Scintillators are functional materials that immediately emit low-energy photons after absorbing high-energy ionizing radiations. Glasses have advantages as a material form for scintillator use, such as low manufacturing costs, ease of formability, high optical transparency, and high compositional tunability. A ⁶Li-glass scintillator doped with Ce is a commercial glass scintillator, and it has been used for thermal neutron detections owing to ⁶Li(n,α)³H neutron capture reactions. In general, the ³He counter is used in practice for neutron detections; however, the depletion of the ³He supply has driven vigorous research and development of alternative materials. ¹⁰B-contained glasses have an advantage of their large thermal neutron capture cross section (3840 barn). In this study, we focused on MgO-P₂O₅-B₂O₃ glass systems. They are composed of light elements; hence, neutron detection signals can be easily distinguished from noise due to X- and γ-rays. Furthermore, alkali-earth-embedded P₂O₅-B₂O₃ glasses realize high optical transparency with good chemical durability. As luminescence centers, Eu was selected. Eu exists in two different states: Eu²⁺ in reduction states and Eu³⁺ in oxidation states. The former shows broad emission bands with fast decay times of ~μs, whereas the latter shows sharp emission bands with slow decay times of ~ms. In general, Eu³⁺ is governed in the glasses prepared in an air atmosphere; however, alkali-earth-embedded P₂O₅-B₂O₃ glasses can exhibit Eu²⁺ owing to the localized reducing atmosphere generated by NH₃ derived from the raw material (NH₄H₂PO₄). Here, Eu-doped MgO-P₂O₅-B₂O₃ glasses with various concentrations of Eu were synthesized by the conventional melt quenching technique in air, and their optical, photoluminescence (PL), and scintillation properties were examined.

Undoped and 0.1%, 0.3%, 1.0%, and 2.0% (in mole fraction) Eu-doped 25MgO-30P₂O₅-45B₂O₃ (MPB) glasses were synthesized by the melt quenching method. Raw materials were homogeneously mixed and transferred into an alumina crucible. The powders were melted at 1100 ℃ for 1 h with an electrical furnace in air. The melt was flowed onto a preheated stainless-steel plate to quench, and pressed into batches. After that, the glasses were annealed at 400 ℃ for 1 h to remove thermal and mechanical strains. The surfaces of the glasses were polished for the following optical and scintillation measurements. The glass transition temperature (T_g) of the undoped sample was measured with a TG-DTA system. The powder X-ray diffraction (XRD) patterns were measured using a diffractometer. Diffuse transmission spectra were measured using a spectrophotometer (Shimadzu, SolidSpec-3700). PL excitation and emission spectra, PL quantum yields (QYs), and PL decay curves were measured using a Quantaurus-QY and Quantaurus-τ. Scintillation spectra, scintillation decay curves, and afterglow curves under X-ray irradiations, and pulse height spectra of ²⁴¹Amα-rays and ¹³⁷Cs γ-rays were measured with our original setups.

The appearances of undoped and Eu-doped MPB glasses were transparent and included some bubbles. Under ultraviolet light at 360 nm, Eu-doped samples showed bluish-red light. Some parts of the glasses were crushed into powder and used for the XRD measurements. Precipitations of crystalline phases were not observed in the XRD patterns; hence, the prepared samples formed glass phases with no periodical structures. T_g of the undoped glass was estimated to be 535 ℃. The transmittances of all the glasses were 70%-90% at 400-700 nm. Both the undoped and Eu-doped samples showed absorption peaks at 200-250 nm; hence, this can be due to the hosts. In addition, absorption peaks emerged at 250-400 nm in Eu-doped samples. The absorption bands at 250-280 nm and 280-400 nm originated from the 4f⁷-4f⁶5d¹ (T_2g and E_g) transitions of Eu²⁺, respectively. An absorption peak due to the ⁷F₁-⁵D₃ transitions of Eu³⁺ was confirmed at 415 nm in the spectra of 2% Eu. A broad emission band due to the electronic transitions of Eu²⁺ was observed at 400-600 nm under excitation bands at 250-410 nm in the Eu-doped samples. Generally, emissions due to Eu²⁺ are difficult to observe in the glasses prepared in air because of the oxidizing atmosphere. The emissions appeared owing to the localized reduction atmosphere, which derived from NH₄H₂PO₄ and (NH₄)₂O·5B₂O₃·8H₂O. When excitation and monitored wavelengths were respectively set to 340 nm and 420 nm, the PL decay curves were fitted by a sum of two exponential functions. The PL decay time constants of both the fast (0.2-0.5 μs) and slow (0.7-1.2 μs) components were similar to those of other Eu-doped phosphors; hence, they are reasonable as 4f⁶5d¹(T_2g)-4f⁷ and 4f⁶5d¹(E_g)-4f⁷ transitions of Eu²⁺. Eu-doped samples showed emission peaks at 350-500 nm under X-ray irradiations. These emission wavelengths were consistent with those of the PL spectra. Afterglow levels (AL) of undoped and 0.1%, 0.3%, 1.0%, and 2.0% Eu-doped glasses at 20 ms passed after X-ray irradiation of 2 ms were respectively estimated to be 1700×10^-6, 4500×10^-6, 3400×10^-6, 800×10^-6, and 160×10^-6. Pulse height spectra of ²⁴¹Am α-rays (5.5 MeV) were measured using the prepared MPB glasses. Eu-doped glasses showed clear full energy absorption peaks. Light yields (LY) of 0.3%, 1.0%, and 2.0% Eu-doped MPB glasses were respectively calculated to be 70, 150 photons/5.5 MeV, and 40 photons/5.5 MeV.

Eu-doped MPB glasses were synthesized by the conventional melt quenching method. All the samples showed halo peaks with no periodic patterns in XRD measurements. The transmittances were 70%-90%, and absorptions due to electronic transitions of Eu²⁺ and Eu³⁺ were observed. All the samples showed luminescence, which originated from the 4f⁶5d¹-4f⁷ transitions of Eu²⁺. PL QYs of 0.1%, 0.3%, 1.0%, and 2.0% Eu-doped samples were respectively calculated to be 46.7%, 37.2%, 20.4%, and 9.3% when monitored at 350-560 nm under excitation at 320 nm. ALs at 20 ms passed after X-ray irradiations of 2 ms were obtained to be 1700×10^-6, 4500×10^-6, 3400×10^-6, 800×10^-6, and 160×10^-6 in undoped, 0.1%, 0. 3%, 1.0%, and 2.0% Eu-doped samples, respectively. Pulse height spectra of ²⁴¹Am α-rays were measured using the prepared samples. LYs of 0.3%, 1.0%, and 2.0% Eu-doped samples were respectively determined to be 70, 150 photons/5.5 MeV, and 40 photons/5.5 MeV.