Available simulation software lacks the ability to produce in-motion detector responses for detector systems that may be used to detect the illicit trafficking of nuclear materials. In this thesis, a simulation tool is developed that uses static measured data as a basis set for producing in-motion detector responses with the ability to vary many parameters in each simulated trial. Once the basis set is measured and loaded into the simulation tool, the user interface allows the user to enter variations to speed, source height, source-to-detector distance, background exposure rate, which source(s) are present, their relative strength and shielding configuration, and whether the source is moved past a fixed detector, or the detector is moved past a fixed source. The simulation tool outputs data to a standard data format, as well as a specific data format for the detector system being tested so that the system's threat identification algorithm (TIA) can be evaluated for the simulated results.
The output from the simulation tool was validated against in-motion measurements by comparing the count rate profile within an energy range region of interest (ROI) specific to the isotope being measured to see how the time-series for the simulated results matched up against the measured data. To validate the simulation as interpreted by the TIA, the results of the TIA were compared to see if the algorithm performed equally when looking at simulated data vs. measured data. The validation measurements were made with bare sources, sources behind simple shields, and sources placed in various locations inside of a ``standard'' car. In an attempt to better control speed and source-to-detector distance, an additional set of validation measurements were made using a motor controlled gantry that operates at a fixed speed and is mounted on a track so that there is no lateral movement.
Of the 17 drive-by measurement configurations, 10 had simulated data fall within the error bars of the measured data, with 5 of the 7 cases falling outside of this being source in vehicle trials. Source in vehicle results are expected to have the most uncertainty due to the non-uniform nature of the shielding that is encountered during a drive-by trial.
For the 8 gantry validation configurations, only 1 had agreement within the error bars between the simulated and measured count rate ROI profiles, but this was due to an unexpected background suppression that was caused by the flatbed trailer that the measurements were made on. By adjusting the measured profiles to compensate for the background suppression, 6 of the 8 configurations showed agreement between the simulated and measured results. Adding background suppression as a feature to the simulation tool will be addressed in future work.
The intended purpose of this simulation tool is to inform and reduce the effort required for a testing and evaluation campaign, so a 10-20% uncertainty in the simulated results is acceptable to provide useful information about what should or shouldn't be included in a measurement campaign. The ratio of the signal-to-noise ratio (SNR) for the simulated results was compared to the measured results and the average value was is 1.07 ± 0.03. This SNR value is an output from proprietary TIA, and not SNR in a traditional sense. This shows that on average the TIA has an easier time identifying the simulated results than the experimental, but only by 7% which is very good. The flexibility in the tool to be able to provide simulation results quickly and reliably for a large variety of configurations reduces its accuracy to some extent, but greatly increases its value to a program that is interested in evaluating how a detector system will perform in a variety of environments and configurations.
Simulation, Detector Response, in-motion
NSDD (The Office of Nuclear Smuggling Detection and Deterrence)
Level of Degree
First Committee Member (Chair)
Dr. Adam Hecht
Second Committee Member
Dr. Cassiano de Oliveira
Third Committee Member
Dr. James Toevs
Jennings, Brian. "Developing a simulation tool for evaluating in-motion detector systems (LA-UR-18-29139)." (2019). https://digitalrepository.unm.edu/ne_etds/84