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Japanese Journal of Applied Physics 52 (2013) 082401
http://dx.doi.org/10.7567/JJAP.52.082401

Comparison of Luminescence Properties of CsI Crystals Activatedwith Ag , Au , and Tlþ Ions at Room Temperature
Taketoshi Kawai1 , Satoru Nagata1 y, and Takeshi Hirai2
1

Department of Physical Science, Graduate School of Science, Osaka Prefecture University, Sakai 599-8531, Japan
Department of Physical Sciences, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan
E-mail: buri@p.s.osakafu-u.ac.jp
2

Received February 28, 2013; revised May 6, 2013; accepted May 20, 2013; published online July 12, 2013
We have investigated the luminescence properties of CsI crystals activated with Ag , Au , and Tlþ ions at room temperature. When the activatedCsI crystals are irradiated by light with a wavelength shorter than the absorption edge of CsI bulk crystals, luminescence bands related to theactivated centers are observed in the visible wavelength region, in addition to intrinsic luminescence bands due to self-trapped excitons. InCsI:Au and CsI:Tlþ , the visible luminescence bands exhibit decay profiles with a decay time of about 1 s. On the other hand, the visible
luminescence band in CsI:Ag consists of two luminescence components with decay times of 25 and 250 s. Their luminescence properties are
discussed from the viewpoint of their application as a scintillator material. # 2013 The Japan Society of Applied Physics

1. Introduction

CsI crystals have been widely used as scintillator materials
for -rays because of their high stopping power for -rays
due to the relatively high density of 4.51 g/cm3 and the large
atomic number of CsI.1–4) To improve luminescenceeﬃciency, CsI crystals are usually activated with impurity
ions such as Naþ and Tlþ . Therefore, CsI:Tlþ and CsI:Na
crystals have also been used as a scintillator material for-rays. From the sensitive wavelength of optoelectronic
devices such as amorphous silicon detectors, CsI:Tlþ
crystals have been applied as a scintillator material for
nondestructive testing and medicine. Although it is easy tohandle CsI:Tlþ in comparison with CsI:Na owing to the low
deliquescence of CsI:Tlþ , the scintillator material contains a
Tlþ ion, which is a heavy metal ion with toxicity. Recently,
the use of products containing toxic heavy metals such as
thallium (Tl) and lead (Pb) has been avoided owing to asurge of environmental awareness. Therefore, CsI crystals
activated with ions with no toxicity are demanded as a newscintillator material.
Ag and Au anions are potential candidates for activatedmetal ions with no toxicity, because these anions havethe outermost s2 electronic conﬁguration and belong to
the family of Tlþ -type ions.5,6) CsI crystals doped withthese anions exhibit several absorption bands formed byintraionic transitions from the s2 -to-sp electronic conﬁguration and intense luminescence bands in the energy region
below the band edge of the CsI host crystal. The luminescence properties of CsI:Ag and CsI:Au crystals have been
investigated mainly at low temperatures and under excitationin the energy region below the band edge of CsI.7–11) In thisstudy, we have investigated the luminescence properties ofCsI:Ag and CsI:Au under excitation in the energy region
above the band edge of CsI at room temperature (RT). The
luminescence properties of CsI:Ag and CsI:Au areevaluated and compared with those of CsI:Tlþ .
2. Experimental Procedure

Since Ag and Au ions are not naturally present in alkali
halide crystals, a special treatment is needed to produceyPresent aﬃliation: Konica Minolta Business Technologies, Inc.

Ag and Au ions in such crystals. First, the Agþ or Auþions were added into pure single CsI crystals purchasedfrom Union Materials Inc. (Japan) by a diﬀusion reactionof noble-metal halide powders with the crystals. Second,
the noble metal cations were converted to anions by heating
the crystals in potassium vapor or an electrolytic coloration technique. Third, in order to remove F centers andisolate impurity ions, the samples were wrapped in analuminum foil and heated to about 600  C for several
minutes and subsequently quenched to RT. CsI:Tlþ crystals
were grown by the Bridgman method from CsI powder witha TlI concentration of 4  102 mol %, which corresponds
to a Tlþ ion concentration of 4  1018 number/cm3 in thecrystals.
Optical measurements up to the vacuum ultraviolet
wavelength region were performed at the BL-1B line ofUVSOR (Institute for Molecular Science, Okazaki, Japan).
The synchrotron radiation from a 750 MeV electron storage
ring was monochromatized through a 1 m Seya-Namiokatype monochromator for sample excitation. Photoluminescence spectra were measured using a grating monochromator(Action SpectraPro-300i) equipped with a CCD camera(Roper Scientiﬁc LN/CCD-100EB-GI). Excitation spectra
were measured using an optical detection system composed of the grating monochromator and a photomultiplier(Hamamatsu R636-01). The intensity of excitation light wascorrected using the excitation spectrum of sodium salicylate.
To measure temporal proﬁles of the luminescenceintensity, an ArF excimer laser (Lambda Physik COMPex102) was used as the excitation light source in ourlaboratory. The time-resolved luminescence spectra weremeasured using a detection system (ORIEL Instruments
INSTASPECÔ V) consisting of a monochromator(MS257Ô) and an intensiﬁed CCD detector (Model77193). The ArF excimer laser has a wavelength of
193 nm and a pulse duration of about 15 ns. Temporal
proﬁles of the luminescence intensity at a speciﬁc wavelength were obtained from the time-resolved luminescence
spectra at various delay times after the laser excitation.
All the measurements were performed at RT.
3. Results and Discussion

Figure 1 shows the absorption spectra of CsI crystals
082401-1# 2013 The Japan Society of Applied Physics

5.0

4.5

T. Kawai et al.

Photon Energy (eV)
4.0
3.5

3.5

Luminescence
CsIRTAbsorption3.0Photon Energy (eV)2.5
2.030(a) CsI:AgRT

5.0 4.0

10-CsI:Ag
RTfor 500 nm

RT× 10Absorption Coefficient

CsI:Ag-100

200300

CsI:Au--CsI:TlCsI
250

300
350
Wavelength (nm)400

ex. 193 nm
ex. 330 nm

-1

20 cm

Luminescence Intensity (arb. unit)

Lumi. Int.

Jpn. J. Appl. Phys. 52 (2013) 082401

400Fig. 1. Absorption spectra of CsI:Ag , CsI:Au , and CsI:Tlþ crystals at

RT. The top and bottom spectra are the luminescence and absorption spectraof nondoped CsI crystals, respectively.

activated with Ag , Au , and Tlþ ions, the luminescence
properties of which are compared later in this section. The
luminescence and absorption spectra of nondoped CsI
crystals are also shown in the top and bottom of Fig. 1,respectively. The nondoped CsI crystals exhibit no absorption band at a wavelength longer than 235 nm, whichcorresponds to the fundamental absorption edge of CsI. In
the activated CsI crystals, the absorption bands due to theactivated centers are observed at a wavelength longer than
235 nm. The pronounced absorption bands peaking at
340 nm for CsI:Ag and at 265 nm for CsI:Au correspondto the C absorption band formed by the intraionic transitions
from the 1 S1 ground state to the 1 P1 excited state in activatedions.7–11) From the intensities of the C absorption bands, theconcentrations of Au and Ag ions in the crystals were
estimated to be on the order of 1  1017 /cm3 using theSmakula–Dexter formula.12,13)
The A absorption band formed by the intraionic transition
from the 1 S1 ground state to the 3 P1 excited state is observedwith a peak at 380 nm for CsI:Au and at 300 nm forCsI:Tlþ . In CsI:Tlþ , the absorption bands observed at awavelength shorter than the A absorption band are also
related to the Tlþ ions. Although various models of theabsorption bands have been proposed, there has been nodeﬁnitive model. We assigned these absorption bands to thecharge transfer or perturbed exciton bands from the spectralchanges of the absorption bands in CsBr1x Ix mixedcrystals.14) It should be noted that the absorption bands
due to the activated centers are covered with a broad
luminescence band due to self-trapped excitons (STEs) in
nondoped CsI crystals, which is shown in the top of Fig. 1.
This ﬁnding implies that activated centers can reabsorb theSTE luminescence of CsI bulk crystals.
Figure 2 shows the luminescence spectra of the CsI
crystals activated with Ag , Au , and Tlþ ions in the visible
wavelength region at RT. The broken and solid lines
correspond to the spectra under excitation at 193 nm and at30(b) CsI:AuRT

105.0 4.0-CsI:Au
RTfor 468 nm

×5100200300

400

ex. 193 nm
ex. 260 nm(c) CsI:TlRT30+

5.0 4.0for 540 nm

ex. 193 nm
ex. 300 nm

400

10100200500600
Wavelength (nm)CsI:Tl+RT300

400700Fig. 2. (Color online) Luminescence spectra of the (a) CsI:Ag ,(b) CsI:Au , and (c) CsI:Tlþ crystals at RT. The solid and broken lines
correspond to luminescence spectra under excitation at 193 nm by an ArF
excimer laser and at the absorption bands due to the activated centers,respectively. The insets show excitation spectra for luminescence bands due
to activated centers in the CsI:Ag , CsI:Au , and CsI:Tlþ crystals in thevacuum ultraviolet wavelength region at RT.

the absorption band due to activated centers, respectively.
Broad luminescence bands peaking at 540, 500, and 450 nm
are observed in CsI:Tlþ , CsI:Ag , and CsI:Au , respectively. The 500 nm luminescence band in CsI:Ag isattributed to the radiative transition from the relaxed excitedstate of 3 P1 and called the A0 luminescence band.8,9) The 560
nm luminescence band in CsI:Tlþ is attributed to the STE
perturbed by a Tlþ ion15–17) and/or the radiative recombination of an electron in a Tl0 center with a neighboring VKcenter.18,19) The origin of the 450 nm luminescence bandin CsI:Au is unknown. However, we consider the origin to
be the radiative transition from the relaxed excited statesof the Au ion located at the oﬀ-center position in thelattice site.10)082401-2# 2013 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 52 (2013) 082401(a)

10 3RTex. 193 nm+CsI:Tlfor 540 nm

10 2

CsI:Aufor 468 nm

10 1

10 00

Luminescence Intensity (arb. unit)

The luminescence spectra under excitation at 193 nm,
which is shorter than the band gap of CsI, are not identical tothose under excitation at the absorption bands due to theactivated centers. Excitation in the wavelength region of theband-to-band transition of CsI could bring about luminescence bands due to other impurity ions and/or defects inaddition to activated centers. Nevertheless, visible luminescence bands associated with activated centers dominate
even under excitation at 193 nm. This ﬁnding implies thatactivated centers play an important role as luminescencecenters.
The excitation spectra for the visible luminescence bands
in the CsI:Tlþ , CsI:Ag , and CsI:Au crystals are shownin the inset of Fig. 2. The excitation bands observed ata wavelength longer than 235 nm originate from theabsorption bands due to the activated centers. The excitation
spectra at a wavelength below 235 nm exhibit no distinct
diﬀerence among CsI:Tlþ , CsI:Ag , and CsI:Au . Since
the CsI crystals have a larger absorption coeﬃcient than
105 cm1 in the wavelength region of the band-to-band
transition, the penetration depth of the incident light islimited to the surface region, which is comparable to thatin the case of the bulk crystal. Therefore, the number ofactivated centers excited directly by the incident lightis extremely small compared with that under excitation
at the absorption bands due to the activated centers.
Nevertheless, visible luminescence bands are observed
even under excitation in the wavelength region of theband-to-band transition. This ﬁnding implies an eﬃcientenergy transfer process from CsI host crystals to activatedcenters.
Figure 3 shows the temporal proﬁles of the luminescencebands due to the activated centers in the CsI crystals at RT.
In CsI:Tlþ , the temporal behavior of the visible luminescence band, which is shown by open triangles in Fig. 3(a),exhibits a slight increase in the early state and a single
exponential decay. The decay time of 940 ns is obtained
from the slope of the single exponential decay. This value
coincides with those reported previously.3,4,16) The increasein temporal behavior is caused by the recombination ofelectrons, which are thermally released from isolated Tl0centers, with Tlþ –Vk complex centers.18) The temporal
behavior of the visible luminescence band in CsI:Au ,which is shown by ﬁlled circles in Fig. 3(a), is slightly fasterthan that in CsI:Tlþ . The decay proﬁle consists of two decay
components having decay times of about 280 and 960 ns,
respectively.
As shown by ﬁlled squares in Fig. 3(b), excitation at
193 nm on the CsI:Ag crystal induces the temporal proﬁleswith decay times of 25 and 250 s. In general, the decay
time constant of the A0 luminescence band of Ag centersin alkali halides is a few tens of s at RT, reﬂecting
the radiative transition probability of the 3 P1 excitedstate.20,21) Therefore, the 25 s decay component in CsI:Ag
is attributed to the radiative transition from the 3 P1 state.
The 250 s decay component might be associated withnearby defects created by excitation above the band gapof CsI.
Lastly, we would like to evaluate the CsI:Ag andCsI:Au crystals from the standpoint of their application as anew scintillator material. The visible luminescence fromLuminescence Intensity (arb. unit)

T. Kawai et al.

10 3

123Time (µs)

(b)

45RT
ex. 193 nm

10 2-CsI:Agfor 500 nm
10 1

10 00

0.1

0.2
Time (ms)

0.3

0.4

Fig. 3. Temporal proﬁles of the luminescence bands in the CsI:Tlþ ,CsI:Au , and CsI:Ag crystals at RT. Results of the CsI:Tlþ and CsI:Au
crystals are shown by open triangles and ﬁlled circles in (a), respectively.
Data of the CsI:Ag crystal are shown by ﬁlled squares in (b). The ﬁlled
squares correspond to the excitation at 193 nm by the ArF excimer laser.

scintillator materials under the X-ray excitation is converted to an electric signal using popular amorphous silicondetectors. Amorphous silicon detectors have spectral sensitivity to light of visible wavelength and are particularly
sensitive to light with a wavelength range from 550 to600 nm. The wavelength region is a good ﬁt for the visible
luminescence in the CsI:Tlþ crystals. Although the visible
luminescence bands in the CsI:Ag and CsI:Au crystals
are located at a wavelength shorter than that in CsI:Tlþ ,the spectral sensitivity of amorphous silicon detectors
covers the visible luminescence bands in the CsI:Ag andCsI:Au crystals. Therefore, the CsI:Ag and CsI:Au
crystals are not severely ﬂawed as scintillator materials.
Under excitation at 193 nm, the visible luminescence inCsI:Ag has a decay time of several hundreds of s. Thisﬁnding implies that the CsI:Ag crystal is not an adequate
scintillator material for the radiation detector to achieve afast response. Since the decay of the visible luminescenceband in CsI:Au is slightly faster than that in CsI:Tlþ , theCsI:Au crystals might be preferred as a new scintillator
material.

082401-3# 2013 The Japan Society of Applied Physics

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T. Kawai et al.
1) C. W. E. van Eijk: Nucl. Instrum. Methods Phys. Res., Sect. A 460

4. Conclusions(2001) 1.

We have investigated the luminescence properties of theCsI crystals activated with Ag , Au , and Tlþ ions atRT. Under excitation above the band gap of CsI crystals,the CsI:Ag , CsI:Au , and CsI:Tlþ crystals exhibit visible
luminescence bands peaking at 500, 450, and 540 nm,respectively. The temporal proﬁles of the visible luminescence bands in CsI:Au and CsI:Tlþ have a decay time
constant of about 1 s. The visible luminescence band inCsI:Au decays slightly faster than that in CsI:Tlþ . On
the other hand, the visible luminescence band in CsI:Ag
exhibits a 25 s decay component arising from the radiativetransition probability from the 3 P1 excited state and a 250 s
decay component associated with nearby defects. The
existence of the slow decay components implies that theCsI:Ag crystals are inferior to the CsI:Tlþ and CsI:Au
crystals in terms of quality as a scintillator material.

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Acknowledgments

The optical measurements in the ultraviolet wavelength
region were supported by the Use-of-UVSOR Facility
Program of the Institute for Molecular Science, Japan.
Part of this research was supported by the Adaptable andSeamless Technology Transfer Program through TargetDriven R&D, Japan Science and Technology Agency.

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# 2013 The Japan Society of Applied Physics