FoCal – A high granularity electromagnetic calorimeter for forward direct photon measurements

• Published in: Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment Vol. 845; pp. 542 - 547
• Main Author: Zhang, C.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 11.02.2017
• Subjects: Forward Calorimeter (FoCal) prototype; High granularity; Small Molière radius; Two-shower separation; Forward Calorimeter (FoCal) prototype; High granularity; Two-shower separation; Small Molière radius
• ISSN: 0168-9002; 1872-9576
• DOI: 10.1016/j.nima.2016.06.075

The measurement of direct photon production at forward rapidity (y∼3−5) at the LHC provides access to the structure of protons and nuclei at very small values of fractional momentum (x∼10−5). FoCal, an extremely-high-granularity Forward Calorimeter covering 3.3<η<5.3 is proposed as a detector upgrade to the ALICE experiment. To facilitate the design of the upgrade and to perform generic R&D necessary for such a novel calorimeter, a compact high-granularity electromagnetic calorimeter prototype has been built. The corresponding R&D studies are the focus of this paper. The prototype is a Si/W sampling calorimeter. It was instrumented with 24 layers of Monolithic Active Pixel Sensors, a total of 39M pixels. We report on performance studies of the prototype with test beams at DESY and CERN in a broad energy range. The results of the measurements demonstrate a very small Molière radius (∼11mm) and good linearity of the response. Unique results on the detailed lateral shower shape, which are crucial for the two-shower separation capabilities, are presented. We compare the measurements to GEANT-based MC simulations, which additionally include a modeling of charge diffusion. The studies demonstrate the feasibility of this high-granularity technology for use in the proposed detector upgrade. They also show the extremely high potential of this technology for future calorimeter development.