  Thick back-illuminated deep-depletion CCDs have superior quantum efficiency
over previous generations of thinned and traditional thick CCDs. As a result,
they are being used for wide-field imaging cameras in several major projects.
We use observations from the Pan-STARRS $3\pi$ survey to characterize the
behavior of the deep-depletion devices used in the Pan-STARRS1 Gigapixel
Camera. We have identified systematic spatial variations in the photometric
measurements and stellar profiles which are similar in pattern to the so-called
"tree rings" identified in devices used by other wide-field cameras (e.g.,
DECam and Hypersuprime Camera). The tree-ring features identified in these
other cameras result from lateral electric fields which displace the electrons
as they are transported in the silicon to the pixel location. In contrast, we
show that the photometric and morphological modifications observed in the GPC1
detectors are caused by variations in the vertical charge transportation rate
and resulting charge diffusion variations.
