Analysis of the Coupling of Wavelength-Shifting Fibers to Organic Liquid Scintillator Filled Fluoropolymer Tubes for Industrial and Nuclear Security Applications

Industrial and nuclear security applications continue to push radiation detection development into new and exciting frontiers. In this work, an innovative detection module is developed and tested for use in a cosmic ray imaging (CRI) system designed for oil field characterization and is evaluated for its potential use in a fast neutron detection system for nuclear security applications. By measuring density changes in the reservoir, the CRI system will provide real-time information about steam chamber development during the enhanced oil field recovery process known as steam assisted gravity drainage (SAGD). The ability to monitor the development of the steam chamber region has the potential to provide important information, which could be used to optimize the growth and uniformity of the underground steam chamber and minimize costs. The organic liquid scintillator based detection modules also detect fast neutrons. During the initial characterization of an unidentified radioactive source, it is important to have the capability to determine if special nuclear material (SNM) is present and if it is configured to produce a nuclear yield. The emission of multiple neutrons during a single fission makes it possible to use this unique timing characteristic to identify SNM. The number of specialists trained to handle nuclear devices is limited making this determination a critical step in properly responding to the situation. The detector module consists of a 5 mm diameter by 2-meter long fluoropolymer tube filled with organic liquid scintillator (OLS), optically read-out using wavelength shifting (WLS) fibers. The 1:400 ratio of diameter to length makes light collection from the organic scintillator very challenging. Over ten configurations of OLS, fluoropolymer tubes, and WLS fibers were tested. The final configuration consisted of two 2mm BCF-91A WLS fibers optically coupled to the outside of an optically transparent fluorinated ethylene propylene (FEP) tube filled with a commercial OLS (EJ-309). Cosmic ray muons produce large light pulses in the OLS of which a portion reaches the external WLS fibers. The WLS fibers re-emits the light at longer wavelengths and acts as a multi-mode light guide channeling the signal to photomultiplier tubes located at each end of the WLS fibers. This module demonstrated excellent detection efficiency with less than 5% signal reduction, at any point along the module, due to optical attenuation. Timing analysis of the WLS fiber signals also provided coarse position determination, 40 cm, which opens design options not previously available. An important characteristic required of neutron detectors for nuclear security applications is the ability to discriminate fast neutron and gamma ray events. Initial tests have demonstrated the capability of our module to discriminate neutron and gamma rays by applying the rise time pulse shape discrimination (PSD) method to the WLS fiber signals. EJ-309 is well known for its PSD capabilities. Coupling this desirable characteristic with loss free, low attenuation optical read-out through a WLS fiber has the potential to broaden significantly liquid scintillator applications

Feasibility Analysis of a Compton Spectrometer System for Identification of Special Nuclear Material

Current operational needs require the deployment of radiation detection equipment with the ability to accurately and reliably identify special nuclear materials and their byproducts without dependence on cryogenics. This requires a resolution of 0.5% or less over a range of 200 to 700 keV. The feasibility of a Compton spectrometer to achieve this resolution is examined. The Compton spectrometer system used consists of two detectors. The Compton scatter event occurs in a CdTe detector where the Compton electron energy is collected. Gamma rays scattered out of the CdTe at an angle determined by a conical collimator, are collected in a NaI(Tl) detector. Coincidence electronics determine correlated events and allow the Compton electron and scattered gamma ray energy spectra to be collected. Experimental and modeling techniques are used to evaluate the system\u27s resolution and efficiency and provided reasonable agreement. Expected experimental results based on previous work were not reproduced and the source of the difference remains unknown. Results suggested strict requirements of collimation will make some low areal count rate applications impossible

Exploring Laser Induced Breakdown Spectroscopy (LIBS) for Post-Detonation Nuclear Forensics Debris Analysis

In the unlikely but catastrophic event of a nuclear terrorist attack our government leadership will need reliable information to rapidly inform critical decisions. This research explores the use of Laser Induced Breakdown Spectroscopy (LIBS) as a potential analysis tool in the National Technical Nuclear Forensics process. The current state of post detonation nuclear forensics requires ground and air samples be collected and shipped to state-of-the-art laboratories for radiochemical analysis. The samples undergo many measurements and useable data is produced as these measurements are completed. This data flows back into the process to guide additional measurements and inform the process of narrowing down the origin of the nuclear materials. This is a time-consuming process in need of new analytical methods that can be performed in situ. It is clear that LIBS will not be able to perform all of the measurements needed but the intent of this project is to explore where a LIBS system deployed with a ground collection team could provide meaningful data more quickly than the traditional radiochemistry processes. My research will include calibrating and optimizing LIBS system at the United States Military Academy and conducting analysis of Trinitite (glass like debris from the first nuclear weapon test) and a surrogate material produced by University of Tennessee at Knoxville. The intent of the surrogate material is to be used during post-detonation nuclear forensics exercises. The analysis will include optimizing collection parameters for the glass-like samples, comparison of key constituents in Trinitite and the surrogate material, and characterizing the effects of sample non-homogeneity

Applying Machine Learning to Neutron-Gamma Ray Discrimination from Scintillator Readout Using Wavelength Shifting Fibers

Advances in machine learning have found wide applications including radiation detection. In this work, machine learning is applied to neutron-gamma ray discrimination of an organic liquid scintillator (OLS) readout using wavelength shifting (WLS) fibers. The objective of using WLS fiber is to enable the transfer of the light signal from the scintillation medium, with almost any active volume geometry, to a low-profile photomultiplier. This is a common practice in high-energy physics research and has proven to be very effective for such applications. The drawback of this approach is the light pulses carried to the photomultiplier through the WLS fibers do not perfectly replicate the original OLS light pulses’ intensities or timing. This drawback causes traditional pulse shape discrimination algorithms applied to the degraded light pulses to fail to discriminate between neutron and gamma ray events. However, differences in the degraded light pulses for neutrons and gamma rays still exist and various machine learning algorithms can be applied to identify these differences. An experimental system was constructed to simultaneously capture part of the scintillation medium signal and the corresponding signal through the WLS fibers. Using the known neutron-gamma ray discrimination characteristics directly measured in the scintillation medium to provide the ground truth, supervised machine learning algorithms were applied to the corresponding light pulses carried to the photomultiplier through the WLS fibers. The results indicate that this approach will enable enhanced recovery of neutron-gamma ray discrimination information. This research effort will focus on two aspects of the OLS-WLS system: 1) developing an experimental system to create machine learning training data and 2) applying and evaluating various machine learning algorithms

Analysis of the optical coupling of wavelength-shifting fibers to organic liquid scintillator filled fluoropolymer tubes for industrial and nuclear security applications

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 129-133).Industrial and nuclear security applications continue to push radiation detection development into new and exciting frontiers. In this work, an innovative detection module is developed and tested for use in a cosmic ray imaging (CRI) system designed for oil field characterization and is evaluated for its potential use in a fast neutron detection system for nuclear security applications. By measuring density changes in the reservoir, the CRI system will provide real-time information about steam chamber development during the enhanced oil field recovery process known as steam assisted gravity drainage (SAGD). The ability to monitor the development of the steam chamber region has the potential to provide important information, which could be used to optimize the growth and uniformity of the underground steam chamber and minimize costs. The organic liquid scintillator based detection modules also detect fast neutrons. During the initial characterization of an unidentified radioactive source, it is important to have the capability to determine if special nuclear material (SNM) is present and if it is configured to produce a nuclear yield. The emission of multiple neutrons during a single fission makes it possible to use this unique timing characteristic to identify SNM. The number of specialists trained to handle nuclear devices is limited making this determination a critical step in properly responding to the situation. The detector module consists of a 5 mm diameter by 2-meter long fluoropolymer tube filled with organic liquid scintillator (OLS), optically read-out using wavelength shifting (WLS) fibers. The 1:400 ratio of diameter to length makes light collection from the organic scintillator very challenging. Over ten configurations of OLS, fluoropolymer tubes, and WLS fibers were tested. The final configuration consisted of two 2mm BCF-91A WLS fibers optically coupled to the outside of an optically transparent fluorinated ethylene propylene (FEP) tube filled with a commercial OLS (EJ-309). Cosmic ray muons produce large light pulses in the OLS of which a portion reaches the external WLS fibers. The WLS fibers re-emits the light at longer wavelengths and acts as a multi-mode light guide channeling the signal to photomultiplier tubes located at each end of the WLS fibers. This module demonstrated excellent detection efficiency with less than 5% signal reduction, at any point along the module, due to optical attenuation. Timing analysis of the WLS fiber signals also provided coarse position determination, 40 cm, which opens design options not previously available. An important characteristic required of neutron detectors for nuclear security applications is the ability to discriminate fast neutron and gamma ray events. Initial tests have demonstrated the capability of our module to discriminate neutron and gamma rays by applying the rise time pulse shape discrimination (PSD) method to the WLS fiber signals. EJ-309 is well known for its PSD capabilities. Coupling this desirable characteristic with loss free, low attenuation optical read-out through a WLS fiber has the potential to broaden significantly liquid scintillator applications.by Chad C. Schools.Ph. D