C. Ferguson

MGGPOD: a Monte Carlo Suite for Modeling Instrumental Line and Continuum Backgrounds in Gamma-Ray Astronomy

Intense and complex instrumental backgrounds, against which the much smaller signals from celestial sources have to be discerned, are a notorious problem for low- and intermediate-energy γ-ray astronomy (~50 keV-10 MeV). Therefore, a detailed qualitative and quantitative understanding of instrumental line and continuum backgrounds is crucial for most stages of γ-ray astronomy missions, ranging from the design and development of new instrumentation through performance prediction to data reduction. We have developed MGGPOD, a user-friendly suite of Monte Carlo codes built around the widely used GEANT (ver. 3.21) package, to simulate ab initio the physical processes relevant for the production of instrumental backgrounds. These include the build-up and delayed decay of radioactive isotopes as well as the prompt de-excitation of excited nuclei, both of which give rise to a plethora of instrumental γ-ray background lines in addition to continuum backgrounds. The MGGPOD package and documentation are publicly available online. We demonstrate the capabilities of the MGGPOD suite by modeling high-resolution γ-ray spectra recorded by the Transient Gamma-Ray Spectrometer (TGRS) on board Wind during 1995. The TGRS is a Ge spectrometer operating in the 40 keV-8 MeV range. Because of its fine energy resolution, these spectra reveal the complex instrumental background in formidable detail, particularly the many prompt and delayed γ-ray lines. We evaluate the successes and failures of the MGGPOD package in reproducing TGRS data and provide identifications for the numerous instrumental lines.

Monte Carlo simulations and generation of the SPI response

In this paper we discuss the methods developed for the production of the INTEGRAL/SPI instrument response. The response files were produced using a suite of Monte Carlo simulation software developed at NASA/GSFC based on the GEANT-3 package available from CERN. The production of the INTEGRAL/SPI instrument response also required the development of a detailed computer mass model for SPI. We discuss our extensive investigations into methods to reduce both the computation time and storage requirements for the SPI response. We also discuss corrections to the simulated response based on our comparison of ground and inflight calibration data with MGEANT simulations.

First identification and modelling of SPI background lines

On Oct. 17, 2002, the ESA INTEGRAL observatory was launched into a highly elliptical orbit. SPI, a high resolution Ge spectrometer covering an energy range of 20-8000 keV, is one of its two main instruments. We use data recorded early in the mission (i.e. in March 2003) to characterize the instrumental background, in particular the many gamma-ray lines produced by cosmic-ray interactions in the instrument and spacecraft materials. More than 300 lines and spectral features are observed, for about 220 of which we provide identifications. An electronic version of this list, which will be updated continuously, is available for download at CESR. We also report first results from our efforts to model these lines by ab initio Monte Carlo simulation.

In-flight performance of the IBIS calibration unit

We describe the in-flight performance of the on-board calibration unit for the IBIS telescope on INTEGRAL. Both intrinsic performance and the quality of the calibration signals provided to the IBIS detector planes are discussed. The calibration unit intrinsic performance is assessed based on the diagnostic information in IBIS housekeeping. The flux of tagged photons, i.e. those arriving at the detector planes in coincidence with a calibration strobe, is assessed from in-flight data, and analysed in conjunction with a detailed simulation. Proposed usage of the tagged photon flux is discussed, and the expected calibration accuracy derived. The effect on science data from untagged calibration unit photons is assessed based on the predicted rate and distribution of such photons.

Simulating the Background Noise in the Legri CZT Detectors

A Monte-Carlo model of the MINISAT 01 satellite has been built. This model, based on the GEANT software suite, is used to study the background noise induced in the cadmium zinc telluride (CZT) in the LEGRI detector. We find that the background noise count rate at the poles is ∼ 50% higher than at the equator. This increase is due to the effects of geomagnetic rigidity cut-off. We also simulate the effects of passages through the South Atlantic Anomaly (SAA) with simulations showing an increase of ∼ 0.5 counts cm−2 sec−1 after the SAA, in good agreement with observational data.

Modelling the effects of a solar flare on INTEGRAL

The delayed effects of a large solar flare proton flux on the instruments on-board INTEGRAL have been modelled. We simulated exposing INTEGRAL to a flux of 1.5×108 protons/cm2 over a period of five days. The induced count rates due to this proton flux over an energy range of 30 MeV–2 GeV one minute after the end of the flare are 4170±30 c/s for IBIS (the imager) and 190±2 c/s for SPI (the spectrometer). We show that lowering the minimum incident proton energy range in the simulations below 30 MeV has no effect on the delayed count rate. Energy spectra indicate that there is little evolution of the spectral shape of the induced background up to 24 hours after the flare.