The planned International X-ray Observatory (IXO) was the designated successor to the extremely suc- cessful XMM Newton and Chandra X-ray observatories. Aiming to provide high sensitivity, high spatial, spectral and timing resolution observations in the 0.1 keV – 40 keV energy range, the mission would have greatly extended our knowledge of the universe. The international project was canceled due to the pullout of NASA in 2010. Subsequently, ESA pursued an European-only mission: A Telescope for High ENergy Astrophysics (ATHENA).
Both missions foresaw the use of a DePFET-based Wide Field Imager (WFI) for spectroscopic imag- ing observations in the energy range between 0.1 keV – 15 keV. The WFI would offer a high quan- tum efficiency, high spatial (≤ 10arcsec), moderate spectral (∆E ≈ 70eV@1keV) and high timing resolution (< 20μs) for imaging observations. The planned high sensitivity of ≈ 10−17 erg cm−2 s−1 for a 100ks observation translates into a maximum cosmic particle-induced background rate of ≈ 10−4 cts keV−1 cm−2 s−1 – 10−3 cts keV−1 cm−2 s−1. A rate which at the second Lagrangian point of the Earth-Sun system is only feasible if an optimized shielding concept and efficient background detection and reduction algorithms are employed. The study and optimization of these background reduction con- cepts, using a Geant4 Monte-Carlo simulation and the accompanying software development, is the core topic of this work.
After an introduction to X-ray astronomy in which the requirements for a next-generation X-ray ob- servatory are discussed and an overview of the IXO and ATHENA missions is given the simulation envi- ronment and the Geant4 Monte-Carlo toolkit are introduced. In the course of doing so the requirements for the simulation are defined and it is asserted that the radioactive decay simulation of Geant4 does not provide adequate functionality for an X-ray astronomy application. In particular, it is found that the code is not well verified and validated and that an appropriate means of simulating the cosmic-ray induced delayed background is not provided. As a response to this problem an extensive verification and self-consistent validation effort on radioactive decays in Geant4 was undertaken which has resulted in a new radioactive decay code for Geant4. This software is more accurate and significantly faster than the existing code. It includes a novel statistical sampling approach which appreciates the fact that for most experiments radioactive decays and the resulting radiation are a statistical observable. Furthermore, a self-consistent long-term activation simulation which requires minimal user input is included. The new code was extensively verified with Evaluated Nuclear Structure Data File (ENSDF) data and validated with High Purity Germanium (HPGe) detector measurements.
As a concrete application example this code was included into the IXO/ATHENA simulation environ- ment. Using this environment which was validated with XMM Newton EPIC-pn background measure- ments and cosmic ray activation measurements from the Space Shuttle STS-53 mission an extensive characterization of the prompt and delayed on-orbit background was performed. These studies resulted in an optimized graded-Z shielding design which is needed for a fluorescence emission free background; flexible pattern detection and rejection algorithms with > 99% rejection efficiency; as well as a novel approach utilizing an electric field to accelerate secondary electrons to energies above the detection limit thereby additionally reducing the background by ≈ 50%.
In conclusion, a background estimate of (6.42 ± 2.03) × 10−4 cts keV−1 cm−2 s−1 has been obtained for the ATHENA WFI. If a 200 kV cm−1 accelerating field is used a lower rate of (2.70 ± 2.67) × 10−4 cts keV−1 cm−2 s−1 can be achieved. The contribution of the delayed background component was estimated at (0.21 ± 0.05) × 10−4 cts keV−1 cm−2 s−1 after a 10 year mission time. All rates are within the WFI background requirements.
The background studies reported upon in the work are deemed applicable and beneficial for a wide range of silicon pixel detector applications. The code development work which has resulted in a new radioactive decay simulation for Geant is considered useful for an even broader range of experiments including applications in low background detectors, material sciences, radiation safety, nuclear non- proliferation studies, medical physics and homeland security
