Given:

Energy of the X-ray= 6.4 keV

E= 6.4 KeV =

Mass M an iron atom = 9.3 × 10^–26 kg.

Momentum of the photon is given by

=3.41×10^-24 kg/sec

According to conservation of momentum,

momentum of photon= momentum of atom

Hence, momentum of the atom

p=3.41× 10^-24 kg/sec

Recoil K.E of the atom, given by

Recoil K.E of the atom = 3.9 eV