Introduction

A modification of the Fricke solution system to make it suitable for low-level dosimetric studies was proposed about three decades ago [1]. A xylenol-orange metal-ion indicator is added to the standard Fricke reagent allowing the radiation effects to be detected by a visible colour change. Ferric ions form a complex with xylenol whose maximum absorption is at approximately 550 nm, whilst ferrous ions do not form the same complex. Hence, changes in the ferric ion concentration result in proportional changes in the light absorption. This modified chemical dosimetry system was infused into agarose gel and the resulting visible changes were suggested to as an alternative to the MRI readout for 3-D dosimetry [2].

In this poster, we describe our method of producing the colour-change Fricke Xylenol-Orange Gelatin (FXG) gel and examine the following issues concerning its properties:

• How much more sensitive is the colour-change gel than its original UV-active predecessor?

• How stable is the gel to natural oxidation?

• How reproducible are the results?

Investigation into the radiochromic FXG gel dosemeter: stability and uncertainty in optical measurements

Acknowledgements

MB is sponsored by the Radiation Protection Department, Atomic Energy Commission of Syria, Damascus

References

[1] B L Gupta, IAEA-sm-160, 421 (1972)[2] A Appleby, A Leghrouz, Med. Phys. 18, 309 (1991)[3] M J Day, Phys. Med. Biol. 35, 1605 (1990)

Conclusions

The radiochromic gel detector clearly has the advantage of being much more sensitive than other Fricke systems proposed to measure radiation dose distributions in three-dimensions. Unwanted side effects are its sensitivity to light, temperature and storage time. Nevertheless, in a long-term study results the dose-response of the system was reproducible to 10% despite the use of different batches of chemicals.

S M A Bero, S J Doranand W B Gilboy

Department of Physics,University of Surrey,Guildford, GU2 7XH, UK

A= 0.0007t + 0.1151

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Figure 2: Change in the optical absorbance of FXG gel with time when stored under three different conditions

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Figure 3: Repeatability study for different batches of the FXG gel made using nominally the same production technique. Chemical batches and suppliers varied.

• relatively low melting point and hence very little loss ofdissolved oxygen during gel manufacture;

• high degree of transparency (others materials, e.g., agarose and agar, are translucent);

• important role in the enhancement of the dosemeter chemical yield of Fe3+ (hence no need for the addition of any other organic substances, such as benzoic acid).

Materials and methods

Gelatin powder (C17H32N5O6)x was used for almost all studies and was employed because it has important properties for Fricke colour-change measurements:

The other reagents used in our preparations were as follows: ferrous ammonium sulphate hexahydrate, Fe(NH4)2(SO4)2 6H2O; the sodium salt of xylenol orange, C31H28N2O13 SNa4; sulphuric acid, purified water, obtained from a Milli-Q ion-exchange purification system (Millipore UK-Limited, Watford, England). The quality of the water was found to have a significant effect on the gel response.

The system is usually made of two parts, the gel part, which provides 75% of the final volume and the active chemicals that make up the other 25%. The first part contains 5% (by weight of the final solution) gelatin powder, mixed with 75% of the dosemeter volume water, and is left for about 15 minutes to absorb. Then the water-gelatin mixture is heated and continuously stirred until the solid particles have completely dissolved and a clear solution is obtained. This occurs at about 40oC, but in practice, heating is continued up to 45oC. The active chemical part normally contains the following concentrations of reagent (calculated with respect to the final volume of the gel dosemeter): 0.5mM Fe2+, 0.25mM H2SO4, 0.1mM xylenol orange. The two parts were mixed together at about 35oC and the resulting solution left to solidify in a fridge.

Results

Dose-Response: Fig. 1 shows a comparison between the optical absorbance dose-response curves for FXG (measured at 585 nm), non-radiochromic Fricke gel (at the “traditional” UV wavelength of 304 nm) and the conventional ferrous-sulphate solution dosemeter (also at 304 nm). The changes in the optical absorption are approximately 23 times larger in the FXG compared with that of the standard ferrous sulphate solution and it has been suggested [4] that using such a system, doses as small as 0.01Gy might be measureable.

Stability under different storage conditions: It is well known that ferrous ions are naturally oxidised to ferric in the presence of dissolved oxygen and this process depends on the concentrations of ferrous ions as well as the oxygen in the aerated aqueous medium. It has been reported that the natural

oxidation effect is proportional to the square of the initial Fe2+ concentration and to the first power of dissolved oxygen. Over long periods, the formation of the colour complex is strongly influenced by the ambient temperature and light, as can be seen from Fig. 2. In this study, a change in the optical absorbance of about 710-4 cm-1 per hour of storage time was observed for samples kept in the dark at 4oC.

Repeatability of measurements: A long term study has been undertaken over three years, involving the manufacture of 25 separate batches of gel. In each case, 10 samples of a batch of gel were irradiated at each of a number of doses. Typical coefficients of variance of optical absorption within a gel batch were 0.14% at 440 nm and 1.3% at 585 nm. However, studying different dosemeter batches prepared with the same procedure revealed much larger variations in the dosemeter sensitivity to ionising radiation, as shown in Fig. 3. The overall coefficients of variance were 11% and 10% for wavelengths 440nm and 585nm respectively. Average dose response curve slopes were -0.035 cm-1 Gy-1 and 0.077 cm-1

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Figure 1: Comparison between the optical dose response for “traditional” Fricke solution and Fricke gel (both at 304 nm, the standard UV spectrophotometry wavelength) and FXG gel at 585 nm, the peak of the absorption curve