Study on the large area MCP-PMT glass radioactivity reduction

The Jiangmen Underground Neutrino Observatory (JUNO) will install approximately 18,000 20-inch Photomultiplier Tubes (PMTs) in the central detector. From the full detector Monte Carlo (MC) simulation, the PMT glass has one of the largest contribution to the natural radioactive background due to its large mass. Various techniques have been developed to reduce radioactivities introduced in the raw material and during the production process. Efforts such as low background raw material selection, dust and dirty water isolation and continuous production monitoring were taken to reduce the glass background. The radioactivity in the glass sample was measured using a low background gamma ray spectrometer equipped with a high-purity germanium (HPGe) detector. The average level of  238U,  232Th and  40K in total MCP-PMT glass now reach 2.5 Bq/kg for  238U, 0.5 Bq/kg for  232Th and 0.5 Bq/kg for  40K, at least a factor of 2.5 lower than other type of 20-inch PMTs.