National Aeronautics and Space Administration

[VB10.03] The ISOMAX Time-of-Flight System

S. Geier, L.M. Barbier, E.R. Christian, S.K. Gupta, J.F. Krizmanic, J.W. Mitchell, J.F. Ormes, R.E. Streitmatter (NASA/GSFC), A.J. Davis, G.A. DeNolfo, R.A. Mewaldt, S.M. Schindler (Caltech), H. Goebel, M. Bremerich, T. Hams, M. Hof, W. Menn, M. Simon (U. of Siegen)

A state-of-the-art time-of-flight (TOF) system has been developed for the ISOMAX balloon-borne cosmic ray instrument, flown in August, 1998. The TOF system is made up of three layers of fast Bicron BC-420 plastic scintillator viewed by Hamamatsu R2083 photomultipliers through multi-segment adiabatic light guides. The outer layers are 1 m x 1 m in area and are separated by 2.6 m. A 69 cm x 69 cm layer is located 2 m below the top. Particle flight times are measured using an amplitude-corrected leading-edge discrimination technique with dual thresholds. For relativistic Z=1 particles, the TOF system has demonstrated a time resolution of about 120 ps. This is expected to improve considerably for higher charges. The design of the TOF system and its performance for ground level muons and in-flight nuclei will be presented. The full ISOMAX instrument and details of the other detector systems are discussed in other papers at this meeting. ISOMAX was supported by NASA: RTOP 353-87-02 (GSFC) and grant NAGW-1919 (Caltech), and in Germany by the DFG and the BMFT.