Applications of 20-inch photomultiplier tubes in neutrino and cosmic ray experiments

The development of the 20-inch photomultiplier tube (PMT) began in the late 1970s, motivated by the extreme demand for a large photon collection area in large-scale particle physics detectors. To maximize detection of weak Cherenkov radiation, Masatoshi Koshiba from ICRR, Tokyo, Japan, proposed an ambitious concept for a photodetector with an aperture far larger than any existing design at the time. Hamamatsu (Hamamatsu Photonics, Hamamatsu City, Japan) successfully met the challenge, overcoming critical technological hurdles such as forming large glass envelopes and achieving production of uniform large-area photocathodes. In 1981, the first 20-inch PMT (R1449 type) was produced. This breakthrough enabled the Kamiokande experiment to detect neutrinos from SN 1987A, marking the start of neutrino astronomy and earning Koshiba the 2002 Nobel Prize.

Building upon Kamiokande's success, the larger Super-Kamiokande deployed over 11,000 improved 20-inch PMTs (Figure 1). Their high photon collection capability enabled the 1998 discovery of atmospheric neutrino oscillations, confirming that neutrinos have non-zero mass and earning Takaaki Kajita the 2015 Nobel Prize in Physics. Subsequently, 20-inch PMTs evolved into advanced variants such as the dynode-type R12860 and the North Night Vision Technology (NNVT, Nanjing, China) microchannel plate (MCP)-based PMTs, which continue to drive progress in fundamental physics.

Why 20 inches?

The deployment of 20-inch PMTs in large-scale neutrino and cosmic ray experiments is driven by a compelling combination of performance benefits and cost-effectiveness. Their primary role is to detect weak, rare flashes of light from neutrino interactions or extensive air showers.

In large-scale experiments such as Jiangmen Underground Neutrino Observatory (JUNO) and Large High Altitude Air Shower Observatory (LHAASO), a single 20-inch PMT provides a substantial light collection area, providing low number of readout channels and associated electronics. This simplification reduces complexity and cost, and enhances reliability.

Accurate reconstruction of particle properties, such as energy, arrival direction, and type, depends on collecting a sufficient number of photons from each event. The large photocathode of a 20-inch PMT inherently increases the probability to detect photons that pass long distances through the detection medium. This enhanced photon collection area along with the large coverage area, is crucial for improving energy resolution, lowering the energy threshold of the experiment, and enabling clear event identification.

While a single 20-inch PMT has a higher unit cost than a smaller one, the total system cost is often substantially lower. Considering the costs of structures, cables, electronics, and installation, it is overwhelmingly more economical to use fewer large photodetectors than smaller ones to achieve the same overall coverage area. This cost efficiency is a crucial factor for development of large detectors, such as the 20 kton JUNO experiment.