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