Chance an atom survives three seconds

A plutonium atom decays at an average rate of once per second (an exponential waiting time). The probability that it does not decay during the next $3$ seconds equals $e^{a}$. Find $a$.

$a=-3$. The waiting time $T$ is $\text{Exp}(1)$, so $P(T>3)=e^{-3}$.