  Photo-generated charge in thick, back-illuminated, fully-depleted CCDs is
transported by electric fields from the silicon substrate to the collecting
well at the front gate of the CCDs. However, electric fields transverse to the
surface of the CCD -with diverse origins such as doping gradients, guard rings
around the imaging area of the sensor, and physical stresses on the silicon
lattice- displace this charge, effectively modifying the pixel area and
producing noticeable signals in astrometric and photometric measurements. We
use data from the science verification period of the Dark Energy Survey (DES)
to characterize these effects in the Dark Energy Camera (DECam) CCDs. The
transverse fields mainly manifest as concentric rings ("tree rings") and bright
stripes near the boundaries of the detectors ("edge distortions") with relative
amplitudes of about 1% and 10% in the flat-field images, respectively. Their
nature as pixel size variations is confirmed by comparing their photometric and
astrometric signatures. Using flat-field images from DECam, we derive templates
in the five DES photometric bands (grizY) for the tree rings and the edge
distortions as a function of their position in each DECam detector. These
templates can be directly incorporated into the derivation of photometric and
astrometric solutions, helping to meet the DES photometric and astrometric
requirements.
