Detection Efficiency of the CRIS Scintillating Optical Fiber Hodoscope

Measurements of the detection efficiency of a Scintillating Optical Fiber Trajectory (SOFT) detector for 660 MeV/n Carbon, Boron, Beryllium and Lithium nuclei and for 300 MeV/n Fe nuclei are described. Detection efficiencies of 99.8%, 98.7%, 97.5%, and 93.8% for C, B, Be, and Li respectively were obtained for a single layer of fibers at an angle of incidence of 30°. These fibers had a black coating of extramural absorber (EMA) to optically decouple the fibers. Somewhat better detection efficiencies were obtained for optically coupled fibers (no EMA). The detection efficiencies for iron nuclei were ~99.7%. We have used these results to estimate the event detection efficiencies of the fiber hodoscope which is planned for use in the Cosmic Ray Isotope Spectrometer (CRIS) (Stone, et al., 1989) experiment on the Advanced Composition Explorer (ACE). We find that for the highest energy Lithium nuclei which will stop in CRIS, the detection efficiency is about 89% for 6 plane events and about 99% for 5 plane events at 30°.