User manual

131
APPENDIX C
Post-Exposure Change
Exposure of radiochromic film to ionizing radiation starts a solid-state polymerization in
crystals of the active component. Polymer grows within the crystal matrix of the
monomer. Interatomic distances in the polymer are shorter than in the monomer
causing the gap between the end of the growing polymer chain and the next monomer
molecule to increase as polymerization progresses. Consequently the rate of
polymerization decreases with time. Based on measurement, the response is linear with
log(time-after-exposure) as shown in the figure below. This means that an error in the
dose-response function could result if calibration films are scanned at different times-
after-exposure. Since the calibration protocol requires exposed films to be scanned
together at the same time the time-after-exposure for the films will be different.
However, if the timing difference is small, i.e. the films are exposed within a narrow time
window, any error caused by the timing difference will diminish rapidly as the ratio of the
timing difference to the time-after-exposure decreases.
y = -475.61x + 41669
R² = 0.9676
y = -478.84x + 32849
R² = 0.9914
Scanner Response
Log10(time-after-exposure, minutes)
Post Exposure Changes in
EBT2/EBT3 Film
69cGy
163cGy