  A single photon counting system has been developed for efficient detection of
forward emitted fluorescence photons at the Experimental Storage Ring (ESR) at
GSI. The system employs a movable parabolic mirror with a central slit that can
be positioned around the ion beam and a selected low noise photomultiplier for
detection of the collected photons. Compared to the previously used system of
mirror segments installed inside the ESR the collection efficiency for
forward-emitted photons is improved by more than a factor of 5. No adverse
effects on the stored ion beam have been observed during operation besides a
small drop in the ion current of about 5% during movement of the mirror into
the beam position. The new detection system has been used in the LIBELLE
experiment at ESR and enabled for the first time the detection of the
ground-state hyperfine M1 transition in lithium-like bismuth (209Bi80+) in a
laser-spectroscopy measurement.
