Nuclear Instruments and Methods in Physics Research A 793 (2015) 31–34
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in
Physics Research A
journal homepage: www.elsevier.com/locate/nima
Scintillation characterization of thallium-doped lithium iodide crystals
Sajid Khan a, H.J. Kim a,n, Y.D. Kim b
a
b
Department of Physics, Kyungpook National University, Daegu 702-701, South Korea
Center for Underground Physics, Institute for Basic Science (IBS), Daejon 305-811, South Korea
art ic l e i nf o
a b s t r a c t
Article history:
Received 6 November 2014
Received in revised form
28 April 2015
Accepted 28 April 2015
Available online 8 May 2015
The paper discusses scintillation and luminescence properties of thallium-doped LiI crystals, grown by the
Bridgman technique. X-ray induced emission spectrum is obtained between 380 nm and 600 nm, and is
attributed to the Tl þ ion. The photoluminescence measurement with the excitation wavelength of 305 nm
revealed a similar emission spectrum. Light yield, energy resolution and scintillation decay time profiles
were studied under 662 keV (137Cs) γ-ray excitation. A maximum light yield of 14,00071400 ph/MeV and
two exponential decay time components were obtained.
& 2015 Elsevier B.V. All rights reserved.
Keywords:
Decay time
Energy resolution
Lithium iodide
Luminescence
Scintillation properties
X-ray excitation
1. Introduction
Alkali halide scintillators are among the popular materials for
scintillation applications and are widely used in the areas such as
high energy physics, nuclear physics, medicine, geology, astrophysics and security [1,2]. Among the alkali halides, NaI(Tl) was
the first inorganic single crystal scintillator developed by Hofstadter [3]. LiI crystals have attracted attention owing to the presence
of 6Li nuclei that have a large capture cross-section for thermal
neutrons. LiI crystals have been grown with different activators
and tested for scintillation properties. A LiI(Sn) scintillator was
used for studying the detection of thermal neutrons [4]. The
growth of LiI crystals with Tl and Ag doping were reported in
[5,6]. The previous work has been focused on the response of the
grown crystals to the thermal neutrons. Activation of LiI phosphor
with Eu, Sm and Tl was reported in [7]. The investigations
performed in [7] included measuring the β-luminescence and
thermoluminescence at room and low temperatures. Among all of
the activated LiI crystals, LiI(Eu) is the most intensively studied
scintillator for neutron detection and γ-ray spectroscopy. The first
Eu-doped LiI crystal and its luminescence properties were proposed by Schenck during early 1950s [8]. Later, Murray [9]
reported LiI crystals activated with Eu for detecting neutrons with
energies ranging from 1 to 14 MeV, at various temperatures.
Recently, Syntfeld et al. [10] reported on the use of LiI(Eu) in
n
Corresponding author. Tel.: þ 82 539505323; fax: þ 82 539561739.
E-mail address: [email protected] (H.J. Kim).
http://dx.doi.org/10.1016/j.nima.2015.04.061
0168-9002/& 2015 Elsevier B.V. All rights reserved.
neutron and γ-ray spectroscopy. Although LiI(Eu) has reasonable
light yield ( 15,000 ph/MeV) [10], its slow decay time (1.2 ms) [9]
is a limiting factor for high count rate applications.
Despite of the interest in activation of LiI with various dopants,
to the best of our knowledge, a detailed study on the crystal
growth of LiI with optimal Tl concentration and investigation of its
scintillation properties has not been performed. Therefore, for
better understanding of the scintillation properties of LiI, more
detailed studies on the dependence of these properties on Tl
concentration are necessary.
In this paper, we report the crystal growth as well as the
scintillation and luminescence properties of LiI(Tl) crystals. Optimal Tl concentration was determined by growing various crystals.
Room temperature studies of scintillations properties include:
X-ray excited emission spectrum, UV-luminescence, energy resolution, light yield and decay time analysis.
2. Experimental details
2.1. Crystal growth
Single crystals of LiI with various Tl concentrations (0.02%,
0.05%, 0.1%, 0.5% and 1% by mole) were grown by using two zone
vertical Bridgman technique [11]. LiI and TlI powders from Alfa
Aesar with 4 N and 5 N purity respectively were loaded into quartz
ampoules located inside a glove box in Ar atmosphere. The
ampoules were sealed under dynamic vacuum of 10 7 Torr.
The crystals were grown with a speed of 0.5 mm/h. The obtained
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S. Khan et al. / Nuclear Instruments and Methods in Physics Research A 793 (2015) 31–34
crystals were crack-free and transparent. The concentrations of Tl
in the LiI hosts were not determined after the growth of the
crystals. Fig. 1 shows a photograph of the grown crystals in their
corresponding quartz ampoules. The samples used in this work
were cylinders (10 mm in diameter and 5 mm in height) and were
cut from the middle parts of the crystal ingots and polished in Ar
environment. Owing to the hygroscopic nature of lithium iodide,
the grown crystals were stored in mineral oil.
2.2. Equipment
The photoluminescence excitation and emission spectra of the
LiI(Tl) crystals were measured at room temperature by utilizing a
Horiba Flurolog-3 (light source-450 W Xenon lamp) [12] in the
spectral range of 250 to 800 nm. The X-ray luminescence was
measured by using an X-ray tube from DRGEM. Co. having a W
anode at room temperature. The power setting parameters of the
tube were 80 kV and 1 mA. The emission spectra were acquired by
using a fiber optic spectrometer (QE65000, Ocean Optics).
For measuring the pulse height spectra, the samples were
wrapped with Teflon tape and directly coupled to the entrance
window of a photomultiplier tube (PMT, Hamamatsu R6233) using
index matching silicon oil. The measurements were performed at
room temperature in the glove box with Ar atmosphere, and
samples were excited by using a 137Cs γ-source. The analog signals
from the PMT were shaped by using a Tennelec TC-245 spectroscopy amplifier. The output signals were then fed into a 25-MHz
flash analog-to-digital converter (FADC) [13]. The FADC output was
recorded to a personal computer by using a USB2 connection and
the recorded data were analyzed by using a C þ þ data analysis
program [14].
The decay time measurements were performed under 662 keV
γ-rays excitation (137Cs-source). The signals from the PMT were
fed into a 400-MHz FADC and the decay time spectra were
acquired from the recorded pulse shape information. A detailed
Fig. 1. (a) The LiI crystals with 0.02%, 0.05%, 0.1%, 0.5% and 1% Tl concentration
(from left to right) are shown in their quartz ampoules. (b) The LiI(0.5% Tl) crystal,
after cutting and polishing (10 mm diameter and 5 mm height). Since LiI is
hygroscopic, the shown sample is immersed in mineral oil. (For interpretation of
the references to color in this figure legend, the reader is referred to the web
version of this article.)
description of the experimental set-up for measuring the decay
time can be found in Ref. [15].
3. Results and discussion
3.1. X-ray and UV luminescence spectra
X-ray induced emission spectra of the LiI crystals doped with
0.02%, 0.05%, 0.1%, 0.5% and 1% Tl are shown in Fig. 2. Broad
emission bands were observed ranging from 380 nm to 600 nm,
peaking at 470 nm. The observed emission bands are attributed to
the transition between the ground state and excited state of the
Tl þ ion [16,17]. A similar broad emission spectrum was reported
for LiI(0.02% Tl) excited by β particles from 90Sr [18]. A faint
shoulder at 360 nm and a low intensity peak at 610 nm were
also observed in the emission spectra of all crystals. The emission
at 360 nm corresponds to the intrinsic luminescence of LiI and it
was earlier reported for pure LiI crystals in [19]. The small peak at
610 nm was also observed previously in the emission spectra of
LiI(0.1% Ag) and LiI(0.04% Yb) [18], but its origin remains unknown.
We found an increase in emission intensity while going to
higher Tl concentrations. This behavior has also been observed in
other alkali halides scintillators [20,21]. The maximum emission
intensity was obtained for the sample with 0.1% Tl and no further
increase was observed for the sample doped with 0.5% of thallium.
At the Tl concentration of 1% the emission intensity showed its
minimum value. The concentration quenching effect [22] might
serves as explanation for this observation. Fig. 3 shows the
emission intensity as a function of Tl concentration in the host
lattice. A small shift in the emission maximum towards the longer
wavelengths with increasing Tl concentration was observed. This
could be attributed to the re-absorption effects between Tl ions. A
similar effect with increasing dopant concentration has also been
reported for other scintillating crystals [23,24].
The UV-luminescence spectrum of each LiI(Tl) sample is shown
in Fig. 4. All LiI(Tl) crystals were excited at 305 nm and similar
broad emission bands were observed between 380 nm and
600 nm. Emission spectra under X-ray and UV excitation showed
similar spectral range with typical Tl þ emission. Compared with
the X-ray induced emission spectra, no bands at 360 nm and
610 nm were found in the UV-excitation spectra of the different
LiI(Tl) samples.
Fig. 2. X-ray induced luminescence spectra of LiI(Tl) crystals with various Tl
concentrations, measured at room temperature. The intensity was normalized to
the emission intensity of the 0.1% Tl sample. (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of this article.)
S. Khan et al. / Nuclear Instruments and Methods in Physics Research A 793 (2015) 31–34
33
Table 1
Summary of the scintillation properties of LiI(Tl) single crystals.
Tl
concentration
(mole %)
Energy resolution at
ΔE/E (FWHM) %
Light yield
(ph/MeV)
0.02
19
3750 7 375
0.05
17
0.1
13
0.5
8.5
Decay time
(relative
contribution)
6 ns (9%), 167 ns
(56%), 1.4 ms (35%)
5850 7 585
168 ns (64%),
893 ns (36%)
9600 7960
177 ns (81%),
887 ns (19%)
14,0007 1400 185 ns (88%),
1.089 ms (12%)
Fig. 3. Dependence of X-ray emission intensity on the Tl concentration in the LiI
crystals, at room temperature. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
Fig. 5. The γ-ray spectrum of the LiI(0.5% Tl) sample for irradiation with 662 keV γrays from a 137Cs source, measured at room temperature.
Fig. 4. Photoluminescence excitation (left) and emission (right) spectra obtained
from the LiI(Tl) crystals at room temperature. (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of this
article.)
3.2. Energy resolution and light yield
The pulse height spectra of the LiI(Tl) crystals were measured
under 662 keV γ-rays excitation from a 137Cs source. The energy
resolution of each sample was determined by a Gaussian fit to the
respective peak. Results are listed in Table 1. The best energy
resolution of 8.5% full width at half maximum (FWHM) was achieved
for the LiI(Tl) sample with Tl concentration of 0.5%. The spectrum
obtained for this crystal is shown in Fig. 5. The light yield values of
the LiI(Tl) crystals were determined by using a Bi4Ge3O12 (BGO) as a
reference crystal having a light yield of 8500 ph/MeV [25]. BGO is an
Fig. 6. Pulse height spectra of BGO and LiI(0.5% Tl) crystals for irradiation with
662 keV γ-rays from a 137Cs source, measured at room temperature. The photopeak
positions are proportional to the light yield.
ideal reference crystal due to its very similar emission wavelength
region: the BGO crystal exhibits emission in the region of (360–680)
nm, peaking at 480 nm [26], whereas the emission spectra of LiI(Tl)
crystals are in the region of (380–600) nm, peaking at around
470 nm. The light yield was estimated by comparing the photopeak
channel numbers of BGO and LiI(Tl) crystals under 662 keV γ-rays
excitation, under similar conditions of PMT high voltage and shaping
time, as shown in Fig. 6. The determined γ-ray light yield values of
the different samples are listed in Table 1. Since the 0.02% Tl sample
revealed a long decay time component (1.4 μs), the amplifier shaping
time was chosen to be 6 ms for this sample. For the other samples,
3 ms shaping time of the amplifier was selected for the light yield
measurements, as is also mentioned in Table 1. The systematic error
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S. Khan et al. / Nuclear Instruments and Methods in Physics Research A 793 (2015) 31–34
doping concentrations were grown by using the two zone vertical
Bridgman technique. X-ray excited emission spectra exhibited a
typical Tl þ emission at room temperature. The emission disappeared for Tl concentration of 1%. Energy resolution of 8.5%
(FWHM) for irradiation with 662 keV γ-rays from a 137Cs source
was measured for the 0.5% Tl sample. Among all the LiI(Tl)
samples, the 0.5% Tl crystal had the maximum light yield of
14,000 71400 ph/MeV at the shaping time of 3 ms under γ-rays
excitation. Decay time spectra exhibited two components and the
contribution of the faster component (185 ns) became dominant
with increasing Tl concentration. The fast decay time of the LiI(Tl)
crystals offers higher count rate capabilities in comparison to LiI
(Eu) crystals.
Fig. 7. Decay time profile of the LiI(0.5% Tl) crystal measured for irradiation with γrays from a 137Cs source. Inset shows the decay time spectrum of LiI(0.02% Tl). Red
line shows the best fit to the data. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of this article.)
was 10% and was dominated by the uncertainties in the absolute
light yield of the BGO reference crystal. The highest light yield of
14,00071400 ph/MeV was obtained for the LiI(0.5% Tl) sample. The
light yield of this LiI(Tl) crystal is comparable to that of LiI(Eu)
(15,00071500 ph/MeV) [10].
3.3. Scintillation decay time
The scintillation decay times of the different LiI(Tl) samples
were studied under γ-rays excitation at room temperature, and
the results are shown in Fig. 7. The decay times of the LiI(Tl)
samples can be described by two exponential components, except
for the sample with thallium concentration of 0.02%, where the fit
found three decay components instead. The decay time curves
ð t Þ
ð t Þ
ð t Þ
were fit by a function f ðtÞ ¼ A½e τ1 þ B½e τ2 þC½e τ3 , in which
A, Band C are related to the intensity contributions and τ1 , τ2 , and
τ3 are the decay constants for the different light emission
components. The decay time constants of all samples with their
relative contribution to the total light yield are listed in Table 1.
The ultra-fast decay time of 6 ns was observed for the 0.02% Tl
sample. The decay of Tl þ luminescence is not very fast. This is
owing to the spin-forbidden nature of the (3P1-1S0) luminescence
transition of the Tl þ ions [27]. Therefore, the fast decay component (6 ns) is believed to arise from pure LiI and could only be
observed in the LiI(Tl) crystals with very low doping levels. The
long decay time component ( ms) became less significant with
increasing Tl concentration. The origin of this long decay time
component is not known. In comparison to NaI(Tl), for which the
major decay time component is 230 ns, the major decay time of LiI
(Tl) is attributed to the Tl þ ions [28]. The dominant decay
component (185 ns) of LiI(Tl) is about 7 times faster than that of
LiI(Eu) (1.2 ms) [9]; therefore, LiI(Tl) crystals are expected to be
suitable for high count rate applications.
4. Conclusion
In this work we investigated the luminescence and scintillation
properties of thallium-doped LiI crystals. Crystals with different Tl
Acknowledgments
These investigations have been supported by the National
Research Foundation of Korea (NRF) funded by the Korean government (MEST) (2012R1A2A1A03010330) and IBS-R016-D1-2014-a00.
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