We present here an ellipsoidal timing detector in Radioactive Ion Beam Line in Lanzhou (RIBLL). The photons induced by radioactive beam ions passing through a thin plastic-scintillator foil BC422, emit from the foil center corresponding to one focal point of an aluminum ellipsoidal mirror and are reflected to another focus point at which the cathode of a photomultiplier tube locates. A time resolution of about 115ps is achieved for 12N and the counting rate up to 108 pps is allowed. The simulation was carried out using GEANT4 Monte Carlo toolkit. The photons total collection efficiency following projectile from different position, photon collection efficiency and time resolution of photon to photocathode of 3 different cases were calculated. Also the main factors influencing the detector’s time resolution and some proposals are given.

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