Introduction Background Methodology Results and Discussion Conclusion

Simulation of Prompt Gamma Emission Tomography by

Compton Scattering

and

The Implementation of a Neutron Tomography System

Nathaniel Kaneshige1

Mentor: Dr. Heather Chen-Mayer2

1University of Hawai’i at MÄnoa - SURF 2018

2National Institute for Standards and Technology (NIST)

8 August 2018

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Introduction Background Methodology Results and Discussion Conclusion

Outline

1 Introduction

The NIST Center for Neutron Research

In a nutshell

2 Background

Prompt Gamma Activation Analysis (PGAA)

Compton Camera

Neutron Tomography

Research Objective

3 Methodology

Geant4 Simulation

Reconstruction Methods

Neutron Tomography Apparatus

4 Results and Discussion

Prompt Gamma Compton Camera (PGCC) Simulation

Comparing Recon Methods

Neutron Tomography Results

5 Conclusion

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Introduction Background Methodology Results and Discussion Conclusion

In a nutshell

Top Level Summary:

Combine three techniques to understand the structural composition of a material.

Three Techniques

Technique Energy Resolution Spatial Resolution

Germanium Detector (HPGe) High None

Compton Camera Med Med

Neutron Tomography None High

Figure: Germanium Detector with

Compton suppressor. (NIST)

Figure: Compton Camera. (Kim et.

al. 2013)

Figure: Neutron tomography

schematic. (Manescu et. al.

2013)

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Introduction Background Methodology Results and Discussion Conclusion

In a nutshell

Application:

Studying Chloride transport in concrete. Structural degradation.

Hydrogen ion diffusion through metals. Hydrogen blistering and embrittlement of

metals (Fuel Cells and Fusion Reactors).

Figure: Chloride attacking concrete structure. (Nielsen

2004)

Figure: Instrument diagram of the NCNR. (Zhang 2016)

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Introduction Background Methodology Results and Discussion Conclusion

Prompt Gamma Activation Analysis (PGAA)

Characteristic gamma-radiation

emitted under neutron capture

Gammas typically in the MeV range

(eg. 2.22 MeV for 1H)

Bulk elemental composition analysis

for a sample

Non-destructive

Figure: Neutron activated prompt gamma process. (Frontier Tech C.)

1g sample 24 hour measurement

Range (μg) Elements

0.01-0.1 B, Cd, Sm, Gd

0.1-1 H, Cl, In, Nd

1-100 Na, S, K, Ti, and more

Figure: PGAA station at the NCNR. Neutron beam with

HPGe detector and lead shielding.

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Introduction Background Methodology Results and Discussion Conclusion

Compton Camera

Figure: Schematic view of Compton scattering. (MIT

OCW)

1

E0

−

1

E

=

1

mec

2

(1 − cos θ), (1)

Figure: Compton Camera. (Kim et. al. 2013)

Compton scattering applied to imaging

Energy and spatial resolution

Commonly used in astronomy

Improved resolution with Cadmium

Zinc Telluride (CZT) detectors and

miniaturized on-board electronics

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Introduction Background Methodology Results and Discussion Conclusion

Neutron Tomography

Images generated by transmitting

neutrons through a sample.

Inverted contrast to X-ray tomography

(insensitive to high Z-materials).

3D reconstruction using computed

tomography.

Figure: Flower in lead shielding showing the sensitivity of

neutron imaging to hydrogen. (NIST)

Figure: Basic idea for computed tomography. (ZvolskÃœ

2014)

Figure: Neutron tomography schematic. (Manescu et. al.

2013)

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Introduction Background Methodology Results and Discussion Conclusion

Research Objective

My Role:

Continued development of detector simulation to aid the design of a Compton

Camera

Detector parameters (plane separation, position, orientation)

Spatial resolution

Energy resolution

Assemble and evaluate a basic neutron tomography setup

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Introduction Background Methodology Results and Discussion Conclusion

Geant4 Simulation

Geant4 is a C++ toolkit for simulating

particle transport through matter.

Developed for High Energy Physics at

CERN, expanded application to

medical and nuclear engineering.

Expanding work by Ben Riley (SURF

2017)

Outputs basic detector response for

reconstruction

Figure: geant4 logo. (CERN)

Figure: Particle tracks for Geant4 simulation of n-capture

and CZT Detector.

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Introduction Background Methodology Results and Discussion Conclusion

Conic Back-projection

Figure: Compton Camera. (Kim et. al. 2013)

Basic method for reconstructing sample

geometries.

Takes the basic detector output from

experiment or simulation.

Back-projection looks at the conic sections at a

plane of interest.

Different methods available for energy and

spatial resolution.

Figure: Animation of conic back-projection with the number of ideal events.

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Introduction Background Methodology Results and Discussion Conclusion

Assessment of Reconstruction Methods

Other methods we can use:

Filtered Back-projection - Apply a

frequency filter.

Maximum Likelihood Estimation

Method (MLEM) - Iterative process

using guess and check like method.

Stochastic Origin Ensemble (SOE) -

Iterative process applying

Metropolis-Hastings Algorithm.

Figure: Animation of 4 cones projected into a 3D cube.

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Introduction Background Methodology Results and Discussion Conclusion

Neutron Tomography Apparatus

Figure: Sketch of the Neutron Tomography Apparatus for basic testing.

Construct a test neutron tomography apparatus

Uses a 6LiF:ZnS scintillator sheet to convert neutrons to light.

Coordinate rotating stage with CCD

Evaluate apparatus at

PGAA station (uncollimated neutrons)

BT-2 Neutron Imaging Facility (collimated neutrons).

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Introduction Background Methodology Results and Discussion Conclusion

Neutron Tomography Apparatus

Figure: Completed Neutron Tomography Apparatus for basic testing.

Construct a test neutron tomography apparatus

Uses a 6LiF:ZnS scintillator sheet to convert neutrons to light.

Coordinate rotating stage with CCD

Evaluate apparatus at

PGAA station (uncollimated neutrons)

BT-2 Neutron Imaging Facility (collimated neutrons).

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Introduction Background Methodology Results and Discussion Conclusion

Prompt Gamma Compton Camera (PGCC) Simulation

480x speedup

Skips n-capture process for

performance speedup

Implemented gamma emmissions

from random locations in samples

1.8e6 Compton events per 20 min

Compared to 3500 Compton events

per 20 min for 25 meV neutrons and

0.126 g of water

Speedup achieved for back-projection

reconstruction with multithreading

Figure: Figure showing the direct to gamma geant4

method for disk geometries.

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Introduction Background Methodology Results and Discussion Conclusion

PGCC Energy Output Comparison

Experimental data obtained from wax

disk in neutron beam.

Geant4 data is a simulation using the

direct to gamma process.

2.22 MeV 1H peak clearly visible,

Compton edge

Figure: Comparison of energy spectrum’s for geant4

PGCC simulation, Compton Camera, and HPGe.

Figure: Comparison of energy spectrum’s for geant4

PGCC simulation, Compton Camera, and HPGe.

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Introduction Background Methodology Results and Discussion Conclusion

Back-Projection Methods

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Introduction Background Methodology Results and Discussion Conclusion

Iterative Reconstruction Methods

U. of Maryland Maximum Likelihood Estimation Method (MLEM) and Stochastic Origin

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Introduction Background Methodology Results and Discussion Conclusion

Neutron Tomography Results

PGAA vs. BT2 Comparison

PGAA station uses uncollimated neutrons.

BT2 uses highly collimated neutrons.

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Introduction Background Methodology Results and Discussion Conclusion

Conclusion

Key Results:

480x speedup using direct to gamma simulation

Parallelization of back-projection reconstruction software

Corresponding simulation and experimental data

Tested n-tomography apparatus

Future Work:

Prompt Gamma Compton Camera Simulation

Generalize for more complete detector response

Further study detector parameters

Neutron Tomography

Characterize PGAA station beam

Investigate PGAA beam collimation

Assess CCD damage

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Introduction Background Methodology Results and Discussion Conclusion

Acknowledgements

A special thanks to:

Heather Chen-Mayer, Jake LaManna,

Ryan Fitzgerald (MML/NCNR/PML)

Joe Dura, Julie Borchers, Brandi

Toliver (NIST SURF)

Jeremy Polf, Haijen Chen (UMD)

Ben Riley (SURF 2017)

Fellow NCNR SURFers!

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Introduction Background Methodology Results and Discussion Conclusion

CCD Damage

Radiation damage to

CCD

Characterized by

individual pixels and

columns of offset by

some value pixels

Maybe correctable in

post-processing

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Introduction Background Methodology Results and Discussion Conclusion

Sample Geant4 Process

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Introduction Background Methodology Results and Discussion Conclusion

Simulation Geometry

14 cm 5 cm

1 cm 1.5 cm

32 cm

x

y

. z

Radius: 2 cm 

Centered at Origin

**Not to Scale**

CZT Det1 CZT Det2

Disk 1/2

z

2 cm

14 cm 5 cm

1 cm 1.5 cm

32 cm

x

. y

Thickness: 1 mm 

Centered at Origin

**Not to Scale**

CZT Det1 CZT Det2

Disk 1/2

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Introduction Background Methodology Results and Discussion Conclusion

Compton Emphasis

Figure: Gamma interactions. (Smet 2011)

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Introduction Background Methodology Results and Discussion Conclusion

Complication

Figure: Complication(Smet 2011)

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