- Author(s): Pavlovsky, Ryan Thomas
- Advisor(s): Vetter, Kai
- et al.
Ryan Thomas Pavlovsky
Doctor of Philosophy in Engineering - Nuclear Engineering
University of California, Berkeley
Kai Vetter, Chair
Advanced radiation detector sensors and detector systems have promoted the idea of
Gamma-Ray Mapping, the fusion of maps generated by contextual sensors with radiation
data. Gamma-Ray Mapping seeks to discover sources or distributions of radioactive isotopes
in the mapped measurement environment. As this suggests, there are two components to a
Gamma-Ray Mapping system: the map generation and Gamma-Ray imagers. This concept
has been motivated heavily by the availability of advanced gamma-ray detectors as well as
commercial sensors such as Global Positioning System (GPS), Light Imaging, Detection and
Ranging (LIDAR), Inertial Measurement Unit (IMU), etc., which can facilitate Simultaneous
Localization and Mapping (SLAM).
The role of Compton Imagers for Gamma-Ray Mapping has been explored for some
time . These systems have good angular resolution, however they suffer from
imaging ambiguities in the Compton cones and require an ensemble of gamma-ray events to
reconstruct the source location. Electron Track Compton Imagers (ETCI) seek to break the
symmetry of Compton cones and eventually approach event-by-event gamma-ray momentum
computation. Additionally, these detectors provide the possibility of new imaging modalities
that rely only on electron tracks to recover the angular location and energy of a source .
ETCI has been shown to be conceptually possible  in high resolution 10.5 μm Super Novae
Acceleration Probe (SNAP) Charge Coupled Device (CCD) silicon detectors. However, there
are many practical issues with the implementation of CCD detectors as Compton imagers,
namely that time resolution in these devices is conservatively the Frame Read Time (FRT).
In typical SNAP devices the FRT can be ∼ 1 s/MPxl. The lack of time resolution limits
the use of CCD electron trackers in Compton imagers as well as in other radiation detector
In order to circumvent the limit established by the FRT, CCD-strip was proposed. CCD-
strip devices conceptually provide time resolution on the order of the electron drift time in
n-type CCD detectors by application of strip segmentation to the CCD backside. Time
stamps would be correlated through the coarse strip spatial coding and the high resolution
electron tracks in the CCD pixel plane. Backside strips imply that double-sided, micrometer
alignment would be required for CCD-strip fabrication. Significant effort was required in the
coordination of the CCD fabrication facility Teledyne DALSA and strip detector fabrication
facility SINTEF. A small batch of CCD-strip devices were fabricated and tested in the
Lawrence Berkeley National Laboratory (LBNL) engineering test stand for screening pur-
poses. The bulk of this work’s contribution is in the construction and demonstration of the
newly fabricated devices in a custom test stand. The DALSA control wafers were the first
of this batch to produce electron tracks, and the device was characterized in our testbed.
The custom cryostat and measurement stand allowed us to reduce the leakage current in
CCD-Strip by a factor of 1000. The reduction removed voltage transients with equivalent
charge of about 3-5 MeV at the output of the preamplifiers. Ultimately the output baseline
had an Equivalent Noise Charge (ENC) of 400 keV-RMS per strip. The magnitude of the
ENC is large enough that strip operation is yet to be demonstrated.
Benchmarked simulations for the improvement of electron tracking and imaging algo-
rithms are also presented. From these simulations follows an investigation of the nuclear
scattering effect in electron trackers. Nuclear scattering is an important design consideration
for electron trackers as it scrambles the Compton kinematic information without producing
ionization, or signal, in electron trackers. From an examination of the nuclear scatter limit,
diamond was identified as a very interesting detector material for electron tracking. Dia-
mond is a material that minimizes nuclear scattering while maintaining photon efficiency.
This boosts the amount of ionization signal obtained from the initial portion of the electron
track. Electron-nuclear scattering is a large source of kinematic information loss in ETCI
Beyond complex detectors, augmentation of monolithic sensors with contextual sensors
to provide SLAM has been investigated for Gamma-Ray Mapping. Given the many types
of instruments which may be of interest, we constructed Localization and Mapping Plat-
form (LAMP), a data collection and demonstration platform for detectors and contextual
sensors. Here we used Google Cartographer as a SLAM solution. SLAM provides the 6
Degrees of Freedom (DOF) trajectory of a system, while simultaneously generating 3D mod-
els. These allow for ranging of gamma-ray emitters and the correction of detector responses
to provide better association of flux with physical objects. We demonstrated handheld and
Unmanned Aerial System (UAS) measurements as configurations for making these measure-
ments. LAMP, in handheld and flight configurations, provides a portable, robust indoor
and outdoor 3D SLAM solution for Gamma-Ray Mapping. LAMP was fitted with com-
mercial radiation detectors as a proof of concept. The radiation and scene data can be
fused by utilization of the SLAM output poses, trajectory and 3D model with simple back-
projection. The 3D backprojection scheme demonstrated incorporates simulated detector
angular responses for better resolution of hotspots than just backprojection alone. LAMP
has demonstrated that there is utility in coupling contextual sensors to augment simple com-
mercial detectors. Implementations that use commercial radiation detectors are important
to Gamma-Ray Mapping in that they represent the low end of the cost versus complexity for
mapping. We also demonstrate uses where GPS is either insufficient in function or accuracy.
LAMP will serve in the future as a demonstration platform for many kinds of detectors.
The development of ETCI and LAMP systems continues to expand the Gamma-Ray
Mapping application space. The developments in complex Gamma-Ray Imagers, coupled
with contextual platforms, consider the necessary components for Gamma-Ray Mapping.
This work has presents progress toward understanding the applicability and information
that ETCI systems provide, and we note that there is substantial work to be done toward
the goal of mapping with ETCI devices. The LAMP demonstration platform is crucial to
comparing technologies and understanding the complex problems that Gamma-Ray Mapping
poses. These parallel developments both share a part in enabling Gamma-Ray Mapping.