Key Technology Study Of Multi-gap Resistive Plate Chamber For Muon Tomography

Muon tomography is an innovative High-Z material detection technology developed in recent years.This technology is based on the measurement of multiple Coulomb scattering of muon passing through investigated materials.In this technology,the most crucial part is the position resolution detectors for the muon’s track detection.Tsinghua University built the TUMUTY（Tsinghua University Muon Tomography Facility）based on the Multi-gap Resistive Plate Chamber（MRPC）in 2012 and obtained a series of research results.This non-destructive imaging technology is now being developed into industrial applications.According to the requirements of car detection imaging and the High-Z material detection imaging system of a research institute,a second-generation large sensitive area position resolution MRPC was developed.Compared with the first-generation TUMUTY MRPC,it has a larger sensitive area（up to 1 m² for single detector）and a more compact structure（single detector for X-Y two-dimensional readout）.Considering the stability of the signal transmission of the detector,the device for data encoding readout was placed into the MRPC detector’s aluminum gas box.The performance of the detector was studied in detail based on data acquisition method of waveform under the encoding readout mode,and quantitative research results were given.After the production of twelve detectors for the muon tomography system,we carried out the test work.The test results show that under the encoding readout mode,all MRPC detectors’efficiency is higher than 93%,and the detectors also have a position resolution less than 800μm.Freon and SF6,the traditional working gas mediums of RPC-type detectors,have high GWP（Global Warming Potential）values.Since 1980s,the European Union has banned fluorine-containing gases.At present,many research groups are engaged in the research of environment-friendly gases of RPC-type detectors.Due to the disadvantages of high price and the research results to eco-gas are immature.We studied and designed the third generation eco and low cost SMRPC（Sealed MRPC）type detectors regarding that the detection object of the Muon tomography system is cosmic rays and has low flux which makes MRPC work under extremely low gas flow mode or no gas exchange mode.Through the precise design of the sealing bar and carbon film glass sealing scheme,an SMRPC detector capable of long-term stable operation at an extremely low gas flow rate of 0.5 ml/min was developed.Studies have also shown that the detector can operate stably for more than 60 hours without gas exchange.After injecting a certain volume gas at this time,the SMRPC detector can normally work again.The average working gas flow operating in this mode was reduced to 0.05 ml/min.For next step,we expect to realize the no gas exchange working mode based on the SMRPC muon tomography system through accurate dark current monitoring devices and gas flow control systems.The working medium of MRPC detector is a gas mixture of 90%Freon（134a）+5%sulfur hexafluoride（SF₆）+5%isobutane(i-C₄H₁₀).Gas properties can be easily influenced by external environment temperature.Considering the outdoor working environment of the industrial muon tomography system,we gave a detailed study on the temperature effect of MRPC detectors.Taking industrial applications into consideration,performance of the detector in pure Freon gas mode was also studied in detail at the same time.A method of high voltage compensation was proposed to compensate for the performance loss of MRPC detectors due to temperature changes.The precise parameters of electric field compensation are given:the electric field at working point changes by320 V/mm for every 5℃temperature drift from the standard 25℃.Based on the simulation framework of ANSYS Maxwell+GENAT4 established by our group,a detailed simulation study of the position resolution performance of MRPC was carried out.The width of the readout strips,the ratio width of copper in strips,and the number of gas gaps of the RPC detector was studied.Then we give some suggestions on how to achieve the best performance design of position resolution detectors for future.