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 S M A Bero S J Doran W B Gilboy Department of Physics, University of Surrey, Guildford, GU2 7XH, UK

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

Historical perspective Chemical dosimetry has a long history (Fricke and Hart “Radiation Dosimetry 1966) [Fe 3+ ] originally estimated by titration, then by UV spectrophotometry Modification by using colour-change reaction first suggested in 1972 (Gupta, IAEA-sm-160, , 1972) Initial applications to imaging of radiation dose in early 1990’s (e.g., Appleby and Leghrouz, Med. Phys. 18, , 1991) Other investigations into dosimeter and its application (e.g., Gambarini, Kelly, Jordan, Wolodzko, Bero)

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 Fe 3+ Fricke chemicals  ferrous ammonium sulphateFe(NH 4 ) 2 (SO 4 ) 2.6H 2 O0.5mM  sulphuric acidH 2 SO 4 25mM  xylenol orange (sodium salt)C 31 H 28 N 2 O 13 SNa 4 0.1mM  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.

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

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

Lowering the acid concentration increases the slope of the dose-response characteristic significantly. Dose-response of gel: (3) Effect of acid concentration Absorbance at 585 nm / cm -1 Dose / Gy  10mM 25mM 50mM 100mM

Stability of FXG gel: (1) Acid content As the dose response improves, the stability degrades significantly. An appropriate compromise is about 50 mM. Dose / Gy Absorbance / cm mM H 2 SO 4 50 mM H 2 SO 4 “Immediate” 24 hr 48 hr

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.

Reproducibility of gel dose-response Intra-batch variability in dose-response (  /  ) = 1.3% at 585 nm Inter-batch variability = 10%, but includes changes in batches of raw materials FXG Batch Number Absorbance DRC slope at585nm DRC slope at440nm

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.