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Sensors and Materials, Vol. 30, No. 7 (2018) 1564–1575
MYU Tokyo
S & M 1612

Evaluation of Scintillation and Thermally Stimulated
Luminescence Properties of Cs2CdCl4 Single Crystals
Takumi Sakai,1 Masanori Koshimizu,1* Yutaka Fujimoto,1Daisuke Nakauchi,2 Takayuki Yanagida,2 and Keisuke Asai1
1

Department of Applied Chemistry, Graduate School of Engineering, Tohoku University,
Sendai 980-8579, Japan
2
Graduate School of Materials Science, Nara Institute of Science and Technology,
Ikoma, Nara 630-0192, Japan(Received January 5, 2018; accepted March 27, 2018)

Keywords: thermally stimulated luminescence, s–p transition, Cs2CdCl4

The optical and scintillation properties of undoped, Sb-doped, and Tl-doped Cs2CdCl4
single crystals were investigated. We chose Sb3+ and Tl+ as the dopants of (ns)2-type electronic
configurations. In the photoluminescence and scintillation spectra of undoped Cs2CdCl4, threeemission peaks were observed. The scintillation decay-time constants were small (1.2 and 13ns) and light yields were low (several tens of photons/MeV). In contrast, Sb3+- and Tl+-doped
Cs2CdCl4 showed s–p transitions of the dopant ions, and the light yield was larger than thatof undoped Cs2CdCl4 (625 and 1000 photons/MeV, respectively). Tl-doped Cs2CdCl4 showed
relatively large scintillation decay-time constants (270 ns) because of slow energy transfer fromthe host to Tl+. On the other hand, Sb-doped Cs2CdCl4 showed significant afterglow behavior
and three thermally stimulated luminescence (TSL) glow peaks (93, 168, and 238 ℃) with good
linearity with respect to the X-ray dose. In conclusion, the light yield of undoped Cs2CdCl4 canbe enhanced by doping with Sb3+ and Tl+; TSL characteristics are achieved by doping Sb3+.

1.

Introduction

In general, luminescent materials, such as those utilized in radiation detectors, showeither of two types of luminescence behavior: immediate luminescence upon irradiation andluminescence upon external stimulation after irradiation. Luminescent materials exhibiting
such behavior can be used as scintillators and dosimeters, respectively. Dosimeters store
ionizing radiation energy as trapped electrons and holes and exhibit luminescence upon external
stimulation by light (optically stimulated luminescence, OSL) or heat (thermally stimulated
luminescence, TSL). They are mainly used for personal dose monitoring, and should exhibit
suitable sensitivity, linear response to dose, and low fading. On the other hand, scintillatorsconvert the absorbed energy of ionizing radiation immediately into low-energy photons.
Scintillation detectors are composed of scintillation materials and a photomultiplier tube (PMT),
avalanche photodiode, or Si-PMT, and play important roles in medical imaging,(1) undergroundresources exploration,(2) and security inspections.(3)*Corresponding author: e-mail: koshi@qpc.che.tohoku.ac.jp
http://dx.doi.org/10.18494/SAM.2018.1920
ISSN 0914-4935 © MYU K.K.
http://myukk.org/

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Alkali metal halides are promising scintillation materials for the detection of high-energy
photons such as X-rays or gamma rays because of their efficient light yields, high energy
resolution, optical transparency, and low manufacturing costs, all of which make them favorable
for many practical applications. In fact, NaI:Tl and CsI:Tl exhibit high light yields (~40000
and ~50000 photons/MeV, respectively)(4) originating from 6sp → 6s2 transitions of Tl+ ions,and they have been applied to X-ray and gamma ray detections for many years. However, bothtypes of material have certain disadvantages as scintillators. For instance, NaI:Tl exhibits highhygroscopy in ambient atmosphere, which limits its applications. Its manufacture requires
controlled humidity and special handling. CsI:Tl is free from the high hygroscopy exhibited by
NaI:Tl; it exhibits a long afterglow attributed to the thermal ionization of trapped electrons (Tl0)followed by radiative recombination with trapped holes [VkA (Tl+)](5) and possesses large decay
time constants (τ1 = 600 ns and τ 2 = 3.4 μs) under gamma ray excitation,(6) which leads to pulse
pile-up in high-count-rate applications. Although the afterglow can be suppressed by codopingwith Eu2+, Sm2+, Yb2+, or Bi3+,(7–10) the drawbacks of slow decay components have not yet been
completely resolved. Thus, new scintillators having minimal hygroscopy, high light yield, andsmall decay time constants are required.
Cs2CdCl4 is a transparent nonhygroscopic crystal. The Cs2CdC14 crystal is tetragonal andcrystallizes in a K 2NiF4-type structure having the space group I4/mmm with a = 5.26 Å andc = 16.88 Å, and two equivalent molecules per unit cell.(11) The divalent metal ions (Cd2+) aresurrounded by a tetragonal-type octahedron, slightly compressed [(Cd–Cl (1) = 2.64 Å and Cd–
Cl (2) = 2.51 Å)] along the fourfold axis, which coincides with the crystallographic c-axis.(12)This condition gives rise to a ligand field of D4h symmetry on the divalent site. According to aprevious paper,(12) Mn2+-doped Cs2CdC14 crystals exhibit intense 3dn–3dn transitions of Mn2+;
detailed spectrum analysis indicated that Mn2+ substitutes for Cd2+ in the site that occupies thecenter of the octahedron. This result indicates that Mn2+-doped Cs2CdCl4 may exhibit efficient
scintillation. However, 3dn–3dn transitions are not appropriate for scintillator applications
because of their very long lifetime (~ms). Alternatively, ns2-type ions (e.g., Tl+, Sb3+) exhibit
s–p transitions in a crystal field with octahedral symmetry, such as KCl and KI.(13,14) The s–
p transitions of ns2-type ions, especially for Tl+, are an attractive mechanism for scintillators
because of their efficient light yield(1) and fast decay under UV excitation (~several tens of
nanoseconds).(15) In addition to Tl+, Sb3+ ions are another possible dopant owing to theirsimilar electron configuration to that of Tl+. According to a previous paper,(16) Sb3+ can behave
as a luminescence center in Cs2NaMCl6 (M = Sc, Y, La), the crystal structure of which has
octahedral symmetry as in Cs2CdCl4. Thus, Sb3+ ions are expected to function as luminescencecenters in Cs2CdCl4. On the other hand, it has been reported that Sb-doped Sr2SnO4 shows TSL
glow peaks due to the s–p transitions of Sb3+ after irradiation with UV light.(17) This indicatesthat Sb-doped Cs2CdCl4 may behave as a dosimeter.
In this study, the photoluminescence (PL) and scintillation properties of undoped, Sb-doped,
and Tl-doped Cs2CdCl4 crystals were investigated at room temperature using X-ray-excited
luminescence spectra, pulse height spectra, and scintillation decay curves. The dosimeter
properties of the Sb-doped Cs2CdCl4 crystal were also investigated using TSL measurements.

Sensors and Materials, Vol. 30, No. 7 (2018)

2.

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Experimental Procedure

Single crystals of undoped, Sb-doped, and Tl-doped Cs2CdCl4 were grown via theBridgman–Stockbarger method. Prior to crystal growth, raw powders of CsCl (99.999%, HighPurity Chemical, Japan), CdCl2 (99.9%, High-Purity Chemical, Japan), and SbCl3 (99.999%,
Wako, Japan) or TlCl (99.999%, Wako, Japan) were thoroughly mixed in a stoichiometricratio and dried at 423 K for one day to remove water. The crystals were grown under vacuum
in sealed quartz ampules. The molar concentrations of Sb or Tl in the melt were 0.1 mol%.
Crystals larger than 1 mm were used for optical and scintillation characterizations. Powder
X-ray diffraction (XRD) measurements revealed that all the samples had the tetragonal
Cs2CdCl4 structure.
PL measurements were carried out using a spectrofluorometer (Hitachi, Japan, F-7000)
in air at room temperature. The PL spectra of undoped Cs2CdCl4 were measured usingsynchrotron radiation in the vacuum ultraviolet (VUV) region as the excitation source at BL7B at UVSOR, IMS, Japan. The X-ray-induced radioluminescence (RL) spectra were obtainedusing an X-ray tube operated at 80 kV and 1.2 mA as the excitation source. The spectra
were recorded using a Peltier-cooled charge-coupled device (CCD; Andor, US, DU-920-P)
combined with a monochromator (Andor, US, SR163i-UV).(18) The pulse height spectra of thesamples were obtained using a photomultiplier tube (Hamamatsu, Japan, R7600U) under 662
keV γ-ray irradiation from 137Cs.(19) The scintillation temporal profiles were obtained via atime-correlated single-photon counting method with a pulsed X-ray excitation source.(20) To
investigate the dosimeter properties, we measured the TSL glow curve with an original setup
consisting of a heater (SAT0983a, Sakaguchi, Japan), a thermostat (SCR-SHQ-A, Sakaguchi,
Japan), and a photomultiplier tube (PMT; H11890-210, Hamamatsu, Japan). The heating rate
for the TSL measurements was set to 0.5 ℃/s, and the sample was heated from 36.5 to 290℃ to measure the glow curve. Furthermore, the TSL spectrum was measured using a CCDbased spectrometer (QE pro, Ocean Optics, US), while the sample was heated by an electricheater (SCR-SHQ-A, Sakaguchi, Japan, E. H. Voc) at a constant temperature. The sample wasimmediately heated to each temperature after irradiation.

3.

Results and Discussion

3.1

Samples

Photographs of the undoped, Sb-doped, and Tl-doped Cs2CdCl4 crystals are shown in Fig. 1.
The samples had cracks and were not hygroscopic.
3.2

Photoluminescence properties

The room-temperature PL spectra of an undoped Cs2CdCl4 crystal with excitations at 58 and
200 nm are shown in Fig. 2(a). Emission peaks at 320 and 600 nm are observed with excitations
at 58 and 200 nm, and an emission peak at 430 nm is observed only with excitation at 58 nm.
The origins of the emission bands at 320 and 430 nm are unclear at present; the possibilities are1568

Sensors and Materials, Vol. 30, No. 7 (2018)

Fig. 1.

(Color online) Photograph of undoped, Sb-doped, and Tl-doped Cs2CdCl4 crystals.

(a)

(b)

(c)

Fig. 2. (Color online) (a) VUV emission spectra of undoped Cs2CdCl4 crystal, and PL spectra (dotted lines) andPL excitation spectra (solid lines) of (b) Sb-doped and (c) Tl-doped Cs2CdCl4 crystals at room temperature. For
undoped Cs2CdCl4, the excitation wavelengths are (1) λex = 200 nm and (2) λex = 58 nm. For Cs2CdCl4:0.1%Sb, theexcitation and emission wavelengths are (1) λem = 516 nm and (2) λex = 277 nm. For Cs2CdCl4:0.1%Tl, the excitationand emission wavelengths are (1) λem = 350 nm and (2) λex = 250 nm.

electronic transitions at defects, self-trapped excitons (STEs), and Auger-free luminescence (AFL).
AFL is the radiative transition of an electron from the valence band to the uppermost core level,
which can be observed only with excitation energies higher than the binding energy of the corelevel, e.g., 14 eV for CsCl.(21) In the case of undoped Cs2CdCl4, the two emission peaks did notSensors and Materials, Vol. 30, No. 7 (2018)1569

show a threshold-like behavior in the excitation energy. For this reason, AFL is excluded fromconsideration as the origin.
The excitation and PL spectra of Sb- and Tl-doped Cs2CdCl4 are shown in Figs. 2(b) and2(c), respectively. In the case of the Cs2CdCl4:Sb crystal, the excitation spectrum recorded for
λem = 516 nm consists of five bands: a triplet at 254, 277, and 296 nm, and a doublet at 336 and
350 nm. According to previous studies,(13,16,22) the triplet and doublet bands are ascribed to the1
T1u → 1A1g and 3T1u → 1A1g transitions of Sb3+, respectively. A similar splitting of excitation
peaks has been observed for alkali metal halide crystals doped with ns2-type ions such as
Pb2+,(23–25) Tl+,(26) and Sn2+(27–29) and has been explained in terms of thermal lattice vibrations.(29,30)
The adiabatic potential energy surface (APES) of the excited state is split for general distorted
lattice configurations, although it is degenerate in the equilibrium configuration of the groundstate. Thus, thermal vibrations, i.e., the dynamical Jahn–Teler effect, cause instantaneous
splitting of the excitation bands,(29,30) and the observed splitting in the excitation bands in Fig.
2(b) is attributed to this phenomenon. In the emission spectrum recorded at λex = 277 nm, a
broad peak at 516 nm is observed. According to a previous study of Sb3+-doped Cs2NaMCl6 (M= Sc, La, Y),(16) this broad peak is attributed to the 3T1u → 1A1g transition of Sb3+. In general,
ns2-type ions doped into alkali metal halide crystals exhibit two types of emission peaks, theso-called Ax and AT emissions, which arise from two minima on the APES of the 3T1u relaxed
excited state. These minima are due to the Jahn–Teller coupling to the ɛg and τ2g lattice modes.(15)
However, the shapes of these emission peaks excited at the other excitation wavelength (254,
296, 336, and 350 nm) are almost the same. This result indicates that the Jahn–Teller interaction
working on the 3Tlu APES does not lead to two emissions (the Ax and AT emissions). A
similar case was reported for Sb-doped elpasolite-type crystals such as Cs2NaMCl6 (M = Sc, Y,
La).(16,31) In Cs2NaMCl6, the Sb3+ ion substitutes at the M3+ ion site with octahedral symmetry.
Considering that the PL properties of Cs2NaMCl6:Sb3+ and Cs2CdCl4:Sb3+ are similar, the Sb3+
ion should substitute at the Cd2+ site, which occupies the center of octahedral symmetry as in[SbCl6]3−.
In the emission spectrum of the Cs2CdCl4:Tl crystal recorded at λex = 250 nm, a narrow
peak and a broad peak are observed at 350 and 490 nm, respectively. Considering that the
wavelengths of the emission peaks are similar to those of CsCl:Tl,(32–35) the narrow peak (350nm) and the broad peak (490 nm) are ascribed to the 3P1 → 1S0 transition of Tl+ and off-center
STEs, respectively. Accordingly, the excitation peak recorded at λem = 350 nm is attributed to
the 1S0 → 3P1 transition of Tl+.
3.3

Scintillation properties

Figure 3 shows the X-ray-induced RL (XRL) spectra of the undoped, Sb-doped, and Tldoped Cs2CdCl4 crystals. In the case of undoped Cs2CdCl4, three narrow peaks at 320, 360,
and 400 nm, a weak band at 450 nm, and a broad peak with a maximum intensity at 600 nm areobserved. In the PL spectra with VUV excitation, the emission peaks at 320 and 360 nm may
not be resolved. The emission peaks in the RL spectrum have the same origin as those in the
PL spectra.

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Fig. 3. (Color online) Room-temperature X-ray-excited luminescence spectra of undoped, Sb-doped, and Tldoped Cs2CdCl4 crystals. The lines show undoped Cs2CdCl4 (red-orange line), Cs2CdCl4:0.1%Sb (blue line), andCs2CdCl4:0.1%Tl (light blue line).

In the spectrum of the Cs2CdCl4:Sb crystal, a broad peak at 520 nm, which is ascribedto the 3T1u → 1A1g transition of Sb3+, is observed. On the other hand, in the spectrum of theCs2CdCl4:Tl crystal, in addition to the peak at 350 nm, which is also seen in the PL spectrum,
we observed a broad intense peak at 600 nm, which is not seen in the PL spectra. This case issimilar to that of CsI:Tl. In the PL spectra of CsI:Tl,(36) there are four peaks; the two weak ones
were ascribed to Ax (375 nm) and AT (401 nm) emissions of Tl+, while the other two intense
peaks (486 and 550 nm) were ascribed to off-center configurations of STEs perturbed by Tl+.
In the XRL spectra of CsI:Tl,(5) an intense broad peak at 550 nm was observed and attributed
to the thermal ionization of trapped electrons (Tl0) followed by their radiative recombination
with trapped holes. Thus, as for Cs2CdCl4, a similar mechanism may result in the difference
between the PL spectra and the XRL spectra.
The pulse height spectra of the undoped, Sb-doped, and Tl-doped Cs2CdCl4 crystals areshown in Fig. 4. To estimate the light yields of these crystals, a Gd 2SiO5:Ce (GSO) crystal,
whose light yield is 10000 photons/MeV, was used for comparison. The peak at channel 630
corresponds to the full-energy peak of GSO. The course gain was 10 times larger for the
undoped Cs2CdCl4 and 50 times larger for Cs2CdCl4:Sb and Cs2CdCl4:Tl than for GSO. The
full-energy peaks of the undoped Cs2CdCl4 crystal were not clearly visible, so the light yieldof undoped Cs2CdCl4 cannot be estimated. On the other hand, shoulders are observed atchannels 250 and 180 in the pulse height spectra of the Cs2CdCl4:Sb and Cs2CdCl4:Tl crystals,
respectively, although they are not well resolved. Assuming that these shoulders correspond to
the full-energy peaks, and considering the quantum efficiencies of the bialkali photocathode of
the PMT for GSO (28% for 430 nm), Cs2CdCl4:Sb (10% for 520 nm), and Cs2CdCl4:Tl (2.5% for
600 nm), the light yields of Cs2CdCl4:Sb and Cs2CdCl4:Tl crystals are estimated to be 600 and1000 photons/MeV, respectively.
The scintillation temporal profiles of the undoped, Sb-doped, and Tl-doped Cs2CdCl4
crystals are shown in Fig 5. The scintillation decay-time constants were deduced from the sumsof two or three exponential decay functions. Table 1 summarizes the decay-time constants andrelative intensities. The decay-time constants for the undoped Cs2CdCl4 crystal are 1.2 andSensors and Materials, Vol. 30, No. 7 (2018)

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Fig. 4. (Color online) Pulse-height spectra of undoped, Sb-doped, and Tl-doped Cs2CdCl4 crystals. Thelines show GSO (dotted red line), undoped Cs2CdCl4 (light green line), Cs2CdCl4:0.1%Sb (blue line), andCs2CdCl4:0.1%Tl (light blue line).

(a)

(b)

Fig. 5. (Color online) Scintillation temporal profiles of (a) an undoped Cs2CdCl4 crystal and (b) Sb-doped and Tldoped Cs2CdCl4 crystals.
Table 1
Decay-time constants and relative intensities of scintillation temporal profiles of undoped, Sb-doped, and Tldoped Cs2CdCl4 crystals. The words “fast”, “middle”, and “slow” in this table refer to “fast components”, “middle
components”, and “slow components” of the crystal, respectively.

Decay-time constant τ (ns)
Relative intensity (%)

Undoped
fast
slow
1.2
13
66
34

fast
0.8
28

Sb-doped
middle
11
53

slow
270
18

Tl-doped
fast
slow
6
200
63
37

13 ns. On the other hand, the decay-time constants for the Sb-doped or Tl-doped Cs2CdCl4
crystals are composed of a fast component due to host luminescence (0.8 and 11 ns for the Sbdoped sample and 6 ns for the Tl-doped sample) and slow components due to each dopant ion (270
ns for the Sb-doped sample and 200 ns for the Tl-doped sample). According to the intensity
ratio, the emissions due to the s–p transitions of the dopant ions do not contribute significantly
to the scintillation.

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3.4

Sensors and Materials, Vol. 30, No. 7 (2018)

Dosimeter properties

To investigate the origin of the afterglow of Cs2CdCl4:Sb, TSL glow curves after irradiation
with X-rays at doses of 1, 10, and 100 Gy are shown in Fig. 6. The Cs2CdCl4:Sb crystal shows
glow peaks near 91, 166, and 242 ℃. The activation energies are derived using the initial risemethod(37) on the assumption of first-order kinetics. The intensities at the glow peaks increased
proportionally with dose with a negligible change in the glow curve. Therefore, it is appropriate
to consider the TSL process as first-order, although first-order kinetics have not been confirmedin this study.(38) The peaks at higher temperatures were analyzed without eliminating thepeak at lower temperature because the glow peaks were well separated in temperature. The
estimated values are summarized in Table 2. Figure 7 shows the TSL spectra near the glow
peaks (93, 168, and 238 ℃). The TSL spectra show the same peak with a maximum intensity
near 520 nm, which is attributed to the 3T1u → 1A1g transition of Sb3+. Thus, it is impliedthat the afterglow also resulted from the 3T1u → 1A1g transition of Sb3+. Figure 8 shows therelationship between the TSL intensity and X-ray dose, namely, the dose response curve. The
dose response curves for all the glow peaks have good linearity. This result indicates that the
Sb-doped Cs2CdCl4 may be used for dosimetry. In addition, the significant TSL may be areason for the small light yield of Sb-doped Cs2CdCl4. Figure 9 shows the TSL glow curvesof the undoped Cs2CdCl4 crystal after X-ray irradiation from 1 to 100 Gy. A dominant peak isobserved at 93 ℃ with a small shoulder at 170 ℃. From the comparison of the glow curves of
undoped and Sb-doped Cs2CdCl4, the glow peaks at 93 and 168 ℃ for Sb-doped Cs2CdCl4 are
attributed to the trapping sites in the Cs2CdCl4 host; on the other hand, the glow peak at 238 ℃
is attributed to Sb3+-related centers.

Fig. 6.

(Color online) TSL glow curves of a Cs2CdCl4:0.1%Sb crystal after X-ray irradiation from 1 to 100 Gy.

Table 2Glow peak temperature and estimated activation energies of a Cs2CdCl4:0.1%Sb crystal.
SamplePeak temperature (℃) Activation energy (eV)
90.5
0.22
165.5
0.51
Cs2CdCl4:0.1%Sb
242
0.64

Sensors and Materials, Vol. 30, No. 7 (2018)

Fig. 7. ( C o l o r o n l i n e ) T S L s p e c t r a o f a
Cs2CdCl4:0.1%Sb crystal measured at 93 (red dottedline), 168 (red-orange dotted line), and 238 ℃ (redsolid line). The inset expands the 300–700 nm
regions.

Fig. 9.

4.

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Fig. 8. (Color online) Dose response cur ve ofCs2CdCl4:0.1%Sb crystal. The TSL signals used here
are the peak intensities observed at 90.5 (diamonds),
165.5 (squares), and 242 ℃ (triangules).

(Color online) TSL glow curves of an undoped Cs2CdCl4 crystal after X-ray irradiation from 1 to 100 Gy.

Conclusion

Undoped, Tl-doped, and Sb-doped Cs2CdCl4 crystals were synthesized using the Bridgman–
Stockbarger method, and their optical, scintillation, and dosimeter properties were evaluated.
In the room-temperature PL spectra, the undoped Cs2CdCl4 crystal exhibited three types ofemission peaks, and the AFL was excluded as the origin of the emission. In the case of a Sbdoped Cs2CdCl4 crystal, the splitting of excitation bands due to the dynamical Jahn–Teller
effect and the A x emission of Sb3+ were observed. These results are similar to those in the caseof Sb-doped Cs2NaMCl6 (M = Sc, La, Y) and indicate that Sb3+ ions substitute for the Cd2+
site, which occupies the center in octahedral symmetry. For the Tl-doped Cs2CdCl4 crystal,emissions due to 3P1 → 1S0 transitions of Tl+ and off-center STEs were observed. Under X-ray

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Sensors and Materials, Vol. 30, No. 7 (2018)

irradiation, the undoped and Sb-doped Cs2CdCl4 crystals showed emission peaks similar
to those in the PL spectra. On the other hand, the Tl-doped Cs2CdCl4 crystal exhibited an
additional emission peak, which is attributed to off-center configurations of STEs perturbed by
Tl+ or the thermal ionization of trapped electrons (Tl0) followed by radiative recombination withtrapped holes. According to the pulse height spectra, the light yields of undoped Cs2CdCl4 wereslightly enhanced by doping with Sb3+ and Tl+ (625 and 1000 photons/MeV, respectively). The
scintillation decay-time constants for the undoped Cs2CdCl4 crystal were small (1.2 and 13 ns).
The Sb- or Tl-doped Cs2CdCl4 crystals exhibited, in addition to the fast component due to hostluminescence, slow components due to the dopant ions as a relatively small contribution (270 ns
and 18% for the Sb-doped sample, and 200 ns and 37% for the Tl-doped sample). Furthermore,significant afterglow was observed for the Cs2CdCl4:Sb crystal. The TSL glow curves of
Cs2CdCl4:Sb are composed of three glow peaks at 91, 166, and 242 ℃. The TSL is attributed tothe 3T1u → 1A1g transition of Sb3+ according to the TSL spectra. In addition, the dose response
curves for all the glow peaks have good linearity.

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