Thick GEMs Research Articles

Systematic investigation of critical charge limits in Thick GEMs

We present discharge probability studies performed with a single Thick Gas Electron Multiplier (THGEM) irradiated with alpha particles in Ar-CO2 and Ne-CO2 mixtures. We observe a clear dependency of the discharge stability on the noble gas and quencher content pointing to lighter gases being more stable against the development of streamer discharges. A detailed comparison of the measurements with Geant4 simulations allowed us to extract the critical charge value leading to the formation of a spark in a THGEM hole, which is found to be within the range of (3 to 7)×106 electrons, depending on the gas mixture.Our experimental findings are compared to previous GEM results. We show that the discharge probability of THGEMs exceeds the one measured with GEMs by orders of magnitude. This can be explained with simple geometrical considerations, where primary ionization is collected by a lower number of holes available in a THGEM structure, reaching higher primary charge densities and thus increasing the probability of a spark occurrence. However, we show that the critical charge limits are similar for both amplification structures.

New (TH)GEM coating materials characterized using spectroscopy methods

In this work GEM and single-hole Thick GEM structures, composed of different coating materials, are studied. The used foils incorporate conductive layers made of copper, aluminium, molybdenum, stainless steel, tungsten and tantalum. The main focus of the study is the determination of the material dependence of the formation of electrical discharges in GEM-based detectors. For this task, discharge probability measurements are conducted with several Thick GEM samples using a basic electronics readout chain. In addition to that, optical spectroscopy methods are employed to study the light emitted during discharges from the different foils. It is observed that the light spectra of GEMs include emission lines from the conductive layer material. This indicates the presence of the foil material in the discharge plasma after the initial spark. However, no lines associated with the coating material are observed while studying spark discharges induced in Thick GEMs. It is concluded that the conductive layer material does not play a substantial role in terms of stability against primary discharges. However, a strong material dependence is observed in the case of secondary discharge formation, pointing to molybdenum coating as the one providing increased stability.

The gain in Thick GEM multipliers and its time-evolution

In the context of a project to upgrade the gas photon detectors of COMPASS RICH-1, we have performed an R&D programme aimed to develop photon detectors based on multi-layer arrangements of thick GEM electron multipliers coupled to a CsI photoconverter. For this purpose, thick GEMs have been characterised in detail including the gain performance, its dependance on the geometrical parameters and its time-evolution, a feature exhibited by the gas detectors with open insulator surfaces. The variation due to this evolution drammatically depends on the parameters themselves.In the present article we summarise the outcomes of the studies dedicated to the thick GEM gain and its evolution versus time. We also include a qualitative model which accounts for the peculiar details of the observed thick GEM gain time-evolution.

Study of Ion Back Flow suppression with thick COBRA GEM

Ion Back Flow (IBF) suppression is essential to avoid a space-charge distortion of the electric field under a high rate condition in the Time Projection Chamber (TPC). A GEM technology is one possible solution to achieve a small IBF and to keep a good performance in terms of particle tracking and particle identification at high rates in TPC. We developed Thick COBRA GEMs to investigate the capability of further IBF suppression. It was found that the COBRA GEM can suppress IBF more effectively compared to a standard GEM. IBF reaches about 0.1–0.5% with a stack configuration consisting of one standard GEM facing to the drift field and two COBRA GEMs. In this paper, the current status of development of COBRA GEM is described.

Detection of single photonswith ThickGEM-based counters

The photon detectors based on MPGD (MicroPattern Gaseous Detector) represent the new generation of gaseous photon detectors.We report about an R&D programme dedicated to study both the principles and the engineering aspects of photon detectors based on Thick GEMs (THGEM) electron multipliers coupled to a CsI photoconverting coating.The goal is the development of a gaseous detector of single UV photons, able to stably operate at high gain and high rate, to provide good time resolution and insensitive to magnetic fieldto be used in the Cherenkov imaging counter RICH-1 of the COMPASS experiment at CERN SPS.

GEMs With Double Layered Micropattern Electrodes and Their Applications

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We have developed and tested several new designs of GEM detectors with micropattern electrodes manufactured by microelectronic technology. In one design, the inner layer of the detector's electrode consists of thin metallic strips and the outer layer is made of a resistive grid manufactured by a screen printing technology. In other designs, the electrodes were made of metallic strips fed by HV via micro-resistors manufactured by a screen printing technology. Due to these features, the new detectors have several important advantages over conventional GEMs or ordinary thick GEMs. For example, the resistive grid (in the first design) and the screen printed resistors (in other designs) limited the current in case of discharges thus making these detectors intrinsically spark-protected. </para>

Thick GEM versus thin GEM in two-phase argon avalanche detectors

The performance of thick GEMs (THGEMs) was compared to that of thin GEMs in two-phase Ar avalanche detectors, in view of their potential application in coherent neutrino-nucleus scattering, dark-matter search and in other rare-event experiments. The detectors comprised a 1 cm thick liquid-Ar layer followed by either a double-THGEM or a triple-GEM multiplier, operated in the saturated vapor above the liquid phase. Three types of THGEMs were studied: those made of G10 and Kevlar and that with resistive electrodes (RETHGEM). Only the G10-made THGEM showed a stable performance in two-phase Ar with gains reaching 3000. Successful operation of two-phase Ar avalanche detectors with either thin- or thick-GEM multipliers was demonstrated at low detection thresholds, of 4 and 20 primary electrons respectively. Compared to the triple-GEM the double-THGEM multiplier yielded slower anode signals; this allowed applying a pulse-shape analysis to effectively reject noise signals. Noise rates of both multipliers were evaluated in two-phase Ar; with detection thresholds of 20 electrons and applying pulse-shape analysis noise levels as low as 0.007 Hz per 1 cm2 of active area were reached.

First tests of thick GEMs with electrodes made of a resistive kapton

We have developed a new design of a GEM-like detector with electrodes made of a resistive kapton. This detector can operate at gains close to 10 5 even in pure noble gases (Ar and Ne) and if transited to discharges at higher gains, due to the high resistivity of electrodes, they do not damage either the detector or the front-end electronics. Gains ∼ 10 6 can be achieved in a cascaded mode of operation. The detector can operate without gain degradation at counting rates of at least up to 10 4 Hz / cm 2 and it could thus be very useful in many applications requiring safe high-gain operation, for example, in RICH, TPCs, and calorimetry.