Updated Contact Dose Rate Conversion Factors for Encapsulated Gamma Sources Including Secondary Electron Emission

Transcription

1 Updated Contact Dose Rate Conversion Factors for Encapsulated Gamma Sources Including Secondary Electron Emission Eric Heritage Supervisor: Dr. Ed Waller

2 Outline Background Objectives PHITS Simulations Experimental Work Conclusions and Recommendations 2

3 Background Fig 1: Schematic of encapsulated gamma source [1] [1] Industrial Radiation Sources Product Information, Eckert & Zieger Isotope Products, CA USA,

4 Background Fig 2: Injury from accidental contact with encapsulated gamma source [2] Accurate dose estimation is important for treatment planning Secondary electrons contribute significantly to contact dose [2] The Radiological Accident in Yanango. International Atomic Energy Agency, Vienna,

5 Background: NCRP-40 Fig 3: NCRP-40 contact dose table [3] [3] Protection Against Radiation From Brachytherapy Sources (NCRP Report No. 40). National Council on Radiation Protection and Measurements,

6 Background: Radiation Dosimetry Vol. 3 SSSS FFFFFFFFFFFF = γγ dddddddd + ssssssssssssssssss eeeeeeeeeeeeeeee dddddddd γγ dddddddd Fig 4: Secondary electron contribution to the contact dose rate as a function of encapsulating material for a Ra-226 encapsulated source [4] [4] R. J. Shalek and M. Stoval, Dosimetry in implant therapy, in Radiation Dosimetry, F.H. Attix and E. Tochilin, Eds. NewYork: Academic Press, 1969, vol.3, ch. 31, pp

7 Background: Benner (1931) Encapsulated Ra-226 source Helmholtz coil magnet Ion chamber Fig 5: Schematic of Benner s experimental setup to measure relative contribution of secondary electrons for various encapsulation materials [5] [5] S. Benner, On secondary ββ-rays from the surface of radium containers, Acta Radiologica, pp ,

8 Objectives Expand and Correct NCRP-40 Table of Contact Dose Conversion Factors New contact dose conversion factors using Monte Carlo simulations Determine if secondary electron correction factors from old experiments are applicable 8

9 PHITS Monte Carlo radiation transport code External magnetic fields B Fig 6: Magnetic field bending the secondary electrons from an encapsulated Cs-137 source 9

10 NCRP-40 PHITS Model Encapsulation Tissue Fig 7: PHITS NCRP-40 contact dose geometry NCRP-40 encapsulation: stainless steel (type 304) ¼ inch outside diameter 1/32 inch thick walls Surrounded by a tissue layer for dose calculations 10

11 PHITS Contact Dose Table 1: PHITS contact dose conversion factors for first 0.07mm of tissue including secondary electrons (msv/h assuming a 1MBq source) Table 2: PHITS contact dose conversion factors for first 1mm of tissue including secondary electrons (msv/h assuming a 1MBq source) PHITS values are significantly less than those in NCRP-40 11

12 PHITS Secondary Electron Correction Factors Table 3: Secondary electron correction factors for first 0.07mm and 1mm in tissue calculated with PHITS Nuclide PHITS 0.07mm PHITS 1mm NCRP-40 Cs Co Ir Ra Secondary electron contribution decreases with tissue depth 0.07mm factors much greater than NCRP-40 factors Some 1mm factors are greater than the NCRP-40 range of

13 PHITS: Expansion of Contact Dose Tables Eight more gamma sources simulated in PHITS Commonly used encapsulated gamma sources Values calculated for both first 0.07mm and 1mm of tissue Table 4: Contact dose rates to the first 0.07 mm of tissue for 1 MBq sealed gamma sources calculated in PHITS 13

14 Experimental Setup B Fig 8: Electron flux from encapsulated Cs- 137 source in the center of the Variable Gap Magnet s 7000G B-field Fig 9: Modernized version of Benner s experimental setup to measure secondary electron correction factor for stainless steel encapsulated Cs

15 Experimental Results Table 5: Experimentally measured secondary electron correction factors Five minute measurements Five minute Measurements with Build-up cap Ninety Second Measurements Ninety Second Measurements with Build-up cap 1.13 ± ± ± ± 0.1 Five minute charge collections gave lower standard deviations Measurements with and without build-up cap did not have appreciable difference 15

16 PHITS Model of Experiment With Magnet Without Magnet Fig 10: PHITS geometry of experiment. Cross sectional (left) and 3-d (right) views Tagging Secondary Electrons Table 6: Summary of PHITS results Correction Factor without Build-up cap Correction Factor with Build-up cap 1.05 ± ± 0.04 Magnetic Deflection 1.05 ± ±

17 Tagging Secondary Electrons Observations Table 7: Comparison of PHITS simulation and experiment Correction Factor without Build-up cap Correction Factor with Build-up cap 1.05 ± ± 0.04 Magnetic Deflection 1.05 ± ± 0.04 Experimental Result 1.13 ± ± 0.04 Measurements without Build-up cap PHITS results without BC are within error of experimental results Scattering has no impact on measured SE correction factors Measurements with Build-up cap Scattering decreases measured SE factor by 6% Measured SE correction factor corrected from 1.13 to

18 Tagging Secondary Electrons Observations Table 7: Comparison of PHITS simulation and experiment Correction Factor without Build-up cap Correction Factor with Build-up cap 1.05 ± ± 0.04 Magnetic Deflection 1.05 ± ± 0.04 Experimental Result 1.13 ± ± 0.04 Good agreement between experimental results and simulation of experiment Gives confidence in PHITS contact dose simulations 18

19 Discussion 1. Experimental SE correction factors less than PHITS contact dose simulations 2. Part of secondary electron spectrum is not detected No BC BC No BC BC Fig 11: Photon and electron spectra emitted from encapsulated Cs-137. Obtained using PHITS Fig 12: Electron range in ion chamber wall 19

20 Discussion 3. SE correction factor is highly geometry dependant Table 8: Comparison of Cs-137 secondary electron correction factors for different geometries PHITS SE Correction Factor 0.07mm PHITS SE Conversion Factor 1mm Experimental Conversion Factor

21 Conclusions NCRP-40 overestimates contact dose NCRP-40 underestimates secondary electron contribution Benner s experiment not applicable to contact dose 21

22 Recommendations for Further Research Recommend using simulated dose conversion factors and SE correction factors Recommend calculating these factors for other encapsulation designs 22

23 Thank you 23

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27 Photon Interactions 27

28 Literature Review: Quimby (1939) Corrected for gamma shielding Relative measurements Verified by skin irradiations 1 mm absorption depth in tissue Fig 7: Quimby et al. experimental results [6] [6] E.H. Quimby, L.D. Marinelli, and J.V. Blady, Secondary filters in radium therapy, Am. J. Roentgenol, vol. 41, no. 5, pp ,

29 Ion Chamber Basics 29

30 A12S Ion Chamber and Build-up Cap 30

31 A12S Ion Chamber Response as a Function of Energy 31

32 Custom Designed Source Holder 32

33 Hall Effect Probe From: 33

34 PHITS Model of Encapsulated Cs-137 Source used in Experiment 34

35 Magnetic Deflection of Electrons z [cm] x [cm] 35

36 PHITS Magnetic Deflection Calculations Magnetic field only simulated in vacuum Charged particle trajectories based on Lorentz Force Gyro radius used to calculated trajectories FF = qq vv BB rr gg = mmmm qq BB Note: vv is velocity component perpendicular to magnetic field 36

37 Range of Electrons in Air National Institute of Standards and Technology. (1998) Stopping-power range tables for electrons, protons, and helium ions. [Online]. Available: 37

38 Skin Layers Epidermis (first mm in palms) Stratum corneum (dead cells) Basal layer (living cells) Dermis (next mm) Image taken from: 38

39 Range of Electrons in Tissue Range (cm) Kinetic Energy (MeV) National Institute of Standards and Technology. (1998) Stopping-power range tables for electrons, protons, and helium ions. [Online]. Available: 39

40 ICRP Soft Tissue Element Weight Fraction H C N O Na Mg P S Cl K Ca Fe Zn From: Compendium of Material Composition Data for Radiation Transport Modeling. U.S. Department of Homeland Security (2011) 40

41 PHITS: Pseudo Random Number Generator (PRNG) PHITS PRNG based on Linear Congruential scheme of Lehmer II nn+1 = GGII nn + CC mmmmmm 2 MM, nn = 0,1, 41

42 PHITS Error σσ = NN ii=1 xx ii ww 2 ii NN XX ww 2 NN(NN 1) NN is the total history number xx ii and ww ii are tally results and source weight of each history XX and ww are the mean values of the tally results and source weights Ratio of σσ to XX is relative error given in output file 42

43 PHITS Error Simulation of 1mm tissue dose for encapsulated Cs-137 Simulation used in thesis: Energy deposited (MeV/source): Relative error calculated by PHITS: Simulated 40 times with different seeds: Average energy deposited (MeV/source): Average relative error (std dev divided by average): Encapsulation Tissue 43

44 PHITS Simulation of Experiment Tagging Secondary Electrons (with Holder) Tagging Secondary Electrons (without Holder) Correction Factor without Build-up cap Correction Factor with Build-up cap 1.05 ± ± ± ± 0.04 Magnetic Deflection 1.05 ± ± 0.04 Experimental Result 1.13 ± ±

45 Including Beta Emission PHITS contact dose conversion factors for first 0.07mm of tissue for encapsulated Cs-137 (msv/h assuming a 1MBq source) Without Beta Emission With Beta Emission Total Dose Photon Only Dose Secondary Electron Dose Secondary Electron Correction Factor 4.94 ± ± ± ± ± ± ± ± 0.3 Cs-137 also emits betas with max energies MeV (94.6%), and MeV (5.4%) Simulations show results are similar (within error) 45

46 Including Source Material PHITS contact dose conversion factors for first 0.07mm of tissue for encapsulated Cs-137 (msv/h assuming a 1MBq source) Without Source Material Defined With Source Material (Cs) Total Dose Photon Only Dose Secondary Electron Dose Secondary Electron Correction Factor 4.94 ± ± ± ± ± ± ± ± 0.2 Simulations show results are similar (within error) 46

47 Additional Sources: Gamma Energies Nuclide Gamma Energies (kev) Am-241 <60 Ba-133 <384 Co , 136 Eu Gd-153 ~100 Na Se Yb