Replacing Photomultiplier Tubes with Avalanche Photodiode Arrays
Photomultiplier tubes (PMTs) have long been used by the NNSS to detect and amplify light signals. However, they are becoming obsolete in the commercial industry: photonics company Hamamatsu recently announced it would be discontinuing the PMT most frequently used by the NNSS. This PMT was considered unmatched in terms of gain and linearity and worth its expensive price tag. However, its discontinuation has left a gap at the NNSS in terms of what could replace PMTs going forward.
Principal investigator James Mellott believes that solid state is the future of radiation detection. In his Site-Directed Research and Development (SDRD) project “Digitized Nanosecond Silicon-Germanium Photo-Multipliers for Prompt Radiation Detection,” James aims to design an avalanche photodiode array (APD) and read out circuitry that are fast enough to measure prompt radiation. If successful, James’ APDs could replace PMTs with small and relatively inexpensive application-specific integrated circuit designs and microcontrollers.
In the first year of this project, James and his team primarily tested and reported on existing silicon germanium (SiGe) chips at the University of Nevada, Las Vegas (UNLV). They used SiGe APDs to increase the quantum efficiency of silicon APDS. The APDs are designed to be small, fast, and quickly resettable so that they can be used for prompt detection. APDs that are passively quenched result in higher dark counts and longer reset times, so they designed actively quenching structures to mitigate this issue. The team completed testing on 108 passively quenched structures and plans to test the actively quenching structures as the project progresses.
James’ efforts are unique because commercial industry isn’t pursuing the same innovations that he and his team are—commercial solid-state replacements being designed are much too slow for the NNSS’ prompt radiation detection needs. Without the team’s innovation, the NNSS would have to rely on poor performance PMTs or slow commercial PMT replacements in the next ten years.
Because James and his team work closely with UNLV, they have been able to leverage student support for their efforts. So far, an undergraduate and a graduate student have worked on the project, with the graduate student staying on as a 2024 summer intern. By working with the students at UNLV and establishing a pipeline from the university to the NNSS, James’ project contributes to the SDRD goal of workforce development.
In fiscal year (FY) 2025, James and his team plan to design and fabricate several arrays consisting of at least two active quenching and two passive quenching arrays. Their goal is to have a functioning prototype submitted for fabrication in FY 2025.
We wish James and his team the best of luck as they continue to work toward a viable PMT replacement for the NNSS!
