But you don't tell us what type you have now.
18 years ago IIRC most of them were of the radioactive type using the
radioisotope
Americium-241, which has a half-life of 432 years.
So after 18 years its radioactivity would have decayed by about 3%
so if anything you would expect a slight decrease in its ionisation
capability
with a corresponding decrease in sensitivity.

--
Graham.

%Profound_observation%

Sorry.
Those figures are total arse-gravy, I didn't read the wiki I was quoting
from properly.

http://en.wikipedia.org/wiki/Americium
--
Graham.

%Profound_observation%