Synchrotron VUV-UV and positron lifetime spectroscopy study of vacancy-type defects in reactor neutron-irradiated MgO·nAl₂O₃ (n = 2)

Abstract

We investigated neutron-irradiation-induced point defects in spinel single crystals using a synchrotron VUV-UV source and positron lifetime spectroscopy. Photoexcitation (PE) spectra near 230 nm and their corresponding photoluminescence (PL) spectra at 475 nm were attributed to F-centers. With increasing irradiation temperature and fluence, PE efficiency and PL intensity decreased dramatically. Positron lifetimes (PLT) of neutron-irradiated and non-irradiated samples were measured to identify the cation vacancies. A PLT measurement of 250 ps was obtained in a neutron-irradiated (20 K) sample which is tentatively attributed to an aluminum monovacancy. Decreasing PLT with higher irradiation indicates the formation of oxygen-vacancy complex centers.

Keywords:

• neutron irradiation
• F-center
• vacancy
• positron lifetime
• magnesium aluminate spinel

Public Interest Statement

We study the effects of radiation on magnesium aluminate spinel, an insulator with potential applications in fusion reactors, by exposing spinel crystals to high-intensity neutron and electron beams at multiple temperatures. Radiation alters the crystals by removing oxygen anions, yielding F-centers, or by removing aluminum cations, yielding V-centers. We then irradiated the crystals with ultraviolet light. Excitation and emission peaks near 230 nm and 475 nm, respectively, were observed, which are characteristic of F-centers. We measured the V-center by exposing the crystals to positrons. Positrons can be absorbed into the cation holes, where their lifetime is greater. We found that moderate radiation yields high F- and V-center densities. However, higher radiation doses, especially at high temperatures, cause these to quench, perhaps due to the formation of more complex centers.

Acknowledgements

We would like to thank the staffs of KURRI and JMTR for their technical support during irradiation of the samples. We also thank Prof. Dr. Ye Tao and Shuaishuai Sun of the VUV group, IHEP, CAS for their support during the VUV-UV spectroscopic measurement. We would like to express our gratitude to Prof. Dr. Long Wei for his kind support.

Dedication

This work is dedicated to the memory of Prof. Dr. Masuo Nakagawa (1936–2015).

Additional information

Funding

This work was partially supported by the NSFC [grant number 91026006], [grant number 10835006], and [grant number 11375201]; CAS Fellowships for Young International Scientists [grant number 2012Y1JB0007]; and HIR grant, UM from Ministry of Higher Education, Malaysia [project number UM.C/625 /1/HIR/ MOHE/ENG/26].

Notes on contributors

Abu Zayed Mohammad Saliqur Rahman

A Z M S Rahman and his coauthors have a wide range of research interests in material physics including irradiation-induced defect in materials, electronic & luminescent materials, nuclear reactor materials, positron spectroscopy, optical properties of solid, mechanical properties of materials, energy conversion, and high energy physics. Experimental findings of this article may be useful for an eventual understanding of the mechanism related to evolution of point defects into more complex defect center or colloid formation.