GEM-based plasma radiation diagnostics development: design aspects affecting its performance

The Gas Electron Multiplier (GEM) was first introduced by Fabio Sauli in 1997, which is a thin layer of an insulating polymer, coated on both sides with copper and chemically perforated with a high density of microscopic holes. The GEM detectors, which are built using GEM foils, have been utilized for various applications due to their excellent spatial resolution, high rate capabilities and flexibility in design. Large area of triple GEM detectors are planned in experiments as it is the case in the upgrade of the CMS forward muon system. During the assembly of these detectors, the GEM foils are manually stretched to maintain the required gap configuration throughout the detector area. This stretching procedure can introduce local variation in the size or the shape of the perforated holes in GEM foil which can further vary the operational characteristics of the foil as well as the detector. Therefore, the distribution and size of the holes over a GEM foil should be uniform to achieve homogeneous functionality of the detector over the active surface. We can prevent the deformation of the perforated holes by optimizing the stretching force which is used during the assembly.

Visual investigation of possible degradation in GEM foil under test

An optical Quality Assurance (QA) system has been developed at the Detector Laboratory of the Helsinki Institute of Physics. The diameters of the holes in a Gas Electron Multiplier (GEM)-foil, both in the copper and in the polyimide, can be measured with the system. The system is utilized for the QA of the GEM foils of the TPC Read-Out Chambers (ROC) being assembled for the upgrade of the TPC readout chambers of the ALICE experiment at CERN. The correlation between the GEM hole size variation and the corresponding gain variation has been studied with several different gas compositions and operating voltages. A clear correlation has been shown to exist between the size of the GEM holes and the gain of the foil. Furthermore the relative gain of the foil can be estimated within 10 % based solely on the GEM hole size variation. The current correlation results are shown, as well as the progress towards a quantitative prediction of the gain of individual GEM foils. The possibility of predicting the gain of a full GEM stack is discussed. This study has been made in collaboration between Helsinki Institute of Physics and Wigner Research Centre in Budapest as part of the QA effort of the ALICE TPC upgrade project.

Gas Electron Multiplier (GEM) detector is a gas device with high gain and high efficiency. These detectors use chemically perforated 65 μm thick copper clad Kapton polyimide foils. Given its potential for detecting X‐rays and other radiations, GEM detectors may be used in an environment with high radioactivity. The Kapton foils manufacturer, Du Pont Inc., claims that the foils are radioactive resistant. To verify whether the GEM detector performance is affected by the exposure to radiation, several GEM foils were irradiated to a 60Co source at the gamma‐ray irradiation facility at Sterigenics, Tustin, CA. Four sets of GEM foils were exposed to the level of 10 kGy, 100 kGy, 1,000 kGy and 10,000 kGy. The output signal from the GEM detectors with irradiated GEM foils were measured and compared to the detector with no irradiation. We observed that the shapes of the peaks from 5.9 KeV 55Fe X‐ray were distorted and that the detector gain increased compared to that of the un‐irradiated detector. In particular, the detector with 10,000 kGy irradiation appeared to have the biggest peak distortion and increased gain. It was also found from that additional electrons from radiation‐induced free radicals in the Kapton film contribute to output signal of the irradiated GEM detectors. Further studies are needed to explain the mechanism of these detector performance changes.

With the planned upgrade of the ALICE Time Projection Chamber (TPC) the current readout technology will be replaced by a Gas Electron Multiplier (GEM)—based readout technology in order to allow for a continuous operation at high interaction rates up to 50 kHz. A stack of four GEM stages with non-standard field configuration was chosen to achieve a suppression of the ion backflow below 1%, while maintaining a good energy resolution below σ / E= \\unit[12]% for 55 Fe. A discharge probability of 10−10 for α-particles was confirmed for this low ion backflow field configuration. This is comparable to standard triple GEM detectors in low discharge settings. To upgrade all the Inner and Outer Readout Chambers of ALICE, 576 GEM foils will be needed. Only GEM foils that fullfill the highest quality criteria can be used. Therefore a quality assurance scheme has been developed that includes a large set of quality assurance measurements.

Production and validation of industrially produced large-sized GEM foils for the Phase-2 upgrade of the CMS muon spectrometer

The LHC will undergo a major upgrade, named the High-Luminosity LHC (HL-LHC), that will deliver instantaneous luminosity as high as 7×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">34</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . To cope with the higher event rates and with the increased radiation doses, the Compact Muon Solenoid (CMS) experiment will undergo several upgrades including the installation of additional muon detectors based on the Gas Electron Multiplier (GEM) technology.GEM foils, the core of GEM-based detectors, are currently produced with either double-mask or single-mask etching. Single-mask etching is a promising method, but results in an asymmetry in the shape of the holes. The effect of the hole asymmetry on the detector performance was studied with a twofold approach. Experimental data were collected using GEM foils with different hole geometries and detailed simulations were performed matching the experimental conditions. We present an overview of GEM detectors in CMS and describe a study to assess the impact of the GEM hole asymmetry on the detector performance.

The Gas Electron Multiplier (GEM) is one of the most popular and powerful technologies for gaseous ionization detectors used in high energy physics, medical physics and other applications. One of the fundamental elements of a GEM is foils with micro perforations; the qualities of such perforations are basic for an optimal performance of the GEM. In this work we study different computational methods (implemented in java programming language), in order to determine the quality of GEM-foils from high resolution images of the foils. This computational method will provide an automatic and high precision alternative to a procedure which, at the moment, is very expensive, slow and imprecise, being a limitation for the development and application of this important technology of detectors.

Operation of gas electron multiplier (GEM) with propane gas at low pressure and comparison with tissue-equivalent gas mixtures

A Gas Electron Multiplier (GEM) based detector system has been envisaged for use in a muon tracker in the CBM experiment at FAIR. The choice of the detector technology is mainly guided by the stringent requirements of rate capability and radiation tolerance in the harsh CBM environment, where the particle rates reach upto about 1 MHz/cm2. Several 10 cm × 10 cm triple GEM prototypes have been built at VECC and tested with radioactive sources and with proton and pion/muon beams. Owing to high rate requirements, all the detectors in CBM will have to be operated in a self triggered mode of readout. The readout electronics also have to be appropriately tuned to cater to high rates. A detailed investigation of the self triggered readout electronics including its appropriate setting parameters have been carried out. Prototype tests based on self triggered mode of readout showed a charged particle detection efficiency of more than 90%. The Muon detector modules would employ large area GEM foils. First attempts to stretch and frame large area GEMs, foils of 31 cm × 31 cm have been carried out locally using thermal techniques. A relative gain variation of less than 10% has been observed.

Gem detectors for X-ray astronomy

Thin gaseous ionization detectors have been developed based on a current-integration mode for measurements of high-energy hadron beams. In the present detector R&D, two different types of prototype detectors with an active area of 16 × 16 cm2, each equipped with 256-signal processing channels, were manufactured and tested with 43-MeV protons provided by the MC50 proton cyclotron at the Korea Institute of Radiological and Medical Science (KIRAMS). The first one was equipped with a single gas electron multiplier (GEM), and the second one was a thin-plane ionization detector without the GEM foil loaded. The linearities of the detector responses for both detectors were examined for various proton-beam intensities. The quantitative accuracies for the channel-response data and for the total detector responses measured for 43-MeV protons were 0.4% and 0.34%, respectively. We conclude from the beam test that operating both types of detectors in the current-integration mode will allow quality measurements of dynamic-mode hadron beams to be performed with accuracies of better than 1%.

Fast 2-D soft X-ray imaging device based on micro pattern gas detector

A forward modeling technique is developed for determining the characteristic features of observed MHD modes from the line-of-sight data of the soft x-ray (SXR) diagnostics with 64 vertical lines-of-sight in theTokamak à Configuration Variable tokamak. Using diagnostics with excellent spatial resolution, this technique is shown to evaluate the poloidal mode numbers m, radial location and ballooning character of the MHD modes. In the first stage, the poloidal mode structures are modeled by the radially localized Gaussian-shaped emission regions rotating along the magnetic flux surfaces. In the second stage, the space structures of observed 95–100 kHz toroidal Alfvén eigenmodes (TAEs) are investigated numerically using HELENA, CSCAS and MISHKA codes. The calculated TAE eigenfunctions are then used in our forward modeling similarly to the approach of contrast imaging diagnostics (Edlund et al 2009 Phys. Rev. Lett. 102 165003) and SXR diagnostics (Piovesan et al 2008 Nucl. Fusion 48 065001). A ballooning structure of the observed n= 1 TAE mode can be analyzed more easily due to the low poloidal mode number m in contrast to the high-m modes analyzed by a similar SXR technique in the W7-X stellarator (Dreval et al 2021 Plasma Phys. Control. Fusion 63 065006).

A one-coordinate gaseous detector of soft X-ray photonsfor wide-angle X-ray scattering (WAXS) studies is being developed.The detector operates in a counting mode and is based on amulti-stage Gas Electron Multiplier (GEM). A full size detectorhas been assembled. It has a 67-degree aperture at 350 mm from thesource, a readout multi-strip structure with 2048 strips with a0.2 mm pitch. There are some readout electronics in its centralpart. Main parameters of the detector have been measured with a8 keV X-ray beam on the VEPP-3 synchrotron ring. A spatialresolution of 470 μm (FWHM) has been demonstrated. That willallow distinguishing diffraction spots at 0.1 degree.