Measurement of ion backflow fraction in GEM detectors

Abstract

A systematic study is performed to measure the ion backflow fraction of the GEM detectors. The effects of different voltage configurations and Ar/CO${}_2$ gas mixtures, in ratios of 70:30, 80:20 and 90:10, on positive ion fraction are investigated in detail. Moreover, a comparative study is performed between single and quadruple GEM detectors. The ion current with detector effective gain is measured with various field configurations and with three proportions of gas mixtures. The ion backflow fraction for the GEM is substantially reduced with the lower drift field. A minimum ion backflow fraction of 18% is achieved in the single GEM detector with Ar/CO${}_2$ 80:20 gas mixture, however, a minimum ion backflow fraction of 3.5%, 3.0%, and 3.8% are obtained for a drift field of 0.1 kV/cm with Ar/CO${}_2$ 70:30, 80:20 and 90:10 gas mixtures, respectively for quadrupole GEM detector. Similar values of effective gain and ion backflow fraction have been found by calculating the current from pulse height spectrum method, obtained in the Multi Channel Analyser.

Introduction

The concept of the Gas Electron Multiplier (GEM) detector was introduced in 1997 by Fabio Sauli [1], [2]. Some of the major advantages of the GEM detectors are its excellent spatial resolution, good energy resolution, high particle rate capability and long term stability [3], [4], [5], [6]. In recent years, Gas Electron Multiplier (GEM) detector has been a preferred choice for many High Energy Physics Experiments like COMPASS experiment at CERN [6], STAR, and PHENIX experiment at RHIC [7], [8]. GEM has also been proposed for components of the International Heavy Ion Collider [9] and the Facility for Antiproton and Ion Research (FAIR) at GSI [10], [11], [12]. These GEM detectors are also being included in the upgrade of the CMS and ALICE experiments [13], [14]. Ion backflow (IB) refers to the flow of the positive ions, that result from the electron avalanches inside the gas, towards the drift region. Thus the electric field, that should remain uniform for most of the cases, is distorted due to the presence of the positive ions in the drift region.

The definition of ion backflow fraction [15] is the ratio of the cathode current due to positive ions to the anode current due to electrons. The positive ions flowing back into the drift region create a space charge effect that affects the normal operation of the detector and results in performance degradation of the detector [16], [17], [18]. So, the reduction of ion backflow fraction is important for stable detector operation. The Time Projection Chamber (TPC) and gas-filled photomultiplier (GPMT) are a few examples in which the detector is sensitive to IB. For example, in TPC the particle tracks are reconstructed by assuming that the drift field is uniform for estimating the x, y, z position of each voxel. The specific energy loss (dE/dx) and the position resolution depend on the amount of IB. Similarly, in case of GPMT the IB is an obstacle for its operation at higher gains. Therefore, an active gate electrode is introduced between the successive GEM elements for blocking the avalanche-induced ions.

GEM can operate with different gas mixtures and can reach effective gain in the order of 10${}^2$–10${}^3$, for a single layer amplification. To achieve further high gain, multiple layers of GEM can be coupled with the advantage of minimum discharge probability and lower spark rate [19], [20]. These result in maximising the avalanche process in the last GEM foil. Most of the created ions are collected on the top copper layer of the GEM foil during their drift path. Hence, fewer ions are capable of reaching the cathode and this results in minimising the ion backflow fraction.

We have used both single and quadruple GEM detectors to study the ion backflow fraction for these two different geometries and also with different Ar/CO${}_2$ mixture.

The paper is organised as follows: In Section 2, the experimental configuration with different configurations of detectors is described in detail. The results, for the optimisation of gain and ion backflow fraction values for both single and quadruple GEM prototype, are discussed in Section 3. Here, the effects of the gas mixing ratio on detector performance are also presented. Finally, the conclusion is given in the last section.