Interaction Processes Flashcards
what is absorption
transfer to a medium
what is scatter?
radiation which travels in a different direction compared to the primary beam
what is TMAC
Total Mass Attenuation Coefficient
a fraction of the x-ray beam is removed, enabling comparison between the materials
it is independent within the material density
what are the interaction processes?
- Photoelectric absorption
- Compton scatter
- Elastic scattering
- Pair production
With each interaction it has its own MAC
Explain TMAC?
- it is used to avoid TLAC
- divide TLAC by density
- u
- p
- value given for a medium which is independent of the density medium
- TMAC = TLAC/ density
Describe the process of elastic scattering?
photon collides with an electron within an atom, as it has no energy the electron can’t be released, so the photon rebounds in a different direction, causing scattering, with the atom recoiling.
There is NO energy transfer
State the scatter/ absorption for ES
- forward scatter
- ONLY SCATTER NO ABSORPTION
where is ES found
at low energies
Describe the process of PEA?
it is when the BE is equal to the photon energy (or a tiny bit greater), all the energy if transferred to the electron. The electron is ejected from an atom (photoelectron has its own KE). This has ionised an atom with a vacancy, the vacancy is filled (characteristic radiation)
what is the x-ray production process within PEA?
characteristic radiation
what happens within PEA if all energy is given up by the photon?
it no longer exists
What does PEA depend on?
ENERGY DEPENDENT (large energy difference will cause it to be transparent, e will remain in the orbital)
number of protons, as it links with the number of energy levels/ electron shells. The photon must have the same or tiny but greater energy than the outer electron. The photon is absorbed by the inner electron
State the scatter/ absorption for PEA?
NO SCATTER, ONLY ABSORPTION
describe PEA in soft tissue?
low proton number = low IE
- low energy photons via CR, quickly absorbed
- increase in internal energy and temp increase
describe PEA in high Z
- most likely interaction
- low BE
- electron ejected from inner shell
- CR
- dependent on Z
what is the by product of photoionisation?
photoelectrons
what is the relation with PEA and Z
PEA is directly proportional to Z^3
What is PEA used in?
mammography
what is attentuation
absorption + scatter
what is the density in white regions
high density
what is the density in dark regions
low density, less absorption
what is the total dose
absorption of primary beam + absorption of scatter
is the energy in a secondary beam higher or lower than a primary
lower
what type of beam does PEA occur at
low energy beams
What happens if the x-ray photon energy is too low
there is no PEA
what happens in PEA if the energy is above the BE
PEA is reduced, incidence decreases as energy increases
when does absorption occur
at high Z
what happens when the photon energy increases
the likelihood of ionisation reduces until the next threshold is reached and an electron can be ejected at that level. A gradual fall leads to a sudden increase in PEA likelihood as photon energy increases until final IE is reached. Further increases results in a final decrease in PEA incidence.
are soft photons good or bad
bad
when does absorption energies occur?
when the attenuation suddenly increases, with the photon energy = BE
This is dependent on Z
describe the relation with PEMAC?
PEMAC is inversely proportional to PE^3
small increase in PE produces a large decrease in PEA
describe absorption edges
- interaction with the M shell: x-ray photon = BE M-shell
- PEA occurs
- when photon energy> BE, PEA falls
- until the energy = BE of L, then PEA occurs causing a peak in PEA
- when energy > BE of L, PEA falls
- until photon energy = BE of K then PEA occurs
when does an absorption edge occur?
PE = BE
why are absorption edges useful?
in detectors, contrast agents and planning
does bone have a high or low Z
high, more likely to be absorbed
state the relation with PEA and TMAC
PEA is part of TMAC
PEMAC is part of TMAC
PEMAC = t/ p (density)
t= PEA
how does a high Z link with attenuation
they have more energies that a proton can react with, increasing the chances for a photon to remove an electron. A slight change in proton number causes a large change in beam intensity
state the equation for PEMAC & Z
PEMAC directly proportional to Z^3
state the equation for PEA and photon energy
PEA is inversely proportional to photon energy cubed
what happens when there is a high Z
PEA is high
what is the relationship between CS and energy
CS is inversely proportional to energy
what is CS proportional to
electron density (dependent on density) greater electron density, greater CS
independent of Z
what is uptake like with CS
fairly even with all tissues
what direction of scatter is most common with CS
forward
what is the Compton recoil electron?
the ejected electron may ionise other atoms before coming to rest
what happens when PE increases?
CS decreases as photons are less likely to be defected off the path. Less CS occurs
what happens to the photon in CS once it collides?
the photon changes direction, causing scattering, the loss of energy depends on the scattering angle
what is the greatest loss in energy angle?
180
describe the CS process?
- the x-ray photon energy> BE (much greater)
- photon transfers KE to the electron
describe the scattering with CS
when the beam energy is high then forward scatter occurs, as less energy is transferred to the electron. It does occur in all directions up to 100 keV
Above 100 KeV CS decreases
Forward direction- over 1 MeV
describe the relation with CEMAC
CEMAC = sigma (CS)/ p(density ) which is proportional to electron density/ energy
describe the process of PP
- high energy photons interact with the nuclear coulomb field producing a negatron and positron pair
- this energy is converted into the rest mass and KE of negatron and positron
- the mass of the positron and negatron is converted into two photons - ANNIHILATION ENERGY
What is the energy of the positron and negatron for PP
1.02 MeV
What is the photon energy within the PP
0.51 MeV each
what happens when Z increases in PP
PP increases, more photons create a stronger columb field which chances of interaction increasing
what is PP proportional to?
Z and E
what is TMAC?
sum of all individual mass coefficients
which interaction processes are energy dependent?
CS and PEA
Is PEMAC less important with higher energy
yes
Is CEMAC less important with higher energy
no
what is the highest energy used in RT
PPMAC
what happens to the dosimeter readings as attenuating materials increase in thickness
it reduces
how is an exponential curve created
X-ray tube -> attenuating medium -> radiation measuring device
Aluminium is 0.5mm thick (each)
1. first recording is with no Al
2. from then onwards add 0.5 mm each time recording the reading and thickness
what is the total linear attenuation coefficient
quantity related to attenuating properties of medium, different materials have different values
do larger atoms have a greater chance of attenuation or lower
greater chance
does more or less electrons increase attenuation chances
more
what is the exponential constant
2.72
