Donald Gunter

First-generation hybrid compact Compton imager

At Lawrence Livermore National Laboratory, we are pursuing the development of a gamma-ray imaging system using the Compton effect. We have built our first generation hybrid Compton imaging system, and we have conducted initial calibration and image measurements using this system. In this paper, we present the details of the hybrid Compton imaging system and initial calibration and image measurements

Simulation of light output from narrow sodium iodide detectors

The use of narrow scintillators in imaging devices raises the question of whether there is enough light output that can yield reasonable energy and spatial resolutions. Compared to scintillation within large-area detectors (such as those used in conventional gamma cameras), scintillation photons within narrow detectors are expected to undergo more reflections because of the proximity of the detector surfaces. In this study, we use simulation methods to estimate the light output from long, narrow sodium iodide crystals, and to investigate the effects of detector geometry and detector surface reflection properties on the fraction of scintillation photons that are able to leave the crystal through the exit window (on one of the long, narrow sides). Our simulations show that the light output from a 3.5-mm wide by 10-mm high by 300-mm long detector can result in reasonable energy resolutions. Our simulations also suggest that, in conjunction with the use of external reflectors, a crystal that has its sides polished to a smooth finish results in better light output than one with sides grounded to a rough finish.

A Filtered Back-Projection Algorithm for

Compton imaging is a gamma-ray imaging technique useful for photons with energies in the range of a hundred keV to several MeV. Measuring gamma rays with a Compton camera results in cone data that needs to be mathematically inverted to determine the incident flux distribution. In the past, filtered back-projection solutions for Compton telescope data required sums of spherical harmonics or stereographically mapping the back-projection, which can result in imaging artifacts. We present a solution to this inversion problem that removes these complexities by embedding the 2-D directional image on the surface of a sphere S2 into R3 where it is easily solvable. In this manner we relate 2-D Compton 4π imaging to the 3-D Radon transform, which has known solutions. To accomplish this, the cone data is converted to planar data. Additionally we show how the planar geometry can be used to produce a computationally efficient implementation. This reconstruction is demonstrated with a two-plane, double-sided strip, HPGe Compton camera.

4\pi

We present a feasibility study of real-time radioactive source localization in which the effects of low count rates on source localizations with a moving coded-aperture detector system are addressed. The conventional crosscorrelation method with the installed binary mask was not reliable enough to filter out background noise at low count rates in this study. To improve the cross-correlation performance, we adopted a new binary mask design method for future work. Offline data processing to mimic on-line data processing was based on multi-CPUs and multi-GPUs (graphics processing units) parallel processing. We also show an iterative list-mode localization method using background-free simulated data.

Compton Camera Data

When evaluating detectors and algorithms for nuclear threat detection in populated environments, introducing actual sources is usually neither feasible nor economical. It is more practical to move the detector, (which is either airborne, vehicle-borne, or human portable) through the test environment and then to later artificially superimpose either measured or simulated source signatures on the recorded background data. We present a source injection algorithm that can be used to inject either measured or simulated source data into list-mode background data. It is designed to accommodate cases where measured “injection data” are available only at a limited set of locations. We describe a sampling scheme suitable for obtaining measured source injection data. We then use these data to demonstrate the source injection algorithm applied to list-mode data collected with a helicopter-borne detector system, where arbitrary detector poses and trajectories are possible. Stochastic methods are used both to select source events from a dataset containing both source and background events, and to scale the number of selected events to match the field conditions of detector-source distance, air attenuation, acquisition duration, and the relative strength of the measured and injected sources. This algorithm is planned to be used as part of the Airborne Radiation Enhanced-sensor System (ARES) Advanced Technology Demonstration.

Real-time radioactive source localization with a moving coded-aperture detector system at low count rates

Gamma-ray imaging utilizing Compton scattering has traditionally relied on measuring coincident gamma-ray interactions to map directional information of the source distribution. This coincidence requirement makes it an inherently inefficient process. We present an approach to gamma-ray reconstruction from Compton scattering that requires only a single electron tracking detector, thus removing the coincidence requirement. From the Compton scattered electron momentum distribution, our algorithm analytically computes the incident photons correlated direction and energy distributions. Because this method maps the source energy and location, it is useful in applications, where prior information about the source distribution is unknown. We demonstrate this method with electron tracks measured in a scientific Si charge coupled device. While this method was demonstrated with electron tracks in a Si-based detector, it is applicable to any detector that can measure electron direction and energy, or equivalently the electron momentum. For example, it can increase the sensitivity to obtain energy and direction in gas-based systems that suffer from limited efficiency.

List-mode source injection algorithm for detectors with arbitrary pose and trajectory

Gamma-ray imaging facilitates the efficient detection, characterization, and localization of compact radioactive sources in cluttered environments. Fieldable detector systems employing active planar coded apertures have demonstrated broad energy sensitivity via both coded aperture and Compton imaging modalities. However, planar configurations suffer from a limited field of view, especially in the coded aperture mode. To improve upon this limitation, we introduce a novel design by rearranging the detectors into an active coded spherical configuration, resulting in a

Gamma-ray momentum reconstruction from Compton electron trajectories by filtered back-projection

4 {\pi }

A Spherical Active Coded Aperture for

isotropic field of view for both coded aperture and Compton imaging. This paper focuses on the low-energy coded aperture modality and the optimization techniques used to determine the optimal number and configuration of 1-cm3 CdZnTe coplanar grid detectors on a 14-cm diameter sphere with 192 available detector locations.

4\pi

Gamma-ray imaging allows for efficient detection, characterization, and localization of compact radioactive sources in cluttered environments. Fieldable detector systems employing active planar coded masks demonstrate broad energy sensitivity via both coded aperture and Compton imaging modalities. Because planar configurations suffer from a limited field-of-view, we introduce a novel design by rearranging the detectors into an active coded spherical configuration, thereby facilitating an isotropic 4π field-of-view with both coded aperture and Compton imaging. This report focuses on the low-energy coded aperture modality and the optimization techniques used to determine the optimal number and configuration of 1 cm3 CdZnTe coplanar grid detectors on the inner surface of a ∼14 cm outer diameter sphere with 192 available detector locations. We also explore the improvements in image reconstruction with the addition of depth-of-interaction information in each detector.