The $$

particles emitted in the decay of $3$H

$($tritium$)$ interact with matter to create light in a glow$-$in$-$the$-$dark exit sign. At the time of manufacture, such a sign contains $13\mathrm{.}5$Ci

of $3$H

$.$ The atomic mass of tritium is $3\mathrm{.}016049$u

and its half$-$life is $12\mathrm{.}3$ years