PHOTON INTERACTIONS

With MATTER

There are 6 types of Interactions Photons can undergo with Matter.

• Only 3 of these types are important to Radiation Therapy and Diagnostic Radiology

1. COHERENT SCATTER (also known as classical, simple, or unmodified scatter)

There are two types of coherent scattering, Thomson scatter and Rayleigh scattering.

• In Thomson scattering only one electron of the atom is involved in the interaction.
• With Rayleigh scatter all the elecbons of the atom, sometimes called the electon cloud, are involved in a cooperative effort in the interaction with the photon
• With coherent scattering the incident photon interacts with the electron or electron cloud in the atom, in such a way that the they will oscillate (that is vibrate) at the same frequency as the incident electromagnetic wave.
• Then the electron or electron cloud will reradiate the energy at the same frequency without absorbing any of the energy of the incident photon
• The electron motion is same after as it was before the interaction.
• This causes the scattering of photon at a small angle.
• Probability of coherent scatter increases with high Z atomic number materials and low energy photons.
• NOT A INTERACTION IN RADIATION THERAPY OR DIAGNOSTIC RADIOLOGY

2. PHOTOELECTRIC EFFECT

It was found both experimentally and theoretically that 80% of the photoelectric absorption processes takes place in the K shell, provided that the incident photon energy ( hv ) exceeds the K shell binding energy (Evans Pg 695)

The photo electric effect could be broken up into five parts:

1.The photon must first have an energy equal to or greater than the binding energy of electron in the atom.

2.The incident photon must be completely absorbed by the electron.

3.The electron is then ejected from the atom.

4.The excess energy over the binding energy is given to the electron in the form of kinetic energy (which is the speed of the electron).

5.The hole left in the atom is now filled by an outer shell electron or a free electron with the emission of characteristic radiation.

The biological material of the body is largely made of low atomic number elements. Because of this the characteristic radiation that is emitted will also be low in energy and will therefore be locally absorbed.

The approximate energy for K characteristic radiation for:

• calcium - 4.00 KeV
• oxygen - .54 KeV
• carbon - .28 KeV
• hydrogen – 13.6 ev

The Photoelectric Effect Depends on 2 things:

1.Energy of the Photon (E)

2.Atomic number of the absorber (z)

The shorthand way of writing the probability of the photoelectric effect is t (tau)

1. Energy of the Photon:

Equation for the probability of the photoelectric effect occurring :    

 

• Inverse relationship
• If energy goes up the probability of the photoelectric effect (t)  occurring goes down

 

2.Atomic Number of the absorber:

Equation for the probability of the photoelectric effect occurring:   

• Direct relationship
• If the Z of the material goes up then the probability of the photoelectric effect (t) occurring goes up

 

Probablity of Photoelectric Effect = (t)= attenuation coefficient

3. COMPTON INTERACTION

Almost all scatter in diagnostic radiology comes from Compton Scattering

• Causes film to fog (degraded image)
• most common type of interaction in Radiation Therapy
• it is the ideal interaction because it is independent of Z (no preferential bone absorption)

 

In the Compton Interaction a photon interacts with a “free” or outer shell electron

• a portion of the incident energy of the photon will be transferred to an electron in the form of kinetic energy
• he incident photon now called scatter photon will be deflected in a new direction with less energy

NOTE:

• the energy given to recoil the electron is considered abosorbed
• the energy retained by the photon is considered scattered

 

The Compton Interaction is dependent on 1 thing:

1.Energy of the photon

( s ) - probability of Compton interaction

Equation for the probability of the Compton Interaction occurring :  

• Inverse proportion
• As energy goes up the probability of the Compton Interaction occurring goes down

Compton interaction is independent of atomic number because most elements have the same number of electrons per gram (hydrogen is the exception)