Ionizing Radiation StandardsInstitute for National Measurement StandardsNational Research Council Canada

IAEA IDOS Symposium, 9‐12 Nov 2010, Vienna, AT

Ionizing Radiation StandardsInstitute for National Measurement StandardsNational Research Council Canada

IAEA IDOS Symposium, 9‐12 Nov 2010, Vienna, AT

Determination of absorbed dose  to water in megavoltage 

electron beams using a  calorimeter‐Fricke hybrid system

Determination of absorbed dose  to water in megavoltage 

electron beams using a  calorimeter‐Fricke hybrid system

Claudiu D. Cojocaru, Gerhard Stucki,  Malcolm R. McEwen and Carl K. Ross

Claudiu D. Cojocaru, Gerhard Stucki,  Malcolm R. McEwen and Carl K. Ross

2

Primary standard for photon beams ‐

water calorimeter

2

wD c T= Δ

Well established for Co-60 and MV photon beams

3

Electron range

Moving to electron beams is not straightforward 

dref

= 0.6⋅R50

– 0.1 (cm)

R50dref

50%

3

4

H + O2

→

HO2

Fe2+

+ HO2

→

Fe3+

+ HO2-

H+

+ HO2-

→

H2

O2

Fe2+

+ H2

O2

→

Fe3+

+ OH + OH-

Fe2+

+ OH →

Fe3+

+ OH-

•

[Fe3+] ∝

absorbed dose DF c = ρ⋅G(Fe3+)⋅DF

Chemical dosimetry: Fricke solution

irradiationFe2+

Fe3+

H2

OO2

High purity aerated watere-

H OH e-aq

H+

OH-

H2

H2

O2

Fricke ferrous sulfate in solution

4

5

Measurement of concentration

•

OD = -log10

(I/I0

) –

optical density or absorbance

–

I is intensity of light of a given wavelength –

usually 303 nm for Fe3+

–

the optical density is measured using a Cary 400 UV-Vis spectrophotometer

• OD ∝

c OD = ε⋅l⋅c

I0 I

→ ΔOD ∝

[Fe3+] ∝

absorbed dose DF

l

5

6

Conversion of solution absorbance to absorbed dose

•

DF

= ΔOD

measured at 303 nm

–

ΔOD:

measured change in optical density (absorbance) of the

Fricke solution at 25°C irradiation and readout temperatures

–

ε⋅G(Fe3+): product of the molar extinction coefficient and the chemical yield of Fe3+

-

determined by comparison with water

calorimetry -

(for 60Co, ε⋅G(Fe3+) = 3.5060 cm2⋅J-1)

–

ρ: density of Fricke solution at 25°C (ρ

= 1.0227×10-3

kg⋅cm-3)

–

l: optical path length of the spectrophotometer cuvette in cm

ΔODε⋅G(Fe3+)⋅ρ⋅

l

6

7

Chemical dosimetry: Fricke solution

Recipe

•

(NH4

)2

Fe(SO4

)2

⋅6H2

O

0.001 mol/L•

H2

SO4

0.4 mol/L•

NaCl

0.001 mol/L

–

only high purity ingredients are used to make the Fricke solution at NRC (~ 2 L at a time)

–

careful handling at all stages is required to obtain accurate results

7

8

Conversion from dose to Fricke solution to dose to water

•

Dw

= DF ⋅

fw,F ⋅

Pwall ⋅

kdd

–

fw,F

: accounts for the difference in the radiation absorption properties of water and Fricke solution (best obtained using Monte Carlo calculations)

–

Pwall

: corrects for the effect of the container wall/holder on the dosimeter response (best obtained using Monte Carlo calculations)

–

kdd

: accounts for dose non-uniformities over the irradiated solution (obtained from measured dose distributions)

8

9

Experiment description

–

bag size approximately 40x40x3 mm with a Fricke solution volume of 4 cm3

–

half needed for readout (the rest is

used for rinsing the pipette and cuvette)

•

Innovative approach -

irradiate the Fricke in sealed polyethylene bags:

–

minimizes any wall correction–

allows one to custom design the dosimeter

shape to the application

quartz cuvettes

9

10

Es (MeV)

4 6 8 10 12 14 16

G(F

e3+),

norm

alis

ed0.985

0.990

0.995

1.000

1.005

1.010

1999-2003 data2007 data

Fricke dosimetry: 4, 8 and 12 MeV

Dw,Fricke

= ΔOD⋅k/(ε⋅G⋅ρ⋅l)

22 MeV -

calorimetry + Fricke18 MeV -

calorimetry + Fricke

12 MeV -

calorimetry (tough) + Fricke8 MeV -

Fricke

4 MeV -

Fricke

ε⋅G}

METAS

e-

beam

dref

10

11

-

standard uncertainty in the gradient is estimated to be 0.2 %

-

average repeatability on a single dosimeter is 1.1 %

18 MeV

Dose (Gy) Dose (Gy)

Calibration curve for Fricke

11

12

Energy dependence in electron beams for ionization chambers

e-

beam e-

beam

Fricke IC

Dw,Fricke

= ΔOD⋅k/(ε⋅G(Fe3+)⋅ρ⋅l) Dw,IC

= ND,w

⋅M= kQ

⋅N60CoD,w

⋅M= kR50

⋅Pgr

⋅N60CoD,w

⋅M= k’R50

⋅kecal

⋅Pgr

⋅N60CoD,w

⋅M

→

k’R50 dependence on beam quality (R50 ) - experimentally

dref dref

12

13

Energy dependence in electron beams for PTW Roos chamber

- data are normalized at the 18 MeV beam point (R50

= 7

cm)-

standard uncertainty in the experimental determination of the

energy dependence (k′R50

) is estimated to be 0.2-0.5 %

13

14

Conclusions

•

A new calorimeter‐Fricke hybrid system has been developed for dosimetry  standards for high energy electron beams

•

Calibration factors for a PTW Roos parallel‐plate ion chamber have been  obtained for 4, 8, 12, 18, 22 MeV electron beams

•

Initial results confirm the recent Monte Carlo and experimental investigations  of ion chamber perturbation corrections in electron beams

•

Future work:

Standard – carry out comparisons with other established standards world  wide

Fricke –

investigate application to other radiation field (e.g., IMRT)

14

15

Gerhard Stucki (guest scientist)

Malcolm McEwen

Carl Ross

David Marchington

15

Acknowledgements

For more information about the IRS Group see http://irs.inms.nrc.ca