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Time of Flight


 Four detectors, Drift Chamber, Time Of Flight (TOF), Electromagnetic Calorimeter and Muon detector,will be   installed on the new Beijing Spectrometer (BESIII). TOF is placed between DC and EMC(see Fig. 1). It is  made of barrel and endcap. The solid coverage of the barrel TOF is 0.82, and that of the endcap TOF is from 0.85 to 0.95. The radius of barrel TOF is from 81 cm to 92.5 cm, and its effective length is 232 cm.
    The barrel TOF is designed to use two layers of plastic scintillator bar directly coupled with FM-PMT. The endcap TOF will use the fan-shaped scintillators instrumented with PMTs at inner radius. The scintillator of BC-408 by Saint-Gobain will be used for barrel because of its longer attenuation length and BC404 will be for endcap because it is shorter and has larger light output and faster time response.

Fig. 1 Schematics of TOF on BESIII

Contributions to resolution


    TOF is to measure the flight time of charged particles for particle identification (PID) by comparing the measured time against the predicted time, which can be obtained from the charged particle track and the momentum given by Drift Chamber. Its capability of PID is determined by the flight time difference of particles of different types and time resolution. The former is determined by the flight path and the latter is related to many items as listed in table 1.
Table 1 Analysis of time resolution for PID
Item Barrel time reso. Endcap time reso.
Intrinsic time reso. of one TOF layer for 1 GeV muon 80~90 ps 80 ps
Uncertainty from bunch length 15 mm,35 ps 15 mm,35 ps
Uncertainty from bunch time ~20 ps ~20 ps
Uncertainty from Z position 5 mm,25 ps 10 mm,50 ps
Uncertainty from electronics 25 ps 25 ps
Resolution of expected time of flight 30ps 30ps
Time walk 10ps 10ps
Total time reso, one layer of TOF for 1 GeV muon 100~110ps 110~120ps
Total time reso, double layer of TOF for 1 GeV muon 90ps  

Expectable Resolution

    The target of the time resolution of one layer for 1 GeV muon is about 100~110 ps. For kaon and pion, it will increase by 20% because of strong interaction as experienced at BESI, BESII and BELLE. For the double TOF, intrinsic time resolutions are reduced by a factor of . Fig. 2 shows the K/p separation capability for one layer or double TOF. The momentum of 2s K/p separation can go up to 0.8 GeV/c or 0.9 GeV/c for single and double layer of TOF respectively.