Why are X-rays Emitted when electrons are focused on Heavy metals

During X-ray emission, the kinetic energy of a photon is converted into X-rays. 

If the vacancy is created in K shell and electron from the L shell fills the vacancy then the emitted photon is a K_α – X-ray. If a vacancy is created in the K-shell and an electron from the M shell fills this vacancy then the X-ray emitted is known as a k_β X-ray.

So, characteristic x-rays are emitted from heavy elements when their electrons make transitions between the lower atomic energy levels. 

Heavy metals which are used have following characteristics:

• High atomic
• Ability to maintain its strength at high temperatures
• High melting point
• Relatively low rate of evaporation.