The Fermi Gamma-ray Space Telescope is a satellite mission for measuring high-energy gamma-rays from space. The project is an international collaboration, involving institutions from France, Italy, Japan, Sweden and USA. The effective energy range is 10 keV - 300 GeV. The satellite consists of two different instruments, the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM). The former is comprised of 16 identical modules in a 4x4 array, where each module is made up by a tracker tower for direction determination and a calorimeter for energy measurements. Surronding the tracker towers is a segmented Anti-Coincidence Detector, which is used to veto charged particles.
The scientific goals of the Fermi are to:
- Understand the mechanisms of particle acceleration in active galactic nuclei (AGN), pulsars and supernova remnants
- Resolve the diffuse emission and the unidentified sources detected by EGRET
- Determine the high-energy behaviour of gamma-ray bursts and transients
- Probe dark matter and the early universe
The Fermi calorimeter consists of CsI(Tl) crystals, which were manufactured by Amcrys H in Ukraine, and are hodoscopically arranged in the calorimeter modules. This means that the layer-wise direction of the crystal is alternatingly in the x and the y direction. One calorimeter module consists of 8 layers with 12 crystal in each layer. The crystals are optically isolated by a reflective material and the scintillation light is read out with dual PIN diodes (Hamamatsu S8576) at each end. The position of the energy deposition along a crystal is determined by comparing the light yield from the crystal ends. The 16 calorimeter modules give a total calorimeter volume of about 1.6x1.6x0.2 cubic meters.
The Swedish contribution to the Fermi mission is the funding and testing of the CsI(Tl) crystals in the calorimeters. The tests, performed at the University of Kalmar, included mechanical, optical quality and radiation hardness testing. With data from the Fermi satellite, the Swedish Fermi group will be studying the Dark Matter in the Universe and the high energy emission in Gamma-Ray Bursts.
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