Development of Europium-Doped Barium Fluorochloride Translucent Ceramic Scintillators
Authors: Otake Shota , Kato Takumi , Nakauchi Daisuke , Kawaguchi Noriaki , Yanagida Takayuki 
Published: December 2024

In solid-state ionizing radiation detectors, there are two main types of approaches: direct conversion and indirect conversion. The former generally uses semiconductor detectors (e.g., Si photodiodes) that convert radiation directly into electronic signals. The latter, on the other hand, uses phosphors and converts radiation into low-energy photons, which are then detected with a photodetector. Such phosphors are divided into scintillators and storage phosphors. Scintillators emit photons when electrons and holes generated by ionizing radiation recombine at emission centers. These are widely used in security, medical imaging, and basic physics. In storage phosphors, electrons and holes generated by ionizing radiation are temporarily captured in the trapping levels. When the storage phosphors are externally stimulated, these captured electrons and holes are re-excited and recombine at emission centers. Storage phosphors are used as personal dosimeters and imaging plates.

In general, the requirements for scintillators are high light yield (LY), short decay time constants, large effective atomic number (Z
eff), low afterglow levels, and chemical stability. The search for scintillators has a long history, but no material has been developed that fulfills all the required properties. Therefore, various scintillators with different chemical compositions have been developed, and scintillators have been selected and used for different measurement applications.

Single crystals have been the most commonly used scintillator material form because of their high light transfer efficiency at emission wavelengths. However, these are problematic because of the long time and high cost required for their preparation. To solve the problems that single crystals have, translucent ceramics have attracted attention in recent years. Ceramics are synthesized by solid-phase reactions, which generally take a shorter time and cost less than single crystals grown by melt growth techniques. In this context, our group has successfully developed translucent ceramic scintillators with high luminescence efficiency. We reported that Y3Al5O12:Ce [1] and Lu3Al5O12:Ce [2] translucent ceramics showed higher LYs than single crystals of the same composition. Since translucent ceramics have only recently begun to be developed, only a few materials have been fabricated.

In this study, we developed BaFCl:Eu in translucent ceramic form. Ba-based complex anions have been actively studied as scintillators and dosimetric materials because of their high luminescence intensity and high Z
eff (~50). In particular, BaFCl:Eu is known to have an emission wavelength suitable for general photodetectors (~380 nm), a relatively fast decay time constant (~5 μs), and chemical stability [3]. These properties are suitable for scintillators to detect X- and γ-rays. BaFCl:Eu has been fabricated in opaque ceramics and studied in detail with dosimetric properties, but there are still no reports of BaFCl:Eu translucent ceramics. In this study, BaFCl translucent ceramics doped with 0.01, 0.1, 0.5, and 1.0% Eu were synthesized by the spark plasma sintering method. The optical and scintillation properties of the fabricated samples were investigated, and their potential as scintillators was examined.

Photoluminescence (PL) emission spectra and X-ray-induced scintillation spectra of the BaFCl:Eu translucent ceramics were measured, both of which were dominated by an emission peak at 390 nm. According to the report [3], this emission peak was suggested to originate from the 5d–4f transition of Eu2+. Pulse height spectra were measured to determine a scintillation light yield, which was 15,000 ph/MeV at a maximum. This value is about twice that of commercially used Gd2SiO5:Ce (8,000 ph/MeV, [4]).

We will present a detailed attribution of the luminescence in BaFCl:Eu translucent ceramics and the concentration dependence of the samples.

<reference>

[1] T. Yanagida et al., IEEE Trans. Nucl. Sci., 52, 1836 (2005).

[2] T. Yanagida et al., Radiat. Meas., 46, 1503 (2011).

[3] M. Ignatovych et al., Radiat. Prot. Dosimetry, 84, 185 (1999).

[4] I. G. Valais et al., IEEE Trans. Nucl. Sci., 54, 11 (2007).