[Rockhounds] Ice question ?

[Axel Emmermann]

> If the phosphor in the gunsight needs 1/4 of the remaining
> radioactivity to still be useful it will last two half-lives, or about
> 24 years, but it will slowly grow dimmer over that entire period.
 
[Axel] 
Not necessarily! I have a target plate of an old electron microscope. It's
incredibly fluorescent in either cathode rays or UV.
It responds to even the weakest UV-shimmer with a bright greenish yellow
fluorescence. 
Theoretically, the fluorescence would diminish with the square power of the
distance.
So if I put my Superbright at 5 inches from the plate and then double that
distance, the fluorescence would fall to 1/4 of the original luminosity. It
doesn't.
If I then were to increase the distance to 20 inches the fluorescence should
drop to 1/8 of the original strength. It is however still as bright.
If I repeat the experiment starting with say 1 meter, 2 meters, 4 meters....
the theory fits the data. 
Why? There's a threshold above which fluorescence is already at a maximum.
You need to initiate the experiment at the distance at which the UV-flux
meets the maximum possible emission flux. 
I think that a tritium sight has more tritium than strictly needed and that
the luminescence is limited by the maximum output of the fluorescing medium.
It would take a few years before the tritium is depleted below that
threshold. Oh well, that 's how I would make such a sight.

Cheers
Axel