  Using a series of detector measurements taken at different locations to
localize a source of radiation is a well-studied problem. The source of
radiation is sometimes constrained to a single point-like source, in which case
the location of the point source can be found using techniques such as maximum
likelihood. Recent advancements have shown the ability to locate point sources
in 2D and even 3D, but few have studied the effect of intervening material on
the problem. In this work we examine gamma-ray data taken from a freely moving
system and develop voxelized 3-D models of the scene using data from the
onboard LiDAR. Ray casting is used to compute the distance each gamma ray
travels through the scene material, which is then used to calculate attenuation
assuming a single attenuation coefficient for solids within the geometry.
Parameter estimation using maximum likelihood is performed to simultaneously
find the attenuation coefficient, source activity, and source position that
best match the data. Using a simulation, we validate the ability of this method
to reconstruct the true location and activity of a source, along with the true
attenuation coefficient of the structure it is inside, and then we apply the
method to measured data with sources and find good agreement.
