Electron Multiplier Research Articles

Assessment of a space and energy resolved diagnostic based on GEM technology on MAST-U

Abstract A Gas Electron Multiplier (GEM)-based detector was utilized for the first
time on a spherical tokamak, MAST-U, during the 2023 campaign to investigate Soft
X-Ray (SXR) radiation (1-20 keV) emitted from the plasma. GEM detectors, chosen
for their resilience to harsh fusion environments and their ability to provide energy-
resolved SXR emission images with sub-millisecond time resolution, are a relatively
new diagnostic compared to standard semiconductor diodes. In this study, the GEM
detector features a pinhole geometry outside the vacuum chamber and observes the
plasma through a beryllium window. Filled with an ArCO2 mixture, the detector
consists of an Aluminized Mylar cathode, three Aluminum-coated GEM foils, and an
anode made of a 16x16 matrix of 6mm2 pads for 2D readout. It employs custom
GEMINI ASICs (Application Specific Integrated Circuits) for signal readout, enabling
photon-counting techniques with Time over Threshold (ToT) analysis on each detector
channel, reaching a total maximum rate of 256 MHz.
Preliminary findings from the 2023 campaign demonstrate the effectiveness of the
GEM detector in complementing existing SXR camera data in terms of spatial and
temporal resolution. Case studies include the identification of Magnetohydrodynamic
(MHD) instabilities, such as the Snake instability, and the exploitation of energy
capabilities for plasma event energy characterization, as shown in the case of Internal
Reconnection Events. Additionally, the GEM detector allows for estimation of the
Electron Temperature of Maxwellian plasmas. These results underscore the potential
of the GEM-based diagnostic system in advancing tokamak research for comprehensive
plasma characterization and control.

Development of a multi-layer high-efficiency GEM-based neutron detector for spallation sources

Neutron detection is nowadays mostly based on 3He gas detectors, but its shortage and the continuous upgrades of the neutron facilities require new devices to perform experiments with maximum performances. This work presents a new detector based on the Gas Electron Multiplier (GEM) combined with several boron layers. This detector combines the features of GEM technology with the properties of boron as a neutron converter and the device is produced to sustain high neutron fluxes with high detection efficiency. The detector has been characterised at the ISIS Pulsed Neutron and Muon Source (UK). Based on the analysis of our results, the detector has shown a good response to thermal and epithermal neutrons reaching a detection efficiency of 16% at 1.8 Å (25 meV). The good detection efficiency (even increasable with the addition of further boron GEM foils) and the good time resolution, make the detector a unique device for the neutron techniques. In particular, its use can easily be envisaged in techniques involving neutron transmission measurements, that require high fluxes impinging on the detectors, with the added bonus of a 2D-resolved capability due to the padded anode.

Spatial resolution studies using point spread function extraction in optically read out Micromegas and GEM detectors

Optically read out gaseous detectors are used in track reconstruction and imaging applications requiring high granularity images. Among resolution-determining factors, the amplification stage plays a crucial role and optimizations of detector geometry are pursued to maximize spatial resolution. To compare Micro Pattern Gaseous Detector (MPGD) technologies, focused low-energy X-ray beams at the SOLEIL synchrotron facility were used to record and extract point spread function widths with MICRO-Mesh Gaseous Structure (icromegas) and Gas Electron Multiplier (GEM) detectors. Point spread function width of ≈108µm for Micromegas and ≈127µm for GEM foils were extracted. The scanning of the beam with different intensities, energies and across the detector active region can be used to quantify resolution-limiting factors and improve imaging detectors using MPGD amplification stages.

Equivalent Mechanical Model of a Microchannel Plate

Abstract The microchannel plate (MCP) is an electron multiplier with millions of micro through-holes that must withstand strong vibrations and enormous shocks in spaceborne detectors. To ensure the reliability and robustness of the MCP in space applications, we proposed an equivalent mechanical model of the MCP to investigate its mechanical properties, since the direct creation of the finite element model using the finite element method (FEM) is not feasible. Then, we developed a test system to verify the effectiveness of the equivalent mechanical model. The results show that the error of harmonic response analysis and the test result is less than 10 %, which is acceptable. Finally, we conducted parametric studies of the MCPs and obtained the equivalent mechanical model of the MCPs with different geometric parameters. This study will help researchers to optimize the MCP for aerospace-grade detectors.

Laser-fibre interface for single-particle detector amplification and transportation

Abstract Single quantum particle detectors (e.g. photo-multiplier tubes and electron multipliers) produce ns-output pulses that time-correlate to particles that enter them. The detector output pulses usually require amplification at source, before being sent to counting electronics for analysis. A laser-based transport system is detailed here that converts the detector output into light pulses that are injected into single mode fibre. Pulse delivery is then by fibre, so that electrical interference from the laboratory environment and earth grounding loops are eliminated. A fast photodiode and differential amplifier converts the signal exiting the fibre into a voltage pulse for subsequent timing experiments.

Simulation studies of charging-up effect for single GEMs

Many experiments revealed a charging-up effect inside the Gas Electron Multipliers (GEMs) holes, resulting in variation of GEM effective gain. Charge accumulation on the insulator surface inside the GEM hole (charge process) is involved in the present simulation models. With only the charge process in the simulation, a longer and larger gain stability of GEM was observed while comparing the charging-up curves to references, even distinct in the relatively high conductivity of GEM insulator. This observation indicates that charge transportation within the GEM insulator (discharge process) is non-negligible; the charging-up stability depends on the dynamic balance of the charge and discharge process. Therefore, a discharge model is induced and expressed by a piecewise function based on the presence of a simulation framework. Charging-up curves are compared for single GEMs with two different structures under different gain values and input flux; the trend from simulation with the discharge process is consistent with those from references. Although the deviation of simulated charging-up curves is observed at the region started to be stable, indicating improvements in the discharge formulation, it can still predict the trend of stability and values of GEM effective gain. This study improves the present simulation model of the charging-up effect for single GEMs and provides a reference for other charging-up models.

Time-dependent instrumental effects in IXPE: Pressure variation and GEM charging inside GPDs

We present the results of two studies conducted on long-term effects of a Gas Pixel Detector (GPD), central point of the X-ray detection of the Imaging X-ray Polarimetry Explorer (IXPE) mission: the pressure variation inside the sealed gas cells of the instrument and the charge build-up in the dielectric layer of the Gas Electron Multiplier (GEM), used as the amplification stage of the detector.

Effects of sputtering pressure on microstructure and secondary electron emission properties of AlN film prepared by magnetron sputtering

In this paper, aluminum nitride (AlN) thin films were prepared by reactive radio-frequency magnetron sputtering at different sputtering pressures and the crystalline structure, surface morphology and secondary electron emission (SEE) properties of the AlN films were studied in detail. The experimental results reveal that AlN films present the preferred orientation of a-axis (100), and the grain size and surface roughness of the film decrease with the increase of sputtering pressure. Additionally, the SEE coefficient of the film reaches a maximum of 4.3 at 200 eV when the sputtering pressure is 0.55 Pa and is relatively stable under the continuous bombardment of an incident electron with the energy of 200 eV. Thus, the AlN thin film prepared under the optimized sputtering pressure has a relatively high and stable coefficient at a low incident electron energy, which promotes its potential application in the high-gain and long-life electron multipliers.

Two-Dimensional Plasma Soft X-ray Radiation Imaging System: Optimization of Amplification Stage Based on Gas Electron Multiplier Technology.

The objective of the proposed research is to develop plasma soft X-ray (SXR) radiation imaging that includes spectral information in addition to standard SXR tomography for the purpose of studying, for example, tungsten transport and its interplay with magnetohydrodynamics (MHD) in tokamak plasmas in an ITER-relevant approach. The SXR radiation provides valuable information about both aspects, particularly when measured with high spatial and temporal resolution and when tomographic reconstructions are performed. The spectral data will facilitate the tracking of both light and high-Z impurities. This approach is pertinent to both the advancement of a detailed understanding of physics and the real-time control of plasma, thereby preventing radiative collapses. The significance of this development lies in its ability to provide three-dimensional plasma tomography, a capability that extends beyond the scope of conventional tomography. The utilization of two-dimensional imaging capabilities inherent to Gas Electron Multiplier (GEM) detectors in a toroidal view, in conjunction with the conventional poloidal tomography, allows for the acquisition of three-dimensional information, which should facilitate the study of, for instance, the interplay between impurities and MHD activities. Furthermore, this provides a valuable opportunity to investigate the azimuthal asymmetry of tokamak plasmas, a topic that has rarely been researched. The insights gained from this research could prove invaluable in understanding other toroidal magnetically confined plasmas, such as stellarators, where comprehensive three-dimensional measurements are essential. To illustrate, by attempting to gain access to anisotropic radiation triggered by magnetic reconnection or massive gas injections, such diagnostics will provide the community with enhanced experimental tools to understand runaway electrons (energy distribution and spatial localization) and magnetic reconnection (spatial localization, speed…). This work forms part of the optimization studies of a detecting unit proposed for use in such a diagnostic system, based on GEM technology. The detector is currently under development with the objective of achieving the best spatial resolution feasible with this technology (down to approximately 100 µm). The diagnostic design focuses on the monitoring of photons within the 2-15 keV range. The findings of the optimization studies conducted on the amplification stage of the detector, particularly with regard to the geometrical configuration of the GEM foils, are presented herein. The impact of hole shape and spacing in the amplifying foils on the detector parameters, including the spatial size of the avalanches and the electron gain/multiplication, has been subjected to comprehensive numerical analysis through the utilization of Degrad (v. 3.13) and Garfield++ (v. bd8abc76) software. The results obtained led to the identification of two configurations as the most optimal geometrical configurations of the amplifying foil for the three-foil GEM system for the designed detector. The first configuration comprises cylindrical holes with a diameter of 70 μm, while the second configuration comprises biconical holes with diameters of 70/50/70 μm. Both configurations had a hole spacing of 120 μm.

Fourier Transform Ion Trap Mass Analysis by Deciphering Ion Ejection Signals in the Frequency Domain.

Mass analysis in an ion trap is conventionally realized through time domain analysis of the ejected ion current collected from an electron multiplier (EM), in which the ion ejection time is found to have a correlation with the mass-to-charge (m/z) ratio of the ion. In this study, we investigated a new method for mass analysis by examining ion ejection signals in the frequency domain. Theoretical analysis and ion trajectory simulations show that ions of the same m/z ratio are ejected from an ion trap at regular intervals, producing a periodic pulsed signal on the EM. The period of this pulsed ejection signal is directly linked to the m/z values of the ions. To realize this method experimentally, a broadband preamplifier was built and integrated on a miniature ion trap mass spectrometer (the "Brick" series from Nier Inc.) to capture this pulsed ion ejection signal collected from the EM. Experimental results were in good agreement with theoretical and simulation analyses. This method has the potential to improve the mass resolution of an ion trap mass analyzer. As a proof-of-concept demonstration, a peak width of 0.1 Da at a m/z value of 281 was achieved in experiments.

Further investigation of spatially resolved single grain quartz OSL and TL signals

The use of luminescence signals from single mineral grains for optical dating has become a valuable and frequently utilised tool in Quaternary Geochronology. Single grain luminescence dating is particularly beneficial in complex depositional settings, however the ability to measure single grain signals also offers the opportunity to assess intrinsic luminescence properties of individual mineral grains. The use of spatially resolved luminescence technologies such as an electron multiplier charge coupled device is of benefit when making luminescence measurements at single grain scales because they allow stimulation with light emitting diodes, and this offers a number of key benefits related to stimulation power when it comes to the assessment of characteristics such as optically stimulated luminescence (OSL) decay rate and the calculation of parameters such as the fast ratio and photo ionisation cross-sections. In this paper, the intra- and inter-sample variability of sensitised single grain thermoluminescence (TL) and OSL signals is considered. A comparison between TL and OSL signals is undertaken, as well as calculation of the fast ratio, OSL component photo ionisation cross-sections, thermal stability, and characteristic dose for a suite of quartz samples from a range of geographic locations and depositional settings. For these heated signals, key findings include the lack of relationship between OSL signal intensity and dominance of the fast component, the fitting of two components (a fast and slow component) is the most common fit for single grain OSL signals, characteristic doses from fast dominated signals suggesting saturation at c. 150 Gy, and the identification of the ultrafast OSL component. Intra-sample variability across all measured parameters is observed, suggesting that for this suite of samples, variability is the norm rather than the exception, and that the intrinsic luminescence characteristics of quartz are variable and diverse.

SIMS Data Correction Procedure for Quasi‐Simultaneous Arrival (QSA) Under‐counting and Ramifications of Misapplication

Isotope geochemistry requires isotope ratios measured using secondary ion mass spectrometry (SIMS) to be made with optimal precision and accuracy. Under some analytical conditions when using electron multiplier detectors, secondary ions may be under‐counted because of quasi‐simultaneous arrival (QSA) at the first dynode. The relative magnitude of the associated QSA correction to raw measured isotopic ratios can be up to seventy permil or more. Therefore, not applying the correction, or misapplication of it could lead to significant inaccuracies in published isotope ratio data. Examples and ramifications of the latter are described in addition to a straightforward procedure for QSA under‐counting correction.

Evidence for polyimide redeposition and possible correlation with sparks in Gas Electron Multipliers working in CF[formula omitted] mixtures

Research on aging processes of Gas Electron Multipliers (GEMs) is important to obtain insights on how to increase the detector’s longevity, stability, and performance, as highlighted in the latest developments roadmap by the European Council of Future Accelerators (ECFA). One key aspect of the aging process is the deposit formation on the electrodes surfaces. In this work, through the analysis of the molecular content on the surface of a used GEM, we provide evidence for polyimide redeposition as a source of organic material contributing to the formation of insulating layers on the electrodes, which eventually lead to sparks and detector failure. Furthermore, we show that chromium, used to promote adhesion between copper and polyimide, in the device undergoes a diffusion process, effectively blurring the layered structure. We demonstrate the significance of surface-sensitive chemical analysis to investigate the surface deposits on electrodes of gaseous detectors and our results reveal the necessity of standardization and more stringent study protocols.

Spatial resolution improvements with finer-pitch GEMs

Gas Electron Multipliers (GEMs) are used in many particle physics experiments, employing their `standard' configuration with amplification holes of 140 μm pitch in a hexagonal pattern. However, the collection of the charge cloud from the primary ionisation electrons from the drift region of the detector into the GEM holes affects the position information from the initial interacting particle. In this paper, the results from studies with a triple-GEM detector with an X-Y-strip readout anode are presented. It is demonstrated that GEMs with a finer hole pitch of here 90 μm improve the detector's spatial resolution. Within these studies, also the impact of the front-end electronics on the spatial resolution was investigated, which is briefly discussed in the paper.

Разработка датчика для анализа процессов взаимодействия заряженных частиц и контроля содержания радиоактивных элементов

Of great interest are the processes associated with the results of interaction of charged particles accelerated to energies of several giga-electron volts; they can be applied in the most modern science and technology. To obtain maximum information about the interaction of charged particles in accelerators, special radiation sensors are required that operate in the corresponding sections of the spectrum, both simple ones, such as cathode and photo luminescent detectors, and complex highly sensitive vacuum equipment that not only records even very weak radiation, but also significantly amplifies it. In this work, an automated calculation of the electron-optical and amplifying system of the device will be carried out, a photocathode will be selected, a design of the vacuum block of the sensor will be proposed, and its electric power supply modes will be indicated. Materials and methods. Theoretical analysis based on the studied specialized literature and practical work experience. Development of high-tech products using the automated design system. Optimization and system analysis. Results. An automated calculation of the electron-optical and amplifying system of a photomultiplier-type device with micro-channel amplification was performed, a photocathode was selected, and conditions for minimizing noise during radiation monitoring were provided. Based on the calculations performed, a design of the vacuum block of the sensor was proposed, its electrical power supply modes were specified, and technological issues were considered. Discussion. The modes of operation of the radiation sensor were identified, providing conditions for minimizing noise in operation, concerning the energy of photoelectrons approaching the secondary electron multipliers, the configuration of the signal beam. The design of the radiation sensor of the PMT type was developed, providing the required high levels of the main parameters: sensitivity threshold, signal gain coefficient, photoelectron collection coefficient, anode dark current, and response speed. Conclusion. A critical analysis of approaches to the study of the interaction process of charged particles and experimental means was carried out. The design of a highly efficient radiation sensor was developed, its optimization was carried out in terms of increasing the efficiency of interaction of photoelectrons with the amplification system. An assembly drawing of the vacuum block design was developed, the most important technological issues of its manufacture were resolved. Resume. The results of the work can be useful in setting up experiments to study the processes of interaction of particles accelerated to energies of several giga-electron volts, as well as in the development of devices using ultraviolet radiation, including medical ones.

Performance Studies of Aluminium-Based Gas Electron Multiplier Detector.

In this paper, we report on a systematic study of a soft X-ray Gas Electron Multiplier (GEM) detector built with aluminium-clad kapton GEM foils. The primary objective of this research is to comprehend the performance of this type of detector when irradiated with soft energy photons. The results are analysed and discussed with a particular focus on the long-term detector stability, as well as its gas gain and energy resolution uniformity across the detector area. Presented results lead us to the conclusion that the aluminium based GEM detector is a promising device to suppress the X-ray Fluorescence (XRF) background, simultaneously providing very good stability during long-term measurement campaigns.

The grain agglomeration in the decay of secondary electron yield induced by electrons

In this work, the box-and-grid electron multiplier (EM) operating under specific conditions for 120 h was disassembled. The secondary electron yield (SEY) and the surface morphology of the decayed film from dynodes of EM were tested. It was demonstrated that the SEY decay was linked to the change of grain size in the film. The more severe the film’s decay, the larger the grain size in the decayed film. The results of the Energy Dispersive Spectrometer (EDS) indicated that the increase in grain size was not the introduction of impurities but the grain agglomeration. The grain agglomeration will increase the incident depth of the primary electrons and decrease the range of escapable angles for emitted electrons, which is the mechanism of SEY reduction caused by grain agglomeration. Furthermore, the cause of grain agglomeration was explained by the thermal effect on the surface and the electrostatic attraction between grains.

Charge amplification in low pressure CF4:SF6:He mixtures with a multi-mesh ThGEM for directional dark matter searches

The CYGNO collaboration is developing next generation directional Dark Matter (DM) detection experiments, using gaseous Time Projection Chambers (TPCs), as a robust method for identifying Weakly Interacting Massive Particles (WIMPs) below the Neutrino Fog. SF6 is potentially ideal for this since it provides a high fluorine content, enhancing sensitivity to spin-dependent interactions and, as a Negative Ion Drift (NID) gas, reduces charge diffusion leading to improved positional resolution. CF4, although not a NID gas, has also been identified as a favourable gas target as it provides a scintillation signal which can be used for a complimentary light/charge readout approach. These gases can operate at low pressures to elongate Nuclear Recoil (NR) tracks and facilitate directional measurements. In principle, He could be added to low pressure SF6/CF4 without significant detriment to the length of 16S, 12C, and 19F recoils. This would improve the target mass, sensitivity to lower WIMP masses, and offer the possibility of atmospheric operation; potentially reducing the cost of a containment vessel. In this article, we present gas gain and energy resolution measurements, taken with a Multi-Mesh Thick Gaseous Electron Multiplier (MMThGEM), in low pressure SF6 and CF4:SF6 mixtures following the addition of He. We find that the CF4:SF6:He mixtures tested were able to produce gas gains on the order of 104 up to a total pressure of 100 Torr. These results demonstrate an order of magnitude improvement [1] in charge amplification in NID gas mixtures with a He component.

Secondary scintillation yield from GEM electron avalanches in - and --isobutane for CYGNO — Directional Dark Matter search with an optical TPC

CYGNO is an international collaboration with the aim of operating a ▪ optical time projection chamber (TPC) for directional Dark Matter (DM) searches and solar neutrino spectroscopy, to be deployed at the Laboratori Nazionali del Gran Sasso (LNGS). A ▪/▪ (60/40) mixture is used, along with a triple Gas Electron Multiplier (GEM) cascade to amplify the ionisation signal. The scintillation produced in the electron avalanches is read out using a scientific complementary metal–oxide–semiconductor (sCMOS) camera. This solution has proven to provide very high sensitivity to interactions in the few ▪ energy range. The inclusion of a hydrogen-based gas will offer an even lighter target, resulting in a more efficient energy transfer in a DM particle collision, and consequently, a lower detection threshold. Additionally, longer track lengths of light nuclear recoils are easier to detect with a clearer direction. However, the addition of such gas will contribute to quenching the scintillation, jeopardizing the TPC performance. In this work, we demonstrate the feasibility of adding 1% to 5% isobutane to the ▪/▪ (60/40) mixture by measuring the respective absolute scintillation yield output. The overall scintillation produced in the charge avalanches is not drastically suppressed by quenching due to the isobutane addition. The presence of Penning transfer from excited He atoms to isobutane molecules increases the number of electrons in the avalanches, partially compensating for the loss of scintillation due to quenching. For the highest applied GEM voltage, the total number of photons produced in the avalanche per ▪ deposited in the absorption region presents a decrease of only a factor of about three, from 2.30(20)×104 to 8.2(4)×103▪, as the isobutane content increases from 0 to 5%. The quantification of the visible component of the scintillation shows that isobutane quenches both visible and ultraviolet (UV) photons emitted by ▪/▪.

Study of long term stability of a 50 liters TPC, based on TRIPLE-GEM with optical readout, for the CYGNO experiment

The CYGNO project aims to study rare events, as low-mass (few GeV) Dark Matter (DM) particle or solar neutrino interactions, exploiting the approach of the optical readout of the scintillation light produced in the amplification in a multiple Gas Electron Multiplier (GEM) structure, of the primary ionization originated in large volume Time Projection Chamber (TPCs).The volume is filled with an He:CF4 gas mixture at atmospheric pressure. The 3D topology, and therefore direction of the recoils, is reconstructed thanks to the combined use of high-granularity, high sensitivity sCMOS cameras, for the precise tracking of the projection of the recoils on the GEM plane, and of fast light sensors in order to obtain the coordinate perpendicular to the camera plane.To conclude the R&D phase, the 50 L prototype, called Long Imaging ModulE (LIME), was moved underground at the Laboratori Nazionali del Gran Sasso (LNGS) in order to study the performance of the CYGNO experimental approach in a low background environment and to assess the contributions to the background from different sources, also comparing with Monte Carlo simulations.Stability studies and the effects of environmental condition on the light yield will be discussed, focusing on the effect of humidity on the detector response and stability amount and rate of self-sustaining micro-discharges. This is a crucial step towards the development of a large demonstrator.