Diode Dosimetry for Diode Dosimetry for Megavoltage Electron and ...

Diode Dosimetry forDiode Dosimetry for Megavoltage Electron and g gPhoton Beams

Timothy C. Zhu, Ph.D. Department of Radiation Oncology,U i it f P l iUniversity of PennsylvaniaPhiladelphia, PA 19104

June 24, 2009 AAPM Summer School 2009

Educational Objectives

To understand the fundamentals of diode 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 anddependence of dose rate, temperature, and energy.

In-vivo diode dosimetry using diodes with inherent In vivo diode dosimetry using diodes with inherent buildup


Outline

In-vivo patient diode dosimetryIn vivo patient diode dosimetry Construction of diode detectors Fundamentals of diode detector theory Fundamentals of diode detector theory Dosimetric Characteristics of Diode detectors

Dose rate or SDD dependence Dose rate or SDD dependence Temperature dependence Energy dependence Other dosimetric characteristics

Summary


Outline



Summary


Diode as an in-vivo dosimeter

Advantages: Higher relative sensitivity Quick response – (1 – 10 s) Good mechanical stabilityy No external bias needed Small size Smaller energy dependence of mass collision stopping gy p pp g

power ratios (between silicon and water compared to air and water).

Disadvantages:g Dependence on temperature, dose rate, energy

dependence. Require an electrical connection during irradiation


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


TMR(s, d=0.5, 3.0 cm) vs. Collimator Setting

1.15

p)

1.1

10,d

=bui

ldu

x - Co+ - 6 MV

o - 20 MVDashed line – 0.5 cm

Solid line – 3 cm

1

1.05

p)/T

MR

(s=1

x Co

0.95

(s,d

=bui

ldu

0 10 20 30 400.9

collimator setting (cm)

TMR

Field size dependence is minimum if thicker buildup than d is used but


Field size dependence is minimum if thicker buildup than dmax is used, but....

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


In-vivo Dosimetry for High-Energy Electrons

Nominal EnergyDepth 4 6 8 10 12 15 18 20Depth(cm)

4MeV

6MeV

8MeV

10MeV

12MeV

15MeV

18MeV

20MeV

00.51

9597

8999

8693

8892

9294

9496

9597

96981

1.522.53

9762151

999874378

9399988457

92961009786

94969810098

9697989999

9798999999

9899100100993

3.544.55

81

572661

866538154

9891775532

9998958674

9999999793

99989897955

5.566.57

41

321452

7458392211

9386766348

9593898374


7 11 48 74

Outline

In-vivo patient diode dosimetry In vivo patient diode dosimetry Construction of diode detectors

Fundamentals of diode detector theory Fundamentals of diode detector theory Dosimetric Characteristics of Diode detectors

D SDD d d Dose rate or SDD dependence Temperature dependence

E d d Energy dependence Other dosimetric characteristics


Specifications of commercial diode detectorsDiode Type Shape Buildup Material, Total

buildup thickness (g/cm2)Energy Range Manufacturing

period

Sun NuclearIsorad Red (n-type)

Cylinder 1.1 mm Tungsten, 2.8 15–25 MV 1993 - 1998

Sun NuclearIsorad Electron

Cylinder 0.25 mm PMMA, 0.030 Electrons 1993 - 1998

Sun NuclearIsorad-3 Gold #1

Cylinder 1.1 mm Molybdenum, 1.6 6-12 MV 2003 -

Sun NuclearIsorad-3 Gold #2

Cylinder 1.1 mm Molybdenum, 1.6 6-12 MV 2003 -

Sun NuclearQED Gold (n-type)

Flat 2.1 mm Brass, 1.9 6-12 MV 2003 -QED Gold (n type)

Sun NuclearQED Red (n-type)

Flat 3.4 mm Brass, 3.0 15-25 MV 2003 -

Sun NuclearQED Blue (p type)

Flat 3.4 mm Aluminum, 1.0 1–4 MV 1997 - 2002QED Blue (p-type)

Sun NuclearQED Red (p-type)

Flat 3.4 mm Brass, 3.0 15–25 MV 1997 - 2002

Sun Nuclear Flat 0.25 mm PMMA, 0.030 Electrons 1997 – 2002


QED Electron (p-type)

Package Specifications of commercial diode ddetectorsDiode Type Shape Buildup Material, Total buildup

thickness (g/cm2)Energy Range Manufacturing

periodthickness (g/cm2) period

Nuclear AssociatesVeridose Yellow

Flat 1.2 mm Copper, 1.36 5-11 MV 1998-

Nuclear Associates Flat 1.7 mm Tungsten, 3.57 18-25 MV 1998-Veridose Green

ScanditronixEDP 23G

Flat Epoxy (0.50 mm), 0.20 Electrons 2001 -

Scanditronix Flat 0.75 mm Stainless Steel + 4-8 MV 2001 -ScanditronixEDP 103G

Flat 0.75 mm Stainless Steel epoxy, 1

4 8 MV 2001

ScanditronixEDP 203G

Flat 2.2 mm Stainless Steel + epoxy, 2.0

10-20 MV 2001 -

Scanditronix Flat Epoxy (0 5mm) 0 20 Scanning 2001ScanditronixPFD

Flat Epoxy (0.5mm), 0.20 Scanning 2001-

ScanditronixEDP10

Flat 0.75 mm stainless cap + epoxy,1.0

6–12 MV 1990-2001


Schematics of inherent buildup geometry – Cylindrical geometry

IsoradIsorad


Schematics of inherent buildup geometry – flat geometry

Sun Nuclear

Scanditronix


Outline



Summary


n- and p- type Semiconductors

Intrinsic semiconductor (Si) is material with aIntrinsic semiconductor (Si) is material with a narrow energy band width (1.1 eV). Temperature gives enough energy to

d ll f l d h lproduce a small amount of electron and hole (pair); both are conductiveD i “d ” i it ( h h Doping “donor” impurity (e.g. phosphorous or arsenic) produces additional electrons (n-type)type)

Doping “acceptor” impurity (e.g. boron or aluminum) produces additional holes (p-type)


) p (p yp )

Schematics of n- and p-type semiconductors

E EEc

ED

Ec

EAEv Ev

- +

P+ B-


n-type and p-type Diodes

A diode is a p-n junction made by doping the A diode is a p n junction made by doping the semiconductor with donors and acceptors at adjacent junctions.j j

n-type diodes have the high doping level of n-type semiconductors and the low doping leveltype semiconductors and the low doping level of p-type semiconductors. The reverse is true for p-type diodes.p yp


Basic Structure for diode detectors

Incident Radiation

Direct Radiation

p+n Junction Diode

p+

Scatter Radiation

Silicon Dioden

p+

SubstrateBuildup

PhantomDepletion Layer

Substrate

Electric Transport Radiation Transport


Electric Transport


Diode CurrentRadiation

+ J n J p

X=L

++

X=x0+X=x0

-

W

p +n

n

+

L

++

LElectrometer

Ir+Ie Diffusion layers

W LpLn

Only radiationOnly radiation--generated electrongenerated electron--hole pairs in thehole pairs in thedepletiondepletion and diffusion layer contribute to radiation currentand diffusion layer contribute to radiation current

r e Diffusion layers


pp yy

Recombination Processes for radiation generated Excess carrier Concentration

Wh i d t i i di t dWhen a pure semiconductor is irradiated, an“electron-hole” pair is created: n=n=ppIt takes a finite time for these excess carriers

d p p

It takes a finite time for these excess carriers to annihilate via recombination processes

d pdt

p G t gr t

( ) ( )

)1( / t

g is a constant (4.2×1012 pairs/cGy in silicon), i th d t

)1( / tegrnp


r is the dose rate

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.


Excess minority Carrier Lifetime

For n-type diode, p >> p0, p >>p1, and n0 >> n1

)1()( 0 pppnpp

)(00 pnpn pp

For p type diode n = p >> n n =p >>n and p >> pFor p-type diode, n = p >> n0 , n =p >>n1, and p0 >> p1:

)1()( 0

1 nnpn

)

)(1(

00 npnp nn

= n/pWhere: 81.6


n p 81.6

Continuity EquationFor n-type, the diffusion current density for holes, Jp, can be obtained from the continuity equation.can be obtained from the continuity equation.

)(22 pGRp

)(22 trgppGRptp

opp

R: is the Net Recombination RateG: is the Genration Rate(t) i th i t t d tr(t) is the instantaneous dose rate

g0= 4.24.2××10101212 pairs/cGypairs/cGy is a constant


Solution of the continuity equation (Wirth and Roger, 1964)Thus for pulsed radiation with pulse width tp

terfLtrqgJ

pp )(0

0 < t tp

)(

)( ptterfterfLtrqgJ t > tp

)(0p erferfLtrqgJ

p

tp is the width of the rectangular pulse for a pulsed beam and isthe exposure time for the continuous beam.


Radiation current in a radiation pulse


Intrinsic diode Sensitivity

di ddttJA

M 0)(

pt

diode

diodediode

dttrDM

S 0

)(0

KSdi d

)()1( typenp

p

KSdiode

)()1(

)()1(

0

0

typeppn

typenn

n

p


Radiation Transport


Absorbed dose Sensitivity

OHdiodediode

water

diode

diode

diode

water

diode DSDD

DM

DM

S 2waterdiodewater

can be determined by Monte-Carlo (MC) i l ti B G it th

OHdiodeD 2

simulation or Bragg-Gray cavity theory


Outline



Summary


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

p-typen-type

p yp


Recombination Time Increases With Instantaneous Dose Rate If the dose rate is high the ions are produced If the dose rate is high, the ions are produced

at such a high rate that the recombination cannot “keep pace”, and more charge p p , gcarriers escape recombination than at lower dose rates. increases with instantaneous dose rate.

for p-type Si semiconductors is less o p type S se co ducto s s essdependent on the dose rate than n-type Si semiconductors.


Dose Rate Dependence (Saini and Zhu, 2004)

1 08

1.10 Dose Rate Dependence (n-type)

(a)

1 08

1.10Dose Rate Dependence (p-type)

(b)

1.04

1.06

1.08

00) 1.04

1.06

1.08

00)

0.98

1.00

1.02

S/S

(40

0.98

1.00

1.02

S/S

(400

0.92

0.94

0.96

0.92

0.94

0.96

0 1 2 3 4x 104

Instantaneous dose rate (cGy/s)0 1 2 3 4

x 104Instantaneous dose rate (cGy/s)

o-Isorad Gold#1 + - Isorad Red (n-type), - Isorad-3 Gold, - Veridose Green

- EDP103G , x- EDP203G , * - Isorad-p Red, - QED Red (p-type), - QED Blue


x - QED Red (n-type), (p yp ),

SSD Dependence (Ratio at 200 cm)

Diode 6 MV 20 MV Co-60Diode 6 MV 20 MV Co 60

Isorad 1 Gold 0.950 0.957 0.988

Isorad 2 Gold 0.965 0.973 ---

Isorad Red 0.974 0.994 0.987Isorad Red 0.974 0.994 0.987

SPD 1.002 0.995 ---

EDP30 0.995 0.998 0.994


Comparison of Accumulated Dose Dependence for n- and p-type Diodes

Rikner and GrusellRikner and Grusell19831983


Recombination Time Decreases With Accumulated Dose Accumulated radiation introduces additional Accumulated radiation introduces additional

lattice defects, which act as recombination centers for the excess charge carriers g

K10

1

where K is the damage coefficient (smaller for holes than for electrons) is thefor holes than for electrons), is the radiation fluence


Temperature coefficient vs. preirradiation

1.06

1.08QED unirradiated diode

Error

o - Co-60 = 0.34 %/ºC 1.06

1.08QED Red preirradiated diode

Error

o - Co-60 = 0.29 %/ºC

1.02

1.04

ve C

harg

e

+ - 6 MV = 0.27 %/ºC

x - 20 MV = 0.25 %/ºC

1.02

1.04

ve C

harg

e

o Co 60 0.29 %/ C+ - 6 MV = 0.29 %/ºC

x - 15 MV = 0.29 %/ºC

0.96

0.98

1.00

Rel

ativ

0.96

0.98

1.00

Rel

ativ

10 15 20 25 30 35 400.94

Temperature (ºC)10 15 20 25 30 35 40

0.94

Temperature (ºC)

(Saini and Zhu 2002)


(Saini and Zhu, 2002)

Temperature Coefficients for n-type and p-type diodes

6 MV 15 or 20 MV Co-60


Diode Type (%/oC) (%/oC) (%/oC)

Isorad Gold 1, unirradiated 0.06 0.05 (20 MV) 0.45 (T1000)

I d G ld 2 i di t d 0 08 0 10 (20 MV) 0 16 (T1000)Isorad Gold 2, unirradiated 0.08 0.10 (20 MV) 0.16 (T1000)

Isorad Red 0.22 0.21 (20 MV) 0.37 (T1000)

QED unirradiated 0 27 0 25 (15 MV) 0 34 (T Phoenix)QED unirradiated 0.27 0.25 (15 MV) 0.34 (T Phoenix)

QED Blue Diode 0.30 0.31 (15 MV) 0.30 (T780)

QED Red Diode 0.29 0.29 (15 MV) 0.29 (T780)Q ( ) ( )

Scanditronix EDP 10 0.38 0.33 (20 MV) 0.36 (T1000)

Scanditronix EDP 30 0.36 0.34 (20 MV) 0.39 (T1000)


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


Energy Dependence for megavoltage photon

1.3

1.4

1.5Energy Dependence

Isorad ElectronIsorad RedEDP10QED Electron (p-type)QED Blue (p-type) 1 3

1.4

1.5Energy Dependence

EDP103G

EDP203G

EDP23G

PFDVeridose GreenVeridose Yellow

1

1.1

1.2

mal

ized

Sen

sitiv

ity

QED Blue (p-type)QED Red (p-type)

1.1

1.2

1.3

aliz

ed S

ensi

tivity

Veridose YellowVeridose ElectronQED Gold (n-type)QED Red (n-type)Isorad 3 Gold #1Isorad 3 Gold #2

0.8

0.9

1

Nor

m

0.8

0.9

1

Nor

ma

0 5 10 15 200.7

Nominal Accelerating Potential (MV) 0 5 10 15 200.7

Nominal Accelerating Potential (MV)




MC Results

Energy Silicon diode Diode + 1.2mm Cu

Diode + 3 mm Cu

Diode + 1.7 mm W

Diode + 3 mm W

SBnorm B

Co 1.000 ± 5.7% 1.000 ± 6.1% 1.000 ± 6.1% 1.000 ± 6.3% 1.000 ± 6.1%

6 MV 0.979± 5.9% 1.094 ± 6.3% 1.126 ± 6.3% 1.136 ± 6.5% 1.139 ± 6.4%6 V 0.979 5.9% .09 6.3% . 6 6.3% . 36 6.5% . 39 6. %

10 MV 0.999 ± 5.8% 1.175 ± 6.2 % 1.204 ± 6.5% 1.226 ± 6.4% 1.259 ± 6.5%

15 MV 0.943 ± 5.7% 1.222 ± 6.1% 1.404 ± 6.1% 1.349 ± 6.3% 1.481 ± 6.0%

24 MV 0 987± 5 9% 1 371 ± 6 2% 1 590 ± 6 2% 1 815 ± 6 3% 1 932 ± 6 1%24 MV 0.987± 5.9% 1.371 ± 6.2% 1.590 ± 6.2% 1.815 ± 6.3% 1.932 ± 6.1%

The statistical uncertainty corresponds to 1 SD.




Angular DependenceScanditronix Sun Nuclear

* - Isorad-3 Gold, + - Isorad-3 Red, O - QED Gold, x - QED Red

(Rik Th i 1983) (S i i Th i 2007)


(Rikner, Thesis, 1983) (Saini, Thesis, 2007)

Summary

Diode dose rate, temperature, energy dependence , p , gy pcan be explained by modeling the electric and radiation transport.

The diode structure has a large effect on the energy 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 continuitygoverning radiation transport with the continuity equations governing the electric current transport of diode detector.


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