UC Berkeley

An approach to gamma-ray imaging has been developed that enables near

real-time volumetric (3D) imaging of unknown environments thus improving the

utility of gamma-ray imaging for source-search and radiation mapping

applications.

The approach, herein dubbed scene data fusion (SDF), is based on integrating

mobile radiation imagers with real-time tracking and scene reconstruction

algorithms to enable a mobile mode of operation and 3D localization of

gamma-ray sources.

The real-time tracking allows the imager to be moved throughout the environment

or around a particular object of interest, obtaining the multiple perspectives

necessary for standoff 3D imaging.

A 3D model of the scene, provided in real-time by a simultaneous localization

and mapping (SLAM) algorithm, can be incorporated into the image reconstruction

reducing the reconstruction time and improving imaging performance.

The SDF concept is demonstrated in this work with a Microsoft Kinect RGB-D

sensor, a real-time SLAM solver, and two different mobile gamma-ray imaging

platforms.

The first is a cart-based imaging platform known as the Volumetric Compton

Imager (VCI), comprising two 3D position-sensitive high purity germanium (HPGe)

detectors, exhibiting excellent gamma-ray imaging characteristics, but with

limited mobility due to the size and weight of the cart.

The second system is the High Efficiency Multimodal Imager (HEMI) a

hand-portable gamma-ray imager comprising 96 individual cm$^{3}$ CdZnTe

crystals arranged in a two-plane, active-mask configuration.

The HEMI instrument has poorer energy and angular resolution than the VCI, but

is truly hand-portable, allowing the SDF concept to be tested in multiple

environments and for more challenging imaging scenarios.

An iterative algorithm based on Compton kinematics is used to reconstruct

the gamma-ray source distribution in all three spatial dimensions.

Each of the two mobile imaging systems are used to demonstrate SDF for a

variety of scenarios, including general search and mapping scenarios with

several point gamma-ray sources over the range of energies relevant for

Compton imaging.

More specific imaging scenarios are also addressed, including directed

search and object interogation scenarios.

Finally, the volumetric image quality is quantitatively investigated with

respect to the number of Compton events acquired during a measurement, the

list-mode uncertainty of the Compton cone data, and the uncertainty in the

pose estimate from the real-time tracking algorithm.

SDF advances the real-world applicability of gamma-ray imaging for many

search, mapping, and verification scenarios by improving the tractiblity

of the gamma-ray image reconstruction and providing context for the 3D

localization of gamma-ray sources within the environment in real-time.