Intensity calibration including pile-up effect in PHA spectra measurements at W7-X

The pulse-height-analysis (PHA) is a technique used at fusion devices for measurements of content and intensity of the X-ray spectrum. At the stellarator W7-X, this method is applied together with the application of silicon drift detectors for observation of the radiation in the energy range from about 0.5 to 20 keV [1, 2]. To receive quantitative information on plasma impurity concentration, several factors should be taken into account. Firstly, the detector response curve which includes the transmission of filters and detector thickness. In this case, the response was determined from the knowledge of the detector's geometry and characteristics of materials used, as calibration measurements with a standard X-ray radiation source were not possible. Secondly, the geometry parameters which define the observation cone of view, and finally, the pile-up effect which is a crucial point in intensity calibration. The pile-up effect can be minimized by settings of the lower values of the digital filter peaking time in X-ray signal processor (DXP), but often it is a compromise between energy resolution and temporal resolution of the time-dependent X-ray intensity signal. The information about the number of lost photons can be derived from the knowledge of the number of photons incoming to the detector and the number of photons observed in a spectrum. For the PHA system dedicated to W7-X, the loss of the pulses which undergo pileup is determined by values of the parameters: input count rate - ICR and output count rate - OCR, both delivered by the DXP. It is always taken into account in the intensity spectra calibration which allows to study the changes in impurity line intensity and hence to determine the impurity concentration.