  UCGretina, a GEANT4 simulation of the GRETINA gamma-ray tracking array of
highly-segmented high-purity germanium detectors is described. We have
developed a model of the array, in particular of the Quad Module and the
capsules, that gives good agreement between simulated and measured photopeak
efficiencies over a broad range of gamma-ray energies and reproduces the shape
of the measured Compton continuum. Both of these features are needed in order
to accurately extract gamma-ray yields from spectra collected in in-beam
gamma-ray spectroscopy measurements with beams traveling at $v/c \gtrsim 0.3$
at the National Superconducting Cyclotron Laboratory and the Facility for Rare
Isotope Beams. In the process of developing the model, we determined that
millimeter-scale layers of passive germanium surrounding the active volumes of
the simulated crystals must be included in order to reproduce measured
photopeak efficiencies. We adopted a simple model of effective passive layers
and developed heuristic methods of determining passive-layer thicknesses by
comparison of simulations and measurements for a single crystal and for the
full array. Prospects for future development of the model are discussed.
