Performance of 8-stage Multianode Photomultipliers

64-channel Multianode Photomultiplier (MaPMT) have been evaluated as a candidate photo detector for the LHCb Ring Imaging Cherenkov (RICH) counters. The newly available 8-dynode stage MaPMT were tested with particle beams at CERN. The MaPMT signals were read out directly by the Beetle 1.2 chip which is designed for silicon sensors operating at 40 MHz. The photon yield and signal loss in a cluster of 3x3 close-packed MaPMTs were determined and preliminary results matched expectations from simulations. The performance of the 8-stage MaPMT was compared to that of the 12-stage MaPMT which has a larger intrinsic gain. For the read out the Beetle 1.2MA0 chip has been designed to match the dynamic range of the 12-stage MaPMT. In addition by using a LED light source the performance of the MaPMTs was tested in magnetic fields up to 35 mT. In measurements without and with mu-metal shielding we determined the magnetic field in which the tubes still can be operated with 90% signal efficiency.