Redesign And Performance Measurement Of The Thin Gap Chamber Detector

LHC (Large Hadron Collider), which lies at CERN (European Organization for Nuclear Research), is a particle collider with the highest colliding energy in the world. It is hoped to find Higgs particle in its energy region, which is important to verify Higgs Mechanism, a theory that predicts spontaneous symmetry breaking in the Electroweak Standard Model. There are four colliding points in the round tunnel of LHC, corresponding to four experiments:ATLAS, ALICE, CMS and LHCb. The purpose of ATLAS is to find Higgs particle, heavy W and Z'objects and super-symmetric particles and to investigate the structure of fundamental fermions and the CP violation in B-physics. The ATLAS detector is mainly composed by an inner tracker, a calorimetric system, a hadron calorimeter, a muon spectrometer and a trigger and data acquisition (DAQ) system. The High Energy Physics Group of Shandong University once developed and produced 400 sets of T9 type TGC (Thin Gap Chamber) for ATLAS muon spectrometer.TGC is a kind of multiwire detector which works in a saturated proportional mode, designed to detect and trigger muons with high transverse momentum in the end caps of the ATLAS detector. PTGC (Precise TGC) is an upgrade version of TGC. The distance between the two cathode plates of PTGC is 2.8mm. The inner sides of the two cathode plates are coated with copper strips, which are used for the reading out of the signal. There three types of spatial period on the cathode plate:1mm,2mm and 4mm. The anode wires lain in the middle of the cathode plates are gilded tungsten wires with a diameter of 50 microns. They are fixed on an insolated frame with a distance of 1.8±0.025 mm. The 128 anode wires are grouped into 32 sets. In each set, the 4 wires are connected in parallel to form an anode signal channel. The working gas of PTGC is a mixture of 45% n-pentane and 55% CO2. The voltage between the anode wires and the cathode strips is about 2900V. The characteristic of former TGC is that the distance between the cathode plate and the anode wire plane is smaller than the distance between the anode wires. Signals from TGC have shorter rise time, narrower pulse width, higher detection efficiency and faster time response. The properties of the prototype of PTGC are listed as following. The capacitance between an anode wire and a cathode strip is 40pF; that between strips is 25pF and that between a strip and the ground is 48pF. The signal of the cathode strips is positive, and that of the anode wires is negative. The amplitude of the signals is about 9mV. The average width of the signals is 46 ns; the average rise time of the signals is 15 ns; the average fall time of the signals is 89 ns; the detection efficiency operating at 2900V is 92%. Thus PTGC maintains the characteristics of TGC. Besides, PTGC has much higher spatial resolution.Tests show that an avalanche produced by the cosmic ray can induce signals in about 6 strips. According to document [7], the detector can have a good spatial resolution in this situation. In the test of detector spatial resolution, we got a level of 68 microns using one layer of detector; while using there layers of detector, we reach a spatial resolution of 113 microns using one-random-variable related convolution fitting function and reach a spatial resolution of 285 microns using two-random-variable related convolution fitting function. In sum, PTGC has an improved spatial resolution than TGC, and is approaching our objective of 100 microns. In this letter, we also test the fitting function with simulated data. It shows that the resolution we got from fitting and the resolution set when the simulated data are generated are fairly close. There are also lots of data and graphs in this letter, which can be referred to by detector developers and testers.