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Interaction Processes Flashcards

1
Q

what is absorption

A

transfer to a medium

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2
Q

what is scatter?

A

radiation which travels in a different direction compared to the primary beam

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3
Q

what is TMAC

A

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

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4
Q

what are the interaction processes?

A
  • Photoelectric absorption
  • Compton scatter
  • Elastic scattering
  • Pair production

With each interaction it has its own MAC

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5
Q

Explain TMAC?

A
  • 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
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6
Q

Describe the process of elastic scattering?

A

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

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7
Q

State the scatter/ absorption for ES

A
  • forward scatter
  • ONLY SCATTER NO ABSORPTION
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8
Q

where is ES found

A

at low energies

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9
Q

Describe the process of PEA?

A

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)

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10
Q

what is the x-ray production process within PEA?

A

characteristic radiation

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11
Q

what happens within PEA if all energy is given up by the photon?

A

it no longer exists

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12
Q

What does PEA depend on?

A

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

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13
Q

State the scatter/ absorption for PEA?

A

NO SCATTER, ONLY ABSORPTION

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14
Q

describe PEA in soft tissue?

A

low proton number = low IE
- low energy photons via CR, quickly absorbed
- increase in internal energy and temp increase

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15
Q

describe PEA in high Z

A
  • most likely interaction
  • low BE
  • electron ejected from inner shell
  • CR
  • dependent on Z
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16
Q

what is the by product of photoionisation?

A

photoelectrons

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17
Q

what is the relation with PEA and Z

A

PEA is directly proportional to Z^3

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18
Q

What is PEA used in?

A

mammography

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19
Q

what is attentuation

A

absorption + scatter

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20
Q

what is the density in white regions

A

high density

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21
Q

what is the density in dark regions

A

low density, less absorption

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22
Q

what is the total dose

A

absorption of primary beam + absorption of scatter

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23
Q

is the energy in a secondary beam higher or lower than a primary

A

lower

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24
Q

what type of beam does PEA occur at

A

low energy beams

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25
What happens if the x-ray photon energy is too low
there is no PEA
26
what happens in PEA if the energy is above the BE
PEA is reduced, incidence decreases as energy increases
27
when does absorption occur
at high Z
28
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.
29
are soft photons good or bad
bad
30
when does absorption energies occur?
when the attenuation suddenly increases, with the photon energy = BE This is dependent on Z
31
describe the relation with PEMAC?
PEMAC is inversely proportional to PE^3 small increase in PE produces a large decrease in PEA
32
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
33
when does an absorption edge occur?
PE = BE
34
why are absorption edges useful?
in detectors, contrast agents and planning
35
does bone have a high or low Z
high, more likely to be absorbed
36
state the relation with PEA and TMAC
PEA is part of TMAC PEMAC is part of TMAC PEMAC = t/ p (density) t= PEA
37
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
38
state the equation for PEMAC & Z
PEMAC directly proportional to Z^3
39
state the equation for PEA and photon energy
PEA is inversely proportional to photon energy cubed
40
what happens when there is a high Z
PEA is high
41
what is the relationship between CS and energy
CS is inversely proportional to energy
42
what is CS proportional to
electron density (dependent on density) greater electron density, greater CS independent of Z
43
what is uptake like with CS
fairly even with all tissues
44
what direction of scatter is most common with CS
forward
45
what is the Compton recoil electron?
the ejected electron may ionise other atoms before coming to rest
46
what happens when PE increases?
CS decreases as photons are less likely to be defected off the path. Less CS occurs
47
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
48
what is the greatest loss in energy angle?
180
49
describe the CS process?
- the x-ray photon energy> BE (much greater) - photon transfers KE to the electron
50
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
51
describe the relation with CEMAC
CEMAC = sigma (CS)/ p(density ) which is proportional to electron density/ energy
52
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
53
What is the energy of the positron and negatron for PP
1.02 MeV
54
What is the photon energy within the PP
0.51 MeV each
55
what happens when Z increases in PP
PP increases, more photons create a stronger columb field which chances of interaction increasing
56
what is PP proportional to?
Z and E
57
what is TMAC?
sum of all individual mass coefficients
58
which interaction processes are energy dependent?
CS and PEA
59
Is PEMAC less important with higher energy
yes
60
Is CEMAC less important with higher energy
no
61
what is the highest energy used in RT
PPMAC
62
what happens to the dosimeter readings as attenuating materials increase in thickness
it reduces
63
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
64
what is the total linear attenuation coefficient
quantity related to attenuating properties of medium, different materials have different values
65
do larger atoms have a greater chance of attenuation or lower
greater chance
66
does more or less electrons increase attenuation chances
more
67
what is the exponential constant
2.72

Physics (25 decks)

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