Hi jr,
Thanks for your reply - We think it was indeed a thin window rupture - refilling it while the vacuum is sealed makes things work again until we vent, in which case it will start boiling off.
The venting that caused the rupture is, as far as I know, the same as it's always been. The regulator is labelled, and as usual, one venting isn't actually enough to fully release vacuum on our system, we always have to vent twice. The event happened after a single venting so there was still vacuum in the system (we loosened a spectrometer door to see if it moved, but it was still stuck in place). It's been fine this way for 15 years... but hey, 15 years of rock-solid use ain't bad.
The good news is we have a replacement in hand, which has a visually in-tact window. I'll know more about what the old window looks like after we swap units. The EDS unit with the broken still works properly (I didn't test count rates as I don't have any historical data to compare with) as long as the vacuum remains sealed. Within a minute or two of starting venting, however, the high temperature light will come on and the LN will start boiling. As long as we re-pump fast enough, the LN will stop boiling, but if it warms up too much the vacuum wont re-seal past ~3E-2Pa until the entire dewar finishes boiling off (which is why I suspect the dewar was the problem initially).
Our plug didn't get blown out, but that's probably because I found it and moved it before it could seal itself and pressurize. It did make a lot of ice though, which I had to sit around and carefully clean off before it dripped all over the front end electronics.