The voltage applied to an X-ray tube is 18KV. The maximum mass of photon emitted by the X-ray tube will be:

Energy of photon is given by $m{c}^2$now the maximum energy of photon is equal to the maximum energy of electron = eV.
$m{c}^{2}ev\Rightarrow m=\frac{eV}{c^2}=\frac{1.6\times {10}^{-19}\times 18\times {10}^3}{{(3\times {10}^8)}^2}=3.2 x {10}^{-32}kg$