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The thermal neutron imaging technique has been advanced to fulfill the requirements of a variety of applications. To fulfill the requirements, research was intended to find a suitable transparent crystalline compound that could serve the purpose of a scintillation screen for thermal neutron imaging. Depending on the thermal neutron absorption cross-section of a variety of elements, the compounds containing Li, B, and Gd were attracted the most due to their high microscopic absorption cross-section. Considering the 𝛾 -rays of different energies from neutron sources and the environment, it is very crucial to discriminate between thermal neutrons and 𝛾-rays by the scintillator.
Three new glass compositions were tested for their ability to distinguish thermal neutrons from 𝛾-rays. Prior to the proceedings, the thermal neutron facility was characterized to determine the thermal neutron and 𝛾-ray contents in the beam as well as the luminescence properties of the glass samples. The samples were doped with Eu$^{3+}$-ions and showed their characteristic 4f-4f transition emission. BLiY:1.5Eu sample was identified through digital and film-based neutron radiography techniques to distinguish thermal neutrons, successfully rejecting the accompanied 𝛾-rays. As a result, Li$_6$Y(BO$_3$)$_3$ was identified as a candidate for crystal growth that matches the elements exist in the glass sample.
The Czochralski method was used to successfully grow a single crystal of pure Li$_6$Y(BO$_3$)$_3$, which was then doped with 4 mol% Dy$^{3+}$ and 1 mol% Pr$^{3+}$-ions. The doped crystals were characterized by luminescence studies under X-ray and photo-excitation. The scintillation performance of these single-crystal samples was evaluated using 𝛽− and 𝛼-particles, 𝛾-rays, along with moderated thermal neutrons from ${}^{90}$Sr, ${}^{241}$Am, ${}^{60}$Co, and ${}^{252}$Cf radioactive sources, respectively. 4 mol% Dy$^{3+}$ doped Li$_6$Y(BO$_3$)$_3$ was found to have 4500 ± 550 Photons/MeV, or 92.8 % photon counts in comparison to CMO crystal under 𝛽− irradiation. The grown samples were found efficient enough to distinguish thermal neutrons from 𝛾-rays generated by nuclear research reactors as well as radioactive source, 252Cf. Li6Y(BO3)3 was discovered to be promising for future use as a thermal neutron scintillator for imaging.
Thesis supervisor: Prof. Hongjoo Kim