Karl Nelson

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

Detection of radioactive sources in urban scenes using Bayesian Aggregation of data from mobile spectrometers

Mobile radiation detector systems aim to help identify dangerous sources of radiation while minimizing frequency of false alarms caused by non-threatening nuisance sources prevalent in cluttered urban scenes. We develop methods for spatially aggregating evidence from multiple spectral observations to simultaneously detect and infer properties of threatening radiation sources.Our Bayesian Aggregation (BA) framework allows sensor fusion across multiple measurements to boost detection capability of a radioactive point source, providing several key innovations previously unexplored in the literature. Our method learns the expected Signal-to-Noise Ratio (SNR) trend as a function of source exposure using Bayesian nonparametrics to enable robust detection. The method scales well in spatial search by leveraging conditional independence and locality in Bayesian updates. The framework also allows modeling of source parameters such as intensity or type to enable property characterization of detected sources. Approaches for incorporating modeling information into BA are compared and benchmarked with respect to other data fusion techniques. HighlightsBayesian Aggregation fuses multiple observations to detect radiation sources.Source intensity and type information can be incorporated in detection.Properties of detected sources can be inferred for appropriate response.

Imaging performance of the Si/Ge hybrid Compton imager

The point spread function (PSF) of a fully-instrumented silicon/germanium Compton telescope has been measured as a function of energy and angle. Overall, the resolution was 3deg to 4deg FWHM over most of the energy range and field of view. The various contributions to the resolution have been quantified. These contributions include the energy and position uncertainty of the detector; source energy; Doppler broadening; and the 1/r broadening characteristic of Compton back-projection. Furthermore, a distortion of the PSF is observed for sources imaged off-axis from the detector. These contributions are discussed and compared to theory and simulations

Evaluation of coded aperture radiation detectors using a Bayesian approach

We investigate tradeoffs arising from the use of coded aperture gamma-ray spectrometry to detect and localize sources of harmful radiation in the presence of noisy background. Using an example application scenario of area monitoring and search, we empirically evaluate weakly supervised spectral, spatial, and hybrid spatio-spectral algorithms for scoring individual observations, and two alternative methods of fusing evidence obtained from multiple observations. Results of our experiments confirm the intuition that directional information provided by spectrometers masked with coded aperture enables gains in source localization accuracy, but at the expense of reduced probability of detection. Losses in detection performance can however be to a substantial extent reclaimed by using our new spatial and spatio-spectral scoring methods which rely on realistic assumptions regarding masking and its impact on measured photon distributions.

Physics and optimal routing for urban radiation source search

This paper describes an optimization-based planning tool to assist planners in developing strategies for searching an urban area with gamma-ray detectors for the presence of a radiation source. Current search strategies (path and speed) are based on simple rules (e.g., move at normal traffic speed) and constrained by road networks and resources such as detection equipment, personnel, vehicles or time. By exploiting knowledge of the urban area, and simple properties of radiation physics, the planning tool computes the optimal time allocation of detectors to road segments, including search vehicle routes, to achieve better search performance with the same limited resources (e.g., increased probability of detection for the same search assets and time, or less time to cover the same area with a given level of confidence).