diode dosimetry for diode dosimetry for megavoltage electron and
TRANSCRIPT
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
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
Schematic of the Different Doses involved for Photon in-vivo dosimetry
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
Field size dependence is minimum if thicker buildup than dmax is used, but....
Schematic of the Different Doses involved for Electron in-vivo dosimetry
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
Schematics of inherent buildup geometry – Cylindrical geometry
IsoradIsorad
June 24, 2009 AAPM Summer School 2009
Schematics of inherent buildup geometry – flat geometry
Sun Nuclear
Scanditronix
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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)
June 24, 2009 AAPM Summer School 2009
) p (p yp )
Schematics of n- and p-type semiconductors
E EEc
ED
Ec
EAEv Ev
- +
P+ B-
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
Electric Transport
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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.
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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.
June 24, 2009 AAPM Summer School 2009
Radiation current in a radiation pulse
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
Radiation Transport
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
Dose Rate Dependence for n- and p-type Diodes (Rikner and Grusell, 1983)
p-typen-type
p yp
June 24, 2009 AAPM Summer School 2009
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.
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
Comparison of Accumulated Dose Dependence for n- and p-type Diodes
Rikner and GrusellRikner and Grusell19831983
June 24, 2009 AAPM Summer School 2009
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
June 24, 2009 AAPM Summer School 2009
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)
June 24, 2009 AAPM Summer School 2009
(Saini and Zhu, 2002)
Temperature Coefficients for n-type and p-type diodes
6 MV 15 or 20 MV Co-60
(Saini and Zhu, 2002)
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)
June 24, 2009 AAPM Summer School 2009
Energy Dependence (Scanditronix diode) (Rikner and Grusell, 1987)
June 24, 2009 AAPM Summer School 2009
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)
(Saini and Zhu 2007)
June 24, 2009 AAPM Summer School 2009
(Saini and Zhu, 2007)
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.
(Saini and Zhu 2007)
June 24, 2009 AAPM Summer School 2009
(Saini and Zhu, 2007)
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)
June 24, 2009 AAPM Summer School 2009
(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.
June 24, 2009 AAPM Summer School 2009