Improvement of mass resolution of cosmic ray nuclei using a ΔE × E Si detector telescope

Using 5-in.-diameter Si(Li) detectors with a detector thickness of 1, 2 and 3 mm combined with 62 mm × 62 mm position-sensitive Si detectors, an excellent isotope telescope has been constructed for the Geotail mission. The performance of the telescope and the individual detectors has been investigated using 56Fe beams with energies between 430 and 696 MeV/n. The nonuniformity of the detector thickness as measured by exposing the detectors to a monochromatic Fe beam is 0.44% for 1 mm thick detectors, 0.54% for 2 mm thick detectors and 0.15% for 3 mm thick detectors. The secondary isotopes of Al through Fe produced by 430 MeV/n 56Fe in a polyethylene target were clearly resolved by the telescope. The measured mass resolution for Si, Ca and Mn were 0.36 amu, 0.42 amu and 0.44 amu in FWHM, respectively. The telescope has the following two key features: an excellent resolution (better than 0.5 amu) and an exceptionally high sensitivity ( SΩ = 43 cm 2 sr ) for a broad range of species from He through Ni with energies between 2.4 and 210 MeV/n.