Johannes Werthebach successfully completes his PhD

We congratulate Dr. Johannes Werthebach on the successful completion of his PhD. In his dissertation, titled "Acceptance tests of PMTs for the mDOM of the IceCube Upgrade", he studied photomultiplier tubes for the multi-PMT Digital Optical Modules of the upgrade of the IceCube Neutrino Observatory in Antarctica.

The IceCube experiment is currently the largest neutrino telescope in the world. It uses the clear Antarctic ice as a detector medium. When neutrinos interact with nuclei in the ice, they produce secondary charged particles that emit light signals due to the Cherenkov effect. This light can be detected by optical sensors distributed throughout the ice. As part of the IceCube Upgrade during the 2025-2026 Antarctic summer, additional optical sensors were installed in the central region of the detector, improving its sensitivity and calibration capabilities. These sensors are multi-PMT Digital Optical Modules (mDOMs), which each contain 24 photomultiplier tubes (PMTs). The PMTs serve as the light-sensitive elements of the modules.

Before the PMTs can be installed in the ice, their performance must be tested. In his PhD work, Dr. Werthebach developed, built, and commissioned a test bench at TU Dortmund University to test the PMTs for the mDOMs of the IceCube Upgrade. The test bench is designed for automated operation and allows up to 92 PMTs to be tested in parallel in less than 24 hours, corresponding to a throughput of about 500 PMTs per week. For the IceCube Upgrade, more than 10,000 PMTs were tested at two sites, with 4,793 of them tested in Dortmund.

A key result of the work was the identification of unexpectedly high dark rates in almost all tested PMTs, meaning that the sensors recorded signals at a higher-than-expected rate even in the absence of external light. This effect was traced back to elevated levels of radioactive isotopes in the glass envelope of the PMTs. The work also showed that the test bench concept delivered consistent results across two independent testing sites and can be adapted for future detector developments, including IceCube-Gen2, the planned next-generation extension of the IceCube experiment.

With his dissertation, Dr. Werthebach made an important contribution to the quality assurance of optical sensors for the IceCube experiment. We warmly congratulate him on this achievement and wish him all the best for his future career.