Soft X-ray measurements with a gas detector coupled to microchips in laser-plasma experiments at VEGA-2

This work presents an innovative usage of the GEMpix detector for soft X-rays (SXR) measurements aimed to make an estimate of the electron temperature of a Laser Produced Plasma (LPP). The GEMpix is a proportional gas detector based on three Gas Electron Multipliers (GEMs) with a Front-End Electronics (FEE) based on four Timepix chips. This FEE provides the Time over Threshold (ToT) acquisition mode pixel by pixel and then a digital measure of the released charge in the gas mixture. In addition, the charge can be amplified through the GEM foils with 4 orders of magnitude spanning gain offering, in this way, a big dynamic range and adjustable sensitivity. Chip design provides a threshold for each channel. All the thresholds are set in order to cut electronic noise and detect X-rays. In this configuration, a cut on the low amplitude signals is set, but the gain has been tuned in order to observe the main signal due to the soft X-rays reaching the detector. This detector works in an energy range between 2 to 15 keV . It offers good imaging properties, high efficiency and absolute calibration. It offers a good immunity to Electromagnetic Pulse (EMP), as checked at VEGA-2 laser facility (hundreds of TW in about 30 fs). In these experiments, where the formation of warm dense matter produced by blast waves has been studied, a measure of the plasma temperature was required. This measurement was realized applying some filters on the active area of the detector, in correspondence of three chips. With this configuration a study of the GEMpix response due to the photons coming from the coronal plasma produced by the laser on the target has been done for each single shot. GEMpix revealed innovative and attractive features, compared to the state of the art where passive films or detectors based on indirect conversion are used, for SXR imaging and spectral analysis to infer the electron temperature.