Positron annihilation study of vacancy-type defects in fast-neutron-irradiated MgO·nAl 2 O 3 Abu Zayed Mohammad Saliqur Rahman a,∗ , Zhuoxin Li a , Xingzhong Cao a , Baoyi Wang a , Long Wei a , Qiu Xu b , Kozo Atobe c a Key Laboratory of Nuclear Analytical Techniques, Institute of High Energy Physics, Chinese Academy of Sciences 19B Yuquanlu Shijingshan District Beijing 100049, China b Reactor Research Institute, Kyoto University 2, Asashiro-Nishi, Kumatori-cho, Sennan-gun, Osaka 590-0494, Japan c Nuclear Safety Technology Center 9-15, 1-chome, Utsubohonmachi, Nishi Ku, Osaka 550-0004, Japan Abstract The positron lifetimes of fast-neutron-irradiated MgO·nAl 2 O 3 single crystals were measured to investigate the formation of cation vacancies. Al monovacancy was possibly observed in samples irradiated by fast neutrons at ultra-low temperatures. Additionally, vacancy-oxygen complex centers were possibly observed in samples irradiated at higher temperatures and fast neutron fluences. Co- incidence Doppler broadening (CDB) spectra were measured to obtain information regarding the vicinity of vacancy-type defects. A peak at approximately 11 × 10 −3 m 0 c was observed, which may be due to the presence of oxygen atoms in the neighborhood of the vacancies. Keywords: neutron irradiation, positron lifetime, coincidence Doppler broadening, vacancy, MgO·nAl 2 O 3 1. Introduction Magnesium aluminate spinel is a radiation-hard material. It is a candidate material for various applications, such as win- dows and insulators in advanced nuclear reactor technology [1–4]. Neutron irradiation can create both anion and cation vacancies in spinel crystals. By trapping electrons, anion va- cancies can be turned into F-type centers. These centers can be detected through optical techniques [5–12]. Positron tech- niques have many advantages in the study of defects in solids because of their nondestructive nature and minimal sample preparation. Studies that evaluate vacancy-type defects in fast- neutron-irradiated single-crystalline MgO·nAl 2 O 3 by positron- annihilation spectroscopy are rare, to the best of our knowledge. However, defect studies in polycrystalline spinel by positron lifetime have been reported elsewhere [13, 14]. Positron life- time measurements provide information on vacancy sizes and concentrations. Because positrons are normally trapped in cation vacancies, they can be used to investigate the V-type cen- ters in spinel single crystals. The use of positrons as a probe to detect cation vacancies is advantageous to other methods, such as optical spectroscopy. In the present study, we used positron lifetime spectroscopy to investigate the cation vacancies in fast-neutron-irradiated MgO · nAl 2 O 3 single crystals. Aluminum monovacancy was successfully detected in samples irradiated by fast neutrons at low temperature. Information on the vicinity of vacancy-type defects was obtained by using coincidence Doppler broadening ∗ Corresponding author Email address: zayed82000@yahoo.com (Abu Zayed Mohammad Saliqur Rahman) (CDB) spectroscopy in fast-neutron-irradiated MgO·nAl 2 O 3 )(n = 2) for the first time. 2. Experimental Single crystals of undoped non-stoichiometric magnesium aluminate spinel (MgO·nAl 2 O 3 )(n = 2) grown by the Czochral- ski method were obtained from the Furuuchi chemical corpora- tion, Japan. The typical size of the samples was 7 × 5 × 1 mm 3 . The samples were irradiated by fast neutrons (E n >0.1 MeV) at different temperatures and fluences by a facility at the Japan Material Testing Reactor (JMTR), the Low Temperature Loop (LTL) and the Hydraulic Exposure Tube (HET) facilities at the Kyoto University Research Reactor Institute. The total irradiation doses ranged from 1.3 × 10 17 to 1.2 × 10 20 n/cm 2 , which corresponds to 6.9 × 10 −5 to 6.4 × 10 −2 displacement per atom (dpa), estimated by using an average displacement energy of 52 eV [15]. Table 1 shows the irradiation conditions of the samples. Positron lifetime measurements were performed for neutron- irradiated and un-irradiated samples at room temperature, using a conventional fast-slow spectrometer with a time resolution of 177 ps (full width at half maximum). Two BaF 2 scintillator detectors coupled to photomultiplier tubes were used to record the start and stop signals. A 22 Na positron source was used. Each spectrum was accumulated to a total of 2 x 10 6 counts. After subtracting the source and background components, the obtained lifetime spectrum was decomposed into three lifetime components using the LT computer program 9.0: [16] N(t) = n  i=1 I i τ i exp(− t τ i ), (1) Preprint submitted to Nuclear Instruments and Methods in Physics Research B May 29, 2014