Development of the large area MCP-PMT

Abstract

To adapt the specific demands of photomultiplier tubes (PMTs) in the Jiangmen Underground Neutrino Observatory, we design and manufacture a new type of micro-channel plate PMT (MCP-PMT) with the following performance indicators: the PMTs’ glass shell adopts the formula of GG-17 which have no K₂O, and very low background (the contents of ²³²Th, ²³⁸U and ⁴⁰K in the raw materials are less than 40 ppb, 20 ppb and10 ppb, respectively). Its main body is sphere with 500 mm external diameter, 4 mm wall thickness and the tails that using a gradual transition of kinds of low radiation background glasses, then sealing with the Kovar. The photocathode material with lowest dark noise is bi-alkali photocathode which spectral region matches the liquid scintillator emission spectral (400~440 nm). The front and back hemisphere is transmission and reflective photocathode separately. Two sets of double-stack micro-channel plates replacing the dynode chain are used to detect the photoelectrons from both sites. The focusing system makes the photoelectrons hit into the inside of MCPs to the uttermost. The anode and lead make sure charge signal is no distortion. No matter made by transfer equipment or not, the peak value of quantum efficiency of the PMT should reach 30%, and the peak-to-valley ratio of single photoelectron spectrum has surpassed 2.5:1. In a word, the MCP-PMT’s reliability and other characteristics meet the need of Jiangmen Underground Neutrino Observatory.