A Low Noise Front End for the Belle II Forward Electromagnetic Calorimeter Upgrade

The Belle II experiment will operate at the SuperKEKB e+ e- collider, designed to reach a top luminosity of 8 × 10
³⁵
at the Υ(4s) resonance. The high background environment of the accelerator po ses serious challenges to the design of the detector. In particular, the Belle II collaboration is developing an ambitious upgrade program that involves the forward electromagnetic calorimeter. We will use pure CsI crystals, since they have less scintillation time but unfortunately much lower light yield. The electromagnetic calorimeter upgrade is subject to the same detector constraints as the first design; an intense research and development program on photon-detectors and front-end electronics has been initiated by the Italian collaboration. Our preliminary results show that a readout chain using avalanche photodiodes (APDs) to match the detector constraints and a dedicated front-end card for the readout meet the Belle II collaboration requirements. In this paper, we will show the experimental setup and detail the main characteristics of the read-out and power distribution system since the front-end card hosts both a low noise preamplifier and a power regulator to set the single APD bias voltage. The main HV module has been developed for this purpose to deliver a low noise power distribution to the front-end cards. All the relevant parameters of the front-end system can be set and monitored via Ethernet using the Experimental Physics and Industrial Control System. Slow control and monitoring functions in Belle II are performed by a dedicated hardware. The platform has been designed as an uP-based service-oriented platform, capable to offer an integrated solution for all the needs of data acquisition, analysis, and network functions.