Investigation into the radiochromic (FXG) gel dosimeter: stability and uncertainty in optical measurements

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Department of Physics, University of
Surrey, Guildford, GU2 7XH, UK
M A Bero S J Doran W B Gilboy
Investigation into the radiochromic (FXG) gel
dosimeter stability and uncertainty in optical
measurements
M A Bero, S J Doran and W B Gilboy Department of
Physics University of Surrey
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Structure of talk

• Historical perspective
• Gel components and manufacture
• Response of FXG compared with traditional
  Frickedosimeter
• Gel saturation and the effects of XO
  concentration
• Gel stability
• Gel reproducibility over time

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Historical perspective

• Chemical dosimetry has a long history(Fricke and
  Hart Radiation Dosimetry 1966)
• Fe3 originally estimated by titration, then
  byUV spectrophotometry
• Modification by using colour-change reaction
  first suggested in 1972 (Gupta, IAEA-sm-160,
  421-432, 1972)
• Initial applications to imaging of radiation dose
  in early 1990s (e.g., Appleby and Leghrouz, Med.
  Phys. 18, 309-312, 1991)
• Other investigations into dosimeter and its
  application (e.g., Gambarini, Kelly, Jordan,
  Wolodzko, Bero)

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Gel manufacture

• Gel component of dosimeter
• Gelatin (5 by final weight, 300 bloom) 75 of
  total water
• Highly transparent
• Low melting point ? little loss of dissolved
  oxygen
• Macromolecule, enhances g-value for production of
  Fe3
• Fricke chemicals
• ferrous ammonium sulphate Fe(NH4)2(SO4)2.6H2O 0.5
  mM
• sulphuric acid H2SO4 25 mM
• xylenol orange (sodium salt) C31H28N2O13
  SNa4 0.1 mM
• water (25 of total) Milli-Q reverse osmosis
• Method
• Leave gelatin to swell in cold water, dissolve at
  45C, stir for 10 mins.
• Mix Fricke chemicals at room temperature, combine
  with gel at 35 C.

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Dose-response of gel (1) Comparison with
traditional Fricke

• Response of FXG approximately 23 times larger
  than standard Fricke solution
• Highly linear dose response up to 30 Gy

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Dose-response of gel (2) Xylenol orange
concentration

• Changing the concentration of XO does not
  markedly alter the slope of the dose-response
  characteristic.
• However, it does change the point at which
  saturation occurs.

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Dose-response of gel (3) Effect of acid
concentration
10 mM 25 mM 50 mM 100 mM

Absorbance at 585 nm / cm-1
Dose / Gy

• Lowering the acid concentration increases the
  slope of the dose-response characteristic
  significantly.

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Stability of FXG gel (1) Acid content
10 mM H2SO4
50 mM H2SO4
48 hr
Absorbance / cm-1
Immediate
24 hr
Dose / Gy

• As the dose response improves, the stability
  degrades significantly.
• An appropriate compromise is about 50 mM.

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Stability of FXG gel (2) Storage conditions

• Storage conditions affect the gel absorbance
  markedly.
• For a refrigerated gel, in the dark, the change
  in absorbance is highly linear over a period of
  weeks.

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Reproducibility of gel dose-response

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DRC slope at
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Absorbance
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DRC slope at
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FXG Batch Number

• Intra-batch variability in dose-response (s/m)
  1.3 at 585 nm
• Inter-batch variability 10, but includes
  changes in batches of raw materials.

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Conclusions

• The FXG polymer gels are much easier to make than
  polymer gels.
• FXG is much more sensitive than the original
  Fricke solution.
• The concentration of acid is a balance between
  dose-response and stability.
• Storage conditions are important.
• Intra-batch repeatability is very good, but
  intra-batch repeatability has been relatively
  poor.