How Come Early CRTs contained more Ions?

[jeyurkon]

Originally Posted by wa2ise

Did those tubes use lower very high voltage? maybe ions are not an issue at 5KV?

I think Tubesrule gave the right answer for the electrostatic tubes.

As far as HV effect...

I tried to do a literature search, I found some information but I have to jump through too many hoops to access the articles from home. So, I think that what I am going to say is accurate, but also incomplete as an answer.

I suspect that the major ions species are nitrogen and hydrogen since they are not so easy to getter out. There may be some CO2, which the beam would dissociate.

I did a damage simulation using SRIM for 5 and 10 KeV for hydrogen ions and nitrogen ions on Zinc Sulfide. The types of damage that they would cause are lattice dislocations, vacancies and displacements. Zinc sulfide would get broken into the metal zinc and elemental sulphur. Oxygen would also make zinc oxide and volatile sulfates. None of these would scintillate.

Average grains sizes for ZnS are between 1-10 microns. You'll notice that the damage is from a few hundredths of a micron to a few tenths of a micron in depth. Small compared to the grain size.

The emission of light by excited ZnS and the effect of dopants is complicated. ZnS is a semiconductor and much of the light is from recombination of electrons and holes. Impurities, such as iron, not only have an overlapping absorption band, but cause deep wells that can cause a non-radiative recombination of the electrons and holes (no light...)

If you look at the damage it seems to be on the surface of the ZnS grains. The metal zinc and elemental sulphur would be randomly distributed in that surface and would easily capture light before it managed to escape the crystal. They, and sulfates from oxygen could also cause non-radiative recombinations.

What I'm leading up to is that I don't think there's a big difference between 5KV CRTs and 10KV CRTs. At least not more than a factor of 2 in aging rates.

The first two plots are 5 & 10KeV nitrogen on ZnS and the next two are 5 & 10 KeV hydrogen on ZnS. Oxygen would look similar to nitrogen.

The plots show the damage per ion per angstrom of travel versus depth. The peak damage along a track is about the same for 5KV and 10KV. The biggest difference is the depth of the damage. There is a much bigger difference between hydrogen and nitrogen. Hydrogen causes damage deeper into the grain but nitrogen causes much more total damage.

If the explanations I gave for the quenching or capture of the photon emission is correct, then the difference in depth of the damage isn't going to have a big effect. It's just the total amount of damage that matters and it appears that 5KV might last about twice as long as a 10KV CRT assuming that it has the same amount of residual gas to start with.

But, all this hand-waving might be nothing more than hand-waving.

John

[jeyurkon]

Since this thread contain some discussion of ion trap magnets I thought I'd mention this.

The RCA Sept 15, 1949 datasheet for the 12LP4 says that the main pole pieces of the magnet should be longitudinally opposite the internal pole pieces of the gun. Also, the orientation should be such that the north pole is adjacent to pin 6 and the south pole to pin 12.

I set the ion trap magnet that way when I installed the 12LP4 that Scotty rebuilt with a rebuilt gun. It required almost no re-adjustment of the ion trap. It was very close to the proper position to begin with. This seems like a way to minimize any damage that might occur by having a badly adjusted magnet when you do an initial adjustment.

John