An 18 element strip of 1 mm<sup>2</sup> G-APDs for CMS HCAL upgrade results of CERN test beam 2009

Abstract

The Compact Muon Solenoid hadronic barrel (CMS HB) calorimeter consists of 17 layers of scintillator in a brass absorber. Each scintillator consists of 16 tiles comprising one layer of the corresponding towers in pseudo-rapidity. The light from each tile is routed into a single 0.96 mm wavelength shifting fiber. In the current design, the 17 fibers from the full tower depth are directed into one pixel of a multi-channel hybrid photo-diode (HPD). Thus, no longitudinal shower information is readout. In the last year we have developed 18 element MAPD arrays (Micro-pixel Avalanche Photo Diodes) on 8 inch wafers with Zecotek in Singapore. A special prototype 20 pin ceramic package was made by Kyocera in Germany. By using 18 element G-APD strips (Geiger mode Avalanche photo diodes) we could readout each layer separately, so that 17 of these strips replace one HPD. Because of the large linear range requirement in the hadronic calorimeter and radiation hardness needed, we used the new generation high cell density Zecotek MAPDs with 15,000 cells per strip element. Due to the higher PDE (photon detection efficiency) versus the HPD (30% vs. 12%) we got 5 p.e. per layer for a MIP. Due to limited availability of electronic channels for these tests, analog signals from multiple strips were added together providing digital readout from four depth segments. Non uniformity between strips was calibrated using muons and measurements of breakdown voltage of each element of the array. After calibration of the 4 longitudinal depth segments, the linearity and detector resolution was studied using pions with energies from 30 to 300 GeV.