Experimental Study On Ion Backflow Of Micro-pattern Gas Detector

Micro-pattern gas detectors(MPGD)have the advantage of great isotropic for two-dimensional detection in sensitive area,due to its micro structure unit.MPGD are widely used in the track chamber of large-scale physical experiments for particle posi-tion detection.But the resolution of MPGD is not much excellent at long drift distances,cause the positive ions fed back from the readout structure lead to distortion of the drift electric field.The primary task to solve this problem is to suppress that ion backflow effect.Based on the suppression requirements of positive ion feedback of MPGDs,the micro-structure gas detectors combined with GEM and Micromegas are used as research objects to optimize the electric field configuration of MPGDs to achieve positive ion feedback suppression.The paper analyzes the influencing factors of positive ion feed-back one by one,including drift electric field,GEM voltage,transmission electric field and Mi cromegas voltage.In order to study the positive ion feedback of the MPGD sep-arately,a short drift region microstructure gas detector with a 3mm drift region and a 3mm transmission region was built.According to the measurement principle based on positive ion feedback and the measurement requirements of micro current,a micro current measurement scheme and an experimental measurement system are designed.Based on the influencing factors of positive ion feedback,an optimization experiment of positive ion feedback of mi-crostructure detector was designed.For the drift electric field and transmission electric field configuration of the detector,the GEM voltage and the Micromegas voltage con-figuration,a single variable scan is performed one by one to measure the positive ion feedback rate.After the positive ion feedback rate optimization is completed,the de-tector gain feedback factor is calculated,and the gain feedback factor at the appropriate gain is selected according to the gain requirement.Finally,the positive ion feedback suppression of the microstructure detector is maximized by gas optimization and re-placement of high density Micromegas.The experimental results show that the positive ion feedback rate is suppressed under the conditions of low drift electric field,high GEM voltage,low transmission electric field and high Micromegas voltage.The optimal configuration for positive ion feedback rate is 200V/cm for transmission,200V/cm for drift electric field,300V for GEM,400V for Micromegas,and IBF is 0.1%.In Ar/iC4H10(95/5)gas,the elec-tric field in the gas at 200V/cm,the drift electric field at 200V/cm,the GEM voltage at 280V,and the Micromegas voltage at 400V,the gain feedback factor KIBF of the detector reaches 6.2(the detector gain is 5000);In T2K gas(Ar/CF4/iC4H10(95/3/2)),transmission electric field at 200V/cm,the drift electric field at 200V/cm,the GEM volt-age at 260V,and the Micromegas voltage at 400V,the detector’s gain feedback factor KIBF reaches 5.6(at the gain of 5000).After replacing Micromegas with 520 LPI Mi-cromegas,the electric field at 200V/cm in Ar/CF4/iC4H10(95/3/2)gas,the drift electric field at 200V/cm,the GEM voltage at 260V,and the Micromegas voltage at 400V,the detector’s gain feedback factor reaches 4.9(the detector’s gain is 5000).