Realization of
reference air-kerma rate
for low-energy photon sources
Hans-Joachim Selbach
Hans-Michael Kramer
Physikalisch-Technische Bundesanstalt
Braunschweig, Germany
Wes Culberson
Medical Radiation Research Center
University Wisconsin, Madison, WI, USA
Introduction
¾ Radioactive 125I and 103Pd seed implantation is an increasingly
popular treatment for localized prostate cancer
½ Since spring 2005 the treatment is accepted by the
German health insurances
¾ Typical free-air chamber collecting volumes are too small
¾ The National Institute of Standards and Technology (NIST) uses
a wide-angle free-air chamber (WAFAC) since 1993 (Loevinger)
½ Half-angle of 8°
½ Uses the difference between two collecting volumes
¾ New chamber was developed at the Physikalisch-Technische
Bundesanstalt (PTB) in Germany in 2002
½ Large air-filled parallel-plate extrapolation chamber (GROVEX) with thin graphite coated
polyethylene front and back electrodes
½ For low-energy photon emitting sources with energies up to 40 keV
½ Extrapolation chamber measurements and interface effect elimination
Schematic of the GROVEX measuring system
high voltage
electrode
5 mm Pb
shutter
potential
rings
guard electrode
5 mm Pb
measurement
volume
0,1 mm Al
measurement
electrode
10,0 mm Ø
30 cm
0 - 20 cm
Schematic of the GROVEX measuring system
high voltage
electrode
5 mm Pb
shutter
potential
rings
guard electrode
5 mm Pb
measurement
volume
0,1 mm Al
measurement
electrode
10,0 mm Ø
30 cm
0 - 20 cm
Realization of
the reference air-kerma rate,
⋅
K δ , by means of the
extrapolation chamber technique:
⎛W ⎞
⎜ ⎟
⎜ e ⎟
⋅
⎛ d (kI ) ⎞
⎝ ⎠ air
Kδ =
⎜
⎟∏ ki
ρ air Aeff (1 − g air ) ⎝ ds ⎠ i
⎛W
⎜
⎜ e
⎝
⎞
⎟ = 33,97 eV
⎟
⎠ air
Aeff = 7754±11 mm2
⎛ d (kI ) ⎞
⎜
⎟
ds
⎝
⎠
ki
ρ air = 1,2046 kg/m3
g air = 0,0
is the increment of corrected ionization current per
increment of the chamber volume
are corrections to the entire measurement
Front and back view of the GROVEX
shutter
collector electrode and guard ring
potential rings
source position
high-voltage electrode
(hidden from view)
Determination of the measurement volume
¾ electrode separation
¾ electrical field homogeneity
¾ area of measurement electrode
Calculations of the electrical field
distribution by means of finite element methods
40 Potentialringe
Effective Electrode Area
¾ The area of the measurement electrode is difficult to measure
by mechanical means due to the thin (12 µm) foil, which is
graphitized from both sides
¾ As an alternative, the capacitance of the extrapolation
chamber as a function of electrode separation s is
measured, and from these measurements the area of the
electrode is determined
¾ The voltage-step method is used to measure the capacitance
Determination of the effective
electrode area
C0 = ΔQ / ΔU
-2
-3
s
-4
Q
C0 =
ε 0 ⋅ ε r ⋅ Aeff
0
E-10 C
-1
-5
-6
-7
-8
0
5
10
15
20
25
t
1
(ε 0 ⋅ ε r ⋅ Aeff )
s=
C0
s
30
Effective Electrode Area
measure the capacitance of the extrapolation chamber as a function
of electrode separation s
200
mm
electrode separation s
150
R2 = 0.99999
100
slope = ∈r∈oAeff
50
0
0
0.5
1
1.5
2
Inverse capacitance 1/C
2.5
1/pF 3
Correction factors
¾ Correction factors outside the chamber volume were
determined by experiments and Monte Carlo Calculations
¾ Correction factors for attenuation, scattering in the air and
in the walls, secondary electron equilibrium, etc inside the
chamber volume were determined in total as the product of
all single corrections by Monte Carlo Calculations (MCNPx)
¾ Nearly all correction factors are energy dependent
¾ Therefore, for each type of source the spectral distribution
has to be measured
Spectral photon distribution of
two different types of Iodine-seeds
10000
8000
S06
S17
ΦE(E)
6000
4000
2000
0
15
20
25
30
E
35
keV
40
Variation of some correction factors
with the content of silver
Content of silver
0%
5%
10%
20%
Attenuation in the AL-filter
1,0384 1,0409 1,0428 1,0472
Attenuation in the entrance foil
1,0012 1,0012 1,0012 1,0013
Attenuation in the air from
source to measurement point
1,0144 1,0150 1,0154 1,0164
1,0546 1,0578 1,0601 1,0658
Correction factors for effects inside
the chamber volume are determined by MCC
kinside ( s ) = Π ki ( s )
i
lim(Edep ( s ' ) / M ( s ' ) ) d + s
⋅
kinside ( s ) = s '→0
Edep ( s ) / M ( s )
s
with
d +s
1
=
s
k div
MCNPX - Model of the GROVEX
Product of the correction factors
inside the chamber volume, kinside(s)
1.02
1.01
kinside
1.00
0.99
ISO N20
ISO N30
ISO N40
0.98
0.97
0
50
100
100% I
50% I 50% Ag
150
electrode separation s
200
Uncertainty budget of the GROVEX
according to the GUM
Reason of the uncertainty
Ionization current measurement (reproducibility)
Electrode separation
Air density and humidity
Electrode area
Source-to-measurement point distance
Incomplete ion collection
Attenuation in the Al-filter
Attenuation in the entrance window
Attenuation and scatter between source and entrance window
Attenuation and scatter in the chamber volume
Source holder
combined uncertainty (k=1)
Uncertainty U(k=2)
u [%]
0,5
0,06
0,05
0,5
0,035
0,03
0,5
0,12
0,12
0,12
0,06
0,9
1,8
index
32,6%
0,4%
0,3%
33,0%
0,2%
0,1%
27,8%
1,7%
1,7%
1,7%
0,4%
Intercomparisons (2005)
GROVEX / PTB-primary standard for air-kerma PK100
GROVEX (PTB) / VAFAC (UW) / WAFAC (NIST)
Reference: Wes Culberson, Dissertation, University of Wisconsin (2006)
Calibration results
(regardless of anisotropy effects)
Determination of the azimuthal and polar
anisotropy
Szintillator
source
d=80cm
Athimuthal and polar anisotropy
90
1.10
120
source E 07-0007
60
1.05
1.00
150
0.95
30
0.90
polar
0.85
0
0.80
180
0
1.0
330
30
0.85
0.8
0.90
0.95
0.6
210
1.00
0.4
1.05
0.2
1.10
300
60
330
240
300
270
0.0
270
90
0.2
azimuthal
0.4
0.6
120
240
0.8
1.0
210
150
180
Conclusions and outlook
¾ PTB has developed a primary standard for low energy photon sources
½ GROVEX measurement geometry is different than NIST WAFAC
½ Extrapolation measurements instead of two-volume technique
½ Uncertainty for the mesurement of reference air-kerma rate is 1,8 %
¾ intercomparison with PTB air kerma standards agree within ca. 1%
¾ Intercomparison with 125I and 103Pd brachytherapy seeds calibrated
at NIST, PTB and Univ. Wisconsin show agreement better than 1%
¾ The overall uncertainty of a calibration of a specific source is about
3 % due to anisotropy effects
¾ Future activities
½ A key comparison under the leadership of BIPM is desirable
½ An European calibration network should be installed in the near future
½ An European protocol (on the basis of TG43) should be developed
( DIN 6808-2 is just under review )
½ Performance standard on dosemeters for low energy photon sources should be
developed (IEC work for well-type chambers is already in progress)