primary standards of airprimary standards of air kerma for 60co

PRIMARY STANDARDS of AIRPRIMARY STANDARDS of AIRPRIMARY STANDARDS of AIR KERMA for 60CO and X-RAYS

PRIMARY STANDARDS of AIR KERMA for 60CO and X-RAYS

& ABSORBED DOSE in PHOTON

& ABSORBED DOSE in PHOTONABSORBED DOSE in PHOTON

and ELECTRON BEAMSABSORBED DOSE in PHOTON

and ELECTRON BEAMSMalcolm McEwen

Definitions:

Standard – “instrument/measurement/artifact intended to define, realize, conserve or reproduce a

it tit t f ”unit or quantity to serve as a reference”

Primary standard – “standard that is designated or widely acknowledged as having the highestwidely acknowledged as having the highest metrological qualities and whose value is accepted without reference to other standards of same quantity”

N ti l t d d “ d d i d bNational standard – “standard recognized by a national decision to serve as the basis for assigning values to other standards of the quantity concerned” q y

Secondary standard – “standard whose value is assigned by comparison with a primary standard of the same quantity”same quantity

So that’s clear?

“I shall not attempt today to further define [it]……….but I s a o a e p oday o u e de e [ ] buknow it when I see it.”

Justice Potter Stewart, 1964

ENERGY RANGES & QUANTITIES

10-50 keV – low energy x-rays50-300 keV – medium energy x-rays

Air KermaAir Kerma50 300 keV medium energy x rays

Cs-137 & Co-60Co-60

Air KermaAir Kerma

Absorbed DoseLinac photon (x-ray) beams Linac electron beams

Absorbed DoseAbsorbed Dose

1. kV x-rays – air kerma1. kV x rays air kerma

The primary standard for 10-300 kV x-rays is the F Ai Ch bFree Air Chamber

kV x-rays – air kermakV x rays air kerma

The primary standard for 10-300 kV x-rays is the F Ai Ch bFree Air Chamber

g-11

eW

mQ

g-11

eW X = K

air

airair

W/e = 33.97 eV

mair ?

kV x-rays – air kermakV x rays air kerma

Vair = AL The air volume is air

defined mechanically

Corrections correctionsCorrections, corrections …..

1WQ ionpolhumescatt

airair

airair PPKKKK

g-11

eW

VQ = K

• Katt - attenuation of the primary X-ray beam between the aperture and collecting volume

• Ksc - the extra ionization collected from electrons produced by photons scattered within the chamber

• Ke - ionization lost when electrons strike the collecting electrode.

An aside on HumidityAn aside on Humidity

1.0035

We are all familiar with correcting readings for temperature and pressure (forget them here at your peril)

1.0025

1.0030

your peril)

Humidity, however, is generally ignored, but for kVKhum

1.0015

1.0020

Humidity, however, is generally ignored, but for kV primary standards it is an important correction

1.0005

1.0010

50 kVC 60

0 20 40 60 80 1000.9995

1.0000Co-60

Relative Humidity (%)

UncertaintiesUncertainties

Medium Energy Free-Air Chamber Standard Uncertainty

(MEES) Type A Type B

Ionization Current 0.03 0.03

Volume 0.01 0.04

Positioning 0.02 0.01

Correction Factors (excl. kh) 0.02 0.2

Humidity kh 0.01 0.03y

Physical Constants - 0.15

Air-Kerma Rate 0.044 0.257

Combined Uncertainty 0 26Combined Uncertainty 0.26

Alt ti d i f f i h bAlternative designs of free-air chamber1. Attix design

Designed by Attix in 1961

Extensible length of chamber• No guarding-electrode

system• No voltage dividerNo voltage divider• Eliminates stringent power

supply requirementsE lti f fi ld• Errors resulting from field-nonuniformity are eliminated

Atti h b ti i i lAttix chamber operating principle

Front and back move

Alt ti d i f f i h bAlternative designs of free-air chamber2. PTB design

PTB – Physikalisch-Technische Bundesanstalt

First developed in the 1920sp

Coaxial cylindrical design • Smaller for the same

photon energy than standard design

• Potentially transportable• Potentially transportable

Si d ttSize does matter….

Secondary electrons should not reach electrodesreach electrodes

50 kV » 70 mm250 kV » 350 mmCo-60???

2. Cs-137 & Co-60 – air kerma2. Cs 137 & Co 60 air kerma

The primary standard for Cs-137 & Co-60 gamma rays is the Cavity Chamberrays is the Cavity Chamber

Cavity theory before 9 am is a cruel and unusual punishment

O i th h l lOverview - measure the charge, apply a large number of corrections and factors from tablesDetail - consult your textbook

What is required?What is required?

ilhtttenair

air PPKKKKKL1WQ=K

• A h b i h ll

ionpolhumstemanscattgairairgairair

air PPKKKKKg-1

eV

K

,,

• A chamber with a very well defined volume

• W/e & L• W/e & Lg/air

• Monte Carlo

• A lot of time

3. Co-60 – absorbed dose3. Co 60 absorbed dose

An extension of the cavity chamber to determine absorbed dose is possiblep

Beam Q/mair Dair

(W/e) i

Beam Q/mair Dair

(W/e) i

Prepl

(W/e)air

Prepl

(W/e)air

Dg Dw

(L/ρ)g,air

Dg Dw

(L/ρ)g,air

genwen ///

gwgw ,,

genwen ///

gwgw ,,

Uncertainties

Extrapolation ChambersExtrapolation Chambers

• The biggest problem with cavity chambers is the determination of the effective volume of the chamber• Mechanical measurements are the only accurate method but it’s difficult to be sure once the chamber is sealed up• Extrapolation chambers offer an alternative:

1SWQ airairmed,

med A1 S

eW

xQ = D

A volume determination is replaced by an area determination plus a differential length measurementplus a differential length measurement

Extrapolation ChambersExtrapolation Chambers

But…But…

1SWQ=D

The A∆x determination turns out to be not that much

airairmed,med A

ex

=D

• The A∆x determination turns out to be not that much easier than a measurement of volume• The value of S/ρ isn’t very well known except for graphite/ρ y p g p• Long term stability is not likely to be as good as for a fixed-volume chamber (and standards labs love stability).

Extrapolation chambers are used, but t f t l bnot for external beams

4. Linac beams – absorbed dose4. Linac beams absorbed dose

• There is a very obvious change when we move y gto linac beams. • The standards no longer look all the same• Cavity chambers may have different geometries but they’re basically identical

The primary standard for absorbed dose in linac photon beams is the Calorimeterphoton beams is the Calorimeter

Aside – cavity theoryAside cavity theory

• Technically you can use cavity theory to derive absorbed d f hi h bdose for high-energy x-ray beams. • You need a very thick-walled chamber or a very large build-up cap.build up cap.

Tungsten-walled chambers have been builtBuild-up caps begin to look like phantoms

• However, you still don’t have the ‘unique’ situation of Co-60 of accurately knowing the values of W/e and Lg/air. • An MV cavity standard can therefore only be viewed as a• An MV cavity standard can therefore only be viewed as a secondary device.

Absorbed dose calorimetryAbsorbed dose calorimetry

Simple to define, a lifetime (well almost) to realize

Dm = cm ∆T

1. Measure a radiation-induced temperature rise.2 l h f h f h l2. Apply the specific heat capacity for the material

in question.

What’s the problem? It’s not cavity theory!

Wh i i b l t d i diffi lt?

i) Doses of interest are small

Why is measuring absolute dose is difficult?

i) Doses of interest are small ii) The quantity required is the dose in an

undisturbed phantom.iii) Th tit i d i th d t i t iiii) The quantity required is the dose at a point in

this phantom.iv) Scattered radiation contributes a significant

proportion of the absorbed dose v) Optimization of the measurement is difficultvi) Dose is material dependentvi) Dose is material dependent

Absorbed dose calorimeter the basic components

R2R2R2

Absorbed dose calorimeter – the basic components

Vout

R4

R3

2Rth+wire

Vout

R4

R3

2Rth+wire

Vout

R4

R3

2Rth+wire

3 0 10-6Vmthermistor

bridge circuit

3 0 10-63 0 10-6VmVmthermistor

bridge circuit

1.5 10-6

2.0 10-6

2.5 10-6

3.0 10

V out(V

)calorimeter phantom

bridge output signal

1.5 10-6

2.0 10-6

2.5 10-6

3.0 10

V out(V

)1.5 10-6

2.0 10-6

2.5 10-6

3.0 10

V out(V

)calorimeter phantom

bridge output signal

0.0 10-7

5.0 10-7

1.0 10-6

-200 -100 0 100 200

isolation0.0 10-7

5.0 10-7

1.0 10-6

-200 -100 0 100 2000.0 10-7

5.0 10-7

1.0 10-6

-200 -100 0 100 200

isolation

Time (s)Time (s)Time (s)

D = cΔT

ΔT will depend on the material but for radiotherapy dosimetry it’s always small:

Dose = 2 Gy ΔT (water) = 0.5 mKΔT (graphite) = 2.9 mK

Our target uncertainty for ΔT is 0.1%, which means sub-μK precision.sub μK precision.This is why you don’t often find calorimeters in the clinic.

D = cΔTWe’re measuring a temperature rise due to the energyWe re measuring a temperature rise due to the energy absorbed from the radiation beam. We therefore need a very stable background against

hi h thi t t iwhich we can measure this temperature rise.

D = cΔT

Two optionsPassive temperature control (thermal isolation)(thermal isolation)Active temperature control (feedback system)

D = c ∆T

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

calorimeter3 Evaluate an effective value of c for the complete3. Evaluate an effective value of c for the complete

calorimeter in situ

Corrections correctionsCorrections, corrections …..

Things are never that simple:

Dm = cm ∆T Πki

Correction factors are very dependent on the specific calorimeter design (and there are lots) but may ca o ete des g (a d t e e a e ots) but ayinclude:

Perturbation correctionsPerturbation correctionsConversion from one material to anotherBeam uniformity correction (dose averaging)Radiochemistry

The Calorimeter GalleryThe Calorimeter Gallery

A Domen type graphite calorimeterA. Domen-type graphite calorimeter

The calorimetric equivalent of the Farmer chamber

UncertaintiesUncertainties

This is dose to graphite

Dose conversionDose conversion

Utilises the photon-fluence scaling theorem

Requires knowledge of virtual source

i i dposition and corrections for:

ScatterAttenuationAttenuationPair production

Of M t C l if tOf, course, you can use Monte Carlo, if you must.

Water/graphite scalingWater/graphite scaling

Overall uncertainty in Dw ~ 0.5%

B Other graphite calorimetersB. Other graphite calorimeters

Calorimetrists can be quite creative……

“The remarkably simple”

UK, circa 1989,

“The impressivelyThe impressively complex”

France, 2007

“The carefully optimized”

BIPM, 2009

C. Water calorimeter development

Water calorimetry – the big problemsWater calorimetry the big problems

1. Convection 2 R di h i2. 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 thermal3. Careful design coupled with detailed thermal

modelling

D. The NRC water calorimeterCombines designs of Domen and Seuntjens

UncertaintiesUncertaintiesSource and type of uncertainty (in %) 60Co 10 MVType A

Reproducibility dT/MU 0.08 0.15Monitor reproducibility 0 02 0 12Monitor reproducibility 0.02 0.12

Type Bcw,p (specific heat capacity) < 0.005 < 0.005Thermistor sensitivity 0.08 0.08kc (heat loss) 0.10 0.10c ( )kp (vessel perturbation) 0.05 0.05kHD (heat defect) 0.15 0.15kdd (profile non-uniformity) 0.01 0.04Positioning calorimeter, probes and vessel 0.13 0.13g , p

Pdd 0.03 0.04Pion 0.07 0.08Ppol 0.01 0.04P 0 05 0 05PTP 0.05 0.05Humidity 0.06 0.06Positioning chamber 0.06 0.06

Overall ND,w 0.28 0.35

Not a single MC-derived factor

Size matters (II)Size matters (II)

The NRC water calorimeter is big and heavy:

85 cm cube, 50 kg, g

Moving it within the lab is bad enough

Can we make something smaller?

Size matters (II)Size matters (II)

What about electron beams?What about electron beams?

Standards for electron beam dosimetry have lagged y ggbehind photon standards:

1974 – NIST graphite calorimeter1990 NPL th l l hit l i t1990s – NPL therapy-level graphite calorimeter

1988 – first calibration service for linac photon beams1988 first calibration service for linac photon beams1998 – first service for electron beams

Very few institutions presently involved

Problem #1Electron range

Problem #2Net charge

Electron beam calorimetryElectron beam calorimetry

As for photon beams both graphite and water p g pcalorimeters are in use at primary laboratoriesWater calorimetry is particularly tricky due to

t b ti i f t i t l l d ithperturbation issues of containment vessel coupled with electron rangeBut… it has been done, and at a university!But… it has been done, and at a university!

McGill University Water CalorimeterMcGill University Water CalorimeterStewart, Seuntjens, et al

Pancake-type vessel and vertical beam geometry allow measurements down to 6 MeVmeasurements down to 6 MeV

5. Non-calorimetric standards for MV beams5. Non calorimetric standards for MV beams

What is often forgotten in the discussion of standards is that the radiation facilities available can be crucial to the development and use of the detectorsof the detectors.

METAS (the Swiss Primary Laboratory)METAS (the Swiss Primary Laboratory)

Scanditronix MM22 Microtron

METAS (the Swiss Primary Laboratory)METAS (the Swiss Primary Laboratory)

A primary standard based on the Fricke dosimeterA primary standard based on the Fricke dosimeter

G-value determinationG value determination

1.015

sed 1.005

1.010

e3+),

norm

ali s

1.000

G(F

e

0.990

0.995

1999 2000 data

4 6 8 10 12 14 160.985

1999-2000 data2007 data

Es (MeV)

Comparison of standardsComparison of standards

1. kV x-rays – air kerma

1.008

1.010

1. kV x rays air kerma

e to

BIP

M

1.002

1.004

1.006

erm

a, re

lativ

e

0.998

1.000

Air

ke

0.992

0.994

0.996

tralia

ands

olan

d

UK

Italy

ngar

y

nada

ussi

a

man

y

Chi

na

ustri

a

USA

ance

0.990

2. kQ measurements in photon beams2. kQ measurements in photon beams

1.010 Seuntjens et alElekta

0 990

1.000ElektaTG-51 valuesQuadratic fit

0.980

0.990

k Q

0.960

0.970

k

0.950

0.94055 65 75 85 95%dd10x

3. kQ measurements in electron beams3. kQ measurements in electron beams

1 08

1.10

TG-51 k'R50

TRS 398 k

pend

ence

1.06

1.08 TRS-398 kQ,Qint

NPL graphite calorimeterMETAS FrickeMcGill water calorimeterNRC water calorimeter

e en

ergy

dep

1.04

Rel

ativ

e

1.00

1.02

0 2 4 6 8 10 120.98

R50 (cm)

AcknowledgementsAcknowledgementsc o edge e tsc o edge e tsGerhard Stucki – METAS

Kristin Stewart Jan Seuntjens McGill UniversityKristin Stewart, Jan Seuntjens – McGill University

David Burns – BIPM

Carl Ross, John McCaffrey, Hong Shen – NRCCarl Ross, John McCaffrey, Hong Shen NRC

Simon Duane, Alan DuSautoy, Hugo Palmans – NPL

Josian Daures – LNHB

primary standards of airprimary standards of air kerma for 60co

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