Answer
$Nb$
Work Step by Step
According to the hint given in the problem statement, both $Zr$ and $Nb$ can be used.
In case of $Zr$, $E_\alpha< 18.00\;keV < E_\beta$
and in case of $Nb$, $E_\alpha< 18.99\;keV < E_\beta$.
Therefore, $Nb$ is the second best choice.
