This innovation is a hermetically sealed case or very small chamber. A photomultiplier tube (PMT) is mounted inside the chamber. The circuit board is also installed inside the chamber with appropriate feedthroughs for signals. An adapter/floating mount called the phototube to light pipe system mount (PLPSM) allows the hermetic PMT housing to attach to the light guide.
The prior technique involved putting the PMT and its electronics board inside a container and filling the container with epoxy. There were several problems with this idea. The epoxy was messy, as was vacuuming the epoxy to remove air bubbles. In spite of care taken to remove the air bubbles, some air bubbles always remained, causing problems with the high-voltage and corona effects at certain altitudes. The result was PMT failure. Any repair of the tube itself or electronics board was extremely difficult. The epoxy had to be picked out from the tube and electronics components.
The hermetic PMT housing solves several issues. The housing allows a clean installation of the PMT and its electronic board. The housing is mechanically strong to protect the PMT, and allows a strong and positive mounting. The hermetic housing allows simple disassembly to effect changes or make repairs. The hermetic housing was pumped down to good vacuum and back-filled with dry nitrogen, resolving the high-voltage corona problem. Additional electrical feedthroughs could be added. The aluminum case provided some additional shielding, which was considered a positive effect. The hermetic housing is lighter in weight than the epoxy-filled version.
There are several versions of the hermetic PMT housing. The fundamental innovation is the hermetic seal device. An impact test was conducted successfully, with no damage to the PMT, and no loss of seal. The device can be worked on or modified easily, is lightweight and durable, protects and shields the PMT/electronics package, provides a precision housing that is gas-tight, and can be adapted to various mounting systems.
This work was done by Francisco San Sebastian of Goddard Space Flight Center. GSC-17001-1