Gamma-ray Mapping

Abstract/nGamma-ray Mapping/nby/nRyan Thomas Pavlovsky/nDoctor of Philosophy in Engineering - Nuclear Engineering/nUniversity of California, Berkeley/nKai Vetter, Chair/nAdvanced radiation detector sensors and detector systems have promoted the idea of/nGamma-Ray Mapping, the fusion of maps generated by contextual sensors with radiation/ndata. Gamma-Ray Mapping seeks to discover sources or distributions of radioactive isotopes/nin the mapped measurement environment. As this suggests, there are two components to a/nGamma-Ray Mapping system: the map generation and Gamma-Ray imagers. This concept/nhas been motivated heavily by the availability of advanced gamma-ray detectors as well as/ncommercial sensors such as Global Positioning System (GPS), Light Imaging, Detection and/nRanging (LIDAR), Inertial Measurement Unit (IMU), etc., which can facilitate Simultaneous/nLocalization and Mapping (SLAM)./nThe role of Compton Imagers for Gamma-Ray Mapping has been explored for some/ntime [4][21][13]. These systems have good angular resolution, however they suffer from/nimaging ambiguities in the Compton cones and require an ensemble of gamma-ray events to/nreconstruct the source location. Electron Track Compton Imagers (ETCI) seek to break the/nsymmetry of Compton cones and eventually approach event-by-event gamma-ray momentum/ncomputation. Additionally, these detectors provide the possibility of new imaging modalities/nthat rely only on electron tracks to recover the angular location and energy of a source [20]./nETCI has been shown to be conceptually possible [60] in high resolution 10.5 μm Super Novae/nAcceleration Probe (SNAP) Charge Coupled Device (CCD) silicon detectors. However, there/nare many practical issues with the implementation of CCD detectors as Compton imagers,/nnamely that time resolution in these devices is conservatively the Frame Read Time (FRT)./nIn typical SNAP devices the FRT can be ∼ 1 s/MPxl. The lack of time resolution limits/nthe use of CCD electron trackers in Compton imagers as well as in other radiation detector/nsystem uses./nIn order to circumvent the limit established by the FRT, CCD-strip was proposed. CCD-/nstrip devices conceptually provide time resolution on the order of the electron drift time in/nn-type CCD detectors by application of strip segmentation to the CCD backside. Time/nstamps would be correlated through the coarse strip spatial coding and the high resolution/nelectron tracks in the CCD pixel plane. Backside strips imply that double-sided, micrometer/nalignment would be required for CCD-strip fabrication. Significant effort was required in the/ncoordination of the CCD fabrication facility Teledyne DALSA and strip detector fabrication/nfacility SINTEF. A small batch of CCD-strip devices were fabricated and tested in the/nLawrence Berkeley National Laboratory (LBNL) engineering test stand for screening pur-/nposes. The bulk of this work’s contribution is in the construction and demonstration of the/nnewly fabricated devices in a custom test stand. The DALSA control wafers were the first/nof this batch to produce electron tracks, and the device was characterized in our testbed./nThe custom cryostat and measurement stand allowed us to reduce the leakage current in/nCCD-Strip by a factor of 1000. The reduction removed voltage transients with equivalent/ncharge of about 3-5 MeV at the output of the preamplifiers. Ultimately the output baseline/nhad an Equivalent Noise Charge (ENC) of 400 keV-RMS per strip. The magnitude of the/nENC is large enough that strip operation is yet to be demonstrated./nBenchmarked simulations for the improvement of electron tracking and imaging algo-/nrithms are also presented. From these simulations follows an investigation of the nuclear/nscattering effect in electron trackers. Nuclear scattering is an important design consideration/nfor electron trackers as it scrambles the Compton kinematic information without producing/nionization, or signal, in electron trackers. From an examination of the nuclear scatter limit,/ndiamond was identified as a very interesting detector material for electron tracking. Dia-/nmond is a material that minimizes nuclear scattering while maintaining photon efficiency./nThis boosts the amount of ionization signal obtained from the initial portion of the electron/ntrack. Electron-nuclear scattering is a large source of kinematic information loss in ETCI/nsystems./nBeyond complex detectors, augmentation of monolithic sensors with contextual sensors/nto provide SLAM has been investigated for Gamma-Ray Mapping. Given the many types/nof instruments which may be of interest, we constructed Localization and Mapping Plat-/nform (LAMP), a data collection and demonstration platform for detectors and contextual/nsensors. Here we used Google Cartographer as a SLAM solution. SLAM provides the 6/nDegrees of Freedom (DOF) trajectory of a system, while simultaneously generating 3D mod-/nels. These allow for ranging of gamma-ray emitters and the correction of detector responses/nto provide better association of flux with physical objects. We demonstrated handheld and/nUnmanned Aerial System (UAS) measurements as configurations for making these measure-/nments. LAMP, in handheld and flight configurations, provides a portable, robust indoor/nand outdoor 3D SLAM solution for Gamma-Ray Mapping. LAMP was fitted with com-/nmercial radiation detectors as a proof of concept. The radiation and scene data can be/nfused by utilization of the SLAM output poses, trajectory and 3D model with simple back-/nprojection. The 3D backprojection scheme demonstrated incorporates simulated detector/nangular responses for better resolution of hotspots than just backprojection alone. LAMP/nhas demonstrated that there is utility in coupling contextual sensors to augment simple com-/nmercial detectors. Implementations that use commercial radiation detectors are important/nto Gamma-Ray Mapping in that they represent the low end of the cost versus complexity for/nmapping. We also demonstrate uses where GPS is either insufficient in function or accuracy./nLAMP will serve in the future as a demonstration platform for many kinds of detectors./nThe development of ETCI and LAMP systems continues to expand the Gamma-Ray/nMapping application space. The developments in complex Gamma-Ray Imagers, coupled/nwith contextual platforms, consider the necessary components for Gamma-Ray Mapping./nThis work has presents progress toward understanding the applicability and information/nthat ETCI systems provide, and we note that there is substantial work to be done toward/nthe goal of mapping with ETCI devices. The LAMP demonstration platform is crucial to/ncomparing technologies and understanding the complex problems that Gamma-Ray Mapping/nposes. These parallel developments both share a part in enabling Gamma-Ray Mapping.

Advisor: Vetter, Kai