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BGO Calorimeter

In an ionization calorimeter, a particle's energy is deposited in an absorber via a cascade of nuclear and electromagnetic interactions. At each step, the energy of the primary particle is subdivided among many secondary particles. Ultimately, the primary energy is dissipated via ionization and excitation of the material. The integral of the deposited energy over the depth in the absorber is a measure of the energy of the incident hadron. In ATIC, the calorimeter is composed of a stack of 10 trays with forty 2.5 cm by 2.5 cm by 25 cm BGO crystals in each tray. Thus, the calorimeter is about 22 radiation lengths (~1.14 nuclear interaction lengths). One such tray, half filled with crystals, is shown in Figure 1. Each crystal is viewed, through an air gap, by a single Hamamatsu R5611 photomultiplier tube (PMT). The PMT bleeder string base incorporates a three dynode pickoff to cover a dynamic range of crystal energy deposits  from about 10 MeV (~ mip) to about 20 TeV and is designed for low power consumption. The dynode signals are wired to a Front End Module (FEM) board that contains six application specific integrated cir-cuits (ASIC) based on the design used for the ACE mission.  

Each ASIC has 16 input channels and outputs two “trigger”  signals as well as 12 bit digitized pulse heights. LEDs are incorporated into the front end electronics so each PMT can be checked for liveness and stability. The PMTs, with their brass foil shielding, wiring and part of the FEM can be seen in the bottom part of Figure 1.  As each tray is filled with crystals they are stacked as shown in Figure 2. Each layer is rotated by 90 de-grees to provide an X-Y coordinate for determining the shower core trajectory, using the techniques de-scribed in Ganel et al., 1999. The black vinyl coated handles in Figure 2 are used in tray handling (each tray weighs close to 50 kg) and the PMT / FEM boxes that can easily be mounted / dismounted on the tray ends. Cabling from the connectors on the FEM boxes run to the bottom of the electronics bays where the remainder of the readout electronics is mounted. The readout of the ASICs on multiple FEM boards is controlled by an ASIC Control Logic Board (ACLB) which, in turn, passes the data to the Detector Inter-face Module (DIM) that is part of the Detector Control Unit (DCU) computer.


Figure 1: BGO Calorimeter tray half filled with BGO crystals


Figure 2: The BGO Calorimeter Stack

   
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