High accuracy dosimetry with small pieces of Gafchromic films.

Aim: To investigate the influence of several factors on the accuracy of dose measurements and feasibility of application of small Gafchromic detectors for postal audit.

Background: Our experience showed that precision of dose measurements with small pieces of Gafchromic films may be significantly improved.

Materials and methods: Gafchromic films with dimensions of 1 × 1, 2 × 2 and 3 × 3 cm2 were exposed to 6 MV X-rays at dose levels of 50 cGy-210 cGy. The single- and multichannel methods (MM) were used for dose measurements. Detectors were scanned with an Epson V750PRO flatbed colour scanner. For 1 × 1 and larger detector sizes, separate calibration curves were established. The influence of the following factors was investigated: the heterogeneity of Gafchromic detectors group for single- and MM, ambient thermal detector conditions, the dose delivered on the measurement accuracy, application of two separate calibration curves for the smallest and larger pieces of films.

Results: The MM improves significantly the precision of dose measurement. The uncertainty attributed to detector active layer differences and scanner instabilities was about 1 cGy (1 StDev) regardless of dose and detector size. The ambient temperature of the environment in which films were stored after irradiation influenced the dose reading. Significant difference of transmission for detectors sized 1 × 1 and 2 × 2cm2 was observed. The maximal difference between applied dose and dose reading performed was 1.1%.

Conclusions: The MM with a scaling protocol leads to a very high precision of dose measurements. The ambient thermal detector environment causes significant changes of measured signal. The detector size has relevant impact on dose reading.