  We present results of the detailed study of several hundred Hamamatsu H12700
Multianode Photomultiplier Tubes (MaPMTs), characterizing their response to the
Cherenkov light photons in the second Ring Imaging Cherenkov detector, a part
of the CLAS12 upgrade at Jefferson Lab. The total number of pixels studied was
25536. The single photoelectron spectra were measured for each pixel at
different high voltages and light intensities of the laser test setup. Using
the same dedicated front-end electronics as in the first RICH detector, the
setup allowed us to characterize each pixel's properties such as gain, quantum
efficiency, signal crosstalk between neighboring pixels, and determine the
signal threshold values to optimize their efficiency to detect Cherenkov
photons. A recently published state-of-the-art mathematical model, describing
photon detector response functions measured in low light conditions, was
extended to include the description of the crosstalk contributions to the
spectra. The database of extracted parameters will be used for the final
selection of the MaPMTs, their arrangement in the new RICH detector, and the
optimization of the operational settings of the front-end electronics. The
results show that the characteristics of the H12700 MaPMTs satisfy our
requirements for the position-sensitive single photoelectron detectors.
