by
LALman » Fri Jul 19, 2019 10:04 pm
James_Ball wrote:
If you have 300,000 counts for 69 then the EM should still be ok. The higher it tunes thought, the less linear range you will see, but it is still very functional even at 1900V.
The only ramp that seems odd is the entrance lens. Normally I would think it begins near max at left and peaks with just a few volts, then fall off pretty quickly as the voltage increases. We always ran with the entrance lens at 0 and if we had to add voltage we knew we had to clean the source. Two things can happen to the entrance lens, either the pin can come loose, or the brown vespel insulators can become contaminated and need replacing. If the insulators begin conducting you can bleed voltage over to ground or possibly from the ion focus, which will cause things to act strange. Also if you have the two piece insulator instead of the newer one piece, make sure the set screw that holds those lenses in place is not passing between the insulators and grounding a lens or that the pins have shifted and are grounding.
The x-ray lens is the same as the HED on the newer models, high volts on that should be ok, it is just the deflector than turns the ions towards the EM as they exit the quads.
I agree that the entrance lens ramp is the odd one. Its a two piece and the pins are good I think. I'm pretty sure that the set screw is not touching either lens because I pinch them together at that side and then spread them at the two pins just in case they moved. So, it seems likely the 2 piece vespel insulator is being conductive. I've never put them in solvent but perhaps some alumina dust got on them. Perhaps also, the pin is grounding out somehow. I'll break vacuum and check it all out again.
Why is it that the vespel on the repeller can be put in solvent but not the lens insulators?. Different formulations I suppose.
I too am using an ETP Active Film Model LS516 electron multiplier.
Thanks for the help guys.