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 (~ ½ 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.
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Figure 1: BGO Calorimeter tray
half filled with BGO crystals
Figure 2: The BGO Calorimeter Stack |
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