Estimates of in-flight calibration source activities for the SIXS X-ray detectors on board BepiColombo

The calibration sources of the SIXS detectors consist of titanium coated 55Fe. Two different options for the source placement were considered and Ruby′s formula for a parallel-disk source and detector system was used, in addition to the analytical examination of the attenuation and fluorescence of X-rays inside the Ti-coating, to estimate the calibration signal for each option. Given the complexity of the source-detector geometry, approximations were required for the analytical estimations of the attenuation and fluorescence. Two different approximations were applied to each source-detector configuration. Monte-Carlo simulation program was also developed to estimate the SIXS calibration signal more accurately as well as to provide means to estimate the signal in more general source-detector systems. The Monte-Carlo simulation results of each source-detector configuration were consistent with the results of the analytical calculations. The optimal activity, dimensions and placement of the SIXS calibration sources as well as the optimal thickness of the Ti-coating were determined from the simulation results in co-operation with Oxford Instruments Analytical Ltd. On the basis of the results, the design with the radiation source above the aperture stop was adopted for the SIXS X-ray detectors.