UC Berkeley

Abstract

Gamma-ray Mapping

by

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 [4][21][13]. 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 [20].

ETCI has been shown to be conceptually possible [60] 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

system uses.

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

systems.

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.