Measurements of the charging-up effect in Gas Electron Multipliers
Introduction
Detectors based on the Gas Electron Multiplier (GEM) [1] are widely used in particle physics experiments that require high position resolution over large areas in high-rate environments (e.g. COMPASS [2], [3], LHCb [4], TOTEM [5], [6], JLab Hall A [7] as well as ALICE [8], [9] and CMS [10], [11] after their upgrades). The GEM consists of a 50 µm thick polyimide foil which is coated on both sides with 5 µm thick copper layers. In a photolithographic process, holes are etched into this foil in a hexagonal pattern. Standard GEM foils have an inner diameter of approximately 50 µm, an outer diameter of approximately 70 µm and a pitch between two neighboring holes of 140 µm. If a suitable voltage is applied between both copper layers, strong non-uniform electric fields are created inside the holes (of the order of 50 kV/cm), sufficient for incoming electrons to start an avalanche of further ionizations. During this multiplication process, electrons and ions may diffuse to the polyimide part of the GEM and be adsorbed there as shown in Fig. 1. Due to the high resistivity of the material, the charges remain there for a rather long time. These new charges accumulate over time and dynamically change the electric field inside the hole. This is known as the “charging-up effect”. Many publications suggest that the charging-up effect is responsible for a change of the effective gain which is time-dependent and reaches asymptotically a constant value (e.g. in measurements with GEMs [3], [12], [13], [14], [15], in measurements with different micropattern gaseous detectors [16], [17], [18], as well as in simulations [19], [20], [21]). A quantitative understanding of the various effects reported (which e.g. include both an increase [3] and a decrease [15] of the effective gain), however, has not been reached in our opinion. In addition, as we had to experience ourselves, there are many other external effects which may mimic a genuine charging-up effect. Examples are changes in external conditions like temperature and pressure, initial instabilities of X-ray generators or time constants intrinsic to the high-voltage power supply system (see also Section 2). Especially for applications that require the gain to remain very stable over time, e.g. in Time Projection Chambers for d/d measurements [8], [22] or in photon detectors [23], a quantitative understanding of the effect is indispensable.
This work presents a systematic investigation of the charging-up effect under well-controlled and monitored conditions. By using two different methods, we are able to investigate the time-constant over a wide range of rates of X-ray interactions. The first method makes use of a conventional X-ray tube where the amplified ionization currents are sufficiently large to be measured with a picoamperemeter. For the second method, the peak position of the line in an 55Fe spectrum is observed over time. Both types of measurement were conducted using a single GEM as amplification stage [24].
Section snippets
Setup
In order to measure the charging-up effect, a dedicated detector was set up with a single GEM foil as amplification stage. A sketch of the used detector is shown in Fig. 2. It consists of an aluminum vessel with a 100 µm thick Kapton®400FN0221 window on the top side to irradiate the detector with X-rays. The detector was constantly flushed with Ar/CO (90/10) with a flow of 3 L/h. The total gas volume is approximately 4 L.
Measurement method I – Current measurement
The first measurement method makes use of a picoamperemeter (originally developed at TU München and further improved at Bonn University [27]) which was connected to the pad plane of the detector. In its most sensitive mode, the picoamperemeter has a digital resolution of 0.5 pA and an absolute accuracy of 2 pA, given by a known temperature dependence and possible calibration uncertainties. Over a time interval of approximately 10 s, it measures 128 values and sends out the average value as well as
Measurement method I
The results from measurement method I are depicted in Fig. 3, Fig. 4, which show the currents measured at the readout anode as a function of time for different currents of the X-ray tube and for two different GEM voltages (400 V in Fig. 3 and 350 V in Fig. 4. All measured currents exhibit an initial increase with time and a saturation after longer irradiation times. To each data set, a single exponential function of the form was fitted. denotes the measured
Discussion of the results
In the presented measurements, the charging-up effect of a single GEM foil was examined at different X-ray rates and at different GEM voltages. All measurements show an effective gain which initially increases with time and asymptotically reaches a saturation value. Each single measurement can be described by an exponential function, see Eq. (2). This implies that the net accumulation of charges on the polyimide decreases proportionally to the distance from the saturation value, as it is also
Conclusion and outlook
In this work, the charging-up effect of single standard GEM foils was investigated experimentally under well-controlled conditions. In order to study the rate dependence of the effect over a wide range of X-ray interactions in the conversion volume above the GEM foil, we made use of two different measurement methods. For the first method, a conventional X-ray tube was used which provided a high rate of ionization electrons, allowing for the measurement of currents on the readout anode. The
CRediT authorship contribution statement
Philip Hauer: Conceptualization, Methodology, Investigation, Writing - original draft. Karl Flöthner: Investigation, Validation. Dimitri Schaab: Resources. Jonathan Ottnad: Resources. Viktor Ratza: Software. Markus Ball: Supervision, Writing - review & editing. Bernhard Ketzer: Conceptualization, Supervision, Funding acquisition, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We are indebted to Rui de Oliveira and his team at CERN-EP-DT for providing us with high-quality GEM foils and other detector components. We are also grateful to the RD51 collaboration at CERN for interesting and fruitful discussions. This work is supported by the German BMBF .
References (34)
GEM: A new concept for electron amplification in gas detectors
Nucl. Instrum. Methods A
(1997)Micropattern gaseous detectors in the COMPASS tracker
Nucl. Instrum. Methods A
(2002)Construction, test and commissioning of the triple-GEM tracking detector for compass.
Nucl. Instrum. Methods A
(2002)A triple GEM detector with pad readout for high rate charged particle triggering
Nucl. Instrum. Methods A
(2002)The TOTEM T2 telescope based on triple-GEM chambers
Nucl. Instrum. Methods A
(2010)Large size GEM for super bigbite spectrometer (SBS) polarimeter for hall a 12GeV program at JLab
Nucl. Instrum. Methods A
(2015)A time projection chamber for high-rate experiments: Towards an upgrade of the ALICE TPC
Nucl. Instrum. Methods A
(2013)GEM Based detector for future upgrade of the CMS forward muon system
Nucl. Instrum. Methods A
(2013)Large-size triple GEM detectors for the CMS forward muon upgrade
Nucl. Instr. and Meth. A
(2016)Simulation of the dielectric charging-up effect in a GEM detector
Nucl. Instrum. Methods A
(2012)
A large ungated TPC with GEM amplification
Nucl. Instrum. Methods A
GEM-Based photon detector for RICH applications
Nucl. Instrum. Methods A
Garfield recent developments
Nucl. Instrum. Methods A
Modelling and measurement of charge transfer in multiple GEM structures
Nucl. Instrum. Methods A
Design and construction of the triple GEM detector for TOTEM
IEEE Symp. Conf. Record Nucl. Sci.
A continuous read-out TPC for the ALICE upgrade
Nucl. Instrum. Methods A
Development of tracking detectors with industrially produced GEM foils
IEEE Trans. Nucl. Sci.
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