A High-Density Time-to-Digital Converter Prototype Module for BES III End-Cap TOF Upgrade

This paper describes a high-precision time measurement module for the end-cap time-of-flight (ETOF) upgrade of the Beijing Spectrometer (BES III). ETOF is scheduled to be upgraded with the multi-gap resistive plate chambers (MRPCs) instead of a scintillator. The new ETOF of BES III will have 1728 readout channels, which is a big challenge for the readout electronics. The time resolution of the readout electronics system is required to be better than 25 ps. A 9U VME module with 72 time measurement channels is designed to perform the time measurement. The signals, which are produced by the detector and then amplified and discriminated by the attached frontend electronics, are sent to this module. The module, which is named TDIG, uses the CERN HPTDC to achieve high-precision time measurement. There are nine HPTDC chips on the module, and each provides eight channels to realize 72 channels. A series of experiment tests show that the time resolution of the TDIG module is better than 20 ps. We have constructed a beam test system composed of six plastic scintillators, two MRPC modules, and ETOF prototype electronics, which testifies to the time resolution expected. An overall time resolution of less than 45 ps for beam events has been achieved.