NACP-02 perturbation correction factors for the NPL primary

standard of absorbed dose to water in high energy electron beams

NACPNACP--02 perturbation correction 02 perturbation correction factors for the NPL primary factors for the NPL primary

standard of absorbed dose to water standard of absorbed dose to water in high energy electron beamsin high energy electron beams

Paris 9 - 11, May 2007Paris 9 Paris 9 -- 11, May 200711, May 2007

E. Chin1 , J. Seuntjens1, H. Palmans2, A. DuSautoy2, D. Shipley2, M. Bailey2,

F. Verhaegen1

E. ChinE. Chin11 , J. Seuntjens, J. Seuntjens11, H. Palmans, H. Palmans22, A. , A. DuSautoyDuSautoy22, D. Shipley, D. Shipley22, M. Bailey, M. Bailey22, ,

F. VerhaegenF. Verhaegen1 1

111 222

OutlineOutlineOutline

1. Calibration procedure at NPL2. Model of ion chamber for MC simulations3. Validation of MC model with backscatter

simulations and measurements4. Perturbation correction factors in water5. Perturbation correction factors in graphite6. Implications for the NPL electron beam

calibration

1. Calibration procedure at NPL2. Model of ion chamber for MC simulations3. Validation of MC model with backscatter

simulations and measurements4. Perturbation correction factors in water5. Perturbation correction factors in graphite6. Implications for the NPL electron beam

calibration

NPL Calibration Procedure: high energy electrons

NPL Calibration Procedure: high NPL Calibration Procedure: high energy electronsenergy electrons

(1) Define reference depth in water(1) Define reference depth in water

cmRd ww 1.06.0 ,50 −=

(2) Use range scaling to get depth in graphite(2) Use range scaling to get depth in graphite

(3) Calibrate chamber against the calorimeter, in graphite, at the NPL

(3) Calibrate chamber against the calorimeter, in graphite, at the NPL

wgwg RRdd ,50,50=

gref

ggrefD M

DN

,,, =

(McEwen et al 1998)(McEwen (McEwen et al et al 1998)1998)

Calorimeter for high energy electronsCalorimeter for high energy electrons

NPL Calibration Procedure: high energy electrons

NPL Calibration Procedure: high NPL Calibration Procedure: high energy electronsenergy electrons

(4) Theoretical conversion of graphite to water(4) Theoretical conversion of graphite to water

airg

airw

gref

wrefgrefDwrefD s

spp

NN,

,

,

,,,,, =

(5) Compare user and reference chambers at dwin water, at NPL

(5) Compare user and reference chambers at dwin water, at NPL

wuser

wrefwrefDwuserD M

MNN

,

,,,,, =

NPL Calibration Procedure: high energy electrons

NPL Calibration Procedure: high NPL Calibration Procedure: high energy electronsenergy electrons

(McEwen et al 1998)(McEwen (McEwen et al et al 1998)1998)

wallcavQ ppp =

1=cavp

1=wallp

A plane parallel chamber with adequately large guard ring can eliminate the in-scattering effects

A plane parallel chamber with adequately A plane parallel chamber with adequately large guard ring can eliminate the inlarge guard ring can eliminate the in--scattering effectsscattering effects

Scarce amount of data available at the time and large uncertaintiesScarce amount of data available at the time Scarce amount of data available at the time and large uncertaintiesand large uncertainties

Current Protocols: electron perturbation correction factors

Current Protocols: electron Current Protocols: electron perturbation correction factorsperturbation correction factors

For well guarded plane parallel plate ion chambersFor well guarded plane parallel plate ion chambers

Monte Carlo model of NACP ion chamber

Monte Carlo model of NACP Monte Carlo model of NACP ion chamberion chamber

NACP-02 plane parallel ion chamber (NPL report CIRM13)NACPNACP--02 plane parallel ion chamber (NPL report CIRM13)02 plane parallel ion chamber (NPL report CIRM13)

Monte Carlo model of NACP ion chamber

Monte Carlo model of NACP Monte Carlo model of NACP ion chamberion chamber

Monte Carlo model of NACP ion chamber

Monte Carlo model of NACP Monte Carlo model of NACP ion chamberion chamber

rexoliterexoliterexolite mylarmylarmylargraphitegraphitegraphite airairair

MC NACP model for DOSRZnrc and CAVRZnrc (not to scale)MC NACP model for DOSRZnrc and CAVRZnrc (not to scale)MC NACP model for DOSRZnrc and CAVRZnrc (not to scale)

Primary collimator (CONESTACK)

Window (SLABS)

Upper foil (SLABS)

Lower foil (FLATFILT)

Monitor Chamber (SLABS)

Mirror (MIRROR)

Shield (CONESTACK)

Upper and lower jaws (JAWS)

Reticle (SLABS)

Applicator (APPLICAT)

Primary electron beam

Monte Carlo model of linacsMonte Carlo model of linacsMonte Carlo model of linacs

Scattering foil (SLAB)

Monitor ion chamber (CONS3R)

Lead and Steel collimator

(CIRCAPP)

Primary electron beam

NPL Linac (SSD 2m)NPL Linac (SSD 2m)NPL Linac (SSD 2m)

Varian Linac (SSD 1m)Varian Linac (SSD 1m)Varian Linac (SSD 1m)

a) CL2300 energies 6, 9, 12, 15, 18 MeV (tuned within 1.5%)b) CL21A energy 4MeV (buildup tuned within 3%, tail within 2%)c) NPL linac energies 4, 6, 8, 10, 12, 16, 19MeV (R. Zakikhani)

a)a) CL2300 energies 6, 9, 12, 15, 18 MeV (tuned within 1.5%)CL2300 energies 6, 9, 12, 15, 18 MeV (tuned within 1.5%)b)b) CL21A energy 4MeV (buildup tuned within 3%, tail within 2%)CL21A energy 4MeV (buildup tuned within 3%, tail within 2%)c)c) NPL linac energies 4, 6, 8, 10, 12, 16, 19MeV (R. Zakikhani)NPL linac energies 4, 6, 8, 10, 12, 16, 19MeV (R. Zakikhani)

Linac electron energiesLinac electron energiesLinac electron energies

6MeV PDD CL2300

0

20

40

60

80

100

120

0 0.5 1 1.5 2 2.5 3 3.5

cm

PDD measured

MC 6.85MeV

4MeV PDD CL21A

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2cm

pdd Measured

MC 4.12MeV

Backscatter factors wrt air for NACP-02:

• Water• Graphite• Aluminum• Copper

Phantom material:• PMMA (4-12 MeV)• Solid water (15 &

18MeV)

Backscatter factors wrt air Backscatter factors wrt air for NACPfor NACP--02:02:

•• WaterWater•• GraphiteGraphite•• AluminumAluminum•• CopperCopper

Phantom material:Phantom material:•• PMMA (4PMMA (4--12 MeV)12 MeV)•• Solid water (15 & Solid water (15 &

18MeV)18MeV)

Validation of MC: Backscatter experiments and simulationsValidation of MC: Backscatter Validation of MC: Backscatter experiments and simulationsexperiments and simulations

NACP

Phantom

Backscatterplate

dmax

Electron Beam

Applicator

t

experimental setupexperimental setupexperimental setup

Backscatter Experimental SetupBackscatter Experimental SetupBackscatter Experimental Setup

NACPNACP--02 chamber is flush 02 chamber is flush with phantom surfacewith phantom surface

Backscatter setup with aluminum plate covering NACPBackscatter setup with aluminum plate covering NACP--0202

Backscatter Experimental SetupBackscatter Experimental SetupBackscatter Experimental Setup

Backscatter setup with water phantom on top of NACPBackscatter setup with water phantom on top of NACP--0202

Backscatter Experimental SetupBackscatter Experimental SetupBackscatter Experimental Setup

1. Varied graphite density (1.7 –1.8 g/cm3)

2. All options turned on (bound compton scattering, PE angular sampling, Rayleigh scattering, atomic relaxation)

3. Different beam sources (pt src vs. parallel src)

4. Varied window thickness

1.1. Varied graphite density (1.7 Varied graphite density (1.7 ––1.8 g/cm1.8 g/cm33))

2.2. All options turned on (bound All options turned on (bound compton scattering, PE angular compton scattering, PE angular sampling, Rayleigh scattering, sampling, Rayleigh scattering, atomic relaxation)atomic relaxation)

3.3. Different beam sources (pt src Different beam sources (pt src vs. parallel src)vs. parallel src)

4.4. Varied window thicknessVaried window thicknessNACPNACP--02 chamber02 chamber

Tuning NACP-02 parametersTuning NACPTuning NACP--02 parameters02 parameters

CL2300 water BSF

1

1.01

1.02

1.03

1.04

1.05

0 0.5 1 1.5 2 2.5

thickness (cm)

BSF

measuredMonte Carlo

6MeV

12MeV

18MeV

CL2300 water BSF

0.990

1.000

1.010

1.020

1.030

1.040

1.050

0 0.5 1 1.5 2 2.5

thickness (cm)

BSF measured

Monte Carlo

9MeV

15MeV

Backscatter Results: waterBackscatter Results: waterBackscatter Results: water

Backscatter Results: graphiteBackscatter Results: graphiteBackscatter Results: graphiteCL2300 graphite BSF

1

1.01

1.02

1.03

1.04

1.05

0 0.5 1 1.5 2

thickness (cm)

BSF measured

Monte Carlo

6MeV

12MeV18MeV

CL2300 graphite BSF

1

1.01

1.02

1.03

1.04

1.05

0 0.5 1 1.5 2

thickness (cm)

BSF measured

Monte Carlo

9MeV15MeV

Backscatter Results: aluminumBackscatter Results: aluminumBackscatter Results: aluminumCL2300 aluminum BSF

11.021.041.061.08

1.11.12

0 0.2 0.4 0.6 0.8

thickness (cm)

BSF

measured

Monte Carlo

6MeV12MeV

18MeV

CL2300 aluminum BSF

0.981

1.021.041.061.08

1.11.12

0 0.2 0.4 0.6 0.8 1

thickness (cm)

BSF

measured

Monte Carlo

9MeV

15MeV

Backscatter Results: copperBackscatter Results: copperBackscatter Results: copperCL2300 copper BSF

1

1.05

1.1

1.15

1.2

1.25

1.3

0 0.05 0.1 0.15 0.2 0.25 0.3

thickness (cm)

BSF measured

Monte Carlo

6MeV12MeV

18MeV

CL2300 copper BSF

1

1.05

1.1

1.15

1.2

1.25

0 0.05 0.1 0.15 0.2 0.25 0.3

thickness (cm)

BSF measured

Monte Carlo

9MeV

15MeV

Backscatter ResultsBackscatter ResultsBackscatter Results

• Monte Carlo model based on manufacturer’s specs resulted in BSF that were systematically 1-2% greater than measured

• Making the front window of the NACP chamber slightly thicker improved the match between measured and simulated BSF

• Conclude that tuning the chamber model is an important step in the calculation of chamber perturbation correction factors.

•• Monte Carlo model based on manufacturerMonte Carlo model based on manufacturer’’s s specs resulted in BSF that were systematically specs resulted in BSF that were systematically 11--2% greater than measured2% greater than measured

•• Making the front window of the NACP Making the front window of the NACP chamber slightly thicker improved the match chamber slightly thicker improved the match between measured and simulated BSFbetween measured and simulated BSF

•• Conclude that tuning the chamber model is an Conclude that tuning the chamber model is an important step in the calculation of chamber important step in the calculation of chamber perturbation correction factors.perturbation correction factors.

Calculating Electron Perturbations Correction Factors: in water

Calculating Electron Perturbations Calculating Electron Perturbations Correction Factors: in waterCorrection Factors: in water

b

aairwcav D

Dsp =× )( ,

c

bwall D

Dp =

c

aairwQ D

Dsp =× )( ,

(Verhaegen (Verhaegen et al et al 2006)2006)

Electron Perturbation Correction factors: water dref NPL

Electron Perturbation Correction Electron Perturbation Correction factors: water dfactors: water drefref NPLNPL

NACP chamber (Verhaegen et al 2006)NACP chamber (Verhaegen NACP chamber (Verhaegen et alet al 2006)2006)

pwall :• > 1 by 2.3% for 4MeV • > 1 by ~1% for other energies

pcav:• < 1 by ~1% for all energies

pQ : • > 1 by (1.5% for 4MeV, 0.4% for 19MeV)

ppwallwall ::•• > 1 by 2.3% for 4MeV > 1 by 2.3% for 4MeV •• > 1 by ~1% for other energies> 1 by ~1% for other energies

ppcavcav::•• < 1 by ~1% for all energies< 1 by ~1% for all energies

ppQQ : : •• > 1 by (1.5% for 4MeV, 0.4% for > 1 by (1.5% for 4MeV, 0.4% for 19MeV)19MeV)

0.98

0.99

1.00

1.01

1.02

1.03

0 1 2 3 4 5 6 7

R50 (cm)

Per

turb

atio

n Fa

ctpcav

pwall

pQ

NPL

pcav, Ma-Nahum

(a)

Electron Perturbation Correction factors: water dref CL2300

Electron Perturbation Correction Electron Perturbation Correction factors: water dfactors: water drefref CL2300CL2300

NACP chamber (Verhaegen NACP chamber (Verhaegen et alet al 2006)2006)

pwall:• > 1• greatest for 6MeV (1.014)

pcav:• < 1 for all energies

pQ : • > 1 for all energies

ppwallwall::•• > 1> 1•• greatest for 6MeV (1.014)greatest for 6MeV (1.014)

ppcavcav::•• < 1 for all energies< 1 for all energies

ppQQ : : •• > 1 for all energies> 1 for all energies

0.98

0.99

1.00

1.01

1.02

1 2 3 4 5 6 7 8

R50 (cm)

Per

turb

atio

n Fa

ctpcav

pwall

pQ

CL2300(b)

Electron Perturbation Correction factors: water CL2300

Electron Perturbation Correction Electron Perturbation Correction factors: water CL2300factors: water CL2300

0.96

0.98

1.00

1.02

1.04

1.06

1.08

1.10

1.12

1.14

0.0 0.5 1.0 1.5 2.0 2.5 3.0Depth in water (cm)

Per

turb

atio

n Fa

ct pwall

pQ

pcav

6 MeV Cl2300

zref

R50pcav, Ma-Nahum

0.98

1.00

1.02

1.04

1.06

1.08

1.10

1.12

0.0 1.0 2.0 3.0 4.0Depth in water (cm)

Per

turb

atio

n Fa

ct pwall

pQ

pcav

9 MeV Cl2300

zref

R50

0.98

0.99

1.00

1.01

1.02

1.03

1.04

1.05

1.06

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0Depth in water (cm)

Per

turb

atio

n Fa

ct

pwall

pQ

pcav

15 MeV Cl2300

zref

R50

NACP chamber (Verhaegen NACP chamber (Verhaegen et alet al 2006)2006)

0.98

0.99

1.00

1.01

1.02

1.03

1.04

1.05

1.06

0.0 2.0 4.0 6.0 8.0Depth in water (cm)

Per

turb

atio

n Fa

ct

pwall

pQ

pcav

18 MeV Cl2300

zref

R50

Including pQ when converting PDI to PDD leads to a correction as large as 10% of local dose around R50 for 6MeV. However, the change in R50 is less than 1mm.

Including Including ppQ Q when converting PDI to PDD leads to a correction as large when converting PDI to PDD leads to a correction as large as 10% of local dose around Ras 10% of local dose around R5050 for 6MeV. However, the change in for 6MeV. However, the change in RR5050 is less than 1mm.is less than 1mm.

(Verhaegen (Verhaegen et alet al 2006)2006)

Electron Perturbation Correction factors: water

Electron Perturbation Correction Electron Perturbation Correction factors: waterfactors: water

0

0 )()(

,

,,

Qairw

QairwQQ s

sk =

0Q

Q

pp

× wallcavQ ppp =

Comparison of calculated Comparison of calculated kkQ,QoQ,Qo values with Verhaegen values with Verhaegen et al et al (2006). (2006). TRSTRS--398 (Andreo 398 (Andreo et al et al 2000) and Sempau 2000) and Sempau et alet al (2004).(2004).

Electron Perturbation Correction factors: water

Electron Perturbation Correction Electron Perturbation Correction factors: waterfactors: water

0.98

1.00

1.02

1.04

1.06

1.08

0 1 2 3 4 5 6 7 8

R50 (cm)

kQ,Q

0

cl2300NPLTRS-398Sempau et al 2004

wallp

rearwallp

Verhaegen et al 2006, Buckley et al 2006

•1.023 – 1.007 for the lowest to highest beam energies (4MeV NPL – 21MeV Siemens)

Verhaegen Verhaegen et al et al 2006, Buckley 2006, Buckley et al et al 20062006

••1.023 1.023 –– 1.007 for the lowest to 1.007 for the lowest to highest beam energies (4MeV highest beam energies (4MeV NPL NPL –– 21MeV Siemens)21MeV Siemens)

For lowest to highest E:

•1.014 – 1.005 (McEwen et al 2006)

For lowest to highest E:For lowest to highest E:

••1.014 1.014 –– 1.005 (McEwen 1.005 (McEwen et al et al 2006)2006)

(McEwen (McEwen et alet al 2006)2006)

Electron Perturbation Correction factors: water

Electron Perturbation Correction Electron Perturbation Correction factors: waterfactors: water

McEwen et al 2006

Electron Perturbation Correction factors: graphite dref NPL

Electron Perturbation Correction Electron Perturbation Correction factors: graphite factors: graphite ddrefref NPLNPL

EnergyEnergy R50 (cm)R50 (cm) pcavpcav SDOM (SDOM (±±%)%) pwallpwall SDOM (SDOM (±±%)%) pqpq SDOM (SDOM (±±%)%)

66 1.3481.348 0.99340.9934 0.34%0.34% 1.02041.0204 0.37%0.37% 1.01371.0137 0.34%0.34%

88 1.9271.927 0.99240.9924 0.27%0.27% 1.01481.0148 0.30%0.30% 1.00711.0071 0.27%0.27%

1010 2.3742.374 0.98840.9884 0.33%0.33% 1.01471.0147 0.37%0.37% 1.00291.0029 0.33%0.33%

1212 2.8582.858 0.98800.9880 0.30%0.30% 1.01241.0124 0.35%0.35% 1.00031.0003 0.30%0.30%

1616 3.8263.826 0.99520.9952 0.30%0.30% 1.00921.0092 0.35%0.35% 1.00431.0043 0.30%0.30%

1919 4.3944.394 0.99740.9974 0.46%0.46% 1.00471.0047 0.56%0.56% 1.00211.0021 0.45%0.45%

NP L g ra p h ite e le ctro n p e rtu rb a tion fa ctors a t d re f fo r NACP ch a m be r

0.9800.9850.9900.9951.0001.0051.0101.0151.0201.0251.030

1 2 3 4 5b e a m qu a lity R50 (cm )

pertu

rbat

ion

fact

or

pc avpwallpq

Electron Perturbation Correction factors: graphite dref CL2300

Electron Perturbation Correction Electron Perturbation Correction factors: graphite factors: graphite ddrefref CL2300CL2300

EnergyEnergy R50 (cm)R50 (cm) pcavpcav SDOM (SDOM (±±%)%) pwallpwall SDOM (SDOM (±±%)%) pqpq SDOM (SDOM (±±%)%)

44 0.8810.881 0.99380.9938 0.16%0.16% simulations in progresssimulations in progress

66 1.5691.569 0.99380.9938 0.14%0.14% 1.01651.0165 0.20%0.20% 1.01021.0102 0.19%0.19%

99 2.3592.359 0.99530.9953 0.12%0.12% 1.01001.0100 0.18%0.18% 1.00521.0052 0.17%0.17%

1212 3.2353.235 0.99830.9983 0.14%0.14% 1.00691.0069 0.19%0.19% 1.00521.0052 0.18%0.18%

1515 4.1544.154 0.99780.9978 0.14%0.14% 1.00961.0096 0.18%0.18% 1.00731.0073 0.16%0.16%

1818 4.9344.934 1.00041.0004 0.14%0.14% 1.00721.0072 0.18%0.18% 1.00761.0076 0.17%0.17%

CL 2300 g ra ph ite e le ctro n pe rtu rb a tion fa cto rs a t d re f fo r NACP cha m be r

0.990

0.995

1.000

1.005

1.010

1.015

1.020

0 1 2 3 4 5 6

Be a m Q ua lity R50 (cm )

pert

urba

tion

fact

or

pc avpwallpq

Implications for the NPL electron beam calibrationImplications for the NPL Implications for the NPL

electron beam calibrationelectron beam calibration

airg

airw

gref

wrefgrefDwrefD s

spp

NN,

,

,

,,,,, =

EnergyEnergy R50 (cm)R50 (cm) ppref,wref,w/p/pref,gref,g SDOM (SDOM (±±%)%)

66 1.3481.348 0.99530.9953 0.37%0.37%

88 1.9271.927 1.00061.0006 0.31%0.31%

1010 2.3742.374 1.00221.0022 0.36%0.36%

1212 2.8582.858 1.00591.0059 0.33%0.33%

1616 3.8263.826 1.00001.0000 0.33%0.33%

1919 4.3944.394 1.00231.0023 0.47%0.47%

Preliminary results from ongoing investigations

Preliminary results from Preliminary results from ongoing investigationsongoing investigations

CL2300 graphite pcav for NACP chamber

0.980

0.990

1.000

1.010

1.020

1.030

1.040

1.050

1.060

0 1 2 3 4 5 6depth (cm)

pcav

6MeV12MeV18MeV

CL2300 graphite pwall for NACP chamber

0.980

0.990

1.000

1.010

1.020

1.030

1.040

1.050

1.060

0 1 2 3 4 5 6

depth (cm)

pwal

l 6MeV12MeV18MeV

CL2300 graphite pq for NACP chamber

0.980

0.990

1.000

1.010

1.020

1.030

1.040

1.050

1.060

0 1 2 3 4 5 6de pth (cm )

pq

6MeV12MeV18MeV

ConclusionConclusionConclusion

1. Validating Monte Carlo ion chamber model with measurements is important

2. Electron perturbation factors for plane-parallel ionization chambers are not equal to unity.

3. Perturbation factors are greatest for lowest energies.

4. Perturbation factors increase with depth and are very sensitive to chamber model at depths away from dref

5. NPL calibration procedure may need to be updated to include non-unity perturbation factors (simulations for better statistics in progress)

1.1. Validating Monte Carlo ion chamber model with Validating Monte Carlo ion chamber model with measurements is important measurements is important

2.2. Electron perturbation factors for planeElectron perturbation factors for plane--parallel parallel ionization chambers are not equal to unity.ionization chambers are not equal to unity.

3.3. Perturbation factors are greatest for lowest energies.Perturbation factors are greatest for lowest energies.

4.4. Perturbation factors increase with depth and are very Perturbation factors increase with depth and are very sensitive to chamber model at depths away from sensitive to chamber model at depths away from ddrefref

5.5. NPL calibration procedure may need to be updated to NPL calibration procedure may need to be updated to include noninclude non--unity perturbation factors (simulations for unity perturbation factors (simulations for better statistics in progress)better statistics in progress)