LABORATORI NAZIONALI DI LEGNARO
A study of the irradiation effect was conducted in various materials such as recieved ɑ-Al2O3, MgAl2O4 and Sm-doped Na2SO4. All the irradiation experiments including neutron, electron and γ-ray irradiation were conducted using facilities in Kyoto University Research Reactor Institute (KURRI), Japan Material Testing Reactor (JMTR), Institute of High Energy Physics and China Institute of Atomic Energy (CIEA). A vibronic photoexcitation band at approximately 230 nm was found at 13 K in reactor neutron- and electron-irradiated magnesium aluminate spinel ( MgO.nAl2O3). Vibronic structure was found to be temperature dependent and became obscure at over 120 K. Huang Rhys factor S and Debye Temperature were estimated from the temperature dependence of the 230 nm band by curve-fitting method using Debye approximation. Origin of the vibronic photoexcitation band was suggested as F center. Positron lifetime of the neutron-irradiated MgO.nAl2O3 single crystals was measured to investigate formation of cation vacancies. Al monovacancy was observed in samples irradiated at ultra low temperature. Vacancy-oxygen complex centers were observed at samples irradiated at higher temperature and fast neutron fluences. Coincidence Doppler broadening (CDB) spectra were measured to know the information in the vicinity of vacancy type defects. Near-infrared (NIR) spectroscopic study of neutron-irradiated ɑ-Al2O3 was conducted. Scanning-electron and optical microscopic measurements were also done. Sample irradiated to the uence of 2.8 x 1018n/cm2 shows strong emission band at 1180, 1700, 1945 and 2225 nm. Photoluminescence (PL) intensity of these bands reduced greatly with higher neutron fluences. Anomalous temperature dependence of these NIR PL spectra was observed. Highest PL intensity was measured at 260 K whereas the intensity at low temperatures below 220 K decreased significantly. Novel PL bands was also ovserved in neutron irradiated Spinel. Origin of these PL bands are not fulley understood. However, the results may give new insight about formation of F aggergate centers or colloids. Irradiation effects in Na2SO4:Sm phosphors were studied. X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements show that the particle size of as-synthesized phosphors is 58 and 60 nm, respectively. Irradiated phosphors show the conversion of Sm3+ into Sm2+. PL properties of new luminescent composite materials Sm-doped SiO2-Na2SO4 and LDPE- Na2SO4 were also investigated. New luminescent composites were successfully synthesized by using melt-mixing and hot pressing methods. Sm-doped SiO2-Na2SO4 shows highest PL intensity with addition of 10%Na2SO4:Sm phosphor whereas PL intensity in Sm-doped LDPE-Na2SO4 composite increased with concentration of the Na2SO4:Sm phosphor upto 20%. The luminescent composite material shows response upon gamma-ray irradiation.

DATA: 28-06-2016