Groundbased calibration and characterization of t

Authors: E Bissaldi A von Kienlin G Lichti H Steinle P N Bhat M S Briggs G J Fishman A S Hoover R M Kippen M Krumrey M Gerlach V Connaughton R Diehl J Greiner A J van der Horst C Kouveliotou S McBreen C A Meegan W S Paciesas R D Preece C A WilsonHodge

Publish Date: 2009/01/30

Volume: 24, Issue: 1-3, Pages: 47-88

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Abstract

One of the scientific objectives of NASA’s Fermi Gammaray Space Telescope is the study of GammaRay Bursts GRBs The Fermi GammaRay Burst Monitor GBM was designed to detect and localize bursts for the Fermi mission By means of an array of 12 NaITl 8 keV to 1 MeV and two BGO 02 to 40 MeV scintillation detectors GBM extends the energy range 20 MeV to  300 GeV of Fermi’s main instrument the Large Area Telescope into the traditional range of current GRB databases The physical detector response of the GBM instrument to GRBs is determined with the help of Monte Carlo simulations which are supported and verified by onground individual detector calibration measurements We present the principal instrument properties which have been determined as a function of energy and angle including the channelenergy relation the energy resolution the effective area and the spatial homogeneityThe Fermi Gammaray Space Telescope formerly known as GLAST which was successfully launched on June 11 2008 is an international and multiagency space observatory 2 26 that studies the cosmos in the photon energy range of 8 keV to greater than 300 GeV The scientific motivations for the Fermi mission comprise a wide range of nonthermal processes and phenomena that can best be studied in highenergy gamma rays from solar flares to pulsars and cosmic rays in our Galaxy to blazars and GammaRay Bursts GRBs at cosmological distances 11 Particularly in GRB science the detection of energy emission beyond 50 MeV 6 17 still represents a puzzling topic mainly because only a few observations by the Energetic GammaRay Experiment Telescope EGRET 33 onboard the Compton GammaRay Observatory CGRO 12 15 and more recently by AGILE 10 are presently available above this energy Fermi’s detection range extending approximately an order of magnitude beyond EGRET’s upper energy limit of 30 GeV will hopefully expand the catalogue of highenergy burst detections A greater number of detailed observations of burst emission at MeV and GeV energies should provide a better understanding of bursts thus testing GRB highenergy emission models 5 25 30 35 Fermi was specifically designed to avoid some of the limitations of EGRET and it incorporates new technology and advanced onboard software that will allow it to achieve scientific goals greater than previous space experimentsThe main instrument on board the Fermi observatory is the Large Area Telescope LAT a pair conversion telescope like EGRET operating in the energy range between 20 MeV and 300 GeV This detector is based on solidstate technology obviating the need for consumables as was the case for EGRET’s spark chambers whose detector gas needed to be periodically replenished and greatly decreasing 10 μs dead time EGRET’s high dead time was due to the length of time required to recharge the HV power supplies after event detection These features combined with the large effective area and excellent background rejection allow the LAT to detect both faint sources and transient signals in the gammaray sky Aside from the main instrument the Fermi GammaRay Burst Monitor GBM extends the Fermi energy range to lower energies from 8 keV to 40 MeV The GBM helps the LAT with the discovery of transient events within a larger FoV and performs timeresolved spectroscopy of the measured burst emission In case of very strong and hard bursts the GRB position which is usually communicated by the GBM to the LAT allows a repointing of the main instrument in order to search for higher energy prompt or delayed emissionOn the left Schematic representation of the Fermi spacecraft showing the placement of the 14 GBM detectors 12 NaI detectors from n0 to nb are located in groups of three on the spacecraft edges while two BGOs b0 and b1 are positioned on opposite sides of the spacecraft On the right Picture of Fermi taken at Cape Canaveral few days before the launch Here six NaIs and one BGO are visible on the spacecraft’s side Photo credit NASA/Kim Shiflett http//mediaarchivekscnasagovIn order to perform the above validations several calibration campaigns were carried out in the years 2005 to 2008 The calibration of each individual detector or detectorlevel calibration comprises three distinct campaigns a main campaign with radioactive sources from 144 keV to 44 MeV which was performed in the laboratory of the MaxPlanckInstitut für extraterrestrische Physik MPE Munich Germany and two additional campaigns focusing on the low energy calibration of the NaI detectors from 10 to 60 keV and on the high energy calibration of the BGO detectors from 44 to 176 MeV respectively The first one was performed at the synchrotron radiation facility of the Berliner ElektronenSPEIcherringGesellschaft für Synchrotronstrahlung BESSY Berlin Germany with the support and collaboration of the German PhysikalischTechnische Bundesanstalt PTB while the second was carried out at the SLAC National Accelerator Laboratory Stanford CA USASubsequent calibration campaigns of the GBM instrument were performed at systemlevel that comprises all flight detectors the flight Data Processing Unit DPU and the Power Supply Box PSB These were carried out in the laboratories of the National Space Science and Technology Center NSSTC and of the Marshall Space Flight Center MSFC at Huntsville AL USA and include measurements for the determination of the channelenergy relation of the flight DPU and checking of the detectors’ performance before and after environmental tests After the integration of GBM onto the spacecraft a radioactive source survey was performed in order to verify the spacecraft backscattering in the modeling of the instrument response These later measurements are summarized in internal NASA reports and will be not further discussedThis paper focuses on the detectorlevel calibration campaigns of the GBM instrument and in particular on the analysis methods and results which crucially support the development of a consistent GBM instrument response It is organized as follows Section 2 outlines the technical characteristics of the GBM detectors Section 3 describes the various calibration campaigns which have been done highlighting simulations of the calibration in the laboratory environment performed at MPE see Section 34 Section 4 discusses the analysis system for the calibration data and shows the calibration results In Section 5 final comments about the scientific capabilities of GBM are given and the synergy of GBM with present space missions is outlinedThe GBM flight hardware comprises a set of 12 Thallium activated Sodium Iodide crystals NaITl hereafter NaI two Bismuth Germanate crystals Bi4 Ge3 O13 commonly abbreviated as BGO a DPU and a PSB In total 17 scintillation detectors were built 12 flight module FM NaI detectors two FM BGO detectors one spare NaI detector and two engineering qualification models EQM one for each detector type Since detector NaI FM 06 immediately showed lowlevel performances it was decided to replace it with the spare detector which was consequently numbered FM 13 Note that the detector numbering scheme used in the calibration and adopted throughout this paper is different to the one used for inflight analysis as indicated in Table 4 columns 2 and 3 in the Appendix

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