Speaker
Mr
anshun zhou
(Department of modern physics, University of science and technology of China)
Description
1 INTRODUCTION
1) CEPC and ECAL
With the discovery of Higgs boson in LHC, basic physics has come to one of the most exciting crossroads in its history. CEPC (Circular Electron-Positron Collider) is the first phase of CEPC-SPPC. The main purpose is to generate positive and negative electron pairs with a centroid energy of up to 240 GeV, which collide in the ring at a time interval of about 3.5 us, with a brightness of 2*〖10〗^34 〖cm〗^(-2) s^(-1) , and generate a large number of Higgs particles and Z particles.
ECAL (Electromagnetic Calorimeter) s an important part of CEPC concept detector. The electromagnetic energy meter mainly measures 30% of photons in jet. To achieve this goal, the pre research technical scheme adopts the way of scintillator array + SiPM stack, and forms a sampling type energy meter with tungsten plate absorber.
2 Design of the Temperature Compensation System
1) Basic structure
A readout electronics system including ECAL base unit (EBU) and data interface board (DIF) was built to acquire the output of SiPMs. SiPMs and Scintillators are set on the EBU board. We use the SPIROC2E as the acquisition ASIC in this system. This chip can acquire the signal from the SiPMs, and do the A/D conversion with the ADC embedded. So we can get digital signal directly from SPIROC2E. There are 6 SPIROC2Es on each EBU board. EBU board is under the control of the DIF board. The detection results are transmitted to PC through the DIF board.
2) Design of the Temperature Compensation System
We chose to use SiPM as the detector of the ECAL prototype, but SiPM has some features which could be the limitation of the improvement of the performance of the prototype. The Gain of SiPM is sensitive to the environment temperature. According to the datasheet of SiPM by HAMATSU the gain nonlinearity of SiPM could reach 10% per degree Centigrade. To solve this problem we designed an automatic temperature compensation system based on the function of the readout chip we used on the EBU board. 16 temperature probes were set on the EBU board according to the simulation result. Some of the temperature probes are located near the SPIROC2E while the others are located around the SiPMs. We ran the readout system and acquired the output of every temperature probe. The result shows that the maximum temperature difference can reach about 1 degree Centigrade.
The gain of SiPM increases linearly with the operation voltage, so we can change the operation voltage according to the temperature to compensate the nonlinearity. Each one of the SPIROC2E’s 36 channels is embedded with an output DAC for SiPM high voltage adjustment on 5V to tune gain channel by channel. There is an 8bit DAC connected to each input pin of the SPIROC2E which could be configured manually.
The whole temperature compensation system is under the control of the FPGA on the DIF board. During every slow control period FPGA will acquire and analyze the temperature information from the temperature probes. FPGA will configure the DAC embedded to each output pin of the SPIROC2E according to the temperature. If the temperature near a SiPM is higher than the average level FPGA will improve the output of the DAC connected to it, while the temperature is lower FPGA shall do the opposite.
3 Work Situation of Temperature Compensation System
To see if the system works as well as we expect, we did the optical calibration of SiPM. There are 210 SiPMs on each EBU board. We can see the single photoelectron spectrums of the SiPMs. A certain degree of distortion happens in the single photoelectron spectrums between different SiPMs before the temperature compensation, while after the temperature compensation, the result was corrected.
4 Summary
In order to solve the problem that SiPM is sensitive to environment temperature, we built a temperature compensation system. This system could adjust the operation voltage of each SiPM to maintain gain consistency. The test result we got proved that our system works as well as we expected.
Summary
As a new photoelectric detector, SIPM is the inevitable choice of ECAL because of its small size, low working voltage and low cost. However, gain of SiPM is sensitive to environment temperature. In order to solve this problem we design and built a temperature compensation system which could maintain the gain consistency of SiPM automatically. The actual test result proves that our system works well.
Primary author
Mr
anshun zhou
(Department of modern physics, University of science and technology of China)
