Nicholas Myllenbeck is a Senior Member of the Technical Staff at Sandia National Laboratories in Livermore, CA. His research interests are in organic scintillator development and characterization, biomaterial and plastic valorization using synthetic chemistry, and environmental aging of organic materials. Nicholas has developed environmentally aging-resistant plastic scintillators, pulse-shape discriminating organic glass-polymer blends, and advanced processing methods for organic scintillators. He has twelve years of experience in small-molecule and polymer synthesis, including six years of organic scintillator research.
In 2010, light scattering defects were observed in several fielded panels of poly(vinyltoluene) plastic scintillators that had experienced losses in radiation sensitivity. Pacific Northwest National Laboratory first deduced that the defects were caused by trapped water. A subsequent multi-lab team funded by the DHS CWMD office found that the defects were produced when the trapped, supersaturated water caused the plastic matrix to yield. In the following years, using sophisticated optical measurements and chemical synthesis, the team discovered that the degree of light scattering and defect appearance was a function of moisture supersaturation in the plastic, scintillator formulation, and cycling history. This report focuses on the recent technical achievements by the team in 1) further understanding the root causes of the observed sensitivity loss, 2) deploying a computational tool to predict when and at which sites fogging may occur, and 3) developing and characterizing a class of fog-resistant formulations that have been demonstrated at full panel-scale and will serve as replacements for failed and at-risk panels.
Radiation Portal Monitors (RPMs) based on polyvinyl toluene (PVT) scintillators have been in use since the 1970s. Recently it has been discovered that under certain conditions PVT scintillators can fog at low temperatures. In collaboration with researchers at national laboratories, Eljen Technology has developed methods for the production of Intrinsic Anti-Fogging Scintillators. The production methods have been scaled up to allow for castings of rods and large sheets of scintillators that are deployable in RPMs and other extreme locations. The work is ongoing. The presentation will focus on the properties of the new scintillators and on the ongoing testing results from exposure to extreme environmental conditions.
Organic-based scintillators are indispensable materials for radiation detection owing to their high sensitivity to fast neutrons, low cost, and tailorable properties. There has been a recent resurgence of interest in organic scintillators due to exciting discoveries related to neutron discrimination and gamma-ray spectroscopy, which represent capabilities previously thought not possible in these materials. I will discuss our development of crystalline and polymer-based scintillators for these applications. Structure-property relationships related to intermolecular interactions and host-guest electronic exchange will be discussed in the context of energy-transfer pathways relevant to scintillation. An emphasis will be placed on the rational design of these materials, as guided by first principles and DFT calculations.
Two related topics will be discussed:
1) Incorporation of organometallic triplet-harvesting additives to plastic scintillator matrices to confer a 'two-state' (singlet and triplet) luminescence signature to different types of ionizing radiation. This approach relies upon energetic and spatial overlap between the donor and acceptor excited states for efficient electronic exchange. Key considerations also include synthetic modification of the luminescence spectra and kinetics, as well as the addition of secondary additives to increase the recombination efficiency.
2) Design of organotin-containing plastic scintillators as a route towards gamma-ray spectroscopy. Organometallic compounds were selected on the basis of distance-dependent quenching relationships, phase compatibility with the polymer matrix, and the gamma-ray cross sections. This approach is guided by molecular modeling and radiation transport modeling to achieve the highest possible detection sensitivity luminescence intensity.
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