Phosphor burnup

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Richard Hull
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Phosphor burnup

Post by Richard Hull »

I believe this has been touched upon before, but the activating sites in phosphors are not forever. They can be burned up or damaged by radiation, especially.

This is a non-issue with detectors of nuclear radiation (hornyak buttons alpha re-radiators, etc). However, if the radiation is intense and applied over a long enough period of time, the phosphor will die as dead as a doornail!

I have a number of ultra, super, leathally hot aircraft instruments of WWII vintage and in the dark with the dark adapated eye not one photon can be seen. They are shot.
The phosphor probably lasted about 20 years while the Ra has about another 3200 years before it is down a lot.

One particular turn and bank indicator by Longines watch Company has a 1" by 2" backing screen of ZnS (Ag) with mixed in RaBr. It is, of course dead now with a manufacture date of Jun 11 44. At 1" from a cutie pie it reads 4.2 roentgens! Alpha, beta gamma. This thing must have been like a searchlight in the B-17 it was in at the time!

So you see, intimately mixed with a radioactive, phosphors are killed real quick. For them there is no place to run, no place to hide.

Another factor which can ruin or sully phosphors relates to storage. New in jar and left sealed in a cool area, they can last quite a while. Once actually "on th'job", in an aplication, they can deteriorate chemically due to heat or humidity conditions. Most chemicals can suffer this fate, but phosphors are much more suseptable with the required activator being homogeneously mixed. With changes in humidity and temperature the active sites can migrate, combine or fail in a number of ways.

I found out the silver content in ZnS (Ag) alternately written ZnS:Ag is one part in 10,000 molecularly.

Trivia or knowledge, you be the judge.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Tom Dressel
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Re: Phosphor burnup

Post by Tom Dressel »

Isn't phosphor burnup the reason why they invented screen savers?

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Richard Hull
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Re: Phosphor burnup

Post by Richard Hull »

Yes, but not so much to prevent burnup of the screen as burned in images. Modern screen phosphors can take a lot of abuse because the particles hitting them only have a kinetic energy of 20-30kv. The alpha particle scintillating a watch hand has about 5Mev of energy. A lot worse.

As Phosphors are excited time and again, the natural color of the new phosphor is replaced by a brownish coloration. With even illumination, you would never see it and the phosphor would still be fine.

In computers, especially those in businesses the monitors are left on 24 hours a day. They ultimately set on the exact same screen for 99% of there useful life. The phosphor discolors and when you turn the screen off you can see your work screen in dark brown phosphor. Likewise if you go to another screen, the dim image in dirty brown is over what you want to see.

TV sets are not prone to this inspite of 40 or more Kv acceleration potential due to the ever changing images.

The only phosphor I have ever seen absolutely dead is that from military Ra dialed instruments. And, I have seen some pretty horribly burned monitors, but they still work even where they are burned.
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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