Interaction of X-Rays with Matter Flashcards
For Diagnostic radiography X-Radation is incident upon the part of the body being examined as we want the radiation to produce an image on the detector, the ability to do this depends on …
how it interacts with the parts encountered, however the situation is much more complex
X-Ray beam encounters many different types of body part as it transverses the body - each have differences in …
shape, thickness, atomic number, density and radiosensitivity
In radiotherapy you need to give a specific dose of radiation to abnormal (cancerous) tissue however there are healthy organs surrounding cancerous tissue therefore you have to …
restrict the dose to healthy tissues whilst giving a lethal dose to cancerous tissue
When a beam of X-Ray or Gamma Ray photons is incident upon an object, which 3 things can occur
Transmission
Absorption
Scattering
Describe transmission
Photons are fired but they miss everything, they penetrate without giving up energy, nothing happens, goes straight the patient and hits the detector
Describe absorption
photons fired but depending on what they strike in the patient either come to a stop and disappear (absorbed), energy is deposited in medium which absorbs the photons, gives up energy and then disappears
Describe scattering
photons fired, when they hit part of the patient they are not absorbed but scattered, the photons change direction and lose energy and some intensity, they give up energy and change direction
Secondary radiation is …
the radiation after scattering (starts with primary radiation until scattered)
Name the 3 processes that take place when a beam of X/Gamma Rays is incident upon the body
Photoelectric absorption
Compton Scattering
Pair Production (only at greater than or equal to 1.02 MeV - RT)
Define Attenuation
the total reduction of intensity of a beam of radiation as it transverses through a medium
Define Total Attenuation
the reduction due to absorption + the reduction due to scattering (+ reduction due to pair production - in RT only)
Describe photoelectric absorption
this is the process of absorption of an X-Ray beam, an incoming electron must have energy equal to or greater than the binding energy of an inner shell (L or K) electron of the absorber material - if this is the case = energy of the photon is used to eject the inner shell electron, the photon then disappears and the released electron now has kinetic energy equal to the difference between the initial energy of the photon and the electrons binding energy - vacancy remains
Explain the electron transition during photoelectric absorption
An electron from an outer shell fills the vacancy left by the ejected electron, therefore a lower energy X-Ray photon is emitted which is also photoelectrically absorbed by adjacent atoms
Define the Linear Attenuation Coefficient (u)
The fractional reduction in intensity per unit thickness of the absorber - this tells us how much each mm of absorber reduces the intensity at a given photon energy, does this in terms of what fraction of the original beam is absorbed
Describe a problem when using u alone and how its rectified
It doesn’t take into account the density of the material, different latest of the same material can have different densities (e.g. Ice = less dense than water), therefore u/p is used to introduce the materials density
Describe the Mass Attenuation Coefficient
defined as the fractional reduction in intensity per unit mass of the absorber, this depends on the absorber’s atomic number and the photons energy, given as t/p
t/p is proportional to …. and inversely proportional to …
proportional to atomic number cubed = t/p a Z cubed
inversely proportional to photon energy cubed = t/p a 1/KeV cubed
Name the other important feature of the attenuation coefficient- photon energy relationship
An example
it changes abruptly at 1 particular energy = the binding energy of the shell electrons - generally designated as the K/L edge
(e.g. at the K edge, photons with energies slightly greater than the binding energy for the K shell can now interact with the K shell electrons in addition to those in L shell, therefore more electrons in the material that are available for interactions, produces sudden increase in attenuation coefficient at K shell binding energy, theres a similar change in attenuation coefficient at L shell electron binding energy - for most elements this is below 10 KeV and not within useful portion of X-Ray spectrum, photoelectron interactions occur at highest rate when energy of X-Ray photon is just above binding energy of electrons)
Describe Compton Scattering
An incident X-Ray photon strikes the outer shell where there’s a loosely bound electron, the incident X-Ray photon ejects the electron and both move off at the same angle (theta) to initial trajectory - the compton electron gains kinetic energy and the scattered X-Ray photon loses photon energy
What does the loss of energy experienced by a photon depend on
depends on the angle of scatter, the greater the angle through which the photon is scattered, the greater its loss of energy
Give the symbol for mass attenuation coefficient for compton scattering and what its proportional to
o/p - independent of atomic number (Z), proportional to the inverse of the photon energy - o/p a 1/KeV
Photoelectric absorption and compton scattering are …
competing processes - each have influence on how much of the X-Ray beam is attenuated
Total mass attenuation coefficient (u/p) is …
the sum of the mass attenuation coefficients for the 2 processes - u/p = t/p + o/p
Describe with what Pair Production occurs and its significance in DT and RT
occurs with X/Gamma Rays photons whose energy is in excess of 1.02 MeV - significant in RT, relative for Positron Emission Tomography (PET) imaging
Describe Pair Production
incoming photons with energy equal to or greater than 1.02 MeV interacts with the field surrounding the nucleus, the incoming photon is converted to an electron and a positron, any energy in excess of 1.02 MeV is shared between these 2 new particles as Kinetic Energy, any KE that the electron has is dissipated locally within a few mm of soft tissue, any KE the positron has is dissipated locally AND when it comes to rest undergoes the reverse formation reaction creating 2 0.51 MeV photons travelling in opposite directions
Describe the mass attenuation coefficient for the pair production process and what its proportional to
written as H/p, proportional to atomic number Z and to the photon energy once the threshold or 1.02 MeV has been reached
