Sorry about the subject line, I was trying to be concise and cound't find better wording.
We have noticed a phenomenon when pumping down silicon particle detectors in vacuum chambers, and I had a possible explanation.
So, we put large-area silicon strip detectors in a vacuum chanber, and then notice the leakage current when reverse biased goes up for a CONSIDERABLE time, but eventually levels off. The last detector was the biggest we've ever used, about 200 mm diameter, and 1 mm thick.
My theory is that this thing has an insane amount of surface area and very little mass. Also, it has really poor thermal conductivity to the outside world. It is mounted on a piece of PC board material that is then mounted to an inner metal box in the vacuuum chamber. So, as the air is pumped out of the chamber, the air gets cold. Massive objects are not going to cool much, but the things with the greatest surface area/mass are going to cool the most. Then, the detector is going to warm up at a very slow rate. So slow, in fact, that the leakage only stabilizes after more than 8 hours.
Anybody have any experience with this? I know one could calculate the adiabatic cooling due to expansion of the air, but I have NO IDEA how one would calculate the cooling of objects in the chamber as the air is pumped out. It seems there would be some worst-case rate of pumping. Pump really fast and the air is all gone before much heat has left objects, pump REALLY slowly, and the air all maintains thermal equilibrium with the chamber walls. Somewhere in the middle of those extremes is where the most heat is drawn out of objects.
And, of course, the adiabatic cooling heads toward absolute zero as the pressure drops to hard vacuum levels, so that is not a useful point, either.
Thanks in advance for any comments, crazy or experienced!
(I did an experiment about 20 years ago with thermal conductivity in vacuum between hard surfaces without any conduction medium like thermal paste, and found that it is essentially zero! I had a piece of beryllium oxide heat conductor with a water loop thermal epoxied to it, in a vacuum chamber. I clamped an AD 590J temp sensor to it, and turned on the water cooling loop. The temp of the sensor changed a few degrees over tens of minutes. Then, I did it with one tiny drop of thermal grease, and the sensor reached equilibrium in less than 10 seconds. In air, with no grease, it took less than 30 seconds.)
Jon