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Radioactive decays are of concern in a wide variety of applications using Monte-Carlo simulations. In order to properly estimate the quality of such simulations, knowledge of the accuracy of the decay simulation is required. We present a validation of the original Geant4 Radioactive Decay Module, which uses a per-decay sampling approach, and of an extended package for Geant4-based simulation of radioactive decays, which, in addition to being able to use a refactored per-decay sampling, is capable of using a statistical sampling approach. The validation is based on measurements of calibration isotope sources using a high purity Germanium (HPGe) detector; no calibration of the simulation is performed. For the considered validation experiment equivalent simulation accuracy can be achieved with per-decay and statistical sampling.

The simulation of radioactive decays is a common task in Monte-Carlo systems such as Geant4. Usually, a system either uses an approach focusing on the simulations of every individual decay or an approach which simulates a large number of decays with a focus on correct overall statistics. The radioactive decay package presented in this work permits, for the first time, the use of both methods within the same simulation framework - Geant4. The accuracy of the statistical approach in our new package, RDM-extended, and that of the existing Geant4 per-decay implementation (original RDM), which has also been refactored, are verified against the ENSDF database. The new verified package is beneficial for a wide range of experimental scenarios, as it enables researchers to choose the most appropriate approach for their Geant4-based application.

The astroparticle physics experiment CERN Axion Solar Telescope (CAST) aims to detect hypothetical axions or axion-like particles produced in the Sun by the Primakoff process. A Large Hadron Collider (LHC) prototype superconducting dipole magnet provides a 9 T transverse magnetic field for the conversion of axions into detectable X-ray photons. These photons are detected with an X-ray telescope and a novel type of frame-store CCD detector built from radio-pure materials, installed in the optics focal plane. A novel type of cooling system has been designed and built based on krypton-filled cryogenic heat pipes, made out of oxygen-free radiopure copper, and a Stirling cryocooler as cold source. The heat pipes provide an efficient thermal coupling between the cryocooler and the CCD which is kept at stable temperatures between 150 and 230 K within an accuracy of 0.1 K. A graded-Z radiation shield, also serving as a gas cold-trap operated at 120 K, is implemented to reduce the surface contamination of the CCD window and suppress background radiation.

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 fur 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.