IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 50, NO. 4, AUGUST 2003 803 Electron Collection and Ion Feedback in GEM-Based Detectors Fabio Sauli, Steffen Kappler, and Leszek Ropelewski Abstract—We report the results of systematic experimental in- vestigations on electron transmission and ion feedback in a single gas electron multiplier (GEM) detector for the operating region close to unity gain. Critical factors for obtaining good collection are the transverse diffusion coefficient and the hole diameter. A unit gain GEM can be used for instance as first element in a cas- cade to improve collection efficiency and ion feedback suppression in time projection chambers. Index Terms—Gas electron multiplier (GEM), gaseous detectors, ion feedback. I. INTRODUCTION D ETECTORS based on the gas electron multiplier (GEM) [1] find numerous applications in particle physics and in other fields [2]–[5]. The high rate capability, good localization accuracy, and multitrack resolution, together with robustness of operation make GEM devices a good choice for harsh radiation environments. GEM end-cap detectors for the time projection chamber (TPC) are also investigated by several groups, within the TESLA collaboration [6], [7] and for detector upgrade studies at Relativistic Heavy Ions Collider (RHIC) [8]. A GEM-TPC readout offers intrinsic multitrack resolutions of a few ; moreover, due to the detector geometry, distortions in presence of magnetic field are largely reduced. Another attrac- tive feature is the strong suppression of positive ion feedback, a serious problem in high-rate high-multiplicity devices. In multiple cascaded GEM structures, the authors of [9] measured an ion feedback of a few percent; similar values have been reported in other works [10], [11]. In these studies, made with large multiplication factors in each GEM, the quantity called “electron transparency” corresponds actually to the product of the electron transmission and gain. Given the total detected charge, a loss in the collection of the primary ionization can affect energy resolution and efficiency. The scope of the present work is to measure the real electron transparency of several types of GEM electrodes; the method consists in measuring the transport currents at gains close to one and extrapolating at higher gains. At the same time, we have measured the transparency of the foils to ions and deduced the fractional ion feedback in a wide range of external fields and GEM voltages. Making use of the data, one can estimate the charge transfer properties in a detector with multiple cascaded GEM foils. Manuscript received November 14, 2002; revised April 23, 2003. The authors are with CERN, CH-1299 Geneva, Switzerland (e-mail: fabio.sauli@cern.ch; steffen.kappler@cern.ch; leszek.ropelewski@cern.ch). Digital Object Identifier 10.1109/TNS.2003.814543 Fig. 1. Schematics of the single-GEM detector. Reversing the field, one can measure transfer properties both of ions and electrons. II. EXPERIMENTAL SETUP We have used a general-purpose detector assembly [12] to mount a framed GEM foil between a readout board and a drift electrode (Fig. 1). Each electrode in the assembly is connected to a separate high-voltage power supply through high value resistors 1 . Gas ionization is produced with an 8 keV X-ray generator and currents on various electrodes recorded from the reading on power supplies. For measurements at low detector’s gain, a flux around converted photons provides recorded currents of several hundred nA. To avoid ambiguities induced by overlapping ionization currents produced in several gaps, irradiations have been done with the beam parallel to the electrodes. We have also observed that irradiating the detector perpendicular to the electrodes results in instabilities presumably due to surface charging of the insulator exposed to the beam. Proper choice of the voltages permits to measure the charge flow of electron or of ions. The detector was operated with an open gas flow, and the gas mixture monitored with external flow meters. In the following, the quantity electron (ion) transmission represents the fraction of negative (positive) charge transferred through a GEM to the collecting electrode. In the low field region, it represents he transparency of the electrode to drifting charges; when multiplication sets in, the recorded charge is a convolution of transparency and gain, and currents of opposite polarity can overlap and cancel on the intermediate electrodes. We define fractional ion feedback the ratio of the positive charge collected by the drift electrode, to the electron current collected at the anode. 1 Convenient for systematic measurements, this powering scheme is intrinsi- cally dangerous for the devices and its use is discouraged in operational detec- tors. 0018-9499/03$17.00 © 2003 IEEE