The image was obtained by illuminating the face of the detector
with gamma-rays (122 and 136 keV) from a Co-57 source.
Though it is possible to see the scintillator pixellation when illuminated
with 28-35 keV photons from I-125 the image is very
much washed out and individual crystal elements are more difficult
to identify as when using Co-57. This is because of the intrinsic
position resolution of the Hamamatsu PSMTs is directly
proportional to the number of photoelectrons emitted at the photocathode.
Though nearly each crystal can be identified some
distortions exist which are caused by less than ideal selection
of de-coupling resistors in the resistor matrix (see Fig. 3). We
have developed crystal element mapping software to produce a
crystal-to-pixel map that performs better with the flood image
obtained with the Co-57 source. The crystal array is not ideal
for I-125 imaging but was obtained to be a useful experimental
array for various energies.