Presentation M. McEwen

FUNDAMENTALS
OF DOSIMETRY
Malcolm McEwen
Ionizing Radiation Standards Group
WORKING DEFINITION OF DOSIMETRY
¾ Dosimetry is more than the literal definition
(“measurement of dose”)
¾ Dosimetry is generally concerned with
characterizing the effects of ionizing radiation
rather than the properties
¾ Particle properties are of interest – type, energy,
fluence but only in relation to how they interact
¾ Dosimetry is really about measuring final states
TYPES OF IONIZING RADIATION
Incident energies
photons
electrons
protons
neutrons
– 10 keV to 25 MeV
– 50 keV to 50 MeV
– 50 MeV to 250 MeV
– thermal to 20 MeV
PENETRATION
50 kVp
photons
electrons
protons
neutrons
250 kVp
Steep dose fall-off
Need high kV to achieve
reasonable penetration
(e.g. for therapy)
PENETRATION
1.00
0.80
dose (norm alised)
photons
electrons
protons
neutrons
Much deeper penetration
6 MV most commonly
used for radiation
therapy
0.60
6 MV
10 MV
25 MV
0.40
0.20
0.00
0
5
10
d (cm)
15
20
PENETRATION
photons
electrons
protons
neutrons
Sharp dose fall-off is
very useful where only
a certain volume needs
irradiation
(‘tissue sparing’ in
radiation therapy)
PENETRATION
photons
electrons
protons
neutrons
Single Bragg peak not very
useful but Spread Out Bragg
Peak gives very uniform
dose to significant volume.
SOBP requires energy and
intensity modulation
SOBP
SOURCES OF IONIZING RADIATION
x-ray tubes
radioactive sources
Cs-137
Co-60
Ir-192
Sr-90
Am-241
van der Graaf
linear accelerators
cyclotrons
SOURCES OF IONIZING RADIATION
x-ray tubes
radioactive sources
– 10 keV to 400 keV photons
Cs-137
Co-60
Ir-192
Sr-90
Am-241
van der Graaf
Oldest established radiation source technology
Wide range of imaging applications
Also commonly used for radiation therapy
SOURCES OF IONIZING RADIATION
radioactive sources
Cs-137 – 633 keV γ
Co-60 – 1.25 MeV γ
Ir-192 ~ 400 keV γ
Sr-90 ~ 2.5 MeV β
Am-241– 60 keV γ, 5.5 MeV α
Therapy-level Co-60
unit at NRC calibration
laboratory
Ir-192
spectrum
Ir-192 High Doserate
Brachytherapy unit
SOURCES OF IONIZING RADIATION
Clinical linear accelerator
2.0e-6
6 MV
10 MV
25 MV
probability
1.5e-6
linear accelerators
Applications – primarily
radiation therapy but
also radiation processing
Typical bremmstrahlung
spectra for a clinical linac
1.0e-6
– 25.0e-7
MeV to 25 MeV electrons
0.0
0
5
10
photon energy (MeV)
15
20
SOURCES OF IONIZING RADIATION
x-ray tubes
radioactive sources
Cs-137
Co-60
Ir-192
Sr-90
Am-241
van der Graaf
cyclotrons
Clinical proton delivery system for radiation
therapy
Left – cyclotron
Above – treatment couch and rotating gantry
– 50 MeV to 250 MeV protons
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
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)
ENERGY RANGES & QUANTITIES
10-50 keV – low energy x-rays
50-300 keV – medium energy x-rays
Cs-137 & Co-60
Co-60
Linac photon (x-ray) beams
Linac electron beams
Proton beams
Neutron beams
Air Kerma
Air Kerma
Air Kerma
Absorbed Dose
Absorbed Dose
Absorbed Dose
Absorbed Dose
Dose Equivalent
QUANTITIES - DEFINITIONS
1.Shown for an x-ray beam
2.Same basic principle for
electrons
3.For protons and neutrons you
have nuclear collisions and
reactions to consider as well
QUANTITIES - DEFINITIONS
dEtr (energy)
Kerma: K =
dm (mass)
Kerma
QUANTITIES - DEFINITIONS
dEtr (energy)
Kerma: K =
dm (mass)
Kerma
dEab (energy)
Absorbed Dose: D = dm (mass)
QUANTITIES - DEFINITIONS
Conversion of energy
dEtr (energy)
Kerma: K =
dm (mass)
Kerma
Deposition of energy
dEab (energy)
Absorbed Dose: D = dm (mass)
Both quantities have same unit
Energy/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 radiation
X-, gamma, beta radiation, highenergy electrons
Quality factor
(Q)
1
Alpha particles, multiple-charged
particles, fission fragments and heavy
particles
20
Neutrons
10
High-energy protons
10
TYPICAL DOSERATES
Environmental – microGray
Imaging - milliGray
Therapy – 1-100 Gray
Food irradiation – 5 kGy
Sterilization – 25 kGy
Industrial Processing – 100 kGy
1 Gray (Gy) = 1 J/kg
Full-body lethal dose ~ 5 Gy
Background dose to general population ~ 1-2 mSv
Long-haul flight ~ 0.025-0.05 mSv
Why is measuring dose difficult?
i)
ii)
iii)
iv)
v)
vi)
vii)
Doses of interest are small
Dose is material dependent
The quantity required is the dose in an
undisturbed phantom.
The quantity required is the dose at a point in
this phantom.
Scattered radiation contributes a significant
proportion of the absorbed dose
Optimization of the measurement is difficult
The charge of the incident radiation can affect
the measurement system
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 = c ΔT
ΔT will depend on the material but for radiotherapy
dosimetry it’s always small:
Dose = 2 Gy
(radiotherapy)
ΔT (water) =
ΔT (graphite) =
0.5 mK
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 = c ΔT
We’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 = c ΔT
Two options
Passive 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 c
2. Measure c for a sample of the material used in the
calorimeter
3. Evaluate an effective value of c for the complete
calorimeter in situ
Other things to consider
¾ Conversion from one material to another
¾ Perturbation corrections
¾ Radiochemistry
¾ Beam uniformity correction (volume averaging)
Specifics - the NRC water calorimeter
Water calorimetry – the big problems
1. Convection
2. Radiochemistry
3. Containment
Water calorimetry – the solutions
1. Operate at 4 °C
2. High purity water, known composition of dissolved
gases
3. 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 phantom
Outer box controls temperature at 4 °C
The NRC water calorimeter
System is physically large
but can be moved
between facilities within
laboratory
(e.g., linac and Co-60)
SUMMARY
•
•
•
•
•
•
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!
THANK YOU