A test system is developed for the BESIII ETOF/MRPC beam tests of data acquisition, environment monitoring and automatic control. The software framework is based on the CAMAC bus, VME bus and Serial Port,which are responsible for communications with the detectors. The monitor system works well in the beam test.
An automatic clock synchronization method implemented in a field programmable gate array （FPGA） is proposed in this paper. It is developed for the clock system which will be applied in the end-cap time of flight （ETOF） upgrade of the Beijing Spectrometer （BESIII）. In this design, an FPGA is used to automatically monitor the synchronization circuit and deal with signals coming from the external clock synchronization circuit. By testing different delay time of the detection signal and analyzing the signal state returned~ the synchronization windows can be found automatically by the FPGA. The new clock system not only retains low clock jitter which is less than 20ps root mean square （RMS）, but also demonstrates automatic synchronization to the beam bunches. So far, the clock auto-synchronizing function has been working successfully under a series of tests. It will greatly simplify the system initialization and maintenance in the future.
The prototype of a time digitizing system for the BESⅢ endcap TOF （ETOF） upgrade is introduced in this paper, The ETOF readout electronics has a distributed architecture. Hit signals from the multi-gap resistive plate chamber （MRPC） are signaled as LVDS by front-end electronics （FEE） and are then sent to the back-end time digitizing system via long shield differential twisted pair cables. The ETOF digitizing system consists of two VME crates, each of which contains modules for time digitization, clock, trigger, fast control, etc. The time digitizing module （TDIG） of this prototype can support up to 72 electrical channels for hit information measurement. The fast control （FCTL） module can operate in barrel or endcap mode. The barrel FCTL fans out fast control signals from the trigger system to the endcap FCTLs, merges data from the endcaps and then transfers to the trigger system. Without modifying the barrel TOF （BTOF） structure, this time digitizing architecture benefits from improved ETOF performance without degrading the BTOF performance. Lab experiments show that the time resolution of this digitizing system can be lower than 20 ps, and the data throughput to the DAQ can be about 92 Mbps. Beam experiments show that the total time resolution can be lower than 45 ps.
Time measurement plays a crucial role for the purpose of particle identification in high energy physics experiments. With increasingly demanding physics goals and the development of electronics, modern time measurement systems need to meet the requirement of excellent resolution specification as well as high integrity. Based on Field Programmable Gate Arrays（FPGAs）, FPGA time-to-digital converters（TDCs） have become one of the most mature and prominent time measurement methods in recent years. For correcting the time-walk effect caused by leading timing, a time-over-threshold（TOT） measurement should be added to the FPGA TDC. TOT can be obtained by measuring the interval between the signal leading and trailing edges. Unfortunately, a traditional TDC can recognize only one kind of signal edge, the leading or the trailing. Generally, to measure the interval, two TDC channels need to be used at the same time, one for leading, the other for trailing. However, this method unavoidably increases the amount of FPGA resources used and reduces the TDC＇s integrity.This paper presents one method of TOT measurement implemented in a Xilinx Virtex-5 FPGA. In this method,TOT measurement can be achieved using only one TDC input channel. The consumed resources and time resolution can both be guaranteed. Testing shows that this TDC can achieve resolution better than 15 ps for leading edge measurement and 37 ps for TOT measurement. Furthermore, the TDC measurement dead time is about two clock cycles, which makes it good for applications with higher physics event rates.
In order to study the possibility of improving the timing performance of the time of flight （TOF） systems, which are made of plastic scintillator counters, and read out by photomultiplier tubes （PMT） with mesh dynodes and conventional electronics, we have conducted a study using faster PMTs and ultra fast waveform digitizers to read out the plastic scintillators. Different waveform analysis methods are used to calculate the time resolution of such a system. Results are compared with the conventional discriminating method based on a threshold and pulse height. Our tests and analysis show that significant timing performance improvements can be achieved by using this new system.
A GEANT4-based Monte-Carlo （MC） model is developed to study the performance of endcap time- of-flight （ETOF） at BESIII. It＇s found that the multiple scattering effects, mainly from the materials at the MDC endcap, can cause multi-hit on the ETOF＇s readout cell and significantly influence the timing property of ETOF. A multi-gap resistive plate chamber （MRPC） with a smaller readout cell structure is more suitable for the ETOF detector due to significantly reduced multi-hit rate （per channel）, from 71.5% for currently-used scintillator-based ETOF to 21.8% or 16.7% for MRPC-based ETOF, depending on the readout pad size used. The timing performance of an MRPC ETOF is also improved. These simulation results suggest and guide an ETOF upgrade effort at BESIII.
A new prototype of large area Multi-gap Resistive Plate Chamber （MRPC） with long readout strips was built. This Long-strip Multi-gap Resistive Plate Chamber （LMRPC） is double stacked and has ten 250 μm-thick gas gaps. Signals are read out from the two ends of each strip with an active area of 50 cmx 2.5 cm in each. The detector was tested at FOPI in GSI, using the secondary particles of proton beams （E = 3.5 GeV） colliding with a Pb target. The results show that the LMRPC prototype has a time resolution of about 60- 70 ps; the detecting efficiency is over 98% and the ratio of cross-talk is lower than 2%. The detector also has a good spatial resolution of 0.36 cm along the strip direction.
