X-ray Production

speeding electrons

So far we have learned that the generator steps up the voltage that goes to the cathode.  The filament within the cathode becomes super heated and boils off electrons.  The focusing cup collects the electrons which are then accelerated towards the positively charged anode at a high rate of speed.  So what happens at the atomic level when electrons interact with the anode?  X-rays are formed in 2 different ways, x-ray fluorescence (Characteristic), and Brehmstralung.


Characteristic Interaction

x-ray fluorescent 

 

This graph looks a little confusing, but will make sense after explanation.  When a high speed electron interacts with the anode, the electron may actually collide with an orbiting electron in the Tungsten atom.  The electron is ejected from the atom and sent on its way. This causes instability in the atom by creating a vacancy in the shell.  In the above example, the instability is in the K shell.  An atom cannot exist in this state, so an electron from the next shell, the L shell, must fall in to fill the vacancy.  Since the electron in the L shell has more energy than is required to fill the K shell, it must release some its energy before it can occupy the K shell.  This energy is released is in the form of a x-ray photon.  The energy of the x-ray equals the difference between the electron energies of the K shell and L shells. 

To take this one step further, when the L shell fell in to replace the K shell electron, it created an instability in the L shell. An electron in the M shell must now fall in to occupy the vacancy, and in the process must release some of its energy to occupy that shell.  Another x-ray photon is created.  This can happen several more times within the same atom until stability is reached. 

Within this process, the K shell is most likely be the starting point for x-ray production.  The reason is simple.  The K shell electrons have the lowest energy, so it is easier to eject an electron to begin the process.  The higher the electron orbit, the higher the energy required to eject it from its shell.  Most of the time, a high speed electron that interacts with a higher shell electron will transfer some of it's energy to the orbiting electron.  It is not enough energy to eject the electron, but is enough energy that the electron cannot keep it and remain in orbit.  The orbiting electron will release the energy in the form of heat and return back to stability.  Remember, 99% of all energy production in x-ray occurs in the form of heat.


Bremsstrahlung Interaction

bremsstrahlung

 

Bremsstrahlung can be be viewed similarly to a comet that goes around the sun.  A high speed electron from the cathode is accelerated towards the anode.  As it approaches the nucleus of the atom it changes direction, much like a comet what appoaches the sun.  As it changes direction it loses some of its energy.  This loss of energy is released in the form of a x-ray.  Bremsstrahlung is by far the most common form of x-ray production.  This is because, as was discussed in the first section, atoms are made of mostly empty space.