Radiation Detectors Flashcards
Examples of pulsed and continuous radiation
Pulsed: LINAC, CT
Continuous: Isotopes, 60Co, 192Ir
What does dosimeter measure?
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
Properties of an ideal dosimeter
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
What is precision and accuracy and relevance to dosimeters
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
Why do we want tissue equivalent dosimeters?
Want to replicate the scenario in the patient to get an accurate estimate of the dose being delivered
How can tissue equivalence be described?
In terms of atomic composition or density
Or energy absorption coefficient and stopping power, which is more accurate
What do radiochromic films consist of and how do they differ from radiographic films?
A single or double layer of radiosensitive microcrystal monomers on a thin polyester base with transparent coating
Differ as self developing, not processed
Size and dynamic range of radiochromic films
Depends on type:
Gafchromic EBT2: 20 x 25cm x 0.29mm and 0.01 - 5Gy
Applications of radiochromic film
QA in imaging and dosimetry
Verification of patient plans
Dose monitoring of staff - film badges
How does radiochromic film work?
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
Why do some gafchromic films have marker dyes?
To help minimise response variations which can be caused by coating anomalies (for example)
EBT3 appears green
How is radiochromic film read out?
Spectrophotometer (measures light intensity as function of light source wavelength)
Laser scanning densitometer
Flatbed scanners
Advantages of radiochromic films
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
Disadvantages of radiochromic film
Less sensitive than radiographic
Not reusable
Dose linearity of radiographic film
Degree of linearity depends on scanning medium: spectromphotometer more accurate than scanner
Energy dependence of radiochromic film
Weakly dependent on energy (significant dependence at low kV)
Degree of dependence can vary between films, depending on constituents in active layer mostly
Temperature dependence of radiochromic films
Insensitive to range of room temperatures
What are n-type semiconductors doped with?
Group V elements such as phosphorus
Gives extra electrons
Also called electron donors
What are p-type semiconductors doped with?
Group III elements such as boron
Gives rise to excess holes
Also called electron acceptors
Applications of semiconductor diodes
Used in in-vivo dosimetry
Relative dosimetry measurements
How can depletion zone also be described?
Potential hill
Sizes of which can be controlled by bias voltage
Forward and reverse bias
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.
How is pn junction created in reality?
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
What is measured to estimate dose in semiconductor?
Current produced which is proportional to dose
How does radiation detection with semiconductor work?
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
Response sensitivity of semiconductors
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
Temperature dependence of semiconductors
Response sensitive to change in temperature for pulsed and continuous, but linear so can be characterised
Dose rate dependence of semiconductors
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
Energy dependence of semiconductors
Response changes with energy for both p and n type
Pre-irradiation of semiconductors
Semi-conductors are pre-irradiated due to their sensitivity varying over time.
Advantages of semiconductors
Small size, high resolution
No external bias needed
Good mechanical stability
High sensitivity
Instant readout
Simple instrumentation
Disadvantages of semiconductors
Energy dependent
DR dependent
Temperature dependent
Requires cables/ connection during irradiation for readout
Sensitivity changes over time
What is a diamond detector?
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
Application of diamond detector
Small field dosimetry (although very expensive so few places use them)
Response stability of diamond detector
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
Bias voltage dependence of diamond detector
Response is sensitive
Current increase proportional to increase in bias voltage, but response less stable at high voltage
Optimum is 125 p/m 25V
Dose response of diamond detector
Linear
DR dependence of diamond detector
Significant dependence on dose rate
Mainly due to impurities in sensitive volume creating polarizing effect when bias voltage applied across detector
Response sensitivity with accumulated dose of diamond detector
Detector can withstand a significant amount of accumulated dose without showing degradation of response sensitivity, i.e. not susceptible to radiation hardening
Energy dependence of diamond detector
Only weakly energy dependent
Advantages of diamond detectors
Small size, high resolution
Waterproof
Almost tissue equivalent
Sensitive response
Disadvantages of diamond detectors
Weakly energy dependent - must correct
Dependent on dose rate
Requires pre-irradiation
Sensitive to bias voltage
Expensive
What is luminescence?
Process where a material upon stimulation releases trapped energy in form of UV, visible, IR
What does delay between stimulation and emission determine?
<10^-8s, fluorescence
>10^-8s, phosphoresence
Thermoluminescence vs optical stimulated luminescence
If stimulation is done by heat, TL
If stimulation done by light, OSL
Applications of TLDs and form
In-vivo dosimetry
Relative dosimetry
EBR measurements
Brachy measurements
Can have powder, rods, chip, ribbon etc, small
What happens when TLD irradiated?
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
How is signal read out of TLD?
PMT converts light to signal and amplifies so it can be read out
What is glow curve?
Plot of thermoluminescence intensity vs temperature
Single glow peak: single energy trap depth
What is annealing and why is it done?
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
What are uses of heat in cycle of measurement for TLD?
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
Energy dependence of TLDs
Energy dependent response, significant at lower energies, primarily because TLDs not tissue equivalent
Angular dependence of TLDs
No significant angular dependence
Dose linearity of TLDs
Dose response linear. Can become supralinear at higher doses, influence by type and amount of impurities
How do OSLDs work
Same principle as TLDs but use laser light source instead of heat to release trapped energy
Applications of OSLDs
In-vivo dosimetry
Potentially more versatile than TLDs but not used as widely
Temperature dependence of OSLDs
Virtually none (immediate advantage of TLDs)
Angular dependence of OSLDs
None
Dose linearity of OSLDs
Linear in clinically relevant dose range 0-200cGy
Supralinear at high doses
Dose rate dependence of OSLDs
None
Response sensitivity with accumulated dose of OSLDs
Sensitivity stable for accumulated doses up to 20Gy then begins to drop
Energy (beam quality) dependence of OSLDs
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
Advantages of TLDs
Small sizes - high resolution
Various forms, different clinical applications
Not expensive
Many TLDs can be irradiated in single exposure
Resuable
Disadvantages of TLDs
No instant readout (24 hours)
Signal erased during readout
Fiddly
Energy and dose, temperature dependent, must be calibrated well
Advantages of OSLDs
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
Disadvantage of OSLDs
Some energy dependence, need correction factor
Readout not instantaneous (8 minutes)
Only linear up to about 4Gy
More expensive than TLDs
What is a scintillator and what types can you have?
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
Size and application of scintillators
Small, 1x4mm
Can be used in in-vivo dosimetry, brachy, small field dosimetry, radiosurgery
How do scintillators work?
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
Temperature dependence of scintillators
None at ambient temperatures
Dose rate dependence of scintillators
Independent
Dose linearity of scintillators
Linear in clinical range
Reproducibility of scintillators
Stable response with good reproducibility
What is alanine and how does it work?
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
How does EPR work?
Free radical subjected to EM energy
Gets excited
Excitation energy matches frequency of B field, radical resonates
Amplitude of frequency proportional to dose
Size and applications of alanine
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
Energy dependence of alanine
Weakly energy dependent
Temperature dependence of alanine
Some temperature dependence
Response stability of alanine
Signal fades with time depending on dose, storage implications
What is Frick gel?
A gelatine, agarose or PVA phantom which has Fe2+ ions in Ferrous sulphate solution
True 3D dosimeter
How does Fricke Gel work?
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
Advantages of alanine
Almost tissue equivalent
Large dynamic range up to 1000Gy
Disadvantages of alanine
Weakly energy dependent, needs correction factors
Signal fades with time dependent on dose, environmental and storage factors
Not reusable
Low sensitivity
Uses of water for measurement
Used for reference/baseline measurements
Using water equivalent phantoms
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
Examples of equivalent water phantoms
Perspex
Solid water
Virtual water
Plastic water
Polystyrene
