Applied Physics Express 7, 062602 (2014)
http://dx.doi.org/10.7567/APEX.7.062602

X-ray detection capability of a Cs2ZnCl4 single-crystal scintillator
Natsuna Yahaba1, Masanori Koshimizu1*, Yan Sun1, Takayuki Yanagida2, Yutaka Fujimoto2,Rie Haruki3, Fumihiko Nishikido4, Shunji Kishimoto5, and Keisuke Asai1
1

Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Sendai 9800-8579, Japan
Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu 808-0196, Japan3
Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
4
Molecular Imaging Center, National Institute of Radiological Sciences, Chiba 263-8555, Japan
5
Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan
E-mail: koshi@qpc.che.tohoku.ac.jp
2

Received April 19, 2014; accepted May 7, 2014; published online May 27, 2014
The X-ray detection capability of a scintillation detector equipped with a Cs2ZnCl4 single crystal was evaluated. The scintillation decay kinetics canbe expressed as the sum of two exponential decay components. The fast decay component had a decay time constant of 1.8 ns, and its relative
intensity was 95%. The total light output was 630 photons/MeV, and a subnanosecond timing resolution of 0.66 ns was obtained. The detection
efﬁciency of 67.4 keV X-rays was 80% for a detector equipped with a 2.2-mm-thick Cs2ZnCl4 crystal. Thus, excellent timing resolution and highdetection efﬁciency were achieved simultaneously. © 2014 The Japan Society of Applied Physics

ynchrotron radiation is an essential light source formany measurement techniques because it can produce intense narrow beams over a broad energy range
from infrared to hard X-ray. Furthermore, a pulsed beam
can be provided by an electron bunch, making it possible toperform time-resolved measurements. Time-resolved spectroscopy or high count rate measurements using high-energy
X-rays are commonly adopted techniques in synchrotron
radiation facilities, and detectors that show excellent time
resolution, a short dead time, and high detection efﬁciency
for such X-rays are in great demand.
One type of detector with a short dead time and high time
resolution is the silicon avalanche photodiode (Si-APD).1) In
X-ray detection events, electron–hole (e–h) pairs are createdin the depletion layer of APDs by the photoelectric effect.
The conduction electrons then generate further e–h pairs byan avalanche multiplication process. However, it is difﬁcult
to obtain both sufﬁcient detection efﬁciency and excellent
timing properties simultaneously for high-energy X-rays with
the Si-APD. The depletion layer needs to be thin in orderto obtain excellent timing properties, but the photoelectricabsorption probability of X-rays inside such a thin depletion
layer is quite small owing to the low atomic number of
Si; thus, the detection efﬁciency is very low. To achieve ahigh detection efﬁciency, a certain thickness and constituent
elements with high atomic numbers are necessary.
Another candidate is a scintillation detector equipped with
fast scintillators. To realize scintillation detectors having
a short dead time, the scintillation materials should satisfy
the following two conditions: (i) The scintillation decay
time constant should be small, and (ii) the contribution of thelong components and afterglow should be negligible. Plastic
scintillators, which typically have decay time constants on
the order of nanoseconds, are examples. They are composedof a polymer matrix and organic phosphors. A commercial
plastic scintillator, NE-142, is suitable for high-energy X-ray
detection because it is loaded with lead (5 wt %) to enhance
the stopping power for X-rays.2) However, the enhancement
is limited because of either the limited solubility of the loaded
elements or severe degradation in light output at high loading
concentrations. In terms of the detection efﬁciency, inorganic
scintillators are advantageous owing to the high atomic
numbers of their constituent elements. In doped scintillation

Smaterials, relatively short scintillation decay time constants
have been obtained by using Ce or Pr as a dopant. For
example, YAlO3:Ce (YAP:Ce) has a decay time constantof approximately 27 ns, much longer than that of plastic
scintillators, although it exhibits a high detection efﬁciency
and large scintillation yield.3) Another approach is to useundoped insulator crystals. We recently examined the applicability of a BaCl2 single-crystal scintillator.4,5) The fast
scintillation component in BaCl2 may be attributable toself-trapped excitons (STEs).6) Although the scintillator
exhibits excellent timing properties, it still has a long lifetime
component on the order of several tens of nanoseconds. A
scintillation material having a quantum conﬁnement structure
was also tested, and nuclear resonant scattering measurements were successfully performed;7) however, it is quite
difﬁcult to grow a large crystal of the compound.
Another approach to the development of fast scintillationmaterials is the use of the Auger-free luminescence (AFL).8,9)
AFL is a radiative transition from the valence band (VB) tothe outermost core band, and its decay time constant ranges
from subnanoseconds to several nanoseconds. For AFL to beobserved, the energy difference between the top of the VB
and that of the core band must be smaller than the band-gap
energy. AFL has been used as a fast scintillation process.
BaF2 is a representative scintillation material exhibiting AFLwith a decay time constant of approximately 0.8 ns;10) thus,
it is used as a fast inorganic scintillator in a wide rangeof applications. However, BaF2 has a long scintillation component with a decay time constant of 600 ns due to STEs,10)
so it is not suitable for high count rate or short dead timeapplications. In addition, the wavelength of the fast component ranges from the vacuum ultraviolet (VUV) to the deep
UV region, so a photomultiplier tube (PMT) with a quartz
window is necessary. Other crystals exhibiting AFL, suchas CsF,11) CsCl,8) CsBr,8) and CsCaCl3,12) also have long
components, possibly due to STEs, in addition to a fast
scintillation component due to AFL; thus, they are inappropriate for high count rate measurements.
In this paper, we report the X-ray detection capability of a
Cs2ZnCl4 single crystal. A previous paper reported that AFL
occurs in Cs2ZnCl4 on the basis of the excitation spectrum inthe VUV region and that the luminescence bands due to AFL
under VUV excitation are located at 3.3, 4.2, and 4.8 eV.13)

062602-1© 2014 The Japan Society of Applied Physics

Appl. Phys. Express 7, 062602 (2014)

N. Yahaba et al.
1

Normalized intensity

0.80.60.4
0.2
0200300

400500600

Wavelength (nm)Fig. 1. Scintillation spectrum of Cs2ZnCl4 single crystal.

10000019000exp( − t / 1.8ns) + 65exp( − t / 10ns) + 1.75

10000CountsThe long emission wavelength in the UV region, which ischaracteristic of AFL in CsCl-based crystals, makes this material advantageous over BaF2 for application as a scintillationmaterial.13,14) In this paper, the scintillation properties ofCs2ZnCl4 under high-energy X-ray irradiation are discussed,with particular emphasis on the scintillation time proﬁle.
A single crystal of Cs2ZnCl4 was grown in a vacuum
quartz ampule by the Bridgman–Stockbarger method. Before
crystal growth, a stoichiometric mixture of CsCl and ZnCl2
was dried by heating at approximately 500 K under vacuum.
The purity of each raw material was 99.999%. The preparedsample crystal was colorless and transparent, and it was notdeliquescent. The crystal was approximately 5 © 5 © 2.2
mm3 in size.
A scintillation spectrum was obtained using an X-ray tube
as an excitation source. The spectrum was recorded by a
Peltier-cooled charge-coupled device (Andor DU-420-OE)
coupled with a grating monochromator (Oriel Instruments,285 grooves/mm, blaze wavelength: 280 nm).
The X-ray detection capability was characterized usingsynchrotron X-rays at BL-14A, Photon Factory, KEK. Single
bunch mode operation was used, and the period of thesuccessive X-ray pulses was 624 ns. The impinged X-rayenergy was 67.4 keV. The beam spot was approximately
1 mm in diameter. The duration of the X-ray pulse was
140–150 ps. A scintillation time proﬁle was obtained by asingle photon counting technique. The scintillation photons
were detected with a photomultiplier tube (PMT; Hamamatsu
R7400P) and a fast preampliﬁer (Phillips Scientiﬁc 6954).
The rise time of the preampliﬁer was 220 ps. The X-ray
arrival timing signals were supplied by the accelerator, andtheir time resolution was 20 ps. The timing of the signals wasdetermined by a constant fraction discriminator (CFD; Ortec935). The time difference between the signals was converted
into the pulse height by a time-to-amplitude converter(Ortec 566), and the converted signal was accumulated to ahistogram with a multichannel analyzer (MCA; Aptec 5004).
The scintillation time proﬁle was obtained as a histogramof the time difference. In this measurement system, the contribution of components having a decay time constant longer
than the period of successive pulses appears as the timeindependent component. We also characterized a scintillation
detector equipped with the Cs2ZnCl4 crystal. The Cs2ZnCl4
single crystal was attached to the PMT with optical grease.
For the measurement of the pulse-height spectra, the detection signal was ampliﬁed with a charge-sensitive ampliﬁer(Canberra 2005) and a main ampliﬁer (Ortec 572A, Ortec);
a histogram was obtained with the MCA mentioned above.
The shaping time of the main ampliﬁer was set to 0.5 µs. To
estimate the scintillation light yield, the pulse-height spectrum for 1-mm-thick YAP:Ce (OKEN) was also measured forcomparison. A time resolution curve was also obtained with asetup similar to that for the scintillation time proﬁle except
for the use of a fast preampliﬁer (Ortec VT120A). In addition, the detection efﬁciency was determined by comparing
the count rate of the current scintillation detector with that ofa scintillation detector equipped with 5 mm thick NaI:Tl.
Figure 1 shows the scintillation spectrum of the Cs2ZnCl4
single crystal. Three bands were observed, at 390, 280, and
255 nm. The wavelengths of these bands were consistent withthose observed under VUV irradiation.13)

1000
100101-20020406080100

Time (ns)Fig. 2. Scintillation time proﬁle of Cs2ZnCl4 crystal. Solid line represents
function obtained by ﬁtting.

Figure 2 is the scintillation time proﬁle of the Cs2ZnCl4crystal. The proﬁle was reproduced as the sum of two
exponential decay functions. The numerical formula for theﬁtted function is also shown in Fig. 2. The relative intensityof each component was calculated according to the formula.
The intensity of each component was obtained by integrating
the function for 624 ns, which is the period of the successive
X-ray pulses. The decay time constant of the fast component
was 1.8 ns, and its relative intensity was 95% of the total
scintillation amount. The decay time constant of the slow
component was approximately 10 ns, and its relative intensity
was 2.5%. The remaining 2.5% consisted of decay components having much longer lifetimes, which are seen astime-independent components. When the X-rays were turnedoff, the photon count rate soon decreased to the backgroundlevel. Thus, we observed no signiﬁcant afterglow.
Figure 3 shows pulse-height spectra of Cs2ZnCl4 andYAP:Ce. The ampliﬁer gain for the Cs2ZnCl4 crystal was 5
times that for YAP:Ce. In the Cs2ZnCl4 spectrum, a peak wasobserved at channel number 75. On the basis of a comparisonof the peak channel numbers, the photoelectron yield ofthe Cs2ZnCl4 crystal was estimated to be 2.3% of that ofYAP:Ce. Thus, the photoelectron yield is estimated to beapproximately 100 photoelectrons/MeV, assuming the liter-062602-2© 2014 The Japan Society of Applied Physics

Appl. Phys. Express 7, 062602 (2014)

N. Yahaba et al.

1000

Table I.

Cs2ZnCl4

Atomic number

Slow component

Cs2ZnCl4High

Negligible

UV–visBaF2High

Signiﬁcant

VUV (fast)
UV (slow)

PlasticLow

Negligible

UV–vis

YAP:CeCounts

100

Properties of fast scintillators.
Wavelength

10

10200

400

600

800

1000

ChannelFig. 3. Pulse-height spectra of Cs2ZnCl4 single crystal and YAP:Ce for67.4 keV X-rays.

10000
Threshold voltage:20 mVCounts

1000

Threshold voltage:50 mV

100

101-505101520Time (ns)Fig. 4. Time-resolution curves of scintillation detector equipped with
Cs2ZnCl4 crystal. Diamonds and open squares correspond to results withthresholds of 20 and 50 mV, respectively.

ature value of 4300 photoelectrons/MeV for YAP:Ce.15) The
average quantum efﬁciency of the PMT (R7400P) for thescintillation from Cs2ZnCl4 was estimated to be 16% on thebasis of the scintillation spectrum in Fig. 1 and the wavelength-dependent sensitivity of the PMT. The photoelectron
yield and average quantum efﬁciency led to an estimatedlight yield of 630 photons/MeV. The detection efﬁciency
was calculated using the histogram shown in Fig. 3. The
total count in Fig. 3 was approximately 9.4 © 104 counts, andthe measurement time was 100 s. The arrival rate of the X-ray
photons at the detector position was approximately 1.18 ©
103 photons/s. Thus, a detection efﬁciency of 80% was
obtained using a 2.2-mm-thick Cs2ZnCl4 crystal at 67.4 keV.
The detection efﬁciency agrees well with the stopping
probability for 67.4 keV X-rays of approximately 85%,
which was estimated using the mass attenuation coefﬁcientspublished by the National Institute of Standards andTechnology.16)
Figure 4 shows the time-resolution curve of the scintillation detector equipped with the Cs2ZnCl4 crystal at athreshold voltage of 20 mV at the CFD. This threshold level
corresponds approximately to the output of a single photon

detection signal. The full width at half-maximum time
resolution was approximately 0.66 ns. We observed a longtail up to 10 ns, which originated from the long scintillation
components. At a threshold voltage of 50 mV, the time
resolution did not differ signiﬁcantly from that at the 20 mV
threshold. The long tail was considerably suppressed. However, the count rate was approximately 35% less than thatfor the 20 mV threshold. In a crude approximation, the higher
threshold level corresponds to the detection of lower-energy
X-ray photons. The lower count rate at a higher thresholdvoltage indicates that the detection efﬁciency will decreasefor X-rays of lower energy because of the small light yieldof Cs2ZnCl4.
The decay time constant of 1.8 ns for the fast scintillation
component agrees well with the value of 1.6 ns for luminescence under VUV irradiation.13) Thus, the fast scintillation
component is ascribed to AFL. According to a previouspaper,13) in addition to AFL, two luminescence bands wereobserved at 2.4 and 6.2 eV at 10 K and were ascribed to defectluminescence. In addition, in insulators such as Cs2ZnCl4,
STE luminescence often contributes to the long component.
The quite small contribution of the longer scintillation
components in Cs2ZnCl4 may be ascribed to either quenchingof the STEs or defect luminescence at room temperature.
Table I summarizes the properties of several fast scintillators. A Cs2ZnCl4 scintillator with a 2.2-mm-thick crystal
exhibits high detection efﬁciency because of the high atomic
numbers of its constituent elements and a moderate density of
2.93 g cm¹3. In addition, the decay time constant of the main
component of 1.8 ns is comparable to or smaller than thatof plastic scintillators. Thus, as a fast scintillation material forhigh-energy X-ray detection, Cs2ZnCl4 has several advantages over plastic scintillators, although the light output
was rather low. In comparison to BaF2, the relative intensityof the fast component was much higher. In addition, thescintillation wavelength of 250–450 nm shown in Fig. 1
matches well the sensitive wavelength region of genericphoton detectors, in contrast to BaF2, in which the wavelength of the fast component lies in the deep UV region.
These advantages indicate that Cs2ZnCl4 is an appropriate
scintillation material for high-energy X-ray detection withhigh detection efﬁciency, good timing resolution, and a highcount rate.
In summary, the X-ray detection capability of a novel
scintillation material, a Cs2ZnCl4 single crystal, was characterized. The dominant component of the scintillation time
proﬁle was a fast one having a decay time constant of 1.8 ns
and a relative intensity of 95%. The scintillation light yield
was estimated to be 630 photons/MeV on the basis of acomparison with YAP:Ce. A detection efﬁciency of 80% wasachieved with a 2.2 mm thick crystal for 67.4 keV X-rays.
Thus, this scintillation material is promising for high-energy

062602-3© 2014 The Japan Society of Applied Physics

Appl. Phys. Express 7, 062602 (2014)

N. Yahaba et al.

X-ray detection with high detection efﬁciency, good timing
resolution, and a high count rate. This result indicates that thefast scintillation component is due to AFL.
Acknowledgments This research was partially supported by a Grant-inAid from the Adaptable and Seamless Technology Transfer Program throughTarget Drive R&D from the Japan Science and Technology Agency. This study
was performed with the approval of the Photon Factory Advisory Committee(Proposal Nos. 2008G104, 2010G179, and 2012G062). Part of this work wascarried out at the UVSOR facility, and it was supported by the Joint Studies
Program (2011, No. 23-540) of the Institute for Molecular Science.

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062602-4© 2014 The Japan Society of Applied Physics