The ATLAS Zero Degree Calorimeter Brookhaven National Laboratory, USA Sebastian White Brookhaven National Laboratory, USA A position sensitive calorimeter for multi Gigarad applications: Requirements: 1) extend ATLAS coverage to h>8. 2) Characterize Heavy Ion collisions (impact parameter and orientation). 3) Provide a trigger for inclusive and diffractive hard photoproduction in Pb-Pb and Pb-p. 4) Measure 2.75 TeV neutron energy to ~20%. 5) Reconstruct p0, h0 at large xF. Figure 1 Configuration of the ATLAS Zero Degree Calorimeters (ZDC’s). Detectors are located in the TAN absorber housing at +/- 140 m from IP1. Detectors are on the line of site for forward neutral particles. They consist of 4 modules (in depth) some of which have coordinate readout. Figure 4 Annual integrated dose expected for 1 year of LHC running in pp mode with 1033 luminosity. Calculation uses MARS code and full beamline geometry. Figure 7 Digitized waveforms ( 8 PMTs x 7 time slices) through full ATLAS daq (May 15 ’09)/ Challenges: . 1)Highest Radiation dose of any detector at the LHC (-2 GRad/yr at L=1033 cm-2 s-1) 2) Measure energy and position of neutrals to mms. METHOD : Figure 2 Principle of coordinate readout. Sampling is by 11 -1cm plates with layers.of 1.5 mm fused silica rods (strips). In addition 64 (96) 1mm rods penetrate the stack and are projective along the beam. Rods are read out by individual Hamamatsu R1635 PMTs. Figure 5 Transmittance at several wavelengths as a function of dose in a 64 mm long fiber. Figure 8 . Final installation of the sector 1-2 ATLAS ZDC, Radiation Damage Studies: The Zero Degree Calorimeter (ZDC) is a sampling device with alternating 1cm Tungsten radiator plates and 1.5 mm diameter fused silica rod layers (strips). The total calorimeter consists of 4 modules- each 1.14 Lint (29 X0) thick. Modules 1 and 2 are built to reconstruct positions of Electromagnetic and hadronic showers, respectively. In each case1 mm diameter silica rods (rods) pass through holes drilled through the Tungsten Plate in a 1x1 cm pattern. Fused silica (GE-124) is chosen for radiation hardness and a hybrid of strip layers and penetrating rods provide sufficient light signal as well as spatial resolution. Fibers were exposed to an integrated dose of up to 28 Grad at the BNL light Ion Producer. Transmission loss was observed at ~5 Grad. Microscopic inspection shows damage to both bulk and surface. Visible dislocations are likely due to energetic nuclear fragments. REFERENCES: ATLAS Heavy Ion Physics Performance Report 2. ATLAS ZDC Letter of Intent- CERN/LHCC/2007-001 LHCC I-016 3. Grad-level Radiation Damage of SiO2 Detectors N.Simos,S.White et al.-IEEE Nucl.Sci. Figure 6 Pre and post irradiation micrographs of our fused silica fibers. Figure 3. Schematic of position determination method (left) and 2g reconstructed mass spectrum from 1 M simulated Pythia events.