Studies of a hybrid avalanche photo-detector in magnetic field

For the Belle II spectrometer a proximity focusing RICH counter with an aerogel radiator (ARICH) will be employed as a PID system in the forward endcap region of the spectrometer. The main challenge was the development of a reliable multichannel sensor for single photons that operates in the high magnetic field of the spectrometer (1.5 T) and withstands the radiation levels expected at the experiment. A 144-channel Hybrid Avalanche Photo-Detector (HAPD) was developed with Hamamatsu Photonics K.K. and the mass production of ∼480 HAPDs was completed recently. While our first tests of HAPD performance in the magnetic field (before mass production) showed no issues, we lately observed a presence of very large signal pulses (∼5000× single photon signal), generated internally within about 20% of HAPDs, while operating in the magnetic field. The rate of these pulses varies from sample to sample. These pulses impact the HAPD performance in two ways: they introduce periods of dead time and, in some cases, damage to the front-end electronics was observed. Here we present conditions under which such large pulses are generated, their properties and impact on HAPD performance, and discuss possible mechanism of their origin.