Spatial resolution optimization in a THGEM-based UV photon detector
THick Gas Electron Multiplier (THGEM) is considered in many UV photon detector applications. It has the capability of detecting single photon and imaging with high sensitivity. Operating parameters such as choice of gas mixture, pressure, drift field, drift gap, multiplication voltage, induction field and induction gap play an important role in deciding the spatial resolution of the detector. Detailed simulation study enables to optimize the above-mentioned parameters for a given THGEM-based imaging detector and hence to achieve improved performance for the same.
Materials and methods
Simulation, using ANSYS and Garfield++, starts with the release of primary electrons at random coordinates on the photocathode plane. They are tracked as they pass through the drift gap and THGEM hole till the electron cloud reaches anode plane. Distribution of electron cloud on the anode plane along X and Y axis is plotted in histogram and fitted with Gaussian function to determine spatial resolution. Ar/CO2 (70:30) mixture, which shows higher ETE and lower transverse diffusion, is chosen for this simulation study.
Transverse diffusion has a major impact on both ETE and the spatial resolution. Lower transverse diffusion coefficient is always desired for having better resolution as well as for ETE. It is found from the simulation study that higher gas pressure, lower drift field and induction field, smaller drift and induction gap can provide optimum detection efficiency with the best spatial resolution. The simulation method proposed here can also be extended to X-ray imaging detectors.
KeywordsThick gas electron multiplier (THGEM) UV photon detectors Electron transfer efficiency Spatial resolution simulation Garfield++
There are few reported works on spatial resolution measurement and also on its simulations [12, 13, 14, 15, 16, 17]. Most of them deal with traditional GEM but not THGEM. In a GEM/THGEM-based position-sensitive detector, spatial resolution depends on the geometrical parameters like hole diameter, pitch and readout method [12, 13, 14]. Pixelated readout coupled with centre-of-gravity calculation allows in obtaining resolution better than the pixel width. However, the final resolution or position uncertainty is determined by the intrinsic electron diffusion from its creation point. Once the geometrical parameter is fixed, operating parameters play further role in deciding the final resolution. The aim of this work is to study the parameters that affect the spatial resolution. We fixed the THGEM geometrical parameters and simulations carried out to evaluate the dependence of spatial resolution on operating parameters.
Modelling of a THGEM unit cell is done using ANSYS , a software tool based on finite element method. Geometrical parameters of THGEM chosen in this study are: insulator thickness 250 µm, hole diameter 200 µm and pitch 450 µm, similar to the THGEM described in earlier work . Drift and induction gaps maintained are 3 and 2 mm, respectively, unless otherwise specified. Electric field values are calculated at various coordinates, and these field map files are imported to Garfield++  for further simulation.
Distribution of electron cloud is plotted as histogram along x and y directions. Figure 3b and c presents 2D and 3D view of the electron distribution, respectively. The histogram is fitted with a Gaussian function as shown in Fig. 3d. RMS width of the fitted histogram gives the spatial resolution (sigma in µm) .
Results and discussion
Simulations are carried out for various drift and induction parameters which affect electron transport properties such as diffusion and hence spatial resolution. Gas mixture, pressure, drift field, drift gap multiplication voltage, induction field and induction gap are varied to study their effect on detector performance.
Drift field (E D)
Multiplication voltage (ΔV THGEM)
Induction field (E I)
The simulation tools are used to understand various operating parameters that affect the THGEM performance as UV imaging detector. RMS spread of electron cloud (sigma), representation of spatial resolution, is studied in detail. Optimization is required to have best possible spatial resolution with maximum detection efficiency for any high sensitivity application. The effect of operating parameters such as gas mixture, pressure, drift field, drift gap, multiplication voltage, induction field and induction gap on spatial resolution have been simulated. ETE data are compared with spatial resolution as ETE is very important factor in maximizing the detection efficiency in a THGEM-based photon detector with semi-transparent photocathode configuration.
Considering higher ETE and lower transverse diffusion coefficient, Ar/CO2 (70/30) gas mixture is chosen for the present simulation studies. Transverse diffusion coefficient has major impact on both ETE and the spatial resolution. Lower transverse diffusion coefficient is always desired for having better resolution as well as for ETE. Therefore, higher gas pressure, lower drift field, higher induction field, smaller drift and induction gap with sufficient gain may provide optimum detection efficiency of a single photon with best spatial resolution. Effects of multiplication voltage and induction field on spatial resolution are not significantly high, and these voltages can be chosen as per operating gain and collection efficiency requirements, respectively. The method proposed here, can be extended to THGEM-based X-ray detectors also. In X-ray detectors, primary electron cloud, which is energy dependent, has to be considered for simulations instead of single electron.
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