Scintillation Properties of Yb2+-Doped LiBr Single Crystals
Authors: Miyazaki Keiichiro , Nakauchi Daisuke , Kato Takumi , Kawaguchi Noriaki , Yanagida Takayuki 
Published: December 2024

As neutron detectors, 3He proportional counters have generally been used in security, environmental monitoring, medicine, and well-logging fields. The 3He is a product produced during β-decay of 3H. The 3H has generally been used in nuclear weapons, and the supply of 3He decreased due to nuclear disarmament and prohibition of nuclear experiment. However, the demand of the neutron detectors rapidly became high in the security field after September 11th terrorist attacks in 2001, and the demand of 3He became higher than the supply. Therefore, scintillators for neutron detection have been required as alternative detectors for 3He proportional counters. The required properties of scintillators for neutron detection are high light yield (LY), neutron-γ discrimination capability, large cross section for neutron capture, small effective atomic number (Z
eff), non-hygroscopicity, and high transparency etc. 6Li-based inorganic scintillators have been focused because 6Li has a large cross section for neutron capture (940 barn) and high Q-value (4.8 MeV) in the 6Li (n, α) 3H nuclear reaction. In this reaction, α-ray and 3H with the total energy of 4.8 MeV were generated, and these charged particles produce numerous secondary electrons. Finally, the luminescence is caused by recombination of electrons and holes in luminescence centers. Up to now, Ag-doped ZnS/LiF, Ce-doped Li-glass, Eu-doped LiCaAlF6, Eu-doped LiI, and Ce-doped Cs2LiYCl6 have been developed as commercial 6Li-based inorganic scintillators. However, these scintillators do not fully satisfy all the required properties, and research activities have continued to find new scintillators for neutron detection. Lithium halide compounds are one of the promising hosts for neutron detection because of high Li-concentration. Among them, Eu-doped LiI has been intensely researched because of high LY but underutilized compared with other commercial scintillators due to high hygroscopicity. LiBr shows less hygroscopicity and has smaller Z
eff (Z
eff=34) than those of LiI. From the reasons, LiBr is one of promising candidates as neutron scintillators. So far, photoluminescence and scintillation properties of Eu-doped LiBr were evaluated in our previous study. The LY of Eu-doped LiBr was 4,600 ph/n under 252Cf neutron irradiation, and the LY was lower than that of commercial scintillators for neutron detection. Therefore, the improvement of the LY is required by optimization of the luminescence center elements. Yb2+ shows broad band due to 4f135d1–4f14 transition which is electric-dipole allowed for the free ion. Two emission bands are sometimes observed because the excited states of Yb2+ are divided into low energy level with reversed spin states and high energy level preserves spin states. These emission bands are spin-forbidden and spin-allowed transitions, respectively. So far, there are no reports about photoluminescence and scintillation properties of Yb-doped LiBr. In this study, LiBr doped with various concentration of Yb2+ were synthesized by the vertical Bridgman method, and the photoluminescence and scintillation properties were evaluated. According to scintillation spectra under X-ray irradiation, all the samples showed two emission bands at 420 and 450 nm. These emission bands would be ascribed to spin-forbidden and spin-allowed 4f135d1–4f14 transition of Yb2+. In this presentation, we will report results of the luminescence properties under UV and X-ray irradiation and pulse height spectra under γ-ray and neutron irradiation.