Development of a comprehensive PMT optical model for the JUNO experiment

There are 17,612 20-inch photomultiplier tubes (PMTs) installed at the Jiangmen Underground Neutrino Observatory (JUNO). Developing a precise optical model for the PMTs is crucial for enhancing the accuracy of detector simulations and refining the energy response model at JUNO. In this study, we established a comprehensive PMT optical model based on prior studies, taking into account the non-uniformity of photon detection efficiency (PDE) across the PMT surface and the variances in PDE as well as reflections among different PMTs. By collecting reflectance data from 669 PMTs and utilizing PDE data from mass testing systems, we estimated the thickness maps of the photocathode (PC) and the anti-reflective coating (ARC) for each PMT. We also determined the collection efficiency (CE) by decomposing PDE with consideration of the optical processes occurring within the PMTs. The refractive index and extinction coefficient of both the PC and ARC, along with the escape factor, were evaluated over a broad wavelength range of 300nm to 700nm, covering the entire spectrum of interest for JUNO. Compared to the prediction from a simplified PMT optical model, which assumes uniform PC and ARC across all PMTs of the same type, the further developed PMT optical model yields much more reflectance for HPK PMTs and less for NNVT PMTs, and the change in PDE is at the level of a few percent. This comprehensive PMT optical model also provides a valuable reference for other PMT-based applications.

💡 Research Summary

The Jiangmen Underground Neutrino Observatory (JUNO) employs a massive array of 17,612 twenty‑inch photomultiplier tubes (PMTs) together with 25,600 three‑inch PMTs to achieve a photocathode coverage exceeding 75 % in its 20 kton liquid scintillator detector. Precise knowledge of the photon detection efficiency (PDE) of each large PMT is essential for JUNO’s primary physics goals—determining the neutrino mass ordering and measuring oscillation parameters with sub‑percent precision. PDE depends on a combination of optical processes (photon absorption in the photocathode, escape of photoelectrons) and on the collection efficiency (CE) governed by the internal electric field. Moreover, PDE varies with photon wavelength, angle of incidence, and the exact location on the photocathode surface.

Previous work (reference