High Energy Monitoring Instrument

2008

The High Energy Monitoring Instrument (HEMI) is an instrument developed by me and other Penn State SSPL students for the Joint Astrophysics Nascent Universe Satellite (JANUS). It's detects and captures spectral data about gamma-ray bursts (GRB) with minimal footprint and resource usage from the host bus.

The final instrument was conceptualized as a single scintillation crystal mounted to a photomultiplier tube (PMT) as part of a modular unit for integrating into JANUS.

We developed a payload for the High Altitude Student Platform (HASP) to test the instrument architecture and GRB detection in a near-space environment. The original concept was to enclose the electronics and externally mount two PMTs with a central crystal.

We tested prototypes in SSPL's vacuum chamber lab and discovered that the PMT failed when it reached a certain air pressure. At the time, we suspected the failure to be caused by arcing within the PMT's high voltage circuitry, but the PMT manufacturer assured us that the internals were fully potted to prevent this. To meet the looming HASP qualification timeline, I designed, simulated, machined, and tested a pressure vessel to contain the entire payload. This would allow the crystal to still detect GRBs while maintaining atmospheric pressure for the PMT.

The experiment was a success!

We performed a postmortem of the PMT and discovered that, contrary to manufacturer claims, it was not internally potted. The PMT had been experiencing electrical breakdown when the ambient air pressure reached a critical point. At the near-vacuum of space, the PMT would function normally, but it would fail at the air pressure at HASP's altitude.