Abstract The Fermi Gamma-ray Space Telescope Tova Yoast-Hull May 2011 The primary instrument on the Fermi Gamma-ray Space Telescope is the Large Area Telescope (LAT) which detects gamma-rays in the energy range of 20 MeV to more than 300 GeV. The LAT is a pair-conversion telescope which tracks the gamma-ray and resulting electron-positron pair though several layers high-z material and measures the energy of and images the development the particle shower resulting from the electron-positron pair. The main goal of this project will be to understand the design and advantages of such a detector, with supporting calculations. Outline I. Gamma-Ray Detection - Why Fermi? A. Space vs. Ground Based Telescope 1. Brief History of Gamma-Ray Astronomy (a) 1967: Gamma-ray emission from the galactic plane is first detected by OSO III (E > 50 MeV). (b) 1972-3: SAS-2 discovers the diffuse gamma-ray background (20 MeV to 1 GeV). (c) 1975-81: COS-B provides the first complete map of the galaxy in gamma-rays (30 MeV to 5 GeV). (d) 1989: Whipple detects the first galactic TeV gamma-ray source, the Crab Nebula (100 GeV to 10 TeV). (e) 1991-2000: EGRET cataloged more than 250 gamma-ray sources (30 MeV to 10 GeV). 2. Space Telescopes (a) EGRET, Fermi (b) Energy Range: 30 MeV to 300 GeV (c) Detects: electron-positron pair (d) Advantages: No atmospheric effects, wide field-of-view. 3. Ground-Based Telescopes (a) VERITAS, HESS, MAGIC (b) Energy Range: 50 GeV to 50 TeV (c) Detects: Cherenkov light (d) Advantages: Can detect gamma-rays at higher energies. B. Observed Sources 1. Galactic Sources 1
(a) Pulsars, Pulsar Wind Nebulae (b) Supernova Remnants (c) X-Ray Binary Stars, Micro-Quasars 2. Extragalactic Sources (a) Active Galactic Nuclei (AGN), Blazars (b) Starburst Galaxies 3. Diffuse Isotropic Gamma-ray Background (a) Individual gamma-rays sources are viewed against a diffuse background of galactic and extragalactic gamma-ray radiation (Extragalactic Gamma-Ray Background). 4. Example: Starburst Galaxies (a) The main production mechanisms for gamma-ray production are neutral pion decay, bremsstrahlung, and inverse Compton. (b) Above 100 MeV, neutral pion decay is the dominant emission mechanism, as can be seen in the case of starburst galaxies. Fig. 1 shows the expected gamma-ray flux. (c) In the one-year point source catalog, Fermi reports the photon flux for the 1 GeV to 100 GeV range to be F = (1.66069±0.314956) 10 9 photons/cm 2 /s. Integrating the flux from Fig. 1 over the range 1 GeV to 100 GeV, the photon flux from neutral pion decay is F = 1.42 10 9 photons/cm 2 /s, which comes within the one-sigma uncertainty for the integral flux. Figure 1: Gamma-Ray Flux From Neutral Pion Decay For M82 2
II. Physics of the Large Area Telescope A. The Large Area Telescope 1. The large area telescope (LAT) is a 4x4 array of identical towers, each with a silicon tracker/converter, a CsI(Tl) calorimeter, and an electronic module. 2. The tracker modules of the array are covered by the segmented anti-coincidence detector. B. Photon Interactions with Matter 1. Photons interact with matter by (a) Photoelectric Absorption (b) Thomson and Compton Scattering (c) Pair Production 2. Pair production is the dominant process for energies above 100 MeV for low Z, see Fig. 2(a). 3. As photoelectric absorption and pair production are dependent on Z, in high- Z material, the energy range where Compton scattering is important becomes negligible and pair production becomes dominant above 10 MeV, see Fig. 2(b). 4. Thus, to detect gamma-rays above 30 MeV, the gamma-rays should be detected through pair production and track the resulting electron-positron pair. (a) Low-Z Material (b) High-Z Material Figure 2: Cross Section for Photon Interaction C. Anti-Coincidence Detector 1. Expected Background (a) Cosmic rays: protons, helium, electrons, positrons (b) Earth: albedo neutrons, albedo gamma-rays 2. The anti-coincidence detector (ACD) provides charged-particle background rejection. The detector consists of 89 plastic scintillator tiles. 3. Light from the scintillators is collected by wavelength shifting fibers which are coupled to photomultiplier tubes at each end. 3
4. The ACD covers the top and four sides of the tracking detector. The ACD is surrounded by a low-mass micrometeoroid shield to minimize the chance of light leaks. 5. To avoid self-veto created by the backsplash effect (secondary particles from the electromagnetic shower created by a high energy incident gamma-ray can Compton scatter in the ACD and create false veto signals from the recoil electrons), the ACD is segmented into 89 tiles of differing sizes. 6. The idea behind segmenting the ACD is that only the segment near the incident candidate photon will be considered in background rejection, reducing the area of the ACD which can contribute to backsplash. D. Precision Converter-Tracker 1. Each tracker has sixteen layers of high-z material (tungsten foil) in which gammarays can convert to an electron-positron pair. The converter planes are interleaved with position-sensitive detectors which are alternately rotated at ninety degrees. 2. The position-sensitive detectors, two layers (x and y) of single-sided silicon strip detectors, are used to track the electron-positron pair through the detector. This information is then used to reconstruct the direction of the incident gamma-ray. E. Cesium Iodide Calorimeter 1. Electromagnetic Showers (a) High energy photons create electron-photon cascades. (b) After the initial pair production by the incident gamma-ray, the electrons and positrons produce more high energy photons by bremsstrahlung. These photons can then pair produce. (c) The particles and photons continue to bremsstrahlung and pair produce until the average particle energy drops to the critical energy (which is the energy at which ionization becomes the dominant loss mechanism over bremsstrahlung). 2. Each calorimeter has eight layers of twelve CsI(Tl) scintillation crystals with two PIN photodiode readouts at each end. The layers are alternately rotated ninety degrees to form an x-y array. 3. The calorimeter measures the energy deposited by the particle shower resulting from the electron-positron pair and images the shower development profile. 4
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