malcolm mcewen ionizing radiation standards groupsim-metrologia.org.br/presentations/mcewen...
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FUNDAMENTALS OF DOSIMETRY FUNDAMENTALS OF DOSIMETRY
Malcolm McEwenIonizing Radiation Standards Group
WORKING DEFINITION OF DOSIMETRY
Dosimetry is more than the literal definition (“measurement of dose”)
Dosimetry is generally concerned with characterizing the effects of ionizing radiation rather than the properties
Particle properties are of interest – type, energy, fluence but only in relation to how they interact
Dosimetry is really about measuring final states
TYPES OF IONIZING RADIATION
photons – 10 keV to 25 MeVelectrons – 50 keV to 50 MeVprotons – 50 MeV to 250 MeV
neutrons – thermal to 20 MeV
Incident energies
PENETRATION
photonselectronsprotonsneutrons
50 kVp
250 kVp
Steep dose fall-offNeed high kV to achieve reasonable penetration (e.g. for therapy)
photonselectronsprotonsneutrons
0.00
0.20
0.40
0.60
0.80
1.00
0 5 10 15 20d (cm)
dose
(nor
mal
ised
)
6 MV10 MV25 MV
PENETRATION
Much deeper penetration
6 MV most commonly used for radiation therapy
photons electronsprotonsneutrons
PENETRATION
Sharp dose fall-off is very useful where only a certain volume needs irradiation(‘tissue sparing’ in radiation therapy)
photons electronsprotonsneutrons
SOBP
PENETRATION
Single Bragg peak not very useful but Spread Out Bragg Peak gives very uniform dose to significant volume.SOBP requires energy and intensity modulation
SOURCES OF IONIZING RADIATION
x-ray tubes radioactive sources
Cs-137 Co-60Ir-192Sr-90Am-241
van der Graaflinear accelerators cyclotrons
x-ray tubes – 10 keV to 400 keV photons
radioactive sourcesCs-137 Co-60Ir-192Sr-90Am-241
van der Graaf
SOURCES OF IONIZING RADIATION
Oldest established radiation source technology
Wide range of imaging applications
Also commonly used for radiation therapy
radioactive sourcesCs-137 – 633 keV γCo-60 – 1.25 MeV γIr-192 ~ 400 keV γSr-90 ~ 2.5 MeV βAm-241– 60 keV γ, 5.5 MeV α
SOURCES OF IONIZING RADIATION
Ir-192 High DoserateBrachytherapy unit
Therapy-level Co-60 unit at NRC calibration laboratory
Ir-192 spectrum
linear accelerators – 2 MeV to 25 MeV electrons
photon energy (MeV)
0 5 10 15 20
prob
abili
ty
0.0
5.0e-7
1.0e-6
1.5e-6
2.0e-6
6 MV10 MV25 MV
SOURCES OF IONIZING RADIATION
Clinical linear accelerator
Typical bremmstrahlungspectra for a clinical linac
Applications – primarily radiation therapy but also radiation processing
x-ray tubes radioactive sources
Cs-137 Co-60Ir-192Sr-90Am-241
van der Graaf
cyclotrons – 50 MeV to 250 MeV protons
SOURCES OF IONIZING RADIATION
Clinical proton delivery system for radiation therapyLeft – cyclotronAbove – treatment couch and rotating gantry
neutrons - neutrons from reactors
- neutrons from nuclear reactions with charged particles in accelerators
- neutrons from radionuclide sources 1. 241Americium-Beryllium(α,n)
2. 241Americium-Boron(α,n)
3. 252Californium (also moderated)
SOURCES OF IONIZING RADIATION
neutrons - neutrons from reactors
- neutrons from nuclear reactions with charged particles in accelerators
- neutrons from radionuclide sources 1. 241Americium-Beryllium(α,n)
2. 241Americium-Boron(α,n)
3. 252Californium (also moderated)
SOURCES OF IONIZING RADIATION
ENERGY RANGES & QUANTITIES
10-50 keV – low energy x-rays50-300 keV – medium energy x-raysCs-137 & Co-60Co-60Linac photon (x-ray) beams Linac electron beamsProton beamsNeutron beams
Air KermaAir KermaAir Kerma
Absorbed DoseAbsorbed DoseAbsorbed DoseAbsorbed Dose
Dose Equivalent
QUANTITIES - DEFINITIONS
1.Shown for an x-ray beam2.Same basic principle for electrons3.For protons and neutrons you have nuclear collisions and reactions to consider as well
QUANTITIES - DEFINITIONS
Kerma
Kerma: K =dEtr (energy)
dm (mass)
QUANTITIES - DEFINITIONS
Kerma
Kerma: K =dEtr (energy)
dm (mass)
Absorbed Dose: D = dm (mass)dEab (energy)
QUANTITIES - DEFINITIONS
Kerma
Kerma: K =dEtr (energy)
dm (mass)
Absorbed Dose: D = dm (mass)dEab (energy)
Conversion of energy
Deposition of energy
Both quantities have same unitEnergy/mass = J/kg = Gray (Gy)
QUANTITIES - DEFINITIONS
Dose equivalent H*(d)
dose equivalent: product of quality factor, Q, and absorbed dose at point in tissue [unit – Sv]
Type of radiationQuality factor
(Q)
X-, gamma, beta radiation, high-energy electrons
1
Alpha particles, multiple-charged particles, fission fragments and heavy particles 20
Neutrons 10
High-energy protons 10
TYPICAL DOSERATES
Environmental – microGrayImaging - milliGrayTherapy – 1-100 GrayFood irradiation – 5 kGySterilization – 25 kGyIndustrial Processing – 100 kGy
1 Gray (Gy) = 1 J/kgFull-body lethal dose ~ 5 GyBackground dose to general population ~ 1-2 mSvLong-haul flight ~ 0.025-0.05 mSv
i) Doses of interest are small ii) Dose is material dependentiii) The quantity required is the dose in an
undisturbed phantom.iv) The quantity required is the dose at a point in
this phantom.v) Scattered radiation contributes a significant
proportion of the absorbed dose vi) Optimization of the measurement is difficultvii) The charge of the incident radiation can affect
the measurement system
Why is measuring dose difficult?
Example –absorbed dose calorimetry for radiotherapy
Simple to define:
Dm = cm ΔT
1. Measure a radiation-induced temperature rise.2. Apply the specific heat capacity for the material
in question.
D = cΔT
ΔT will depend on the material but for radiotherapy dosimetry it’s always small:
Dose = 2 Gy ΔT (water) = 0.5 mK(radiotherapy) ΔT (graphite) = 2.9 mK
Our target uncertainty for ΔT is 0.1%, which means sub-μK precision.Further constraint - operation around room temperature is required
D = cΔTWe’re measuring a temperature rise due to the energy absorbed from the radiation beam. We therefore need a very stable background against which we can measure this temperature rise.
D = cΔT
Two optionsPassive temperature control (thermal isolation)Active temperature control (feedback system)
D = c ΔT
What is used for the value of the specific heat capacity depends on the calorimeter design.
3 main approaches:1. Apply a value from tables – certain materials (e.g.
water) have a well known value of c2. Measure c for a sample of the material used in the
calorimeter3. Evaluate an effective value of c for the complete
calorimeter in situ
Other things to consider
Conversion from one material to anotherPerturbation correctionsRadiochemistryBeam uniformity correction (volume averaging)
Specifics - the NRC water calorimeter
Water calorimetry – the big problems1. Convection 2. Radiochemistry3. Containment
Water calorimetry – the solutions
1. Operate at 4 °C 2. High purity water, known composition of dissolved
gases3. Careful design coupled with detailed thermal
modelling
Specifics - the NRC water calorimeter
Glass vessel filled with high-purity water (known dissolved gases)2 thermistors measure radiation-induced temperature rise
The NRC water calorimeter
Calorimeter vessel sits in full-scatter phantomOuter box controls temperature at 4 °C
The NRC water calorimeterSystem is physically large but can be moved between facilities within laboratory (e.g., linac and Co-60)
• Dosimetry is the measurement of the result of a radiation beam interacting with matter
• Absorbed dose is material dependent• Unit of absorbed dose is the Gray (Gy = J/kg)• Radiation beams – photons, electrons, protons,
neutrons (10 keV – 100 MeV)• Doses of interest: 1 mGy – 100 kGy• Dosimetry is a challenging area of metrology –
mature but with opportunities develop the science!
SUMMARY
THANK YOU