NACP-02 perturbation correction factors for the NPL primary standard of absorbed dose to water in high energy electron beams

Transcription

1 NACP-02 perturbation correction factors for the NPL primary standard of absorbed dose to water in high energy electron beams E. Chin 1, J. Seuntjens 1, H. Palmans 2, A. DuSautoy 2, D. Shipley 2, M. Bailey 2, F. Verhaegen 1 Paris 9-11, May

2 Outline 1. Calibration procedure at NPL 2. Model of ion chamber for MC simulations 3. Validation of MC model with backscatter simulations and measurements 4. Perturbation correction factors in water 5. Perturbation correction factors in graphite 6. Implications for the NPL electron beam calibration

3 NPL Calibration Procedure: high energy electrons (1) Define reference depth in water dw = 0.6R50, w 0. 1cm (2) Use range scaling to get depth in graphite d g = dw R50, g R50, w (3) Calibrate chamber against the calorimeter, in graphite, at the NPL N D, ref, g = M D g ref, g (McEwen et al 1998)

4 NPL Calibration Procedure: high energy electrons Calorimeter for high energy electrons

5 NPL Calibration Procedure: high energy electrons (4) Theoretical conversion of graphite to water N D, ref, w = N D, ref, g p p ref ref, w, g s s w, air g, air (5) Compare user and reference chambers at d w in water, at NPL N D, user, w = N D, ref, w M M ref, w user, w (McEwen et al 1998)

6 Current Protocols: electron perturbation correction factors For well guarded plane parallel plate ion chambers p = Q p cav p wall p cav p wall =1 =1 A plane parallel chamber with adequately large guard ring can eliminate the in- scattering effects Scarce amount of data available at the time and large uncertainties

7 Monte Carlo model of NACP ion chamber NACP-02 plane parallel ion chamber (NPL report CIRM13)

8 Monte Carlo model of NACP ion chamber

9 Monte Carlo model of NACP ion chamber rexolite graphite mylar air MC NACP model for DOSRZnrc and CAVRZnrc (not to scale)

10 Monte Carlo model of linacs Primary electron beam Primary collimator (CONESTACK) Primary electron beam Scattering foil (SLAB) Window (SLABS) Upper foil (SLABS) Lower foil (FLATFILT) Monitor Chamber (SLABS) Lead and (CIRCAPP) Steel collimator Monitor ion chamber (CONS3R) Mirror (MIRROR) Shield (CONESTACK) Upper and lower jaws (JAWS) NPL Linac (SSD 2m) Reticle (SLABS) Applicator (APPLICAT) Varian Linac (SSD 1m)

11 Linac electron energies 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) 4MeV PDD CL21A 6MeV PDD CL pdd Measured MC 4.12MeV PDD measured MC 6.85MeV cm cm

12 Validation of MC: Backscatter experiments and simulations Backscatter factors wrt air for NACP-02: Water Graphite Aluminum Copper t d max NACP Backscatter plate Phantom Phantom material: PMMA (4-12 MeV) Solid water (15 & 18MeV) Electron Beam experimental setup Applicator

13 Backscatter Experimental Setup NACP-02 chamber is flush with phantom surface

14 Backscatter Experimental Setup Backscatter setup with aluminum plate covering NACP-02

15 Backscatter Experimental Setup Backscatter setup with water phantom on top of NACP-02

16 Tuning NACP-02 parameters 1. Varied graphite density ( g/cm 3 ) 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 NACP-02 chamber

17 Backscatter Results: water CL2300 water BSF MeV 12MeV BSF measured Monte Carlo MeV thickness (cm) CL2300 water BSF BSF MeV 15MeV measured Monte Carlo thickness (cm)

18 Backscatter Results: graphite CL2300 graphite BSF BSF MeV 12MeV 18MeV measured Monte Carlo thickness (cm) CL2300 graphite BSF MeV 15MeV BSF measured Monte Carlo thickness (cm)

19 Backscatter Results: aluminum CL2300 aluminum BSF BSF MeV 12MeV measured Monte Carlo 18MeV thickness (cm) CL2300 aluminum BSF BSF thickness (cm) 9MeV 15MeV measured Monte Carlo

20 Backscatter Results: copper CL2300 copper BSF BSF 1.3 6MeV MeV MeV thickness (cm) measured Monte Carlo BSF CL2300 copper BSF 9MeV 15MeV measured Monte Carlo thickness (cm)

21 Backscatter 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.

22 Calculating Electron Perturbations Correction Factors: in water p ) = cav p = wall ( sw, air D D b p ) = Q ( sw, air c D D D D a c a b (Verhaegen et al 2006)

23 Electron Perturbation Correction factors: water d ref NPL p wall : > 1 by 2.3% for 4MeV (a) p wall NPL > 1 by ~1% for other energies p cav : < 1 by ~1% for all energies Perturbation Fact p Q p cav, Ma-Nahum p cav p Q : > 1 by (1.5% for 4MeV, 0.4% for 19MeV) R 50 (cm) NACP chamber (Verhaegen et al 2006)

24 Electron Perturbation Correction factors: water d ref CL2300 p wall : > > 1 greatest for 6MeV (1.014) p cav : < 1 for all energies Perturbation Fact (b) p wall p Q p cav CL2300 p Q : > 1 for all energies R 50 (cm) NACP chamber (Verhaegen et al 2006)

25 Perturbation Fact Electron Perturbation Correction factors: water CL MeV Cl2300 z ref p cav, Ma-Nahum p Q p wall p cav R 50 Perturbation Fact MeV Cl2300 z ref p Q p wall p cav R 50 Perturbation Fact Depth in water (cm) MeV Cl2300 z ref p Q p wall p cav Perturbation Fact Depth in water (cm) MeV Cl2300 p Q 1.04 p wall 1.03 z ref p cav Depth in water (cm) R NACP chamber (Verhaegen et al 2006) Depth in water (cm) R 50

26 Including p Q when converting PDI to PDD leads to a correction as large as 10% of local dose around R 50 for 6MeV. However, the change in R 50 is less than 1mm. R 50 Electron Perturbation Correction factors: water (Verhaegen et al 2006)

27 Electron Perturbation Correction factors: water k = Q, Q ( s w, air ) 0 ( s ) w, air Q Q 0 p p Q Q 0 p = Q p cav p wall kq,q cl2300 NPL TRS-398 Sempau et al R 50 (cm) Comparison of calculated k Q,Qo values with Verhaegen et al (2006). TRS-398 (Andreo et al 2000) and Sempau et al (2004).

28 Electron Perturbation Correction factors: water p wall Verhaegen et al 2006, Buckley et al for the lowest to highest beam energies (4MeV NPL 21MeV Siemens) McEwen et al 2006 p rearwall For lowest to highest E: (McEwen et al 2006) (McEwen et al 2006)

29 Electron Perturbation Correction perturbation factor factors: graphite d ref NPL NPL graphite electron perturbation factors at dref for NACP cham ber beam quality R50 (cm) pcav pwall pq Energy R50 (cm) pcav SDOM (±%)( pwall SDOM (±%)( pq SDOM (±%)( % % % % % % % % % % % % % % % % % %

30 Electron Perturbation Correction factors: graphite d ref CL2300 perturbation factor CL2300 g raphite electron perturbation factors at dref for NACP chamber Beam Quality R50 (cm ) pcav pwall pq Energy R50 (cm) pcav SDOM (±%)( pwall SDOM (±%)( pq SDOM (±%)( % simulations in progress % % % % % % % % % % % % % % %

31 Implications for the NPL electron beam calibration N D, ref, w = N D, ref, g p p ref, w ref, g s s w, air g, air Energy R50 (cm) p ref,w /p ref,g ref,g SDOM (±%)( % % % % % %

32 Preliminary results from ongoing investigations pcav CL2300 graphite pcav for NACP chamber 6MeV 12MeV 18MeV pwall CL2300 graphite pwall for NACP chamber 6MeV 12MeV 18MeV depth (cm) depth (cm) CL2300 graphite pq for NACP chamber pq depth (cm) 6MeV 12MeV 18MeV

33 Conclusion 1. Validating Monte Carlo ion chamber model with measurements is important 2. Electron perturbation factors for plane-parallel 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 d ref 5. NPL calibration procedure may need to be updated to include non-unity perturbation factors (simulations for better statistics in progress)