BES III Time-of-Flight Readout System

A time-of-flight (TOF) readout system has been developed for a Beijing Spectrometer (BES III). This paper will present an overview of the entire TOF readout system design and its performance in the operation. In a BES III TOF detector, a total of 448 photomultiplier tubes (PMTs) have been installed in barrel and end-cap sections. The corresponding 448 channels' readout electronics aimed precisely to measure the arrival time of PMT signals from the TOF detector is mainly housed in two VME64xP crates, each crate containing 14 FEE modules, 14 FEE rear modules, one Fast Control module (FastCtrl), one Clock module, and one Power-PC interface module. Due to the structure of the spectrometer, the two VME crates with 448 channels of readout electronics are placed in two locations; each is 18 m away from the TOF detector. In order to increase the capability of noise immunization and achieve better signal-to-noise ratio, each channel has a preamplifier equipped to PMT base. The preamplifier drives the differential signals over 18 m of shielded twisted-pair cables to the VME crates. The readout system achieved a timing resolution of 25 ps after time-walk and INL corrections. The amplitude-dependent time-walk correction is accomplished by measuring corresponding pulse height. Achieved pulse-height resolution is better than 10 mV in a dynamic range 180-5000 mV. The whole TOF readout system was installed in the spectrometer and has been in operation steadily since then, with the performance well meeting the requirements of the physics experiments.