# Why apply voltage on an Si detector only on atmosphere or high vacuum

The general instruction when using a silicon detector is to either apply voltage only in atmospheric pressure or in high vacuum. Not in between!

I can't find a physical answer to it. Why is it so important?

• [Citation needed] I've never heard this, and I've been in the detector business for a zillion years. – Carl Witthoft Feb 18 '14 at 12:30
• I also haven't heard of this, but I've never really worked in between atmosphere and high vacuum. – Chris Mueller Feb 18 '14 at 13:05
• @CarlWitthoft : Thank you very much for your comment! I like your approach, because I can't find a reason on why the pressure should be bellow a certain limit. – Thanos Feb 18 '14 at 14:41
• @ChrisMueller : Thank's for your comment! I want to use it on $10^{-1}$ Torr. It's not atmosphere and definitely it's not high vacuum! – Thanos Feb 18 '14 at 15:27
• I on the other hand have come across this phenomenon, in an undergrad lab in fact, where we had to turn off the detector only while pumping down to vacuum. Failure to do this would result in some unhappy equipment and an unhappy grade. – user10851 Feb 18 '14 at 16:40

I just realized what the problem is. It actually doesn't have anything to do with the detector. When working in vacuum systems you have to worry about the dielectric breakdown of the air as the pressure is reduced. It turns out that the breakdown voltage hits a minimum around $\sim 1$ Torr depending on the species of the gas (see the curves below). This phenomenon is known as Paschen's Law; you can read more about in this Wikipedia article.
If you are applying high voltage across your Si detector when this breakdown occurs, you will surely ruin your detector. It looks like you will be OK at $0.1$ Torr, but I make no guarantees :).