GAD dosimeters characteristics

The GAD dosimeters are cast PVC films, yellow in colour, containing azodyes. There are four types of GAD radiochromic film dosimeters:

- GAD 1 - 0.100 mm thick, with 4-methoxy-4'-dimethylazobenzene in concentration 0.25 wt%, useful dose interval 0.05 - 2 kGy, colour changes through orange to red with irradiation;

- GAD 2 - 0 . 1 0 0 mm thick, with 4-dimethylaminoazobenzene in concentration 0.25 wt%, useful dose interval 0.05 - 2 kGy, colour changes through brown to violet with irradiation;

- GAD 4 - 0.055 mm thick, with 4-methoxy-4'-dimethylaminoazobenzene in concentration 0.5 wt%, useful dose interval 0.5 - 5 kGy, colour changes through orange to red with irradiation;

* Work partially supported by IAEA research contract No. 8528.

- GAD 6 - 0.025 mm thick, with 4-methoxy-4'-dimethylaminoazobenzene in concentration 1 wt%, useful dose interval 1 - 1 0 kGy, colour changes through orange to red with irradiation.

The dosimeters are stored at room temperature in light-proof paper packages before and after irradiation.

2.2. Optical measurements

The parameter of the dosimeters under evaluation is optical density measured 24 hours after the irradiation on a spectrophotometer Specord UV-Vis at X=555 nm with accuracy of 1%.

2.3. Calibration

The investigation of the stability of the dosimeter response with time has shown that the GAD dosimeters can be supplied with two calibration curves, constructed 1 hour and 24 hours after irradiation. In the present investigation, calibration of each batch was done 24 hours after irradiation.

The irradiation was done in a laboratory gamma-installation "Issledovatel", with the volume of the irradiation chamber of 4 L. The position of the dosimeters (three for each dose) in the chamber was fixed by a specially constructed device. The dose rate in this position is determined by chlorobenzene dosimetry and by alanine dosimeters supplied by the IDAS of the IAEA. On May 15 1997, the measured dose rate was 1.66±0.07 kGy/h.

The uncertainty of the calibration at room temperatures (20 - 25 °C) is about to 5 %, while the overall uncertainty of the measurement with the dosimetry systems is 10 % at 0.95 confidence level.

2.4. Irradiation

In order to keep the irradiation temperature comparatively constant, especially below 0 °C, a thermos flask was used with double walls filled with alcohol and hermetically closed. The temperature of alcohol was fixed before pouring it into the flask - by heating or cooling. The alcohol temperature during irradiation was monitored by a digital thermometer connected to a thermocouple immersed into alcohol. Its accuracy was ±0.1°C. A series of preliminary experiments were carried out in order to follow the change of the temperature in the chamber with time. It was established that the temperature rise was about 1°C per hour in the low temperatures interval. For irradiation at room temperature, the rise was insignificant. A procedure was established to replace alcohol, after an increase of 2 °C, with a new portion with the appropriate temperature. This had to be done for doses above 2 kGy.

The dosimeters were placed in a glass vessel with a volume equal to that of the calibration volume of the device and with a fixed position in the thermos flask. The vertical deviation was less than

±2%. The films in their packages were placed into the vessel and then the latter was immersed into alcohol. Three dosimeters were used for each irradiation. Several batches from each type of GAD dosimeters were checked.

Figures 1 to 4 present the calibration curves of two batches for each of the four types of dosimetry systems, constructed 24 hours after the irradiation at room temperature.

FIG.l. Calibration curves of GAD I, batch No. 15 (series 1) and 17 (series 2)

FIG. 2. Calibration curves of GAD 2, batch No. 18 (series 1) and20 (series 2)

1.6 -r

1.4 -1.2 -•

1 •-0.8 ••

•§ 0.6 - • J 0.4 - •

Q.

O 0.2 0

-£

re f

2 3 Dose, kGy

FIG. 3. Calibration curves of GAD 4, batch No. 9 (series 1) and 21 (series 2)

1.6 -r

1.4 --1.2 -•

1 -•

JQ

S

f 0.8 0.6 S 0.4

0.2 + 0

0 4 6

Dose, kGy

10

FIG. 4. Calibration curves of GAD 6, batch No. 24 (series 1) and 12 (series 2)

-14

Temperature, C

FIG. 5 Quantitative parameter 6vs temperature of irradiation for GAD 1 series 1 -0.45 kGy, series 2-1 kGy, series 3-2 kGy

FIG 6. Quantitative parameter 6 vs temperature of irradiation for GAD 2 series 1 -0.45kGy, series 2-1 kGy, series 3-2 kGy

-14 -L

Temperature, C

FIG. 7. Quantitative parameter 9 vs temperature of irradiation for GAD 4 series 1-1 kGy, series 2-2kGy, series 3-5 kGy

-14 -I

Temperature, C

FIG. 8. Quantitative parameter 6 vs temperature of irradiation for GAD 6 series 1-2 kGy, series 2-5 kGy, series 3-7.5 kGy

3. RESULTS AND DISCUSSION

To make the quantitative estimation of the effect of the irradiation temperature on the dose measurements, the deviation 6 was calculated for each irradiation for the corresponding temperature as in Eq. (1):

9(%) = (D(T)-D(25°C))/D(25°C) (1) where, D(T) - the dose absorbed at temperature T and calculated from the calibration curve for 25 °C,

D(25 °C) - the dose absorbed at 25 °C and calculated from the calibration curve for 25 °C.

This parameter represents the error introduced in the measured dose value at various irradiation temperatures by the use of the calibration relationship determined at the room temperature (20-25 °C).

Figures 5 to 8 illustrate the values of 0 for the four types of dosimeters.

The sources of uncertainty for the experiments, including the characteristics of the used dosimeter batches, were investigated, and it was established that all the factors affecting the experiments at temperatures 20-25 °C lead to a maximum uncertainty of 7%. Thus, any value of the deviation 0 above 7% could be attributed to the effect of the irradiation temperature.

It can be observed for GAD 1 (batch No 39) and GAD 2 (batch No 41) that all the deviations are below 7 % on the first and second dose levels (doses below 1 kGy). While for the third dose level, the deviations at low temperatures vary from about -10 to -14 %. Similar results have been obtained for the other investigated batches and they are not shown here.

The results for GAD 4 ( batch No 36) and GAD 6 ( batch No 31) are some what similar to those for GAD 1 and 2 for temperatures below 0 °C - below 1 kGy (the first dose level), the quantitative parameter 0 is less than 7 %, while for higher doses its values are -8 to -12.4 % . For temperatures above 0 °C, the values of 0 vary from -2.1 to -11 % for GAD 4 while for GAD 6 they remain below 7 % on the first dose level. For higher doses, 9 continuously decreases with the increase of the temperature from its values at 0 °C . The other investigated batches showed a similar tendency in the behaviour of 0.

4. CONCLUSIONS

The results from the investigation of the effect of the irradiation temperature on the response of the dosimeters and on the dose measured with the dosimetry systems lead to the following conclusions:

• the four types of GAD dosimeters can be used for dosimetric measurements at temperatures below 0 °C;

• a dependence of the response of the dosimeters on the irradiation temperature has been established. It varies with the dose interval and with the batch;

• the calculation of the dose absorbed at temperatures below 0 °C using the calibration curve for 20-25 °C leads to considerable deviations from the true values;

• the introduction of a temperature coefficient is not recommended, since the temperature effect depends on the dose and on the temperature interval;

• calibration of the dosimeters under the conditions of their possible application is recommended;

• the behaviour of the quantitative parameter 9 suggests the possibility of constructing a single calibration curve that is valid for the entire interval of temperatures from -18 to 0 °C

ACKNOWLEDGEMENTS

The authors would like to thank Mr. Kishor Mehta for the most helpful discussions. They also greatly appreciate the financial support by the IAEA Project No. 8528.

IAEA-SM-356/49