M A Bero, S J Doran and W B Gilboy Department of Physics University of Surrey

1. S 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

2. Structure of talk • Historical perspective • Gel components and manufacture • Response of FXG compared with “traditional Fricke”dosimeter • Gel saturation and the effects of XO concentration • Gel stability • Gel reproducibility over time

3. 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 1990’s (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)

4. 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 45°C, stir for ~10 mins. • Mix Fricke chemicals at room temperature, combine with gel at ~35 °C.

5. 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

6. 0.1 mM Absorbance at 585 nm / cm-1 0.05 mM 0.025 mM 0.01 mM Dose / Gy 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.

7. 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.

8. 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.

9. 2.5 Dark in Fridge 2 Dark in lab Daylight in lab 1.5 Absorbance / cm-1 1 A = 710-4 t + 0.11 r2 = 0.993 0.5 0 0 200 400 600 800 Time after Irradiation (hours) 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.

10. Reproducibility of gel dose-response 0 . 12 0 . 1 0 . 08 0 . 06 0 . 04 DRC slope at 585 nm Absorbance 0 . 02 DRC slope at 440 nm 0 1 3 5 7 9 11 13 15 17 19 21 23 - 0 . 02 - 0 . 04 - 0 . 06 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.

11. 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.