Physics
Scientific paper
Aug 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001icrc....6.2123i&link_type=abstract
Proceedings of the 27th International Cosmic Ray Conference. 07-15 August, 2001. Hamburg, Germany. Under the auspices of the Int
Physics
Scientific paper
The Advanced Thin Ionization Calorimeter (ATIC) Balloon Experiment had its first flight from Mcmurdo, Antarctica 28/12/2000 to 13/01/2001, local time, recording over 360 hours of data. The design goal of ATIC was to measure the Cosmic Ray composition and energy spectra from ~50 GeV to near 100 TeV utilizing a Si-matrix detector, a scintillator hodoscope, carbon targets and a calorimeter consisting of a stack of BGO scintillator crystals. The design, the operations and in-flight performance of the scintillator hodoscope and the BGO calorimeter are described. 2 The ATIC design ATIC is designed primarily to measure cosmic ray spectra for elements from hydrogen to nickel. The measurement technique used is ionization calorimetry. Cosmic rays interact with a low Z target, produce secondary pions which then start a shower in the calorimeter. In order to determine these spectra the following quantities need to be measured: 1) The charge of the individual cosmic ray particle, 2) Its energy and 3) The abundance of each species. The ATIC balloon instrument is composed of two major subsystems: the target module and the calorimeter. The target module has 4 functions: 1) Force the incoming cosmic ray to interact 2) Determine the charge of the incoming cosmic ray, 3) Provide a trigger for the instrument 4) Provide tracking in combination with the calorimeter. The calorimeter has 2 functions: 1) Determine the energy of the cosmic ray, 2) Provide tracking in combination with the target module. 2.1 The target module The target module consists of (from top to bottom): a silicon matrix array, a plastic scintillator XY plane, 10cm of carbon target, a 2nd scintillator XY plane, 20cm of carbon target and a 3rd scintillator XY plane. The Silicon matrix is the primary charge detector, supplemented by the topmost scintillator plane. A potential problem for charge determination in the presence of calorimeters are particles back scattered from the shower into the detectors above once the energy of the cosmic ray exceeds a few TeV. Simulations of high energy protons in the ATIC experiment indicate that, indeed, as the proton energy increases the number of "back-splash" particles per unit area increases in all three scintillator planes as well as the silicon matrix detector, potentially adding to the charge signal and degrading the ability to distinguish between protons and Helium. To combat this effect both charge detectors are segmented, thereby decreasing the probability of back scattered particles passing through the same detector element as the cosmic ray. The Silicon matrix consists of 4480 individually read out pixels (Adams et al, 1999). The scintillator detectors consist of individual strips. Fig. 1 shows the bottom half of one scintillator XY plane. It consists of individual strips of plastic scitillator read out by photomultiplier tubes at each end. The front end electronics is mounted directly behind the photomultipliers and consists of preamplifiers, ADCs and discriminator outputs for the trigger logic. One plane consists of two such halfs rotated 90 degrees to each other.
ATIC Collaboration
Isbert Joachim
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