Using a Liquid Ion Chamber for Radiation Dosimetry

1
Using a Liquid Ion Chamber for Radiation Dosimetry

• Kristin Stewart

2
Absolute Dosimetry Protocols

• Air Kerma based
• TG-21
• IAEA TRS 277 and 381
• Absorbed Dose based
• TG-51
• IAEA TRS 398

3
Air Kerma Calibration(TG-21)

4
Absorbed Dose Calibration (TG-51)
5
Dosimeters

• Water calorimeter

Absolute

• Fricke
• Air-filled ion chamber

Relative Absolute

• Liquid ion chamber

• TLD
• Diode
• Diamond

Relative
6
Stopping Power Ratioswater/medium
7
Designing an ideal Detector forAbsorbed Dose
Calibration

• Small sensitive volume
• Medium to water stopping power ratios are beam
  quality independent
• Waterproof and made of water equivalent materials
• Stable over long term use
• Not more difficult to use than conventional air
  filled ion chambers

8
What is a LIC and how does it work?
hn
e-
e-
-

-

-

-
electrodes

insulating liquid
nC
9
What is a LIC and how does it work?
-
-
-
-
electrodes




insulating liquid
nC
10
LIC 9902 mix

• Developed by G. Wickman (Umea,
  Sweden)
• Liquid layer 0.35 mm thick
• Diameter of sensitive volume 2.5mm
• Waterproof

11
Materials

• Body
• Rexolite (water equivalent plastic)
• Electrodes
• graphite
• Liquid
• 60 isooctane 40 TMS by weight

12
Recombination

• Two types of recombination
• Initial recombination
• Ions from the same track recombine
• Independent of dose rate
• Depends on ion density along track
• General recombination
• Ions in overlapping tracks recombine
• Increases with dose rate
• Depends on density of tracks

13
Recombination

• Small (lt 2) in gas filled chambers but larger in
  liquid chambers
• High ionization density (300 times air)
• Low ion mobility (6000 times less than air)
• Liquid chambers do not achieve saturation

14
Saturation curves for air-filled chambers
15
General Recombination Theory for Gasses

• Boag (1950, 1966)
• 2 voltage technique

16
Recombination Theory for LICs
17
General Recombination for LICs in pulsed beams
Johansson et al 1997
?
(
)
i

c

c
E
D
1
2
?
?
(
)
c

c
V
/
d
D
/
D
1
1
2
chamber
o
u

d
chamber
ee
V
p
r
2
o
chamber
i
Q
exp
f


?
?
(
)
exp
i
c

c
V
/
d
D
/
D
theor
1
2
chamber
o
18
TMS General Efficiency
Johansson et al 1997
19
Experiment vs. Theory CL 2300
20
Varying dose rate by changing depth in water
21
Efficiency vs. Dose Rate
22
(No Transcript)
23
Clinac 6-EX
24
Clinac 2300
25
60Co Calibration

• Determine the dose rate for 10 x 10 cm2 field,
  SSD 80 cm, 5 cm depth in water using a calibrated
  EXRADIN A12 waterproof thimble chamber
• Measure response of LIC at same point, at 500 V
  correcting for polarity effects
• Correct for recombination to doserate of 200
  cGy/min

26
kQ Measurements

• Determine dose to water for 10 x 10 cm2 field at
  100 cm SSD,10 cm depth in water with calibrated
  EXRADIN A12 chamber
• Measure response of LIC at same point for 500 V
  correcting for polarity effects
• Correct for recombination to dose rate of
  200 cGy/min

27
(No Transcript)
28
(No Transcript)
29
Whats wrong with this picture?
30
LIC compared to air-filled chambers
Advantages
Disadvantages

• Requires high stable voltage source (500-1000V)
• May not tolerate very high (gtfew kGy) accumulated
  doses (e.g. not useful as linac monitor chamber)
• Operate below 26oC
• Recombination correction larger and more
  difficult to determine

• Smaller sensitive volume (high resolution)
• Stopping power ratio less energy dependent
• Can be used with same electrometer as air filled
  chambers
• Low leakage current, stable and reproducible
  readings

31
Future Work

• Further study of
• Recombination
• Temperature dependence
• kQ values for
• Clinical photon beams
• Clinical electron beams
• Comparison with Monte Carlo calculations

32
Acknowledgements

• Dr. Jan Seuntjens (McGill, Montreal)
• Dr. Carl Ross (NRC, Ottawa)
• Dr. Goran Wickman (Umea, Sweden)

33
Long-term stability of TMS

• Temperature effects
• Extremely small effect on sensitivity (0.03 per
  oC)
• Considerations for thermal expansion
• Air bubble to prevent mechanical stress
• 26oC boiling point
• Sensitivity does not change with time (6 years)
  or increased radiation exposure (10000 Gy)
• No chemical interaction with other chamber
  materials

34
(No Transcript)
35
Possible Advantages

• High resolution
• Beam quality independent

36
Small sensitive volume

• Using a small sensitive volume requires a liquid
  with high ionization density
• Liquids also have lower ion mobility
• This requires
• Thin gap ( 0.3 mm)
• High voltage ( 900 V)
• to keep recombination reasonably low
• Chamber must be carefully constructed

37
(No Transcript)