Development of Silicon Strip Detectors for a Medium Energy Gamma-ray Telescope

We report on the design, production, and testing of advanced double-sided silicon strip detectors under development at the Max-Planck-Institute as part of the Medium Energy Gamma-ray Astronomy (MEGA) project. The detectors are designed to form a stack, the "tracker," with the goal of recording the paths of energetic electrons produced by Compton-scatter and pair-production interactions. Each layer of the tracker is composed of a 3 x 3 array of 500 micron thick silicon wafers, each 6 cm x 6 cm and fitted with 128 orthogonal p and n strips on opposite sides (470 micron pitch). The strips are biased using the punch-through principle and AC-coupled via metal strips separated from the strip implant by an insulating oxide/nitride layer. The strips from adjacent wafers in the 3 x 3 array are wire-bonded in series and read out by 128-channel TA1.1 ASICs, creating a total 19 cm x 19 cm position-sensitive area. At 20 degrees C a typical energy resolution of 15-20 keV FWHM, a position resolution of 290 microns, and a time resolution of ~1 microsec is observed.Comment: 9 pages, 13 figures, to appear in NIM-A (Proceedings of the 9th European Symposium on Semiconductor Detectors

NuSTAR Observations of G11.2–0.3

We present in this paper the hard X-ray view of the pulsar wind nebula in G11.2−0.3 and its central pulsar powered pulsar J1811−1925 as seen by NuSTAR. We complement the data with Chandra for a more complete picture and confirm the existence of a hard, power-law component in the shell with photon index Γ = 2.1 ± 0.1, which we attribute to synchrotron emission. Our imaging observations of the shell show a slightly smaller radius at higher energies, consistent with Chandra results, and we find shrinkage as a function of increased energy along the jet direction, indicating that the electron outflow in the PWN may be simpler than that seen in other young PWNe. Combining NuSTAR with INTEGRAL, we find that the pulsar spectrum can be fit by a power law with Γ = 1.32 ± 0.07 up to 300 keV without evidence of curvature

The MEGA Advanced Compton Telescope Project

The goal of the Medium Energy Gamma-ray Astronomy (MEGA) telescope is to improve sensitivity at medium gamma-ray energies (0.4-50 MeV) by at least an order of magnitude over that of COMPTEL. This will be achieved with a new compact design that allows for a very wide field of view, permitting a sensitive all-sky survey and the monitoring of transient and variable sources. The key science objectives for MEGA include the investigation of cosmic high-energy particle accelerators, studies of nucleosynthesis sites using gamma-ray lines, and determination of the large-scale structure of galactic and cosmic diffuse background emission. MEGA records and images gamma-ray events by completely tracking both Compton and pair creation interactions in a tracker of double-sided silicon strip detectors and a calorimeter of CsI crystals able to resolve in three dimensions. We present initial laboratory calibration results from a small prototype MEGA telescope.Comment: 7 pages LaTeX, 5 figures, to appear in New Astronomy Reviews (Proceedings of the Ringberg Workshop "Astronomy with Radioactivities III"

The Giant Flare of December 27, 2004 from SGR 1806-20

The giant flare of December 27, 2004 from SGR 1806-20 represents one of the most extraordinary events captured in over three decades of monitoring the gamma-ray sky. One measure of the intensity of the main peak is its effect on X- and gamma-ray instruments. RHESSI, an instrument designed to study the brightest solar flares, was completely saturated for ~0.5 s following the start of the main peak. A fortuitous alignment of SGR 1806-20 near the Sun at the time of the giant flare, however, allowed RHESSI a unique view of the giant flare event, including the precursor, the main peak decay, and the pulsed tail. Since RHESSI was saturated during the main peak, we augment these observations with Wind and RHESSI particle detector data in order to reconstruct the main peak as well. Here we present detailed spectral analysis and evolution of the giant flare. We report the novel detection of a relatively soft fast peak just milliseconds before the main peak, whose timescale and sizescale indicate a magnetospheric origin. We present the novel detection of emission extending up to 17 MeV immediately following the main peak, perhaps revealing a highly-extended corona driven by the hyper-Eddington luminosities. The spectral evolution and pulse evolution during the tail are presented, demonstrating significant magnetospheric twist and evolution during this phase. Blackbody radii are derived for every stage of the flare, which show remarkable agreement despite the range of luminosities and temperatures covered. Finally, we place significant upper limits on afterglow emission in the hundreds of seconds following the giant flare.Comment: 32 pages, 14 figures, submitted to Ap

Refactoring, reengineering and evolution: paths to Geant4 uncertainty quantification and performance improvement

Ongoing investigations for the improvement of Geant4 accuracy and computational performance resulting by refactoring and reengineering parts of the code are discussed. Issues in refactoring that are specific to the domain of physics simulation are identified and their impact is elucidated. Preliminary quantitative results are reported.Comment: To be published in the Proc. CHEP (Computing in High Energy Physics) 201

Quantifying the unknown: issues in simulation validation and their experimental impact

The assessment of the reliability of Monte Carlo simulations is discussed, with emphasis on uncertainty quantification and the related impact on experimental results. Methods and techniques to account for epistemic uncertainties, i.e. for intrinsic knowledge gaps in physics modeling, are discussed with the support of applications to concrete experimental scenarios. Ongoing projects regarding the investigation of epistemic uncertainties in the Geant4 simulation toolkit are reported.Comment: To be published in the Proceedings of the 13th ICATPP Conference on Astroparticle, Particle, Space Physics and Detectors for Physics Applications, Villa Olmo, Como, 3-7 October 201

Research in Geant4 electromagnetic physics design, and its effects on computational performance and quality assurance

The Geant4 toolkit offers a rich variety of electromagnetic physics models; so far the evaluation of this Geant4 domain has been mostly focused on its physics functionality, while the features of its design and their impact on simulation accuracy, computational performance and facilities for verification and validation have not been the object of comparable attention yet, despite the critical role they play in many experimental applications. A new project is in progress to study the application of new design concepts and software techniques in Geant4 electromagnetic physics, and to evaluate how they can improve on the current simulation capabilities. The application of a policy-based class design is investigated as a means to achieve the objective of granular decomposition of processes; this design technique offers various advantages in terms of flexibility of configuration and computational performance. The current Geant4 physics models have been re-implemented according to the new design as a pilot project. The main features of the new design and first results of performance improvement and testing simplification are presented; they are relevant to many Geant4 applications, where computational speed and the containment of resources invested in simulation production and quality assurance play a critical role.Comment: 4 pages, 4 figures and images, to appear in proceedings of the Nuclear Science Symposium and Medical Imaging Conference 2009, Orland