Optical Materials 36 (2014) 1930–1933

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Optical Materials
journal homepage: www.elsevier.com/locate/optmat

Scintillation and luminescence properties of a single CsCaCl3 crystal
M. Koshimizu a,⇑, N. Yahaba a, R. Haruki b, F. Nishikido c, S. Kishimoto d, K. Asai aa

Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
Advanced Science Research Center, Japan Atomic Energy Agency, 2-4 Shirakata Shirane, Tokai, Ibaraki 319-1195, JapancMolecular Imaging Center, National Institute of Radiological Sciences, Anagawa 4-9-1, Inage-ku, Chiba 263-8555, JapandInstitute of Materials Structure Science, High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japanba r t i c l ei n f o

Article history:
Available online 22 April 2014
Keywords:
Auger-free luminescenceCore-valence luminescenceCore hole
Scintillator
Self-trapped excitona b s t r a c t
We synthesized a fast scintillation material based on a ternary halide crystal, CsCaCl3, which is known toexhibit Auger-free luminescence (AFL) under core-level excitation. We characterized its luminescence
properties using vacuum ultraviolet (VUV) light for excitation. We observed a broad AFL band centered
at approximately 300 nm for the core hole excitation, with a decay time of 2.3 ns. In addition to AFL,
we observed several luminescence bands for interband and sub-band-gap excitation. The scintillation
light yield was estimated to be 410 photons/MeV. We observed a fast scintillation component due tothe AFL having a decay time constant of 2.3 ns. This fast component accounts for 58% of the total scintillation. These results indicate that a fast scintillation material was successfully fabricated, employing AFL
as the luminescence mechanism based on a ternary halide crystal.
Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction
Auger-free luminescence (AFL), or core-valence luminescence,is a radiative electronic transition between the valence band andthe outermost core band [1]. This luminescence is observed onlyfor insulator crystals in which the band-gap energy is larger thanthe energy difference between the valence band and the outermost
core band. AFL is known to be a fast luminescence process whose
decay time constant is typically of the order of nanoseconds or less.
Therefore, insulator materials that exhibit AFL are applicable as tofast-response scintillators.
Thus far, BaF2 has been the most successful example of scintillators exhibiting AFL. Its scintillation has a fast component corresponding to AFL, whose decay time constant is as short as 0.8 ns[2]. Owing to this fast decay combined with relatively high lightyields, BaF2 has been used for applications in which good timing
characteristics are necessary. However, its scintillation also has along decay component corresponding to self-trapped excitons(STEs). The decay time constant of the long component has beenreported to be 600 ns [2]. Thus, the long component limits theusage of BaF2 in several applications, such as measurements witha high counting rate. AFL has also been observed in other binary
halide crystals [3], such as RbF, CsF, CsCl, and CsBr. In those materials, the light output of AFL is lower than BaF2, and they similarly⇑ Corresponding author. Tel.: +81 22 795 7219.
E-mail address: koshi@qpc.che.tohoku.ac.jp (M. Koshimizu).
http://dx.doi.org/10.1016/j.optmat.2014.04.004
0925-3467/Ó 2014 Elsevier B.V. All rights reserved.

have long components corresponding to STEs. Because the numberof binary halides exhibiting AFL is limited, research should beextended to ternary or more complex halide crystals in order todevelop novel scintillation materials exhibiting AFL.
Recently, AFL has been observed with several ternary halide
crystals, such as LiBaF3 [4], MgBaF4 [4], CsCaCl3 [4,5], Cs2ZnCl4[6,7], Rb2ZnCl4 [8]. Among these, we have focused on the CsClbased ternary crystals, because AFL occurs in the long wavelength
region and so is suitable for detection with photomultiplier tubes(PMTs). In a study reported recently, we found a fast scintillation
component due to AFL in Cs2ZnCl4 [9]. In this paper, we report
the scintillation properties of another halide crystal, CsCaCl3.

2. ExperimentSingle crystals of CsCaCl3 were fabricated using a vertical Bridgman-Stockbarger method. Powders of CsCl (99.999%, Kojundo
Chemical Laboratory Co. Ltd., Japan) and CaCl2 (99.99%, Sigma–Aldrich, Japan) were mixed in a stoichiometric ratio. The mixed
powder was poured into a quartz ampoule and heated at 473 Kin vacuum for 1 day in order to remove water. During the growth,
the temperature gradient was 1.1 K/mm, and the ampoules werelowered at a rate of 1.0 mm/h. Transparent crystals were obtained.
The photo of the sample crystal is shown in Fig. 1. The size wasapproximately 3  5  2 mm2. They showed a slight hygroscopy.
When we left the crystals in air for several days, the surface ofthe crystals turned cloudy.

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Fig. 1. Photograph of the sample crystal.

The luminescence properties were characterized using vacuum
ultraviolet (VUV) light as an excitation source. The measurements
were performed at beamline 7B of the UVSOR facility at the Institute for Molecular Science, Japan. The synchrotron radiation wasmonochromated using a 3-m normal-incidence monochromator.
The samples were irradiated with the monochromated light in a
vacuum chamber. The luminescence spectra were measured using
a charge coupled device (CCD) equipped with a monochromator.
The luminescence was guided to the detector via an optical ﬁber.
The luminescence spectra were measured for various excitation
wavelengths, and the excitation spectra were obtained from theluminescence spectra. In addition, luminescence decay behavior
was observed by a single photon counting method. The singlebunch operation mode was used, and the period between successive pulses was 176 ns. The decay component longer than this period appeared as a time-independent component in the time
proﬁles. The luminescence photons were detected with a multichannel-plate (MCP) mounted PMT (R3809U-50, Hamamatsu,Japan). All measurements were performed at room temperature.
The scintillation properties and X-ray detection capability werecharacterized at BL-14A at the Photon Factory, KEK, Japan. The Xray energy was 67.4 keV. For the characterization of the X-ray
detection capability, the sample crystals were attached to a PMT(R7400P, Hamamatsu, Japan) with a small amount of opticalgrease. The pulse-height spectra were measured using a charge
sensitive preampliﬁer (2005, Canberra, CT, US), a main ampliﬁer(572A, Ortec, TN, US), and a multichannel analyzer (MCA; 5004,Aptec, NY, US). The single bunch operation mode was used forthe measurements of scintillation time proﬁles,. The period
between each successive pulse was 624 ns. The spacing between
the samples and the PMT was 38 mm. The single-photon detection signals from the PMT were ampliﬁed with a fast preampliﬁer(6954, Phillips Scientiﬁc, NJ, US). The timing of the ampliﬁed signals was determined with a constant-fraction discriminator(CFD; 935, Ortec, TN, US), and the timing signals were fed to atime-to-amplitude converter (TAC; 566, Ortec, TN, US) as start signals. The X-ray arrival timing signals were supplied from the synchrotron facility and fed to the TAC as stop signals. The timedifference between the two signals was compiled into a histogramwith the MCA, and yielding the time proﬁles.

Fig. 2. Luminescence spectra of CsCaCl3 for the excitation at 170–200 nm.

peaks. For excitation at 200 nm, we observed a broad band at
480 nm and a shoulder at approximately 290 nm. For shorter excitation wavelengths down to 170 nm, the intensity of the two bands
reversed. In addition, we observed a band at 510 nm for excitation
at 170 nm. The results indicate that the CsCaCl3 crystal had at least
two kinds of defect or impurity cites involved in the luminescence.
One plausible origin is anion vacancies, which is common latticedefect in halide crystals. Another possible origin is cation vacancies
due to a slight deviation of the cation composition fromstoichiometry.
Fig. 3 shows the luminescence spectra of CsCaCl3 excited at 160
and 130 nm. We observed a broad band peaking at approximately
440 nm for both excitation wavelengths or between them. The
photoexcitation at 130 nm corresponds to the interband excitation.
The photon energy at 160 nm is slightly lower than the band-gap
energy, and may correspond to an exciton excitation [5]. Thus, thisbroad band is ascribed, presumably, to STEs.
Fig. 4 shows the luminescence spectra for excitation at 84–
88 nm. Considering the position of the outermost core level of Cs,
14 eV below the conduction band [5,10], these excitation wavelengths correspond to core-level excitation. The spectrum for excitation at 84 nm differs considerably from those for the excitation
wavelengths of 88 and 86 nm. For the excitation wavelength of
88 nm, the luminescence spectrum was quite similar to those forthe excitation wavelengths of 130 or 160 nm shown in Fig. 3. For
the excitation wavelength of 86 nm, in addition to the band centered at 440 nm, a small shoulder was observed at ca. 300 nm.
For the excitation wavelength of 84 nm, a prominent broad band
was observed at 300 nm, and a clear band was also observed at
500 nm. The spectrum for the excitation at 84 nm is quite similar
to that in previous reports [5,10], and a broad band centered at

3. Results and discussion
3.1. VUV spectroscopy
Fig. 2 shows the luminescence spectra of CsCaCl3 excited at
170–200 nm. Considering the band-gap energy of 8.5–8.6 eV
reported previously [5,10], these excitation wavelengths correspond to the sub-band-gap excitation. Thus, the observed luminescence originates from electronic transitions in the band gap, andwe attribute this to impurities or defects. Because we did not use
an optical ﬁlter to remove the scattered excitation photons, thesecond-order and third-order diffraction peaks appeared as sharp

Fig. 3. Luminescence spectra of CsCaCl3 for the excitation at 160 and 130 nm.

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Fig. 4. Luminescence spectra of CsCaCl3 for excitation at 84–88 nm.

Fig. 6. Luminescence time proﬁles of CsCaCl3 for excitation at (a) 84 nm and (b)200 nm.

CsCaCl3
YAP:CeCounts

300 nm can be ascribed to AFL. A peak at approximately 500 nm
may be due to second-order diffraction of the 300 nm band.
Fig. 5 shows the excitation spectra for the 300 nm luminescenceband. In order to analyze the entire range of the broad band at300 nm, the luminescence wavelengths were set at 260, 310, and
350 nm. These wavelength correspond to the short-wavelengthedge, center, and the long-wavelength edge of the broad band,respectively. A steep rise at approximately 84 nm was observed
over the entire range of the broad luminescence band at 300 nm.
These excitation spectra were quite similar to those in previous
reports [5,10], and the result clearly indicates that the broad bandat 300 nm is due to AFL.
Fig. 6 shows the luminescence time proﬁles for excitation at 84
and 200 nm. For the excitation at 84 nm, we observed fast luminescence decay in the entire range of the broad band at 300 nm, andFig. 6(a) illustrates the time proﬁles at short- and long-wavelength
regions of the band. The decay time constant was estimated to be
2.3 ns, which is consistent with the result reported in a previous
paper [5]. In contrast, for the excitation at 200 nm, the time proﬁleswere ﬂat. Because the period between the successive excitation
pulses was 176 ns, this result indicates that the decay time constant of the luminescence is longer than several hundreds ofnanoseconds.

3.2. Characterization of scintillation properties
Fig. 7 shows the pulse-height spectrum of a scintillation detector equipped with the CsCaCl3 crystal for the X-ray energy of
67.4 keV. The pulse height spectrum for YAlO3:Ce (YAP:Ce) is also
shown for comparison. It should be noted that the ampliﬁer gain

ChannelFig. 7. Pulse-height spectra of scintillation detectors equipped with a CsCaCl3 or aYAlO3:Ce (YAP:Ce) crystal for 67.4 keV X-ray. Note that the ampliﬁer gain was tentimes smaller for YAP:Ce.

Fig. 5. Excitation spectra of CsCaCl3.

was ten times smaller for YAP:Ce. A broad shoulder was observed
at approximately 100 ch for CsCaCl3. Assuming the light yield of
YAP:Ce to be 18,000 photons/MeV [11], and considering the wavelength dependent sensitivity of the PMT, the light yield of theCsCaCl3 crystal is estimated to be 410 photons/MeV.
Fig. 8 shows the scintillation time proﬁle of CsCaCl3. The decay
behavior was satisfactorily ﬁtted with a sum of two exponentialdecay components and a time-independent component. The fastcomponent had a decay time constant of 2.3 ns, which is consistent
with that of the luminescence shown in Fig. 6. Thus, a fast scintillation component due to AFL was successfully obtained. The fastcomponent accounts for 58% of the total scintillation. The slow
component had a decay time constant of 14 ns, and thisM. Koshimizu et al. / Optical Materials 36 (2014) 1930–1933

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observed a broad band centered at 300 nm having a decay time
constant of 2.3 ns and ascribed this band to AFL. For interband
excitation and sub-band-gap excitation, we observed several bands
due to radiative decay of STEs and luminescence at defects orimpurities, respectively. The scintillation light yield was estimatedto be 410 photons/MeV. We observed a fast scintillation component having a decay time constant of 2.3 ns and ascribed this component to AFL. This fast component accounts for 58% of the total
scintillation. These results indicate that a fast scintillation material
was successfully fabricated from a ternary halide crystal by usingAFL.
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Fig. 8. Scintillation time proﬁle of CsCaCl3. The solid line represents the ﬁttedfunction.

component accounts for only 4% of the scintillation. The time-independent component accounts for 38%, and this component is considered to have a decay time constant much longer than 624 ns,
which is the period of the successive X-ray pulses. The components
having a long decay time constant are presumably ascribed to theluminescence at defects or impurities and STE emission, whichwere observed for the VUV excitation in Figs. 2 and 3.
4. Conclusion
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