Megavoltage g Electron and Photon Beams

Transcription

1 Diode Dosimetry for Megavoltage g Electron and Photon Beams Timothy C. Zhu, Ph.D. Department of Radiation Oncology, University it of Pennsylvania Philadelphia, PA 19104

2 Educational Objectives To understand the fundamentals of diode dosimetry, i.e., modeling of the transient electric and radiation properties of the diode detectors To understand the basic dosimetric characteristics of commercial diode detectors, especially, the dependence of dose rate, temperature, and energy. In-vivo diode dosimetry using diodes with inherent buildup

3 Outline In-vivo patient diode dosimetry Construction of diode detectors Fundamentals of diode detector theory Dosimetric Characteristics of Diode detectors Dose rate or SDD dependence Temperature dependence Energy dependence Other dosimetric characteristics Summary

5 Diode as an in-vivo dosimeter Advantages: Higher relative sensitivity Quick response (1 10 s) Good mechanical stability No external bias needed Small size Smaller energy dependence of mass collision stopping power ratios (between silicon and water compared to air and water). Disadvantages: Dependence on temperature, dose rate, energy dependence. Require an electrical connection during irradiation

6 Schematic of the Different Doses involved for Photon in-vivo dosimetry

7 TMR(s, d=0.5, 3.0 cm) vs. Collimator Setting p) 1.15 p)/tmr(s=1 10,d=buildu o - 20 MV MV x - Co Dashed line 0.5 cm Solid line 3 cm (s,d=buildu 0.95 TMR collimator setting (cm) Field size dependence is minimum if thicker buildup than d max is used, but...

8 Schematic of the Different Doses involved for Electron in-vivo dosimetry

9 In-vivo Dosimetry for High-Energy Electrons Nominal Energy Depth (cm) MeV MeV MeV MeV MeV MeV MeV MeV

10 Outline In-vivo patient diode dosimetry Construction of diode detectors Fundamentals of diode detector theory Dosimetric Characteristics of Diode detectors Dose rate or SDD dependenced Temperature dependence Energy dependence d Other dosimetric characteristics

11 Specifications of commercial diode detectors Diode Type Shape Buildup Material, Total buildup thickness (g/cm 2 ) Sun Nuclear Isorad Red (n-type) Sun Nuclear Isorad Electron Sun Nuclear Isorad-3 Gold #1 Energy Range Manufacturing period Cylinder 1.1 mm Tungsten, MV Cylinder 0.25 mm PMMA, Electrons Cylinder 1.1 mm Molybdenum, MV Sun Nuclear Cylinder 1.1 mm Molybdenum, MV Isorad-3 Gold #2 Sun Nuclear Flat 2.1 mm Brass, MV QED Gold (n-type) Sun Nuclear Flat 3.4 mm Brass, MV QED Red (n-type) Sun Nuclear Flat 3.4 mm Aluminum, MV QED Blue (p-type) Sun Nuclear Flat 3.4 mm Brass, MV QED Red (p-type) Sun Nuclear Flat 0.25 mm PMMA, Electrons QED Electron (p-type)

12 Package Specifications of commercial diode detectors Diode Type Shape Buildup Material, Total buildup thickness (g/cm 2 ) Nuclear Associates Veridose Yellow Energy Range Flat 1.2 mm Copper, MV Nuclear Associates Flat 1.7 mm Tungsten, MV Veridose Green Scanditronix Flat Epoxy (0.50 mm), 0.20 Electrons EDP 2 3G Scanditronix Flat 0.75 mm Stainless Steel + 4-8MV EDP 10 3G epoxy, 1 Scanditronix EDP 20 3G Flat 2.2 mm Stainless Steel + epoxy, MV Scanditronix Flat Epoxy (0.5mm), 0.20 Scanning PFD Scanditronix EDP10 Flat 0.75 mm stainless cap + epoxy, 1.0 Manufacturing period 6 12 MV

13 Schematics of inherent buildup geometry Cylindrical geometry Isorad

14 Schematics of inherent buildup geometry flat geometry Sun Nuclear Scanditronix

16 n- and p- type Semiconductors Intrinsic semiconductor (Si) is material with a narrow energy band width (1.1 ev). Temperature gives enough energy to produce a small amount of electron and hole (pair); both are conductive Doping donor impurity it (e.g. phosphorous h or arsenic) produces additional electrons (n- type) Doping acceptor impurity (e.g. boron or aluminum) )p produces additional holes (p-type)

17 Schematics of n- and p-type semiconductors E c E c E D E A E v E v - + P + B -

18 n-type and p-type Diodes A diode is a p-n junction made by doping the semiconductor with donors and acceptors at adjacent junctions. n-type diodes have the high doping level of n- type semiconductors and the low doping level of p-type semiconductors. The reverse is true for p-type diodes.

19 Basic Structure for diode detectors p + n Junction Diode Incident Radiation p + n Substrate Depletion Layer Direct Radiation Scatter Radiation Silicon Diode Buildup Phantom Electric Transport Radiation Transport

20 Electric Transport

21 Diode Current Radiation X=x 0 - X=x 0 + X=L p + + J n + + J p + + Electrometer L n W L p n I r +I e r e Diffusion layers Only radiation-generated electron-hole pairs in the depletion and diffusion layer contribute to radiation current

22 Recombination Processes for radiation generated Excess carrier Concentration When a pure semiconductor is irradiated, d an electron-hole pair is created: n= n=p It takes a finite time for these excess carriers to annihilate via recombination processes d p pp G () t gr () t dt p n gr (1 tt / g is a constant ( pairs/cgy in silicon), r is the dose rate e )

23 Recombination Processes via R-G Center Ec Et or or Ev Electron capture Electron emission Hole capture Hole emission Possible electronic transitions between a single-level R-G center and the energy bands.

24 Excess minority Carrier Lifetime For n-type diode, p >> p 0, p >>p 1,andn 0 >> n 1 p ( n 0 p ) p p p (1 n p n 0 0 p ) For p-type diode, n =p >>n 0, n =p >>n 1, and p 0 >> p 1: n 1 n ( p 0 n) n n (1 ) p0 n ( p0 n) = n / p Where: 81

25 Continuity Equation For n-type, the diffusion current density for holes, J p, can be obtained from the continuity equation. p t p 2 2 p p R G p p g o r ( t ) R: is the Net Recombination Rate G: is the Genration Rate r(t) is the instantaneous t dose rate g 0 = pairs/cgy is a constant

26 Solution of the continuity equation (Wirth and Roger, 1964) Thus for pulsed radiation with pulse width t p J qg r( t p ) L erf t 0 < t t 0 p p J p qg 0 r ( t ) L p erf t erf ( t t p ) t > t p t p is the width of the rectangular pulse for a pulsed beam and is the exposure time for the continuous beam.

27 Radiation current in a radiation pulse

28 Intrinsic diode Sensitivity S diode A M dioded 0 D diode t p 0 J ( t) dt r( t) dt S dioded K p p (1 ) ( n type ) n0 n n (1 ) ( p type) p0

29 Radiation Transport

30 Absorbed dose Sensitivity M D diode water M D diode diode D D diode water S diode D H O diode S 2 H O Ddiode 2 can be determined by Monte-Carlo (MC) simulation or Bragg-Gray cavity theory

32 Dose Rate Dependence for n- and p- type Diodes (Rikner and Grusell, 1983) n-type p-type

33 Recombination Time Increases With Instantaneous Dose Rate If the dose rate is high, the ions are produced at such a high rate that the recombination cannot keep pp pace, and more charge carriers escape recombination than at lower dose rates. increases with instantaneous dose rate. for p-type Si semiconductors s is less dependent on the dose rate than n-type Si semiconductors.

34 Dose Rate Dependence (Saini and Zhu, 2004) (a) Dose Rate Dependence (n-type) (b) Dose Rate Dependence (p-type) S/S(40 00) S/S(400 00) x 10 4 Instantaneous dose rate (cgy/s) o-isorad Gold#1 + - Isorad Red (n-type), - Isorad-3 Gold, - Veridose Green x - QED Red (n-type) x 10 4 Instantaneous dose rate (cgy/s) - EDP103G, x- EDP203G, * - Isorad-p Red, - QED Red (p-type), - QED Blue

35 SSD Dependence (Ratio at 200 cm) Diode 6 MV 20 MV Co-60 Isorad 1 Gold Isorad 2 Gold Isorad Red SPD EDP

36 Comparison of Accumulated Dose Dependence for n- and p-type Diodes Rikner and Grusell 1983

37 Recombination Time Decreases With Accumulated Dose Accumulated radiation introduces additional lattice defects, which act as recombination centers for the excess charge carriers K where K is the damage coefficient (smaller for holes than for electrons), is the radiation fluence

38 Temperature coefficient vs. preirradiation Relativ ve Charge QED unirradiated diode 1.08 Error o - Co-60 = 0.34 %/ºC MV = 0.27 %/ºC 1.04 x - 20 MV = 0.25 %/ºC Relativ ve Charge QED Red preirradiated diode Error o - Co-60 = 0.29 %/ºC MV = 0.29 %/ºC x - 15 MV = 0.29 %/ºC Temperature (ºC) Temperature (ºC) (Saini and Zhu, 2002)

39 Temperature Coefficients for n-type and p-type diodes (Saini and Zhu, 2002) 6 MV 15 or 20 MV Co-60 Diode Type (%/ o C) (%/ o C) (%/ o C) Isorad Gold 1, unirradiated (20 MV) 0.45 (T1000) Isorad dgold 2, unirradiated d (20MV) (T1000) Isorad Red (20 MV) 0.37 (T1000) QED unirradiated (15MV) (TPhoenix) 0.34 QED Blue Diode (15 MV) 0.30 (T780) QED Red Diode (15 MV) 0.29 (T780) Scanditronix EDP (20 MV) 0.36 (T1000) Scanditronix EDP (20 MV) 0.39 (T1000)

40 Energy Dependence (Scanditronix diode) (Rikner and Grusell, 1987)

41 Energy Dependence for megavoltage photon Norm malized Sensitivity Energy Dependence 1.5 Isorad Electron Isorad Red EDP QED Electron (p-type) QED Blue (p-type) QED Red (p-type) Norma alized Sensitivity EDP10 3G EDP20 3G EDP2 3G PFD Veridose Green Veridose Yellow Veridose Electron QED Gold (n-type) QED Red (n-type) Isorad 3 Gold #1 Isorad 3 Gold #2 Energy Dependence Nominal Accelerating Potential (MV) Nominal Accelerating Potential (MV) (Saini and Zhu, 2007)

42 MC Results Energy Silicon diode Diode + 1.2mm Cu Diode + 3 mm Cu Diode mm W Diode + 3 mm W SB normb Co ± 5.7% ± 6.1% ± 6.1% ± 6.3% ± 6.1% 6MV 0.979± 5.9% ± 6.3% ± 6.3% ± 6.5% ± 6.4% 10 MV ± 5.8% ± 6.2 % ± 6.5% ± 6.4% ± 6.5% 15 MV ± 5.7% ± 6.1% ± 6.1% ± 6.3% ± 6.0% 24 MV 0.987± 59% 5.9% ± 62% 6.2% ± 62% 6.2% ± 63% 6.3% ± 61% 6.1% The statistical uncertainty corresponds to 1 SD. (Saini and Zhu, 2007)

43 Angular Dependence Scanditronix Sun Nuclear * - Isorad-3 Gold, + - Isorad-3 Red, O - QED Gold, x - QED Red (Rikner, Thesis, 1983) (Saini, i Thesis, 2007)

44 Summary Diode dose rate, temperature, energy dependence can be explained by modeling the electric and radiation transport. The diode structure has a large effect on the energy dependence, which can be explained by MC simulation. For detector to be used as an absolute detector further work is necessary to couple the equations governing radiation transport with the continuity equations governing the electric current transport of diode detector.