Irradiation-induced valence conversion of samarium ions in Na2SO4

Abstract

Na2SO4: Sm phosphors were synthesized by heating pure sodium sulphate with a small amount of SmF3 at 950 degrees C. 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. The samples were irradiated by Co-60 gamma-rays with irradiation doses of 10-50 kGy at room temperature and by electrons with a total fluence of 6.7 x 10(16) e/cm(2) at a flux of 7.8 x 10(11) e/cm(2).sec. After exposure to gamma-ray and high energy electron irradiations, as-synthesized phosphors show the conversion of Sm3+ -> Sm2+. The photoluminescence (PL) spectra of as-synthesized, gamma-ray, and electron-irradiated phosphors obtained under 375 nm excitation consist of narrow emission bands with peaks at 563, 598, 644, and 704 nm, assigned to the (4)G(5/2) -> H-6(J) (J = 5/2, 7/2, 9/2, and 11/2) transitions within 4f(5) electronic configuration of Sm3+. The PL spectra of irradiated phosphors under 570 nm excitation at room temperature consist of a broad band with a peak at 695 nm and an emission line at 685.9 nm on the broad band. The broad band is assigned to the 4f (5)5d -> 4f 6 (7FJ) transition and the emission line to the D-5(0). F-7(0) transition within Sm2+. The PL and photoexcitation (PE) spectra at 10 K were well assigned to energy levels of Sm2+. Annealing at 473 K (200 degrees C) completely erased the Sm2+ emission band within the detection limit at room temperature.