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Radiotherapy Year 2 > Radiation Detectors > Flashcards

Radiation Detectors Flashcards

1
Q

Examples of pulsed and continuous radiation

A

Pulsed: LINAC, CT
Continuous: Isotopes, 60Co, 192Ir

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2
Q

What does dosimeter measure?

A

The amount of ionisation as a result of radiation
The amount of energy deposited in air or tissue as a result of ionisation
Any other related quantities

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3
Q

Properties of an ideal dosimeter

A

Stable, accurate, precise, tissue equivalent
Linear over dose range
Linear with dose rate
Flat energy response (no change in sensitivity with energy)
No angular dependence
Large dynamic range (measure over wide energy range)
High spatial resolution
Independent of radiation type
Perform consistently with environmental changes (T,P)
Require minimal correction
Have direct read out

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4
Q

What is precision and accuracy and relevance to dosimeters

A

Precision: degree of reproducibility of a measurement
Accuracy: how close the mean of measurement is to true value

Precise dosimeter can be calibrated to give accurate reading

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5
Q

Why do we want tissue equivalent dosimeters?

A

Want to replicate the scenario in the patient to get an accurate estimate of the dose being delivered

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6
Q

How can tissue equivalence be described?

A

In terms of atomic composition or density

Or energy absorption coefficient and stopping power, which is more accurate

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7
Q

What do radiochromic films consist of and how do they differ from radiographic films?

A

A single or double layer of radiosensitive microcrystal monomers on a thin polyester base with transparent coating

Differ as self developing, not processed

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8
Q

Size and dynamic range of radiochromic films

A

Depends on type:
Gafchromic EBT2: 20 x 25cm x 0.29mm and 0.01 - 5Gy

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9
Q

Applications of radiochromic film

A

QA in imaging and dosimetry
Verification of patient plans
Dose monitoring of staff - film badges

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10
Q

How does radiochromic film work?

A

When irradiated, energy absorbed by receptive part of photomonomer molecules in active layer
Polmerization results in formation of blue coloured polymer molecules with absoption maxima in red and blue regions of light spectrum
Intensity of maxima proportional to dose
Read out

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11
Q

Why do some gafchromic films have marker dyes?

A

To help minimise response variations which can be caused by coating anomalies (for example)
EBT3 appears green

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12
Q

How is radiochromic film read out?

A

Spectrophotometer (measures light intensity as function of light source wavelength)
Laser scanning densitometer
Flatbed scanners

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13
Q

Advantages of radiochromic films

A

Self developing - no processing or QA
Effectively grainless - very high resolution
Independent of dose rate
Less energy dependence than radiographic film
Large dynamic range: up to 1000Gy

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14
Q

Disadvantages of radiochromic film

A

Less sensitive than radiographic
Not reusable

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15
Q

Dose linearity of radiographic film

A

Degree of linearity depends on scanning medium: spectromphotometer more accurate than scanner

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16
Q

Energy dependence of radiochromic film

A

Weakly dependent on energy (significant dependence at low kV)
Degree of dependence can vary between films, depending on constituents in active layer mostly

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17
Q

Temperature dependence of radiochromic films

A

Insensitive to range of room temperatures

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18
Q

What are n-type semiconductors doped with?

A

Group V elements such as phosphorus
Gives extra electrons
Also called electron donors

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19
Q

What are p-type semiconductors doped with?

A

Group III elements such as boron
Gives rise to excess holes
Also called electron acceptors

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20
Q

Applications of semiconductor diodes

A

Used in in-vivo dosimetry
Relative dosimetry measurements

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21
Q

How can depletion zone also be described?

A

Potential hill
Sizes of which can be controlled by bias voltage

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22
Q

Forward and reverse bias

A

Forward: potential hill lowered, more electrons can now cross junction and current flows
Reverse: potential hill increased, less electrons have energy to climb hill. Some in p side may still be swept across, won’t be replaced.

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23
Q

How is pn junction created in reality?

A

Begin with base type semiconductor, such as p type Si doped with B
Something diffused onto this, eg Ph, some parts of silicon protected by SiO2
Region of n type material created, n+ because very highly doped compared to p type
Because bulk is p type this is called p type

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24
Q

What is measured to estimate dose in semiconductor?

A

Current produced which is proportional to dose

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25
Q

How does radiation detection with semiconductor work?

A

RT radiation provides boost through creation of electrons/holes created when radiation interacts with diode

E/Hs created everywhere

Any electrons in p type that go through depletion region contribute to current, like holes with n type

Diffusion length is function of carrier concentration: diffusion length on very doped side smaller than less doped, mostly ionisations in base that contribute to current

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26
Q

Response sensitivity of semiconductors

A

Sensitivity changes with accumulated dose due to radiation damage, becomes non-linear with dose rate. N-type more sensitive
Due to displacement of Si atoms from lattice sites which leads to recombination centres that siphon off charge carriers

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27
Q

Temperature dependence of semiconductors

A

Response sensitive to change in temperature for pulsed and continuous, but linear so can be characterised

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28
Q

Dose rate dependence of semiconductors

A

DR dependence for p-type less than for n-type at lower energies so p-type used more
Due to saturation of radiation induced traps

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29
Q

Energy dependence of semiconductors

A

Response changes with energy for both p and n type

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30
Q

Pre-irradiation of semiconductors

A

Semi-conductors are pre-irradiated due to their sensitivity varying over time.

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31
Q

Advantages of semiconductors

A

Small size, high resolution
No external bias needed
Good mechanical stability
High sensitivity
Instant readout
Simple instrumentation

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32
Q

Disadvantages of semiconductors

A

Energy dependent
DR dependent
Temperature dependent
Requires cables/ connection during irradiation for readout
Sensitivity changes over time

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33
Q

What is a diamond detector?

A

Detector using diamond to measure dose, works because a diamond with impurities can be used like a semiconductor
Under biasing potential a current is produced that is proportional to dose

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34
Q

Application of diamond detector

A

Small field dosimetry (although very expensive so few places use them)

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35
Q

Response stability of diamond detector

A

Response stable after pre-irradiation of 5-15Gy
Drop in sensitivity due to polarizing effect caused by captured electrons in forbidden zone which creates E field opposed to that applied

36
Q

Bias voltage dependence of diamond detector

A

Response is sensitive
Current increase proportional to increase in bias voltage, but response less stable at high voltage
Optimum is 125 p/m 25V

37
Q

Dose response of diamond detector

A

Linear

38
Q

DR dependence of diamond detector

A

Significant dependence on dose rate
Mainly due to impurities in sensitive volume creating polarizing effect when bias voltage applied across detector

39
Q

Response sensitivity with accumulated dose of diamond detector

A

Detector can withstand a significant amount of accumulated dose without showing degradation of response sensitivity, i.e. not susceptible to radiation hardening

40
Q

Energy dependence of diamond detector

A

Only weakly energy dependent

41
Q

Advantages of diamond detectors

A

Small size, high resolution
Waterproof
Almost tissue equivalent
Sensitive response

42
Q

Disadvantages of diamond detectors

A

Weakly energy dependent - must correct
Dependent on dose rate
Requires pre-irradiation
Sensitive to bias voltage
Expensive

43
Q

What is luminescence?

A

Process where a material upon stimulation releases trapped energy in form of UV, visible, IR

44
Q

What does delay between stimulation and emission determine?

A

<10^-8s, fluorescence
>10^-8s, phosphoresence

45
Q

Thermoluminescence vs optical stimulated luminescence

A

If stimulation is done by heat, TL
If stimulation done by light, OSL

46
Q

Applications of TLDs and form

A

In-vivo dosimetry
Relative dosimetry
EBR measurements
Brachy measurements

Can have powder, rods, chip, ribbon etc, small

47
Q

What happens when TLD irradiated?

A

Radiation ionises electrons - produces electrons in conduction band
Some fall back down to valence but some fall into impurity traps
On application of heat the trapped electron is excited to conduction band
When electron falls to valence band it emits light

48
Q

How is signal read out of TLD?

A

PMT converts light to signal and amplifies so it can be read out

49
Q

What is glow curve?

A

Plot of thermoluminescence intensity vs temperature
Single glow peak: single energy trap depth

50
Q

What is annealing and why is it done?

A

Heating at constant temperature over a fixed period

Controls number of glow peaks - remove first low temperature peaks to make glow curve more stable and predictable

51
Q

What are uses of heat in cycle of measurement for TLD?

A

Low temperature pre-readout anneal to remove low temp peaks
Anneal during readout to induce production of light and signal
High temperature post-readout removes residual signals

52
Q

Energy dependence of TLDs

A

Energy dependent response, significant at lower energies, primarily because TLDs not tissue equivalent

53
Q

Angular dependence of TLDs

A

No significant angular dependence

54
Q

Dose linearity of TLDs

A

Dose response linear. Can become supralinear at higher doses, influence by type and amount of impurities

55
Q

How do OSLDs work

A

Same principle as TLDs but use laser light source instead of heat to release trapped energy

56
Q

Applications of OSLDs

A

In-vivo dosimetry
Potentially more versatile than TLDs but not used as widely

57
Q

Temperature dependence of OSLDs

A

Virtually none (immediate advantage of TLDs)

58
Q

Angular dependence of OSLDs

A

None

59
Q

Dose linearity of OSLDs

A

Linear in clinically relevant dose range 0-200cGy
Supralinear at high doses

60
Q

Dose rate dependence of OSLDs

A

None

61
Q

Response sensitivity with accumulated dose of OSLDs

A

Sensitivity stable for accumulated doses up to 20Gy then begins to drop

62
Q

Energy (beam quality) dependence of OSLDs

A

Some dependence on energy (but better than TLDs)

Difference between photon and electron energies is because e energies give rise to more electrons in storage, affecting sensitivity

63
Q

Advantages of TLDs

A

Small sizes - high resolution
Various forms, different clinical applications
Not expensive
Many TLDs can be irradiated in single exposure
Resuable

64
Q

Disadvantages of TLDs

A

No instant readout (24 hours)
Signal erased during readout
Fiddly
Energy and dose, temperature dependent, must be calibrated well

65
Q

Advantages of OSLDs

A

Small size - high resolution
No temp dependence
No angular dependence
No dose rate dependence
Linear with dose in relevant range
Reusable
Quick readout time compared to TLDs
No cables

66
Q

Disadvantage of OSLDs

A

Some energy dependence, need correction factor
Readout not instantaneous (8 minutes)
Only linear up to about 4Gy
More expensive than TLDs

67
Q

What is a scintillator and what types can you have?

A

A material which fluoresces upon excitation - delay between excitation and light emittance is <10^-8s

Inorganic (eg NaI, BaF), organic (eg plastics)
Inorganic not suitable for dosimetry, high Z and density, plastics more tissue equivalent so acceptable

68
Q

Size and application of scintillators

A

Small, 1x4mm

Can be used in in-vivo dosimetry, brachy, small field dosimetry, radiosurgery

69
Q

How do scintillators work?

A

Absorb energy from incident radiation
Excitation occurs and emission of UV/visible light
Light converted to signal and amplified by PMT

Material in RT often polystyrene, PMMA

70
Q

Temperature dependence of scintillators

A

None at ambient temperatures

71
Q

Dose rate dependence of scintillators

A

Independent

72
Q

Dose linearity of scintillators

A

Linear in clinical range

73
Q

Reproducibility of scintillators

A

Stable response with good reproducibility

74
Q

What is alanine and how does it work?

A

An amino acid which produces free radicals when exposed to ionising radiation
When subjected to electron paramagnetic resonance (EPR), intensity of EPR spectrum proportional to concentration of free radicals
Amplitude of signal proportional to dose

75
Q

How does EPR work?

A

Free radical subjected to EM energy
Gets excited
Excitation energy matches frequency of B field, radical resonates
Amplitude of frequency proportional to dose

76
Q

Size and applications of alanine

A

Most commonly in pellets that are 2.5mm diameter x 5mm thick

Limited clinical use over 1Gy
Used by NPL for calibration at doses <15Gy

77
Q

Energy dependence of alanine

A

Weakly energy dependent

78
Q

Temperature dependence of alanine

A

Some temperature dependence

79
Q

Response stability of alanine

A

Signal fades with time depending on dose, storage implications

80
Q

What is Frick gel?

A

A gelatine, agarose or PVA phantom which has Fe2+ ions in Ferrous sulphate solution

True 3D dosimeter

81
Q

How does Fricke Gel work?

A

Upon irradiation, water decomposition happens and produces hydropoxy radicals
Various reactions then lead to Fe2+ to Fe3+ (Ferrous to Ferric) ions, introducing change in paramagnetic properties
Quantity of ferric ions depends on amount of energy absorbed, change in paramagnetic properties function of dose, measured using NMR

82
Q

Advantages of alanine

A

Almost tissue equivalent
Large dynamic range up to 1000Gy

83
Q

Disadvantages of alanine

A

Weakly energy dependent, needs correction factors
Signal fades with time dependent on dose, environmental and storage factors
Not reusable
Low sensitivity

84
Q

Uses of water for measurement

A

Used for reference/baseline measurements

85
Q

Using water equivalent phantoms

A

Water not always practical or convenient so can get equivalent water phantoms with similar densities and effective atomic numbers

Designed to be durable, consistent and minimise accumulation of charge

Generally used for daily verifications

86
Q

Examples of equivalent water phantoms

A

Perspex
Solid water
Virtual water
Plastic water
Polystyrene

Radiotherapy Year 2 (16 decks)

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