Research On The Gas Electron Multiplier Detector And High Resolution Readout

The Gas Electron Multiplier (GEM), introduced in 1997 by F. Sauli, consists of a set of holes, arranged in a hexagonal pattern (typically 80μm diameter at 140μm pitch), chemically etched through copper-kapton-copper thin-foil composite. Application of a potential difference between the two sides of GEM focuses the field lines into the holes on it, electrons released by the ionization in the gas drift into the holes and multiply in the high electric field (50～70kV/cm), which is high enough for the electrons avalanche.To combine with the Multi-Wire Proportional Chamber (MWPC) or the MicroStrip Gas Chamber (MSGC), the GEM foil can pre-amplify the primary electrons at the order of 102～103, this pre-amplification allows the MWPC (or MSGC) works at the low gain zone, decreases the density of the ion-cloud near the anode wires, improves the response and count ability and the gas gain stability. The gas gain of a double-GEM detector can be larger than 104, there are a lot of advantages of it:1) Compared with the MWPC,the GEM detector uses a set of holes for the the amplification area of instead of the metal wires, this micro-pattern structure can effectively reduce the gas gap and decrease the space charge effect.2) Compared with the MSGC, the readout anode and the foil are independent, the anode keeps staying at the 0V voltage level which can greatly avoid the discharge.3) The pitch and width of the strips on the readout PCB can be much more smaller than the wire space of the MWPC, so that the position resolution (<100μm)of the GEM detector is much more better than that of the MWPC.4) In a larger number of (?)adiation detection environments, it can instead of the Photomultiplier Tube (PMT).5) The GEM foil can be easy manufactured to any size and shape to satisfy the experiment requirement. Because of these outstanding performances, the GEM is not only a big shot in the modern high energy physics area, but also widely used in the synchrotron radiation, medical CT diagnosis, crystallography, etc.Lots of groups are attracted by the GEM thus various R&D experiments for the GEM+MWPC, GEM+MSGC, double-GEM detector and triple-GEM detector are per- formed and show that there are some basic problems need to be solved before we apply the GEMs or GEM detectors in the area which we are interested in. For example, on the detector side, why does the foil geometry greatly effect the gas gain of it; what is the relationship between the GEM material and the gas gain stability;how do the dif-ferent gas mixtures and working voltages control the effective gain of the detector; on the readout side, how to design a suit readout anode and what reconstruction method should be used are attractive topics.In High Energy Physics Group (HEPG) at USTC, I have joined the R&D work of two GEM systems. The first prototype is based on a double-GEM detector whose readout anode is a two layer PCB. The thickness of the readout PCB is 0.2mm, there are strips on both side of it. The strips on the top side collect the avalanche electrons and output a negative pulse. At the same time, this negative pulse can induce a small positive pulse on the bottom layer strips, which can be output as the signal on the bottom side. Our test shows the performances of this GEM system are:·Effective area of the detector:100mm×10mm;·Effective gas gain:5×104;·Count ability:≥105/mm2·s;·Position resolution:<100μm;·Gas gain stability:the fluctuation in two weeks<3%. Based on this configuration, we investigated the GEM micro-structure and the relative readout method and constructed a GEM X-ray imaging system whose image is recon-structed by the Center-Of-Gravity(COG) method.The second system is a triple-GEM detector system, the performances of this system are almost the same as those of the first one except its larger effective gas gain. To adopt the cartesian projective readout PCB and DelayLine reconstruction method, we have performed a detailed research on the parameters of the readout PCB and DelayLine PCB and developed several useful DelayLine PCBs. Based on this DelayLine reconstruction method, the test showed that the position resolution of this GEM system is better than 160μm.The main researches and results are shown and described in this thesis, including:1. Calculation of the electrostatic field and transfer efficiency of the field lines of 4 kinds of GEM foils, a detailed calculation method has been established. The transfer efficiency of the field lines are able to, stands on way of the electrostatic field, show that how does the foil geometry effect the charge transfer process in the detector. The calculated 3D field is quite close to the real one in the GEM foil, the field data could be used for the advanced simulation, e.g. the avalanche and charge transfer processes.2. The system setup and the gas gain test of the double-GEM detector. The effec-tive gas gain of the detector has been detailed investigated while the working gas mixture and working voltage changed, the variation of the gas gain in a long time period has also been tested. the test results show that:1) the effective gas gain of a double-GEM detector is able to reach 104;2) the gas gain stability of the detector is very good, the variation of it is less than 3% during its two weeks work. Based on this GEM detector, we have constructed an X-ray imaging system, clear images have been obtained.3. A triple-GEM X-ray imaging system has been established. Based on this system, a full design method of the Delay-Line PCB and a simulation/design program for the GEM readout anode design have been summarized after the detailed research on the output signal and its propagation of this GEM detector. Our experiments show that this method is accurate and effective, the simulations are quite agree with the experiment results. This Delay-Line design method can be extended to the other detectors which are compatible with it. Based on this Delay-Line readout technology, a GEM detector system for the Muon Telescope Detector (MTD) at BNL/STAR has been designed, the elementary results have been obtained.The GEM is a new particle detector, the performances of it is good enough for the modern experiments of high energy physics and the other application areas, it is a big shot in current detector area. The High Energy Physics Group (HEPG) in USTC is the first group involved in the GEM research (started at 2000) in China, this research got continuous supports (10375062,10575101) from National Natural Science Foundation of China. I joined in the GEM group in HEPG at 2002, the results mentioned in this thesis summarizes my work in the period of 2003-2010.