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Surface Coatings as Xenon Diffusion Barriers for Improved Detection of Clandestine Nuclear Explosions
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis investigates surface coatings as xenon diffusion barriers on plastic scintillators. The motivation for the work is improved radioxenon detection systems, used within the verification regime of the Comprehensive Nuclear-Test-Ban Treaty (CTBT).

One type of radioxenon detection systems used in this context is the Swedish SAUNA system. This system uses a cylindrical plastic scintillator cell to measure the beta decay from radioxenon isotopes. The detector cell also acts as a container for the xenon sample during the measurement.

One problem with this setup is that part of the xenon sample diffuses into the plastic scintillator material during the measurement, resulting in residual activity left in the detector during subsequent measurements. This residual activity is here referred to as the memory effect.

It is here proposed, and demonstrated, that it is possible to coat the plastic scintillator material with a transparent oxide coating, working as a xenon diffusion barrier. It is found that a 425 nm Al2O3 coating, deposited with Atomic Layer Deposition, reduces the memory effect by a factor of 1000, compared an uncoated detector. Furthermore, simulations show that the coating might also improve the light collection in the detector. Finally, the energy resolution of a coated detector is studied, and no degradation is observed.

The focus of the thesis is measurements of the diffusion barrier properties of Al2O3 films of different thicknesses deposited on plastic scintillators, as well as an evaluation of the expected effect of a coating on the energy resolution of the detector. The latter is studied through light transport simulations. As a final step, a complete coated plastic scintillator cell is evaluated in terms of memory effect, efficiency and energy resolution.

In addition, the xenon diffusion process in the plastic material is studied, and molecular dynamics simulations of the Xe-Al2O3 system are performed in order to investigate the reason for the need for a rather thick coating to significantly reduce the memory effect.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 94 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1111
Keyword [en]
Radioxenon, Gas Diffusion Barrier, Plastic Scintillator, Comprehensive Nuclear-Test-Ban Treaty, Atomic Layer Deposition, Al2O3, Molecular Dynamics, Light Transport
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-215562ISBN: 978-91-554-8848-2 (print)OAI: oai:DiVA.org:uu-215562DiVA: diva2:687617
Public defence
2014-02-28, 80121, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2014-02-06 Created: 2014-01-14 Last updated: 2014-02-10
List of papers
1. Investigations of surface coatings to reduce memory effect in plastic scintillator detectors used for radioxenon detection
Open this publication in new window or tab >>Investigations of surface coatings to reduce memory effect in plastic scintillator detectors used for radioxenon detection
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2011 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 656, no 1, 84-91 p.Article in journal (Refereed) Published
Abstract [en]

In this work Al(2)O(3) and SiO(2) coatings are tested as Xe diffusion barriers on plastic scintillator substrates. The motivation is improved beta-gamma coincidence detection systems, used to measure atmospheric radioxenon within the verification regime of the Comprehensive Nuclear-Test-Ban Treaty. One major drawback with the current setup of these systems is that the radioxenon tends to diffuse into the plastic scintillator material responsible for the beta detection, resulting in an unwanted memory effect. Here, coatings with thicknesses between 20 and 900 nm have been deposited onto plastic scintillators, and investigated using two different experimental techniques. The results show that all tested coatings reduce the Xe diffusion into the plastic. The reduction is observed to increase with coating thickness for both coating materials. The 425 nm Al(2)O(3) coating is the most successful one, presenting a diffusion reduction of a factor 100, compared to uncoated plastic. In terms of memory effect reduction this coating is thus a viable solution to the problem in question.

Keyword
Radioxenon, Gas diffusion barrier, Plastic scintillator, Comprehensive Nuclear-Test-Ban Treaty, Atomic layer deposition, Plasma enhanced chemical vapor deposition
National Category
Natural Sciences Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-161431 (URN)10.1016/j.nima.2011.07.038 (DOI)000296129100011 ()
Available from: 2011-11-16 Created: 2011-11-14 Last updated: 2017-12-08Bibliographically approved
2. Effects of surface coatings on the light collection in plastic scintillators used for radioxenon detection
Open this publication in new window or tab >>Effects of surface coatings on the light collection in plastic scintillators used for radioxenon detection
2012 (English)In: Physica Scripta, ISSN 0031-8949, E-ISSN 1402-4896, Vol. T150, 014007- p.Article in journal (Refereed) Published
Abstract [en]

Atomic Layer Deposition coatings are under investigation to reduce diffusion of radioxenon into plastic scintillators. This paper investigates the impact of such surface coating on the light collection efficiency in a cylindrical geometry. A high and uniform light collection efficiency is important to preserve detector resolution. Monte Carlo simulations and measurements have been performed to study the influence of coating thickness, refractive index, and surface quality. It was found important to achieve a smooth coating, and a good optical match between the refractive indices of the coating and the plastic scintillator. Taking these considerations into account, the detector under study could be coated without significant degradation of its resolution.

Keyword
Scintillator detectors, simulations of optical properties, optical ray tracing, thin films, energy resolution
National Category
Physical Sciences
Research subject
Physics with specialization in Applied Nuclear Physics
Identifiers
urn:nbn:se:uu:diva-163278 (URN)10.1088/0031-8949/2012/T150/014007 (DOI)000309605500008 ()
Available from: 2011-12-12 Created: 2011-12-09 Last updated: 2017-12-08Bibliographically approved
3. Memory effect, resolution, and efficiency measurements of an Al2O3 coated plastic scintillator used for radioxenon detection
Open this publication in new window or tab >>Memory effect, resolution, and efficiency measurements of an Al2O3 coated plastic scintillator used for radioxenon detection
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2013 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 714, 128-135 p.Article in journal (Refereed) Published
Abstract [en]

A cylindrical plastic scintillator cell, used for radioxenon monitoring within the verification regime of the Comprehensive Nuclear-Test-Ban Treaty, has been coated with 425 nm Al2O3 using low temperature Atomic Layer Deposition, and its performance has been evaluated. The motivation is to reduce the memory effect caused by radioxenon diffusing into the plastic scintillator material during measurements, resulting in an elevated detection limit. Measurements with the coated detector show both energy resolution and efficiency comparable to uncoated detectors, and a memory effect reduction of a factor of 1000. Provided that the quality of the detector is maintained for a longer period of time, Al2O3 coatings are believed to be a viable solution to the memory effect problem in question.

Keyword
Radioxenon, Gas diffusion barrier, Plastic scintillator, Comprehensive Nuclear-Test-Ban Treaty, Atomic layer deposition, Al2O3 coating
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-203264 (URN)10.1016/j.nima.2013.02.045 (DOI)000319251800019 ()
Available from: 2013-07-09 Created: 2013-07-08 Last updated: 2017-12-06Bibliographically approved
4. Assisted self-healing in ripped graphene
Open this publication in new window or tab >>Assisted self-healing in ripped graphene
2010 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 19, 195434- p.Article in journal (Refereed) Published
Abstract [en]

A monolayer of sp(2)-bonded carbon (graphene) is a material with great technological promise because of, for example, its transport, electrical, optical, and mechanical properties. In this work noble gas diffusion through ripped graphene sheets is explored. The motivation is improved detection systems used worldwide to verify compliance of the Comprehensive Nuclear-Test-Ban Treaty. It is demonstrated that even ripped graphene sheets and/or nonoverlapping graphene flakes inhibit noble gas diffusion. The latter has been shown for He and Xe where an infinitely long rip was constructed to have Stone-Wales edges. It is also shown that the ripped graphene layer self-heal in an alternating pentagon, hexagon, heptagon (5-6-7) and 7-6-5 pattern perpendicular to the rip. Moreover, the noble gas (He and Xe) assists in the healing process of wider rips.

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-139385 (URN)10.1103/PhysRevB.82.195434 (DOI)000284306100008 ()
Available from: 2010-12-27 Created: 2010-12-23 Last updated: 2017-12-11Bibliographically approved
5. Molecular dynamics study of Xenon on an amorphous Al2O3 surface
Open this publication in new window or tab >>Molecular dynamics study of Xenon on an amorphous Al2O3 surface
Show others...
2014 (English)In: Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, ISSN 0168-9002, E-ISSN 1872-9576, Vol. 759, 10-15 p.Article in journal (Refereed) Published
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-215561 (URN)10.1016/j.nima.2014.03.044 (DOI)000338349500003 ()
Available from: 2014-01-14 Created: 2014-01-14 Last updated: 2017-12-06Bibliographically approved

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