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Neutron Scintillation Detectors Based on Polymers Containing Lithium-6 for Radiation Portal Monitor Applications

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Neutron Scintillation Detectors Based on Polymers Containing Lithium-6 for Radiation Portal Monitor Applications University of Tennessee, Knoxville TRACE: Tennessee Research and Creative Exchange Doctoral Dissertations Graduate School 8-2013 Neutron Scintillation Detectors Based on Polymers Containing Lithium-6 for Radiation Portal Monitor Applications Andrew Neil Mabe [email protected] Follow this and additional works at: https://trace.tennessee.edu/utk_graddiss Part of the Materials Chemistry Commons, Nuclear Commons, Polymer Chemistry Commons, and the Radiochemistry Commons Recommended Citation Mabe, Andrew Neil, "Neutron Scintillation Detectors Based on Polymers Containing Lithium-6 for Radiation Portal Monitor Applications. " PhD diss., University of Tennessee, 2013. https://trace.tennessee.edu/utk_graddiss/2453 This Dissertation is brought to you for free and open access by the Graduate School at TRACE: Tennessee Research and Creative Exchange. It has been accepted for inclusion in Doctoral Dissertations by an authorized administrator of TRACE: Tennessee Research and Creative Exchange. For more information, please contact [email protected]. To the Graduate Council: I am submitting herewith a dissertation written by Andrew Neil Mabe entitled "Neutron Scintillation Detectors Based on Polymers Containing Lithium-6 for Radiation Portal Monitor Applications." I have examined the final electronic copy of this dissertation for form and content and recommend that it be accepted in partial fulfillment of the equirr ements for the degree of Doctor of Philosophy, with a major in Chemistry. George K. Schweitzer, Major Professor We have read this dissertation and recommend its acceptance: S. Michael Kilbey II, Jimmy Mays, Dayakar Penumadu, Laurence F. Miller Accepted for the Council: Carolyn R. Hodges Vice Provost and Dean of the Graduate School (Original signatures are on file with official studentecor r ds.) Neutron Scintillation Detectors Based on Polymers Containing Lithium-6 for Radiation Portal Monitor Applications A Dissertation Presented for the Doctor of Philosophy Degree The University of Tennessee, Knoxville Andrew Neil Mabe August 2013 Copyright © 2013 by Andrew Neil Mabe All rights reserved. ii Acknowledgements I would like to acknowledge the others who have worked with me on this project Kyler Thomas, Daniel Henn, John Auxier II, Stephen Young, Matthew Urffer, Dr. Indraneel Sen, Dr. Mariya Zhuravleva, Dr. Rohit Uppal, Dr. Zheng Chang, Dr. Chuck Melcher, Dr. Dayakar Penumadu, and Dr. Larry Miller for their intellectual encouragement and positive attitudes throughout the course of my residency on this project. I believe that I could not have asked for a better group of scholars to work with and learn from. My fellow lab mates Jake Stewart, Brittany Miner, and Deborah Penchoff are appreciated for your camaraderie, as well as Dr. George Schweitzer for whom I am very grateful for his patience and guidance in helping me develop as a research scientist. A significant portion of this work has relied on computer-aided simulations. I would like to acknowledge specifically Matthew Urffer for running all the MCNPX and GEANT4 simulations, Stephen Young for all the microscopy data, John D. Auxier II for all the X-ray diffraction data, and Dr. Chuck Melcher for use of facilities used to characterize fluorescence responses presented in this work. Atlanta Microsystems, Inc., is acknowledged for conducting the elemental analysis described in this dissertation. I would also like to extend the utmost gratitude to my parents Heather Garrett and Britt Godwin and stepfather Jon Garrett without whom this contribution would not be possible and to my wife Rose and daughter Alina for your patience, constant encouragement, and unconditional love. This material is based upon work supported, in part, by the National Science Foundation and the Domestic Nuclear Detection Office under Grants NSF ARI-0833492 and DNDO 003387891. Any opinions, findings, and conclusions or recommendations expressed in iii this material are those of the author and do not necessarily reflect the views of the National Science Foundation or the U. S. Department of Homeland Security. iv Abstract The work presented herein describes an investigation of four main types of thin film polymer scintillators containing 6Li [lithium-6] for neutron detection: polystyrene containing 6LiF [lithium-6 fluoride] and a preblended fluor mixture comprising 2,5-diphenyloxazole and 1,4-bis(5-phenyloxazol-2-yl)benzene; poly(styrene-co-lithium maleate) containing salicylic acid; poly[styrene-co-lithium maleate-co-2-phenyl-5-(4-vinylphenyl)oxazole]; and poly(styrene-co- lithium 4-vinylbenzoate). A variety of chemical and physical characterizations as well as optical and scintillation characterizations were performed to guide the development of optimized compositions of each type of polymer film. The scintillation performances of optimized compositions of each type of polymer film were calibrated using GS20 lithium glass and evaluated against neutron detection and neutron/gamma-ray discrimination criteria established for radiation portal monitors. Thin films were fabricated 2 inches in diameter over a variety of thicknesses using solution-casting methods. Investigation of polystyrene-based films by photoluminescence and scintillation indicated that the optimum concentration of fluor was 5.00%. Optimum neutron/gamma-ray discrimination was achieved for 50 micrometer thick films containing 10% 6LiF [lithium-6 fluoride]. Two transparent lithium-containing polymers were successfully synthesized: poly(styrene-co-lithium maleate) containing salicylic acid and poly[styrene-co- lithium maleate-co-2-phenyl-5-(4-vinylphenyl)oxazole]. To the author’s knowledge, the latter polymer represents the first polymer comprising the matrix, the thermal neutron capture nuclide 6Li [lithium-6], and the fluor that has been synthesized for the purpose of thermal neutron detection. The polymer poly(styrene-co-lithium 4-vinylbenzoate) could not be solvated and decomposed below its melting temperature and was thus considered not useful for this v application. The polystyrene-based materials had the greatest light yields whereas the poly[styrene-co-lithium maleate-co-2-phenyl-5-(4-vinylphenyl)oxazole] material had the best neutron/gamma-ray discrimination properties. All three classes of materials can be used to satisfy the detection criteria in the current radiation portal monitor footprint by implementing a multilayer format. vi Preface It has been shown by Bethe and others that beryllium when bombarded by α-particles of polonium emits a radiation of great penetrating power, which has an absorption coefficient in lead of about 0.3 (cm.)–1. Recently, Mme. Curie-Joliot and M. Joliot found, when measuring the ionisation produced by this beryllium radiation in a vessel with a thin window, that the ionisation increased when matter containing hydrogen was placed in front of the window. The effect appeared to be due to the ejection of protons with velocities up to a maximum of nearly 3 × 109 cm. per sec. They suggested that the transference of energy to the proton was by a process similar to the Compton effect, and estimated that the beryllium radiation had a quantum energy of 50 × 106 electron volts. I have made some experiments using the valve counter to examine the properties of the radiation excited in beryllium. The valve counter consists of a small ionisation chamber connected to an amplifier and the sudden production of ions by the entry of a particle, such as a proton or an α-particle, is recorded by the deflexion of an oscillograph. These experiment have shown that radiation ejects particles from hydrogen, helium, lithium, beryllium, carbon, air, and argon. The particles ejected from hydrogen behave, as regards range and ionising power, like protons with speeds up to about 3.2 × 109 cm. per sec. The particles from the other elements have a large ionizing power, and appear to be in each case recoil atoms of the elements. If we ascribe the ejection of the proton to a Compton recoil from a quantum of 52 × 106 electron volts, then the nitrogen recoil atom arising by a similar process should have an energy not greater than about 400,000 volts, should produce not more than about 10,000 ions and have a range in air at N.T.P. of about 1-3 mm. Actually, some of the recoil atoms in nitrogen produce vii at least 30,000 ions. In collaboration, with Dr. Feather, I have observed the recoil atoms in an expansion chamber, and their range, estimated visually, was sometimes as much as 3 mm. at N.T.P. These results, and others, I have obtained in the course of the work, are very difficult to explain on the assumption that the radiation from beryllium is a quantum radiation, if energy and momentum are to be conserved in the collisions. The difficulties disappear, however, if it be assumed that the radiation consists of particles of mass 1 and charge 0, or neutrons. The capture of the α-particle by the 9Be nucleus may be supposed to results in the formation of a 12C nucleus and the emission of the neutron. From the energy relations of the process the velocity of the neutron emitted in the forward direction may well be about 3 × 109 cm. per sec. The collisions of the neutron with the atoms through which it passes give rise to the recoil atoms, and the observed energies of the recoil atoms are in fair agreement with this view. Moreover, I have observed that protons ejected from hydrogen by the radiation emitted in the opposite direction to that of the exciting α-particle appear to have a smaller range than those ejected by the forward radiation. This is a simple explanation on the neutron hypothesis. If it be supposed that the radiation consists of quanta, then the capture of the α-particle by the 9Be nucleus will form a 13C nucleus. The mass defect of 13C is known with sufficient accuracy to show that the energy of the quantum emitted in this process cannot be greater than about 14 × 106 volts. It is difficult to make such a quantum responsible for the effects observed.
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