Optical and Scintillation Properties of CsCaCl3-Based Crystals
Authors: Saeki Keiichiro , Fujimoto Yutaka , Koshimizu Masanori , Yanagida Takayuki , Tanaka Hironori , Asai Keisuke 
Published: February 2020

A scintillation detector is composed of a scintillator, which converts absorbed ionizing radiation into multiple low energy (~eV) photons, and a photomultiplier tube (PMT), avalanche photodiode, or Si-PMT, which can convert the photons into an electrical signal. The performance of the scintillation detector depends greatly on the characteristics of the scintillator. Inorganic scintillators are widely used for detecting high-energy photons such as X-rays and gamma-rays. Eu-doped CsCaCl3 has been reported to display excellent properties because of the 4f65d1–4f7 transitions of Eu2+ [1]. CsCaCl3:Eu exhibits a 4f65d1–4f7 luminescence peak at 450 nm , with a high light yield (18,000 photons/MeV). Further, undoped CsCaCl3 is known to have a small scintillation decay time constant because of their Auger-free luminescence [2]. In this study, CsCaCl3 is used as the host material and luminescence and scintillation properties of CsCaCl3 crystals doped with impurity ions such as Yb and Tl are investigated. Photoluminescence spectra, X-ray-excited radioluminescence spectra, scintillation temporal profiles, and pulse-height spectra (with 662 keV gamma-ray excitation from 137Cs) of the crystals are analyzed.
Single crystals of Yb- and Tl-doped CsCaCl3 were prepared with the vertical Bridgman–Stockbarger method using a sealed quartz tube. Before crystal growth, stoichiometric mixtures of CsCl and CaCl2 were dried by heating overnight ~500 K in a vacuum. After growth, the crystals were cut and polished.
The Figure shows the X-ray-excited radioluminescence spectra, measured at 300 K, for Yb- and Tl-doped CsCaCl3 crystals. In the spectrum of CsCaCl3:Yb, an intense emission band was observed at 437 nm with a shoulder at 409 nm. The emission band in the radioluminescence spectrum is consistent with that at 439 nm, with a shoulder at 409 nm, in the photoluminescence spectrum. As these emission bands have similarities with emission bands of a SrI2:Yb2+ crystal, the emission bands of the CsCaCl3:Yb crystal are attributed to spin-allowed and spin-forbidden Yb2+ 4f135d–4f14 transitions. In the spectrum of CsCaCl3:Tl, an emission band observed at 327 nm is similar to that at 336 nm in the photoluminescence spectrum. In a previous study of CsCl:Tl, an emission band at 340 nm is attributed to the triplet relaxed excited state of a Tl+ ion [3]. Thus, the emission band at 327 nm can be attributed to the same state. The pulse- height spectra of these crystals were measured with 662 keV gamma-ray excitation from 137Cs. Assuming a light yield of 40,000 photons/MeV for NaI:Tl [4], the light yield is estimated to be 3,800 and 4,300 photons/MeV for CsCaCl3:Yb and CsCaCl3:Tl, respectively. The lower light yields for these crystals can be attributed to the lower concentration of impurity ions. Thus, the light yields of these crystals can be improved by increasing the concentration of impurity ions.
[1] M. Zhuravleva, B. Blalock, K. Yang, M. Koschan, and C. L. Melcher, J. Cryst. Growth 352, 115 (2012).
[2] M. Koshimizu, N. Yahaba, R. Haruki, F. Nishikido, S. Kishimoto, and K. Asai, Opt. Mater. 36, 1930 (2014).
[3] V. Nagirnyi S. Zazubovich, and N. Jaanson, Phys. Stat. sol. 175, (1993) 155.

Figure 1