Journal of Luminescence 157 (2015) 293–296

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Journal of Luminescencejournal homepage: www.elsevier.com/locate/jlumin

Scintillation properties of CdF2 crystal
Takayuki Yanagida a,n, Yutaka Fujimoto b, Masanori Koshimizu b, Kentaro Fukuda cab
cKyushu Institute of Technology, 2-4 Hibikino, Wakamatsu, Kitakyushu, Fukuoka 808-0196, Japan
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, JapanTokuyama Corp., 1-1 Mikage-cho, Shunan-shi, Yamaguchi 745-8648 Japan

art ic l e i nf oa b s t r a c t

Article history:
Received 11 July 2014
Received in revised form
1 September 2014
Accepted 4 September 2014
Available online 16 September 2014

CdF2 single crystal was prepared by Tokuyama Corp. with the μ-PD method to investigate Auger freeluminescence of this material. From optical transmittance spectrum, bandgap wavelength was around280 nm. In X-ray induced radioluminescence spectrum, emission lines appeared around 350 nm and420 nm. Excitation wavelength was investigated and excitation peak was around 250 nm. Photoluminescence and scintillation decay times were evaluated and decay time was few ns. Temperaturedependence of X-ray induced radioluminescence was compared with conventional BaF2 scintillator andscintillation of CdF2 decreased when the temperature increased. Consequently, scintillation of CdF2 ispossibly emission at color centers or exciton related one.
& 2014 Elsevier B.V. All rights reserved.

Keywords:
Scintillator
CdF2
Auger-free
X-ray
Gamma-rays

1. Introduction
Scintillators are widely used in various ionizing radiationdetection applications such as medical, security, and high energy
physics. Most common type of inorganic scintillators are rareearth activated scintillators [1]. This kinds of scintillators havebeen widely used in medical [2], security [3], and high energy
physics [4]. They exhibit high light yield with medium fast decay
times of several tens nanoseconds. On the other hand, intrinsic
luminescence of halide scintillators has also attracted considerableattention. The most famous one is Auger-free luminescence (AFL)of BaF2. AFL is caused by transitions from the valence band to thetop of outermost core levels. In general description, AFL shows afast decay time, a high thermal stability, and a relative high yield.
AFL of BaF2 shows sub-ns decay time and BaF2 was once investigated to apply TOF-PET [5]. Unfortunately, due to a low light yieldand short emission wavelength in vacuum ultraviolet, BaF2 did notreach to practical applications of medical imaging. CsF is also afamous scintillator exhibiting AFL [6]. Compared with BaF2, CsF
had an advantage of visible wavelength emission while hugehygroscopicity prevented to be practically used.
In addition to BaF2 and CsF, CdF2 is predicted to show AFL inultraviolet or visible wavelength theoretically [7]. However, AFL ofCdF2 has not been experimentally conﬁrmed yet. A half-century
ago, one paper suggested the existence of fast luminescence [8]. InnCorresponding author. Tel./fax: þ 81 93 695 6049.
E-mail address: yanagida@lsse.kyutech.ac.jp (T. Yanagida).

http://dx.doi.org/10.1016/j.jlumin.2014.09.013
0022-2313/& 2014 Elsevier B.V. All rights reserved.

1990's, scintillation properties of powder CdF2 was investigatedand 420 nm emission with few and few tens ns decay times werereported [9]. Recently, rare-earth element doped CdF2 was investigated and they also examined X-ray induced radioluminescenceof nondoped CdF2 at 80 K [10]. In [10], presented spectrum
differed with us (described later) possibly due to a difference ofsample production techniques.
In the present work, we investigated optical and scintillation
properties of CdF2 crystal to examine the theoretical prediction
about AFL. Fig. 1 demonstrates a picture of CdF2 crystal prepared inTokuyama by using μ-PD method. The sample was cut andpolished for scintillation study. As shown in Fig. 1, the sample
was colorless. In an evaluation of temperature dependence, BaF2
crystal with a size of 2  7  1 mm3 was used for a comparison.
This reference BaF2 was also fabricated by Tokuyama Corp. Inaddition, excitation spectrum in the vacuum ultraviolet (VUV)
region of CdF2 was observed in order to conﬁrm the occurrence ofthe AFL.

2. Experimental procedures
In-line transmittance was measured by using JASCO V670
spectrometer with a wavelength step of 1 nm from 190 to2700 nm. Photoluminescence (PL) excitation spectrum monitoringat 350 nm was evaluated by using JASCO FP8600 spectroﬂuorometer at wavelength longer than 200 nm. At wavelength shorter
than 200 nm, we evaluated at Synchrtron facility (UVSOR, beamline BL7B) from 50 to 200 nm. In UVSOR, 282 nm and 340 nm data

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T. Yanagida et al. / Journal of Luminescence 157 (2015) 293–296

80100Transmittance (%)

Transmittance (%)

100604020806040200200 220 240 260 280 300 320 340

Wavelength (nm)

0500

1000

1500

2000

2500

Wavelength (nm)Fig. 2. Transmittance of CdF2 crystal. Inset shows the transmittance below 350 nm.

Fig. 1. Photographs of CdF2 crystal.

was also measured for the crosscheck with FP8600 excitation
spectrum. In UVSOR, PL decay time proﬁles under 200 and 282 nm
monitoring at 420 nm were evaluated since we could not detect
350 nm emission due to a low emissivity.
X-ray induced radioluminescence spectrum at room temperature was corrected and the geometry of the setup was described
previously [11]. The supplied bias voltage and tube current for
X-ray generator was 80 kV and 2.5 mA, respectively. Scintillation
decay time proﬁles were evaluated by two instruments. One waspulse X-ray equipped streak camera system which enabled us toobserve wavelength resolved scintillation decay [12]. By using this
instrument, monitoring wavelengths were 350715 and420 715 nm. The other was the pulse X-ray equipped afterglow
characterization system in the fast mode [13]. In this instrument,wavelength was not resolved so that we could observe scintillation
decay similar to practical uses. Temperature dependence of X-ray
induced radioluminescence of CdF2 was compared with conventional BaF2 from  60 to 140 1C since it was a common knowledge
that AFL was relatively stable in the temperature change [14,15].
The sample was put in a constant temperature bath (Espec SH661)
with an optical ﬁber and 80 kV X-ray was irradiated to the sample
through an irradiation port. The X-ray generator and CCD based
spectrometer connected with the optical ﬁber were outside thetemperature bath. Therefore, temperature dependence of scintillator sample itself was observed.

3. Experimental results and discussion
In order to compare the excitation below and beyond (scintillation) the bandgap, optical properties were evaluated at ﬁrst. Fig. 2
represents in-line transmittance spectrum of CdF2. CdF2 showed ahigh transmittance around 90% at wavelength longer than 300 nm
and absorption bands at wavelength shorter than 280 nm wasobserved. This absorption band was longer than the bandgap ofCdF2 (  8 eV ¼ 155 nm) estimated based on reﬂection spectrum[17]. The intense absorption within the bandgap energy strongly
suggests that color centers inﬂuence the optical properties in theIntensity (a. u.)

43.5

3

2.5

250

100

150

200

250

300

350

400

Wavelength (nm)Fig. 3. Excitation spectrum of CdF2 monitoring at 350 nm. Below 200 nm, data wasobtained at UVSOR and beyond 200 nm by FP8600 spectroﬂuorometer.

crystal. PL excitation spectra monitoring at 350 nm is shown inFig. 3. Below 160 nm, no emission was observed and this wavelength corresponded to the bandgap of CdF2. As a result, spectralfeatures roughly overlapped in UVSOR and FP8600 results and theexcitation band appeared around 250 nm. Though we observed
two emission peaks around 350 and 420 nm (described later in
X-ray induced radioluminescence), the excitation band of bothemissions was around 250 nm. Considering the bandgap energy,
this excitation band is located within the bandgap. It should be
noted that the luminescence band was not effectively induced bycore level excitation, which corresponds to the wavelength ofapproximately 100 nm according to the energy split between theconduction band minimum and the outermost core level (Cd2 þ 4dorbital) [18,19]. PL decay time proﬁle of CdF2 under 200 nm
excitation monitoring at 420 nm is demonstrated in Fig. 4. Decay
time was deduced to 2.8 ns. Decay time proﬁle under 282 nm
excitation was quite similar with Fig. 4 and no speciﬁc difference
was observed. When we tried to detect 350 nm decay time proﬁle,low signal intensity prevented us from the measurement.
X-ray induced radioluminescence spectrum of CdF2 is presentedin Fig. 5. Intense emission appeared at 420 nm with a weakemission around 350 nm. Theoretical calculation expected AFL from
300 to 600 nm [7] with 2.3/7.9 ns decay time so that the experimental emission peaks well coincided with the prediction at least inscintillation spectrum. Previously, 420 nm scintillation with 5, 24,
and 78 ns decay time components was reported [9]. Very recently,
only 370 nm scintillation from CdF2 crystal with a decay time of18 ns was reported and they concluded the origin of this emission

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1x104

0.5
0.4

DataFitting

0.3
0.20.1

ex 200 nm

0102030405060708090100

Time (ns)Fig. 4. PL decay time proﬁle of 420 nm emission under 200 nm excitation
at UVSOR.

Intensity (a. u.)

Intensity (a. u.)

T. Yanagida et al. / Journal of Luminescence 157 (2015) 293–296

1000

100

Intensity (a. u.)

3080

050

100150200

Time (ns)3060

Fig. 6. X-ray induced scintillation decay time proﬁle. Wavelength was not resolved.

30403020
3000
200300 400 500 600Wavelength (nm)700Fig. 5. X-ray excited radioluminescence spectrum.

was the exciton [10]. Therefore, our sample showed both reported
emission peaks in one sample. Though we challenged to evaluate
pulse height under 137Cs γ-ray irradiation and we could not detect
clear photoabsorption peak. The upper limit of scintillation lightyield of CdF2 was around 100 ph/MeV and the light yield of CdF2
was smaller than AFL of BaF2 at least one digit.
In Fig. 6, wavelength not resolved scintillation decay time
proﬁle under pulse X-ray excitation is shown. Since we could notobserve the instrumental response of the pulse X-ray equipped
afterglow characterization system, tail part was ﬁtted by double
exponential function. 1.75 and 25.5 ns decay times were deducedby the analysis. In wavelength resolved observations, signiﬁcantdifference with the wavelength not resolved one did not appear.
Two reasons are possible to explain the present result. One
possibility is that the origin of 350 and 420 nm emissions aresame. The other is that wavelength resolution was not enoughsince these two emission bands obviously overlapped in Fig. 5.
Compared with previous results, 25 ns component in powder
sample [9] and similar value of 18 ns [10] were reported recently.
Though the signal intensity ratio generally depends on thewavelength sensitivity of PMT, we calculated relative intensity of
the fast (1.75 ns) and the slow (25.5 ns) components. As a result,the slow emission was dominant  70%.
Temperature dependences of scintillation at  60, 20, and140 1C of CdF2 and BaF2 are depicted in Fig. 7. Since we used todetect scintillation photons via 3 m optical, AFL of BaF2 at 190 nm
was not observed due to an absorption of vacuum ultraviolet
photons by oxygen in air. In BaF2 (Fig. 7(a)), AFL at 220 nm andself-trapped exciton (STE) emission around 300 nm were detected.
Although the tendency of STE was consistent with previous
studies [12], AFL exhibited a temperature dependence at higher
temperature. The root of the common knowledge about thetemperature stability of AFL was based on the report in 1985[16] and the temperature dependence was examined from 198 K(¼  75 1C) to 358 K (¼ 85 1C). We measured spectra at 20 1C stepand till 80 1C AFL at 220 nm was relatively stable. However attemperature higher than 100 1C, AFL rapidly decreased. Therefore,temperature stability is not an evidence of AFL. On the other handin CdF2 (Fig. 7(b)), whole emission monotonically decreased when
the temperature increased and the decreasing rate was an intermediate level between AFL and STE of BaF2. Taking into accountthat the slow component possibly due to STE was dominant inscintillation decay, similar temperature dependence on STE of BaF2
would be valid.
Throughout the present study, PL spectrum and decay curve,
scintillation spectrum and its temperature dependence on acomparison with BaF2, and scintillation decay time proﬁle wereevaluated. In scintillation spectrum, 350 nm emission observed in[10] and 420 nm emission detected in [9] appeared in one sample.
Taking into account excitation wavelengths and scintillation decay
times were quite similar, origin of these two emissions would bethe same. From the theoretical prediction, AFL of CdF2 should
appear around 300–600 nm with 2.3/7.9 ns decay time. In thesepoints, experimental results, 350–420 nm and 1.75/25.5 ns, wereconsistent with the prediction. However, the excitation spectrum
in the VUV region strongly suggests that the AFL hardly occurs inthis crystal. The basis of AFL of previous studies was the light yieldstability against the temperature and this basis was not true evenin conventional BaF2 at higher temperature. Concerning theseresults, a valid scenario of CdF2 emission would be a marginalemission at color centers or exciton related one and these emission wavelengths would be quite overlapped.

4. Conclusion and future prospects
CdF2 single crystal was prepared by Tokuyama Corp. with theμ-PD technique. In optical transmittance, absorption bands wereobserved below 280 nm. Emission peaks appeared around 350 and420 nm in photo- and radio-luminescence spectra. Excitation
bands of these peaks were around 250 nm. Photoluminescenceand scintillation decay time of these emissions were fast few ns.
When the temperature dependence of scintillation light yield ofCdF2 was compared with conventional BaF2, emission intensity ofCdF2 decreased at higher temperature and the same tendency wasobserved in AFL and STE of BaF2. Taking into account optical andscintillation properties in present work, a valid scenario of CdF2
emission would be a marginal emission at color centers or exciton
related one and these emission wavelengths would be quite
overlapped.

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T. Yanagida et al. / Journal of Luminescence 157 (2015) 293–296

51000

1x10STE
4Intensity (a. u.)

Intensity (a. u.)

1x101000

100-60
RT+140-60
RT+140AFL

100

1010CdF2BaF211200 250 300 350 400 450 500 550

Wavelength (nm)200300400500600

700

Wavelength (nm)Fig. 7. Temperature dependence of BaF2 (a) and CdF2 (b) under X-ray excitation.

Acknowledgments
This work was mainly supported by a Grant in Aid for ScientiﬁcResearch (A) 26249147 from the Ministry of Education, Culture,Sports, Science and Technology of the Japanese government(MEXT) and partially by JST A-step. Partial assistance from Nippon
Sheet Glass Foundation for Materials Science and Engineering,Tokuyama Science foundation, Iketani Science and Technology
Foundation, Hitachi Metals Materials Science Foundation, Mazda
Foundation, JFE 21st century Foundation, and The Asahi GlassFoundation, the Cooperative Research Project of Research Instituteof Electronics, Shizuoka University and Collaborative Research
Program of Institute for Chemical Research, Kyoto University(2014-31) are also gratefully acknowledged.
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