FRICKE AND ALANINE
DOSIMETERS
Malcolm McEwen & Carl Ross
Chemical dosimeters
In a chemical dosimeter the absorbed dose is
d
determined
i d from
f
some quantitative
i i change
h
in
i an
appropriate material and any well-characterized
chemical reaction mayy serve as the basis for the
dosimeter.
Fricke – the basics
 The chemical species of interest is the
production of Ferric ions (Fe3+) from Ferrous ions
(Fe2+)
 The Ferrous ions are in solution
 What is irradiated is therefore mainly water
 Ionizing radiation interacting with water
produces a range of ions, radicals and molecules:
H· OH· H2 H2O2
H+
OH-
‐
eaq
Lots of products, therefore a lot of reactions
H  O2  HO2
Fe2  HO2  Fe3  HO2
HO2  H  H2O2
Fe2  H2O2  Fe3  OH  OH
Fe2  OH  Fe3  OH
Note that O2
is needed.
Dose range
g
Oxygen is the limiting factor, rather than the concentration of Fe2+
The maximum absorbed dose that can be measured is about 400 Gy.
Chemistry - preparation
 The Fricke solution consists of 1mM ferrous ammonium sulfate and
1mM sodium chloride in 0.4 M sulfuric acid.
 Contaminants can significantly affect performance therefore care
must be taken to clean all glassware carefully and to use high purity
chemicals.
 Sodium chloride is added to reduce or eliminate any sensitivity to
organic impurities.
Chemistry - preparation
C t i
Containment
t - standard
t d d
Teflon stopper
Lucite holder Containment - specialized
Readout
0.6
Optical den
ensity
0.5
0.4
0.3
0.2
0.1
0.0
200
250
300
350
Wavelength (nm)
400
Peaks at 224 nm and 303 nm
Commercial spectrophotometers usually use both
Readout
OD  log10 (I0 / I)  c  L  
 The yield of the Fe 3+ ion depends on the temperature of
the solution during irradiation (0.12% per degree C)
 The molar extinction coefficient depends on the
temperature of the solution during readout
OD
OD
DF 
3
  G(Fe )    L
Reproducibility
Precision is typically
yp
y better than 0.15 %
Stability of the dosimeter response is only slightly worse.
Energy dependence – photon beams
Energy dependence – electron beams
1.015
3+
G(Fe
e ), normalissed
1.010
1.005
1.000
0.995
0.990
1999-2000
1999
2000 data
2007 data
0.985
4
6
8
10
Es (MeV)
12
14
16
Measurement of dose
OD
DF 
3
  G(Fe )    L
Dw = DF fw,F Pwall kdd
f(Q) = fw,F Pwall
Is Fricke a primary or secondary dosimeter?
OD
DF 
3
  G(Fe )    L
A li ti
Applications
today
t d
Fricke is disappearing from even primary standards
laboratories. Does it have a future?
 Low energy electron beam dosimetry
 Ir
Ir-192
192 dosimetry
Alanine – the basics
Alanine - development
Major players:
NIST
NPL
Latecomers:
NRC
PTB
Alanine pellets
Major players:
NIST
NPL
Alanine – readout
Alanine – readout
Signal Extraction
Holders
Linearity
Relative absorbed dose sensitivity
Alanin
ne dose-to-wa
ater response relative to Co
o-60
MV p
photon beams
1.005
1.000
0.995
0.990
Sharpe and Sephton (2006)
Zeng et al (2004)
Bergstrand et al (2003)
A t ett all (2008)
Anton
0.985
0.980
0.55
0.60
0.65
0.70
TPR20,10
0.75
0.80
0.85
MV p
photon beams
rela
ative resp
ponse
1.005
Meas rement
Measurement
Monte Carlo
1.000
0.995
0 990
0.990
0.985
55
60
65
70
%dd(10)x
75
80
85
Electron beams
Relative absorbed-dose
response
(electrons/60Co)
Standard uncertainty
Bergstrand et al (2004)
0.971
0.015
Zeng et al (2005)
0.987
0.011
McEwen et al (2006)
0.986
0.012
Electron beams
relativ
ve respons
se to 60Co
1.005
0.995
0.985
0.975
Measurement
Monte Carlo
0.965
5
10
15
20
nominal energy (MeV)
25
kilovoltage
Air kerma res
sponse relative to 60Co
1.2
1.1
1.0
0.9
0.8
0.7
0.6
Regulla and Deffner (1982)
Zeng and McCaffrey (2005)
05
0.5
0.4
0.01
0.1
1
Eav ((MeV))
10
Applications - uncertainties
Component
Standard Uncertainty (10 Gy)
1. Calibration in 60Co
60Co
primary standard realization of dose
0 2 % - 0.4
0.2
04%
Irradiation temperature
0.05%
Average mass of 4-6 pellets
0.05%
Precision, set of 4-6 alanine pellets
0.1 % - 0.2 %
Overall (Calibration)
0.2 % - 0.5 %
2 Measure
2.
M
d
dose
i megavoltage
in
lt
b
beam
Calibration reference
0.2 % - 0.5 %
Irradiation temperature
0.1%
Average mass of 4-6 pellets
0 05%
0.05%
Precision, set of 4-6 alanine pellets
0.2 % - 0.3 %
Inter-pellet variations
0.3 % - 0.5 %
Correction
photons)
for
energy
dependence
Overall (Measurement of dose)
(MV
0.2%
0.5 % - 0.8 %
Applications
pp
((I)) - audit
Applications
pp
((I)) - audit
 Dosimeters
D i
and
d
phantom can be
mailed
 Useful as part off
commissioning
process
 Also used in UK
dose audits
Audit - results
Applications
pp
((II)) – Tomotherapy
py
Protocol
Results
Applications
pp
((III)) – electron depth
p dose
Summary
 Alanine and Fricke are unlikely to feature much in clinical
dosimetry
y
and have
 However, theyy are well-established systems
unique properties applicable to special situations
 Keep
p them in mind,, theyy could be useful one day!
y
Acknowledgements
c o edge e ts
Gerhard Stucki – METAS
Ge Zeng – ex NRC
Simon Duane, Peter Sharpe – NPL