I. Introduction
The detection, characterization, localization, and mapping of gamma-ray source distributions are critical in fields such as gamma-ray astronomy, medical imaging, and nuclear security. In the case of nuclear security, free-moving systems are needed to efficiently find sources in potentially cluttered environments or complex shielding scenarios as well as to quickly map radiation fields across large areas. The distinction of a free-moving system is made here to discriminate between portable systems that are designed to acquire data in several static positions and mobile systems designed to acquire gamma-ray data while the system is in continuous motion with no constraints on position or orientation.