Photoluminescence and Scintillation Properties of Ce-Doped SrF2 Transparent Ceramics
Authors: Kato Takumi , Nakauchi Daisuke , Kawaguchi Noriaki , Yanagida Takayuki 
Published: December 2024

A type of phosphors known as scintillators is capable of instantly absorbing incident ionizing radiation and converting its energy into hundreds of ultraviolet/visible photons. A growing variety of novel applications, including medical (X-ray CT and PET), security, oil logging, environmental monitoring, astronomy, and particle physics, has promoted the development of new scintillator materials. Bromides and iodides doped with Tl+, Eu2+, or Ce3+ ions are the most often used scintillators: cesium iodide doped with Tl (CsI:Tl), strontium iodide doped with Eu (SrI2:Eu), and lanthanum bromide doped with Ce (LaBr3:Ce). These crystals are featured by short decay time constants, good energy resolution, and a high light yield (100,000 photons/MeV for SrI2:Eu. The main drawbacks of bromides and iodides are their high hygroscopicity.

Alkaline earth fluorides, on the other hand, have been used as scintillators due to their strong radiation resistance, low phonon energy, and low hygroscopicity. For example, barium fluoride (BaF2) single crystals are known as scintillator materials. According to previous reports, self-trapped excitation (STE) and auger-free luminescence are the causes of the scintillation observed in BaF2 crystals. BaF2, in particular, offers a significant benefit for use in γ-ray detections because of its comparatively large effective atomic number. Single crystals of calcium fluoride (CaF2) have been reported to exhibit scintillation at 270 nm, resulting from STE. Moreover, it has a large band gap energy and relatively high light yield (13,000 photons/MeV), and Eu-doped CaF2 has been investigated for potential as the scintillator in astrophysics.

Nowadays, several scintillators of bulk single crystals are practically employed in practice due to their excellent features, including high transparency, high light yield, and high detection efficiency owing to a large volume. Recent research has made it possible to use transparent ceramics as scintillator materials due to enhanced synthesis technologies that have been developed in tandem with the development of the laser field. Ceramics have many advantages over single crystals, including mechanical strength, cost-effectiveness, and flexible geometric design. Moreover, it has been discovered that some transparent ceramics exhibited comparable or even better scintillator performance than the single crystal form. However, these studies have only dealt with oxide materials; therefore, we have started to investigate the scintillation properties of halogen compound ceramics with transparent or translucent forms. Thus far, our groups have created and assessed transparent ceramics of CaF2, BaF2, CsCl, and CsBr for scintillator applications.

In this work, Ce-doped SrF2 transparent ceramics were fabricated by spark plasma sintering (SPS), and their photoluminescence (PL) and scintillation characteristics were investigated as an extension of our earlier studies. The sintering method and optical properties of SrF2 transparent ceramics have already been studied, but they have not focused on scintillation applications. The scintillation characteristics of transparent ceramics of undoped SrF2 have been first studied by our group [1]. In addition, Eu-doped SrF2 transparent ceramics have also been studied as scintillation materials [2]; however, no information is available on the scintillation characteristics of Ce-doped SrF2 transparent ceramics.

In X-ray-induced scintillation spectra, three peaks were detected at 290, 310, and 330 nm. The peak intensity at 290 nm decreased with increasing Ce concentration. On the other hand, the peak intensities at 310 and 330 nm were maximum when Ce concentration was 0.1%. Compared with the past studies [3,4], the 290 nm peak is due to self-trapped excitation, and the 310 and 330 nm peaks are caused by 5d-4f transitions of Ce3+ ions. These attributions were supported by the measurement results of scintillation decay curves. We will report the other luminescence properties on the day of our presentation.

[1] T. Kato et al., Optik, 168 (2018) 956–962.

[2] T. Kato et al., Sensors Mater., 36 (2024) 531–538.

[3] R. Lindner et al., Phys. Rev. B., 63 (2001) 1–7.

[4] R. Shendrik et al., Radiat. Meas., 56 (2013) 58–61.