Fig. 7 also suggests that the thickness of CsI(Tl) in the
sample on the top is somewhat higher than optimal making
the screen slightly sensitive to gammas. This is evident from the faint presence of Pb disks seen in the top ASTM image.
The second image (on the bottom), obtained with the thinner
CsI layer shows almost no sensitivity to the gammas.
The cadmium phantoms as well as the boron nitride and
cadmium discs in the ASTM phantoms are all very sharply
defined in both the images. These data show that both the
screens have a high level of radiographic sensitivity with
respect to the step block materials. The noise level in the
figure on the top is higher than that in the figure on the
right due to lower scintillation efficiency of CsI layer in that
sensor. Scintillation efficiency is influenced by the deposition
parameters and can vary. Further investigation is
being conducted.