Validation of Compton Scattering Monte Carlo Simulation Models

Several models for the Monte Carlo simulation of Compton scattering on electrons are quantitatively evaluated with respect to a large collection of experimental data retrieved from the literature. Some of these models are currently implemented in general purpose Monte Carlo systems; some have been implemented and evaluated for possible use in Monte Carlo particle transport for the first time in this study. Here we present first and preliminary results concerning total and differential Compton scattering cross sections.Comment: 5 pages, 3 figures, to be published in the Proceedings of IEEE Nuclear Science Symposium 201

Background Simulations of the Wide Field Imager of the ATHENA X-Ray Observatory

The ATHENA X-ray Observatory-IXO is a planned multinational orbiting X-ray observatory with a focal length of 11.5m. ATHENA aims to perform pointed observations in an energy range from 0.1 keV to 15 keV with high sensitivity. For high spatial and timing resolution imaging and spectroscopic observations the 640x640 pixel^2 large DePFET-technology based Wide field Imager (WFI) focal plane detector, providing a field of view of 18 arcsec will be the main detector. Based on the actual mechanics, thermal and shielding design we present estimates for the WFI cosmic ray induced background obtained by the use of Monte-Carlo simulations and possible background reduction measures.Comment: IEEE NSS MIC Conference 2011, Valencia, Spai

Validation of Geant4-based Radioactive Decay Simulation

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

Radioactive Decays in Geant4

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

Photon shot-noise limited transient absorption soft X-ray spectroscopy at the European XFEL

Femtosecond transient soft X-ray Absorption Spectroscopy (XAS) is a very promising technique that can be employed at X-ray Free Electron Lasers (FELs) to investigate out-of-equilibrium dynamics for material and energy research. Here we present a dedicated setup for soft X-rays available at the Spectroscopy & Coherent Scattering (SCS) instrument at the European X-ray Free Electron Laser (EuXFEL). It consists of a beam-splitting off-axis zone plate (BOZ) used in transmission to create three copies of the incoming beam, which are used to measure the transmitted intensity through the excited and unexcited sample, as well as to monitor the incoming intensity. Since these three intensity signals are detected shot-by-shot and simultaneously, this setup allows normalized shot-by-shot analysis of the transmission. For photon detection, the DSSC imaging detector, which is capable of recording up to 800 images at 4.5 MHz frame rate during the FEL burst, is employed and allows approaching the photon shot-noise limit. We review the setup and its capabilities, as well as the online and offline analysis tools provided to users.Comment: 11 figure

Segmented flow generator for serial crystallography at the European X-ray free electron laser

Serial femtosecond crystallography (SFX) with X-ray free electron lasers (XFELs) allows structure determination of membrane proteins and time-resolved crystallography. Common liquid sample delivery continuously jets the protein crystal suspension into the path of the XFEL, wasting a vast amount of sample due to the pulsed nature of all current XFEL sources. The European XFEL (EuXFEL) delivers femtosecond (fs) X-ray pulses in trains spaced 100 ms apart whereas pulses within trains are currently separated by 889 ns. Therefore, continuous sample delivery via fast jets wastes >99% of sample. Here, we introduce a microfluidic device delivering crystal laden droplets segmented with an immiscible oil reducing sample waste and demonstrate droplet injection at the EuXFEL compatible with high pressure liquid delivery of an SFX experiment. While achieving ~60% reduction in sample waste, we determine the structure of the enzyme 3-deoxy-D-manno-octulosonate-8-phosphate synthase from microcrystals delivered in droplets revealing distinct structural features not previously reported

Studies on the Background of the Wide Field Imager aboard the IXO and ATHENA X-Ray Telescopes

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