how x-rays interact with matter

Question 1

define attenuation

Answer 1

the reduction in intensity of a beam of x-rays as it passes through a medium

Question 2

define scattering

Answer 2

the change in direction of a photon without the loss of energy

Question 3

define absorption

Answer 3

the transference of energy from the x-ray beam to the atoms of the medium

Question 4

Describe the photoelectric effect

Answer 4

Occurs at lower X-ray energies (diagnostic imaging, radiotherapy CT localisation)
* The energy of the incident photon is equal to > than the binding energy of the orbital electrons.
* The incident photon interacts with a K-shell electron transferring all the energy to that electron = the incident photon no longer exists
* The transferred energy overcomes the binding energy of the electron and is thus ejected from the atom (known as a photoelectron). The excess energy is converted into kinetic energy to allow the electron to travel through the matter surrounding the atom.

This interaction causes a cascade effect which releases characteristic photons.
* Photo-electric absorption is proportional to the atomic number cubed (Z3)

Question 5

describe Compton scatter

Answer 5

Occurs at both kV and MV energies.
* As the energy of the incident photon increases the low binding energies of the outer orbital electrons become considered insignificant = they’re considered “free”.
* The incident photon releases some of the energy to the free electron kinetically.
* The electron “recoils” from this collision and is ejected at velocity from the atom in any direction.
* The incident photon is deflected from its original path and possess less energy.

Question 6

describe pair production

Answer 6

Pair production.
Uses E=mc2 meaning there is a proportional relationship between mass and energy and only occurs at the megavoltage energies (IMPORTANT FOR LINAC).

Stage 1 – creation of mass from energy
When the high energy X-ray photon passes close to the nucleus it interacts with the electrostatic field of the nucleus.
* The photon then just disappears… but it can’t just disappear the energy is used to create a positron and negatron. They carry opposite charges, so the total charge of the interaction remains zero.

Stage 2 – creation of energy from mass
The positron comes to rest and is very unstable, so it re-joins with the negative electron, this is an annihilation reaction and results in the creation of mass.
* The annihilation produces 2 X-ray photons (annihilation photons) which travel in opposing directions which can be used in the other 2 interactions.