Irradiation temperature and post irradiation storage

The irradiation temperature and post-irradiation storage studies used the Harwell calibration irradiation assembly [6]. The dosimeters were irradiated in sets of four with a dose rate of 1.4 Gys'1. The five irradiation positions and the storage bath in the lead sample shield were both thermostatically controlled. This arrangement, allowed for temperature control both during and after irradiation. The dosimeters were resealed after measurement and returned to temperature controlled storage.

The sensitivity of Gammachrome YR® dosimeters generally decreased with increasing temperature, as found above. However, over the 30 to 50 °C range the effect was larger at the 0.1 kGy than the 3 kGy dose level, not as expected. The apparent inconsistency may be due to the storage for 2 hours at elevated temperatures before measurement.

Table III shows values of km measured at increasing temperature, for 2, 24 and 48 hours storage time. The ratio of km with respect to 30°C is also given for comparison purposes. Gammachrome YR®

dosimeters need 2 hours at room temperature before measurement, to allow full colour development.

5.5 Gy/s, specific abs. change with temperature, dose as

1.1 Gy/s, specific abs. change with temperature, dose as

t

0.5 Gy/s, specific abs. change with temperature, dose as shown

FIG. 1. Gammachrome YR® batch 4 change in k^m with increasing irradiation temperature

j

•8

t£3o

Q.in

'

5.5 Gy/s, specific abs. change with temperature, dose as shown

1.1 Gy/s, specific abs. change with temperature, dose as shown

0.5 Gy/s, specific abs. change with temperature,dose as shown

F/G. 2. Gammachrome 17?® 6oto/j 5 change in km with increasing irradiation temperature.

TABLE III. THE INFLUENCE OF IRRADIATION AND STORAGE TEMPERATURE Irradiation dose 0.1 kGy, values ofk^m, mm"1

Batch Storage Time Irrad/Storage

TABLE IV. THE INFLUENCE OF POST IRRADIATION STORAGE Irradiation dose 0. 1 kGy, values of km , mm"' for the storage times shown

Batch Irrad/Storage

Storage Time Storage Time Ratio 2 Hours 24 Hours 24h/2h

0.180 0.166 0.918 Irradiation dose 3 kGy, values of km, mm"' for the storage times shown

4

The 2 hours storage time at elevated temperature was assumed to be of little significance. However, as noted above, this may not be an entirely legitimate assumption.

Table IV gives values of km for 2, 24 and 48 hours storage time in relation to temperature. The table also shows the ratio to 2 hours for comparison purposes.

3. DISCUSSION

The measurement uncertainty of Harwell Gammachrome YR® dosimeters is quoted as ±3% when applied to specific absorbance values. Therefore, ratios from 0.97 to 1.03 do not indicate significant differences.

3.1 Dose rate and irradiation temperature studies

The effect of dose rate on Gammachrome YR® dosimeters is small as shown in Table I, less than

±3% over the range tested, at -20 °C. This increases at higher temperatures. The dosimeters were generally more sensitive at higher dose rates.

The effect of irradiation temperature was interesting with both batches exhibiting very similar trends, see Figs 1 and 2. The response of the dosimeters at the lowest dose levels, such as 0.1 kGy, was not temperature dependent over the range tested. At higher dose levels of 1 kGy and above, they showed a marked linear trend with negative temperature coefficients ranging from -0.2 to -0.5% per °C.

Biramontri et al [4] reported a similar temperature coefficient above -78°C, of -0.3% per °C for Gammachrome YR® batch 3 at a dose of 2 kGy. The dosimeter shows the highest sensitivity, over the dose range covered, at -20°C. This is a useful property for the irradiation of food at low temperatures.

3.2 Irradiation temperature and post irradiation storage

Post-irradiation storage of dosimeters at elevated temperatures results in a decrease in the specific absorbance, or post irradiation fading, with time. This is much greater at the 0.1 kGy dose level, than at 3 kGy as shown in Table II. Post-irradiation storage at elevated temperatures was the dominant factor affecting the measured response and should be avoided or corrections made.

4. CONCLUSIONS

Harwell Gammachrome YR® dosimeters are affected by irradiation dose rate, but the results are not significant at -20°C over the range tested. Significant dose rate effects occur at 0 °C and above.

The response to temperature over the range -20 to 35 °C showed a linear temperature coefficient between -0.2 and -0.5% per °C, for doses of 1 kGy and above. The dosimeter is more sensitive at -20°C, a useful property for the irradiation of food at low temperatures.

Post irradiation storage at elevated temperature is a very significant source of error. Prolonged storage for 24 or 48 hours will lead to an under estimate of the apparent dose, unless the calibration regime makes due allowances.

These studies reinforce the recommendation that routine dosimeters should be calibrated as close to the conditions of use as possible [2].

REFERENCES

[1] WHITTAKER, B., "A new PMMA dosimeter for low doses and temperatures", Radiat. Phys.

Chem. 35, 4-6, 699, 1990.

[2] GLOVER, K.M., PLESTED, M.E., WATTS, M.F., and WHITTAKER, B., "A study of some parameters relevant to the response of Harwell PMMA Dosimeters to gamma and electron irradiation", Radiat. Phys. Chem. 42, 4-6, pp. 739 -742, 1993.

[3] SOHRABPOUR, M, KAZEMI, A.A., MOUSAVI, H., and SOLATI, K., "Temperature response of a number of plastic dosimeters for radiation processing", Radiat. Phys. Chem., 42, 783, 1993.

[4] BIRAMONTRI, S., HANEDA, N., TACfflBANA, H., and KOJIMA, T., "Effect of low irradiation temperature on the gamma-ray response of dyed and undyed PMMA dosimeters", Radiat. Phys. Chem., 48, 1, pp. 105-109, 1996.

[5] FAIRAND, B., "Calibration of a polymethylmethacrylate routine dosimetry system", Radiat.

Phys. Chem., 52, 1, pp. 523-536, 1998.

[6] WHITTAKER, B., WATTS, M.F., "The influence of ambient temperature and time on the radiation response of Harwell Red 4034 PMMA dosimeters", IEAE-SM-356/51, 1998.

IAEA-SM-356/54 ESR/ALANINE DOSIMETRY: STUDY OF THE KINETICS OF

FREE RADICAL FORMATION — EVALUATION OF ITS CONTRIBUTION