installed close to the internal target position of the ANKE spectrometer, which is situated inside the ultra-high vacuum of the COSY-Jülich light-ion storage ring. The detection and identification of slow protons and deuterons emerging from a deuterium cluster-jet target thus becomes feasible. A good measurement of the energy and angle of such a spectator proton (psp) allows one to identify a reaction as having taken place on the neutron in the target and then to determine the kinematical variables of the ion-neutron system on an event-by-event basis over a range of c.m. energies. The system has been successfully tested under laboratory conditions. By measuring the spectator proton in the pd → pspdπ 0 reaction in coincidence with a fast deuteron in the ANKE Forward Detector, values of the pn → dπ total cross section have been deduced. Further applications of the telescope include the determination of the luminosity and beam polarisation which are required for several experiments.