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 mc2 now the maximum energy of photon is equal to the maximum energy of electron = eV.mc2ev⇒m=eVc2=1.6 × 10−19 × 18 × 103(3 × 108)2=3.2 x 10−32kg