  The ability to map and estimate the activity of radiological source
distributions in unknown three-dimensional environments has applications in the
prevention and response to radiological accidents or threats as well as the
enforcement and verification of international nuclear non-proliferation
agreements. Such a capability requires well-characterized detector response
functions, accurate time-dependent detector position and orientation data, a
digitized representation of the surrounding 3D environment, and appropriate
image reconstruction and uncertainty quantification methods. We have previously
demonstrated 3D mapping of gamma-ray emitters with free-moving detector systems
on a relative intensity scale using a technique called Scene Data Fusion (SDF).
Here we characterize the detector response of a multi-element gamma-ray imaging
system using experimentally benchmarked Monte Carlo simulations and perform 3D
mapping on an absolute intensity scale. We present experimental reconstruction
results from hand-carried and airborne measurements with point-like and
distributed sources in known configurations, demonstrating quantitative SDF in
complex 3D environments.
