AbstractID: 8133 Title: Thermal history causes nonisotropic dose response in radiochromic gelatin gel
Three dimensional optical computed tomography measurements of dose distributions
from a 12 MeV, electron beam dose distributions often revealed a spatial variation in the
gel’s response to dose. The cylinders of gel [4% by mass 300 bloom porcine gelatin, 50
mM sulfuric acid, 0.3 mM ferrous ammonium sulphate and 0.05 mM xylenol orange], 10
cm diameter and 10 cm height, were irradiated through the upper surface and optically
scanned in planes perpendicular to the rotation axis. Changes in optical density with
absorbed dose were normalized at the depth of maximum dose (Dmax) and compared to
depth doses obtained from TG21 corrected RK ion chamber data. Generally, excellent
agreement was obtained from Dmax to greater depths including the low dose ‘tail’ region.
But, at shallower depths, up to Dmax an over-response was often observed with the
deviation generally increasing with shallower depths. Experiments first revealed that
temperature gradients during gel storage were contributing to this nonisotropic gel
response. Subsequent experiments demonstrated that temperature gradients during the
gelation stage were also an important factor. This work demonstrates that different
thermal histories of gels even with minimized thermal gradients can also lead to
nonisotropic response. Gels irradiated within approximately 6 hours of initial gelation
have an excellent isotropic dose response but gels that continued the gelation
‘maturation’ process for 18 to 48 hours developed an nonisotropic response.