What is the maximum practical or theoretical pressure at which x-rays can be produced, as in a Crookes tube or cathode ray tube? I know Crookes tubes operate at 0.1pa and lower, but I need to know if I am at risk of producing x-rays with high voltage (60kv+) discharges at medium vacuum, as low as 5pa. Does the electron mean free path have anything to do with this? Maybe when the e-mean free path equals or exceeds the gap between cathode and anode, x-rays can be produced by electrons traveling across without being slowed by gas molecules? How do I calculate the electron mean free path in air at 5pa and 298k? In the vacuum chamber at maximum vacuum, I can still see a slight movement of tissue paper placed in front of a computer fan, which makes me think there must still be too much air to allow x-rays to be produced, but I want to make sure before it's too late!