Study on the large area MCP-PMT glass radioactivity reduction

TL;DR

This study demonstrates significant reduction in radioactivity levels of large-area MCP-PMT glass, crucial for minimizing background noise in neutrino detection experiments like JUNO.

Contribution

The paper reports a successful development and monitoring of low-radioactivity glass production for MCP-PMTs, achieving substantial reductions in uranium, thorium, and potassium isotopes.

Findings

Radioactivity in glass reduced by factors of 2, 9, and 15 for 238U, 232Th, and 40K respectively.

Measured radioactivity levels now meet the requirements for low-background neutrino experiments.

Monitoring over two months confirmed consistent production of low-radioactivity glass.

Abstract

The Jiangmen Underground Neutrino Observatory (JUNO) will install about 18,000 20-inch Photomultiplier Tubes (PMTs) in the center detector to achieve 3%/sqrt(E(MeV)) energy resolution. From the full detector Monte Carlo (MC) simulation, besides the liquid scitillator (LS) and Acrylic nodes, PMT glass has the largest contribution to the natural radioactive background. Various technologies have been developed in the Chinese industry to control the environment and to improve the production process. We have monitored the glass production for more than two months, and the radioactivity in glass was measured using a low background gamma ray spectrometer equipped with a high resolution HPGe detector. The 238U, 232Th and 40K of the glass bulb are reduced by a factor of 2, 9 and 15 respectively, and now they can reach 2.5 Bq/kg for 238U, 0.5 Bq/kg for 232Th and 0.5 Bq/kg for 40K.