Air-tight vs. Vacuum-tight
I suspect these guys are experimenting with techniques for making semiconductors. Wikipedia has an article about MBE - Molecular-beam epitaxy, a technique for making microscopic crystals. This business of finding a leak is the least technical part of what they are doing, but at least it gives us a bit of look inside the business of making semi-conductors, a business that is almost totally opaque. Or maybe I just don't know where to look.