An activation experiment with laser-accelerated high-energy protons to optimize the graded-z shield design for the IXO/ATHENA satellite missions

The International X-ray Observatory (IXO) and ATHENA are next-generation X-ray satellites that are currently being designed as a successors of XMM-Newton. To ensure a low background for the detectors, the satellite shielding is being optimized with the Monte Carlo code Geant4. The satellite will be impacted by highly energetic protons which can activate the satellite shielding and thus cause additional background signals. Geant4 contains algorithms to simulate the activation and subsequent decay of materials, but the part of the code responsible for modeling these processes has not been thoroughly checked yet and first experiments have shown significant differences between the measurements and the corresponding simulations. To address these modeling issues, we have carried out activation experiments with laser-accelerated protons using the PHELIX high-power laser at the Gesellschaft für Schwerionenforschung in Darmstadt. In these experiments we have created a proton beam via target normal sheath acceleration. The energy spectrum of these protons is similar to typical proton spectra of solar flares and the cosmic background, both in maximum energy and their energy distribution. With the PHELIX laser, we have focused a 100 J, 1 ps pulse onto a primary target, creating protons beams with maximum energies of 17 MeV. We have used these laser-accelerated protons to activate target foils and subsequently measured the gamma spectra of these foils with high-purity germanium detectors; the proton energy spectrum has been measured with radiochromic films. We are currently working on analyzing the data and will use the results to validate physics models of activation processes and radioactive decay processes in Geant4.