Installing the x-ray gun

We input the x-ray gun this morning. Then I measured the resistance between the anode and ground, it's about 50G Ohms. Then I turn on the vacuum pump to evacuate the x-ray gun, I found the resistance decreased to 1M Ohms when the vacuum reached low than 1 Torr. But the resistance was back to 50G Ohms after the vacuum was under 10^-3 Torr. The cacuum was 2x10^-5 Torr when I left lab.