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Sensors and Materials, Vol. 30, No. 7 (2018) 1577–1583
MYU Tokyo
S & M 1613

New Intrinsic Scintillator with Large Effective Atomic Number:Tl2HfCl6 and Tl2ZrCl6 Crystals
for X-ray and Gamma-ray Detections
Yutaka Fujimoto,1* Keiichiro Saeki,1 Daisuke Nakauchi,2Takayuki Yanagida,2 Masanori Koshimizu,1 and Keisuke Asai1
1

Department of Applied Chemistry, Graduate School of Engineering, Tohoku University,
Sendai, Miyagi 980-8579, Japan
2
Graduate School of Materials Science, Nara Institute of Science and Technology,
8916-5 Takayama, Ikoma, Nara 630-0192, Japan(Received January 22, 2018; accepted March 27, 2018)

Keywords: scintillator, crystal growth, Tl2HfCl6, Tl2ZrCl6

We report on the photoluminescence and scintillation characterizations of new Tl2HfCl6 andTl2ZrCl6 crystalline scintillators grown by the vertical Bridgman–Stockbarger method. BothTl2HfCl6 and Tl2ZrCl6 crystals showed an intrinsic emission band peak at 450–470 nm under
UV light and X-ray excitations. The scintillation light yields of Tl2HfCl6 and Tl2ZrCl6 reach24200 and 50800 photons/MeV, respectively. Their energy resolutions at 662 keV are 17.7% for
Tl2HfCl6 and 5.6% for Tl2ZrCl6. The scintillation decay time values of Tl2HfCl6 are 288 and6340 ns; meanwhile, those of Tl2ZrCl6 are 696 and 2360 ns.

1.

Introduction

Inorganic crystalline scintillators have been in use for decades to detect and measure X-ray
and gamma-ray radiations as they provide an efficient means of converting a high-energy
electromagnetic radiation into ultraviolet-visible photons of light that can be detected withphotosensitive sensors such as a photomultiplier tube (PMT) and a photodiode (PD). The
ultraviolet-visible photons are converted into a current pulse signal to be quickly counted andrecorded by the PMT and PD, and thus, the scintillator-based radiation detectors belong to theindirect detector category. The number of scintillation photons as a response to a high-energy
electromagnetic radiation is proportional to the current signal. Thus, the advancement of theradiation detector’s performance cannot be realized without the discovery of new scintillatorswith superior characteristics and the improvement of the existing scintillators. For X-ray andgamma-ray spectroscopy applications, the scintillator material must possess, in general, a large
effective atomic number (Zeff), a high light yield with good energy resolution, and a fast decay
time constant. In particular, the large Zeff provides high photoelectric conversion efficiencies of
X-rays and gamma-rays [the probability of absorption by the photoelectric effect is proportional*

Corresponding author: e-mail: fuji-you@qpc.che.tohoku.ac.jp
http://dx.doi.org/10.18494/SAM.2018.1927
ISSN 0914-4935 © MYU K.K.
http://myukk.org/

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

to ρZeff 3–4 (ρ, density)].(1) Although a classical NaI:Tl (Zeff = 50.8) scintillator and somecurrently developed halide scintillators, such as SrI2:Eu (Zeff = 50.3) and CeBr3 (Zeff = 47.6),
have demonstrated a high light yield and good energy resolution,(2–3) their Zeff values are lessthan those of oxide-based commercial scintillators such as Gd2SiO5:Ce (Zeff = 59.4), Lu2SiO5:Ce(Zeff = 65.4), and Bi4Ge3O12 (Zeff = 75.2). Recent studies of a new scintillator material for X-rays
and gamma-rays focus on Tl- and Hf-based halide scintillators because of the large atomic
numbers of Tl (81) and Hf (72). Burger and co-workers have worked on the development of
new Hf-based intrinsic halide scintillators and reported Cs2HfCl6 and Cs2HfCl4Br2 crystals
having high Zeff values, high light yields, and good energy resolution.(4,5) Other research groups
studied various Tl-based alkali halide elpasolite scintillators activated with Ce3+.(6–9) Amongthe elpasolite scintillators, the Tl2LiYCl6:Ce (TLYC) crystal shows good scintillation properties.
Moreover, the TLYC crystal can distinguish gamma-ray and neutron radiations by pulse shapediscrimination (PSD).(9) Presently, our group has also reported some new halide scintillator
materials with low hygroscopicity and large Zeff values, namely, CsPbCl3,(10) Cs3BiCl6,(11)Rb2HfCl6,(12) TlMgCl3,(13) TlCdCl3,(14) and Cs2HfBr6.(15) In particular, Rb2HfCl6, TlMgCl3,
and Cs2HfBr6 scintillators were very interesting because they show high light yields and goodenergy resolution without any internal doping of an emission center. This study focused on two
intrinsic halide scintillators containing Tl and Hf ions, namely, Tl2HfCl6, and Tl2ZrCl6. To thebest of our knowledge, no other study on crystal growth, photoluminescence, and scintillation
properties of Tl2HfCl6 and Tl2ZrCl6 has been reported so far. The two crystals have large Zeffvalues (72.7 for Tl2HfCl6 and 70.7 for Tl2ZrCl6), resulting in excellent photoelectric conversion
efficiencies of X-rays and gamma-rays. We present and discuss results from the initial study ofTl2HfCl6 and Tl2ZrCl6 crystalline scintillators, grown by the vertical Bridgman–Stockbarger
method, in the following sections.

2.

Experimental Procedure

Samples of Tl2HfCl6 and Tl2ZrCl6 crystals were prepared by the vertical Bridgman–
Stockbarger method. Mixtures of TlCl (99.9%, Mitsuwa Chemicals Co., Ltd.) and HfCl6(99.9%, Mitsuwa Chemicals Co., Ltd.) or ZrCl6 (99.99%, Sigma-Aldrich) raw materials in
stoichiometric ratio were used for the growth of Tl2HfCl6 and Tl2ZrCl6 crystals. The mixtures
were then loaded into precleaned and prebaked quartz ampoules. The mixtures in the quartz
ampoules were dried by heating overnight at 573 K in vacuum for 72 h before sealing. The
crystal growth was carried out under vacuum in the sealed quartz ampoule using the vertical
Bridgman furnace (TBR-30V, Techno Search Corp.). The ampoule was pulled down at a rateof 1.0–3.0 mm/h from top to bottom in a gradient temperature zone. The temperatures of theupper and lower zones in the vertical furnace were 1273 and 1073 ℃, respectively, resulting in
the temperature gradient of 1.3 K/mm across the solid-liquid interface. After the crystal growth
was ﬁnished, the furnace was cooled to room temperature at a speed of 20 ℃/h. As-grown
crystals (shown in Fig. 1) were cut and polished into dimensions of 2.0 × 1.0 × 1.0 mm3 for use
in photoluminescence and scintillation measurements. The measurements were carried out in
air atmosphere because the visual observation indicated that the crystals are less hygroscopic
than SrI2:Eu and LaBr3:Ce crystals.

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

Fig. 1.

1579(Color online) Photograph of as-grown Tl2HfCl6 (left) and Tl2ZrCl6 (right) crystals.

The photoluminescence spectra were evaluated using an F-7000 (Hitachi High-Technologies)
fluorescence spectrophotometer. The photoluminescence decay curves were measured with a
DeltaFlex time-correlated single-photon counting system (Horiba). The crystal samples were
excited at 250 nm using a pulsed NanoLED excitation source. The obtained decay curves werefitted to the appropriate exponential model after the deconvolution of the instrument response
function.
The scintillation spectra were measured by excitation with X-rays from an X-ray generator(RINT2200, Rigaku) equipped with a copper target at power settings of 40 kV and 40 mA.
The scintillation photons from the samples were counted with a SILVER-Nova multichannel
spectrometer (Stellarnet Inc.) that was cooled to −15 ℃ using a Peltier module through an
optical fiber. The crystal samples were optically coupled to the fiber head using optical grease,
and Teflon tape was used as a reflector to enhance the light collection of diffuse reflections.
The scintillation decay time profiles were evaluated using a customized-setup, pulsed X-rayinduced scintillation characterization system (Hamamatsu Photonics).(16) The main modulesof the system are an R7400P-06 PMT (Hamamatsu Photonics) and an X-ray tube (N5084,
Hamamatsu Photonics) controlled by a picosecond laser diode (PLP10-063, Hamamatsu). By
combining the laser diode and X-ray tube, a pulsed X-ray was generated. The PMT was used tocount scintillation photons in the photon-counting mode, and the detectable wavelength regionof the PMT was 160 to 650 nm.
The scintillation pulse height spectra with 662 keV gamma-rays from a 137Cs souce were
evaluated using the sample crystals optically coupled to an R7600-200 PMT (Hamamatsu).
The experimental details were described in our previous report.(17) For better light collection,
crystal samples were wrapped with Teﬂon tape on all sides, except the face in contact with thewindow of the PMT. The PMT was connected to an ORTEC 113 preamplifier, an ORTEC 572
shaping amplifier, and an Amptec 8000A multichannel analyzer (MCA). The spectra wererecorded with a 10 μs shaping time. The quantum efﬁciency (QE) of the PMT was calculatedusing the wavelength response of the PMT and the scintillation spectra. The scintillation lightyield was calculated by comparing the 662 keV gamma-ray-induced photopeak position of thesamples with the position of the peak in spectra collected with a NaI:Tl commercial scintillator (LY= ~40000 photons/MeV).

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

Results and Discussion

Figures 2(a) and 2(b) illustrate the obtained photoluminescence spectra of Tl2HfCl6 andTl2ZrCl6 crystals. The intrinsic emission peak at 450–470 nm was observed upon ultraviolet
light excitation for both Tl2HfCl6 and Tl2ZrCl6 crystals. This indicates that at least theemission process of Tl2HfCl6 is similar to that of Tl2ZrCl6. When monitoring emission at
465 nm, excitation bands at 255 nm for Tl2HfCl6 and at 270 nm for Tl2ZrCl6 were observed.
Similar excitation bands were reported previously for Cs2HfCl6 and Cs2ZrCl6 crystals, and theycorrespond to charge transfer on HfCl62− and ZrCl62− complex ions. Considering the obtained
photoluminescence spectra and referring to some previous studies about luminescence andoptical characterizations of Cs2HfCl6 and Cs2ZrCl6 crystals, the origin of the blue emission
band is possibly self-trapped excitons (STEs) that strongly depend on the Zr4+ impurity in thehost crystal.(18–20) Zr4+ was detected as a major contaminant of the hafnium chloride (HfCl4)
raw material and grown Hf-containing halide crystals,(4,19,21) resulting in the creation of several
types of STE states and the intrinsic luminescence of the Tl2HfCl6 crystal, which is similar tothat of the Tl2ZrCl6 crystal.
The photoluminescence decay curves of Tl2HfCl6 and Tl2ZrCl6 crystals are shown in Figs.
3(a) and 3(b), respectively. From the exponential fitting calculation, the decay time values ofTl2HfCl6 were approximately 324 and 3560 ns, whereas those of Tl2ZrCl6 were found to beapproximately 360 and 3860 ns. The decay time value of Tl2HfCl6 was close to that of Tl2ZrCl6
because of the similar origins of the emission.
Figure 4 illustrates the scintillation spectra of Tl2HfCl6 and Tl2ZrCl6 crystals. The spectrumof Cs2ZrCl6 (black dotted line) is also presented for comparison. The spectra of both Tl2HfCl6
and Tl2ZrCl6 crystals show an emission band peak at 450–470 nm. The spectra are consistent
with the photoluminescence spectra, and thus, the peak can be assigned to the STEs. A similar
emission band was observed for the Cs2ZrCl6 crystal. The slight shift in emission wavelength
may be due to the difference in the monovalent cation between Cs+ and Tl+ since a similar
emission shift was observed previously for Cs2HfCl6 and Rb2HfCl6.(12)

(a)

(b)

Fig. 2. (Color online) (a) Excitation and (b) emission spectra of Tl2HfCl6 and Tl2ZrCl6 crystals (λem = 465 nm; λex= 255 nm for Tl2HfCl6 and 270 nm for Tl2ZrCl6).

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

(a)

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(b)

Fig. 3. (Color online) Photoluminescence decay curves of (a) Tl2HfCl6 and (b) Tl2ZrCl6 crystals (λem = 465 nm,
λex = 250 nm).

Fig. 4. (Color online) X-ray excited scintillation spectra of Tl2HfCl6 (purple squares) and Tl2ZrCl6 (green
triangles) crystals and a Cs2ZrCl6 crystal (black line) for comparison.

The scintillation decay profiles of Tl2HfCl6 and Tl2ZrCl6 crystals are shown in Figs. 5(a) and
5(b), respectively. The decay time values were calculated to be approximately 288 and 6340 ns
for Tl2HfCl6 and 696 and 2360 ns for Tl2ZrCl6. From the measurements, the decay of Tl2HfCl6
and Tl2ZrCl6 crystals was found to be faster than that of Cs2HfCl6 (2200 and 8400 ns) and
Cs2ZrCl6 (1500 and 7500 ns) reported previously.(21) This is possibly caused by the differencein STE type between Cs2MCl6 and Tl2MCl6 (M = Hf, Zr). This difference is dependent on
the crystal structure, crystalline quality, and localized electronic state. Understanding themechanism of the difference in decay time value between photoluminescence and scintillation isclearly beyond the scope of our present study; however, we speculate that it is due to a complex
energy transfer process from the host crystal lattice to emission centers under excitation byionizing radiations.(22)Figure 6 illustrates the scintillation pulse height spectra of Tl2HfCl6 and Tl2ZrCl6 crystals,
and the NaI:Tl commercial scintillator. In the spectra, the 137Cs-662 keV gamma-ray
photopeaks of the Tl2HfCl6 and Tl2ZrCl6 crystals were located at the 250 and 525 channels,
while that of the NaI:Tl commercial scintillator was observed at the 590 channel. Another peakat the 475 channel was also observed for Tl2ZrCl6 owing to the escape of Kβ and Kα X-rays fromTl atoms of the crystal.(23) The QEs of the R7600U PMT at 465 (Tl2HfCl6 and Tl2ZrCl6) and1582

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

(a)

(b)

Fig. 5. (Color online) Decay time profiles of pulsed X-ray-excited scintillation for (a) Tl2HfCl6 and (b) Tl2ZrCl6
crystals.

Fig. 6. (Color online) 137Cs-gamma-ray-irradiated scintillation pulse height spectra of Tl2HfCl6 (purple line) and
Tl2ZrCl6 (green line) crystals and a NaI:Tl scintillator spectrum (black dot-line) for comparison.

415 nm (NaI:Tl) were about 28 and 40%, respectively. Thus, the light yields of Tl2HfCl6 andTl2ZrCl6 crystals were calculated to be approximately 24200 and 50800 photons/MeV on thebasis of a comparison of NaI:Tl. The energy resolutions (FWHMs) of the 662 keV photopeak
are approximately 17.7% for Tl2HfCl6 and 5.6% for Tl2ZrCl6. From the measurements, theTl2ZrCl6 crystal was particularly found to show a high scintillation light yield with good energy
resolution, possibly because crystalline quality depends on the purity of starting materials andthe crystal growth conditions. The poor energy resolution for Tl2HfCl6 is possibly due to low
crystalline quality and chemical composition uniformity because of the high vapor pressureof HfCl4 during melt growth.(24) We expect that further improvement of the quality andcomposition uniformity of Tl2HfCl6 can result in a higher energy resolution.

4.

Summary

Two new intrinsic crystalline scintillators, Tl2HfCl6 and Tl2ZrCl6, were reported. Crystal
samples of Tl2HfCl6 and Tl2ZrCl6 were successfully grown by the vertical Bridgman–
Stockbarger method. For both Tl2HfCl6 and Tl2ZrCl6 crystals, the photoluminescence andscintillation spectra showed an emission band peak at 450–470 nm without any internal dopingSensors and Materials, Vol. 30, No. 7 (2018)1583

of an emission center. The scintillation light yields were calculated to be approximately
24200 photons/MeV with the energy resolution of 17.7% at 662 keV for Tl2HfCl6 and 50800
photons/MeV with the energy resolution of 5.6% at 662 keV for Tl2ZrCl6. For Tl2HfCl6, thescintillation decay time value was estimated to be approximately 288 and 6340 ns, while
Tl2ZrCl6 gave the decay time values of 696 and 2360 ns. Both Tl2HfCl6 and Tl2ZrCl6 crystals
had an emission band peak at 450–470 nm with a high scintillation light yield, which wellmatched the sensitivity wavelengths of the commercial PMT and Si-based photodiodes. Future
efforts toward understanding the origin of these emissions and growing high-quality crystals
are expected to further improve the scintillation characteristics. This study indicates that theTl2HfCl6 and Tl2ZrCl6 crystalline scintillators can be promising candidates for next-generation
X-ray and gamma-ray detectors for various applications.

Acknowledgments
This work was partially supported by the Cooperative Research Project of the ResearchInstitute of Electronics, Shizuoka University.

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