Characterization of the Hamamatsu VUV4 MPPCs for nEXO

In this paper we report on the characterization of the Hamamatsu VUV4 (S/N: S13370-6152) Vacuum Ultra-Violet (VUV) sensitive Multi-Pixel Photon Counters (MPPC)s as part of the development of a solution for the detection of liquid xenon scintillation light for the nEXO experiment. Various MPPC features, such as: dark noise, gain, correlated avalanches, direct crosstalk and Photon Detection Efficiency (PDE) were measured in a dedicated setup at TRIUMF. MPPCs were characterized in the range 163 K <= T <= 233 K. At an over voltage of 3.1 +/- 0.2 V and at T = 163 K we report a number of Correlated Avalanches (CAs) per pulse in the 1 mu s interval following the trigger pulse of 0.161 +/- 0.005. At the same settings the Dark-Noise (DN) rate is 0.137 +/- 0.002 Hz/mm(2). Both the number of CAs and the DN rate are within nEXO specifications. The PDE of the Hamamatsu VUV4 was measured for two different devices at T = 233 K for a mean wavelength of 189 +/- 7 nm. At 3.6 +/- 0.2 V and 3.5 +/- 0.2 V of over voltage we report a PDE of 13.4 +/- 2.6% and 11 +/- 2%, corresponding to a saturation PDE of 14.8 +/- 2.8% and 12.2 +/- 2.3%, respectively. Both values are well below the 24% saturation PDE advertised by Hamamatsu. More generally, the second device tested at 3.5 +/- 0.2 V of over voltage is below the nEXO PDE requirement. The first one instead yields a PDE that is marginally close to meeting the nEXO specifications. This suggests that with modest improvements the Hamamatsu VUV4 MPPCs could be considered as an alternative to the FBK-LF Silicon Photo-Multipliers for the final design of the nEXO detector.