Author’s Accepted Manuscript

Scintillation properties of TGG and TSAG crystalsfor imaging applications

Takayuki Yanagida, Go Okada, Takahiro Kojima,Takeshi Hayashi, Jisaburou Ushizawa, NaokiKawano, Noriaki Kawaguchi

PII: S0921-4526(17)30255-7DOI: http://dx.doi.org/10.1016/j.physb.2017.05.029Reference: PHYSB309953

To appear in: Physica B: Physics of Condensed Matter

Received date: 14 April 2017Revised date: 13 May 2017Accepted date: 15 May 2017

Cite this article as: Takayuki Yanagida, Go Okada, Takahiro Kojima, TakeshiHayashi, Jisaburou Ushizawa, Naoki Kawano and Noriaki Kawaguchi,Scintillation properties of TGG and TSAG crystals for imaging applications,Physica B: Physics of Condensed Matter,http://dx.doi.org/10.1016/j.physb.2017.05.029

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Scintillation properties of TGG and TSAG crystals for imaging applications

Takayuki Yanagida1,*

, Go Okada1, Takahiro Kojima

2, Takeshi Hayashi

2, Jisaburou

Ushizawa2, Naoki Kawano

1, Noriaki Kawaguchi

1

1Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5

Takayama, Ikoma, Nara 630-0192, Japan

2Oxide Corporation, 1747-1 Mukawa, Hokuto, Yamanashi 408-0302, Japan

*Corresponding author: 8916-5 Takayama, Ikoma, Nara 630-0192, Japan. Tel. +81-743-72-6144 /

Fax. +81-743-72-6147. E-mail t-yanagida@ms.naist.jp

Abstract:

Optical and scintillation properties of TGG (Tb3Ga5O12) and TSAG (Tb3Sc2Al3O12) crystals were

investigated, and capabilities to be used as a scintillator screen were demonstrated. In

photoluminescence (PL) spectra, some emission lines due to Tb3+

4f-4f transitions appeared from

500 to 700 nm. PL quantum yields of TGG and TSAG were 6.5 and 50.9%, respectively. When

irradiated by X-rays, these crystals showed intense scintillation, and the emission wavelengths were

the same as those in PL spectra. The scintillation decay times of TGG and TSAG were 94 and 678 ms,

respectively. Further, we have demonstrated X-ray imaging using both TSGG and TSAG crystal plates

and confirmed a capability as scintillator screens.

Keywords:

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TGG; TSAG; X-rays; Scintillation; Scintillator screen

1. Introduction

Scintillators have a function to absorb the energy of invisible ionizing radiations, and convert the

energy to luminescence immediately [1]. This luminescence is generally detected by photodetectors

such as photomultiplier tube (PMT) or some photodetectors composed of Si, and is converted to

electrons. Such kinds of devices consisting of the scintillator and the photodetector are called

scintillation detectors. The range of applications of scintillation detectors are very wide including

nuclear medicine [2], X-ray computed tomography (CT) [3], security [4], oil-logging [5], astrophysics

[6] and particle physics [7]. In the viewpoint of detection techniques, scintillation detectors are

classified to two types such as the photon-integration-type and photon-counting-type detectors.

The integration-type detectors accumulate many radiation detection events and read out signals as

an integrated value typically within several ms. On the other hand, the counting-type detectors

process each radiation detection event and read out the signal by one incident radiation. The

former types are typically used in imaging detectors for X-ray CT [3] and scintillator screens [8-11].

Detectors of the latter type are used in nuclear medicine and high energy physics. In these

scintillation detectors, properties of scintillator materials are essentially important, and continuous

effort has been given to discover new efficient scintillators.

In this study, we will report basic optical and scintillation properties of newly developed

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scintillators such as TGG (Tb3Ga5O12) and TSAG (Tb3Sc2Al3O12) crystals prepared by Oxide Corp.

Tb-based aluminum garnet such as TAG (Tb3Al5O12) has been investigated on the optical isolator

applications using the Faraday rotation [12-14]. To study optical isolator properties, some other

Tb-based garnet materials such as TGG [15] and TSAG [16] have been developed. In addition to

magneto-optical applications, Tb-based materials are applicable for scintillator applications. The

most common Tb-based materials is Tb-doped Gd2O2S (Tb:GOS) [8], and Tb:GOS has been widely

used in integrated-type imaging detectors such as scintillator screen. When Tb-doped materials are

irradiated by ionizing radiation, they show an intense green-yellow emission, and the emission

wavelength is well-suited to Si-semiconductor-based photodetectors. Although Tb is used as an

emission center in the case of Tb:GOS, Tb is relatively free from the concentration quenching so

Tb-based materials can also show efficient luminescence. Actually, one of the common Tb-based

garnet materials, TAG, is reported to have efficient luminescence properties [9, 17-19] when Ce is

activated as an emission center, and Ce-doped TAG thin film is examined in scintillator screen

application [9]. However, no investigations can be found about TGG and TSAG on scintillation

properties and screen application capabilities. In addition, most reports on luminescent properties

of Tb-based garnets have been carried out by Ce-doped TAG, and emission properties due to Tb3+

in

Tb-based garnet materials have not been investigated.

Throughout this work, optical and scintillation properties of TGG and TSAG have been

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investigated. To our knowledge, the report on the scintillation properties of TGG and TSAG is for the

first time. In addition, a demonstration as scintillation screen was performed for potential

applications for X- and g-ray detectors.

2. Experimental

TGG and TSAG crystals were synthesized by Oxide Corp. with the following procedures. TGG and

TSAG single crystals were grown by the Czochralski method with RF induction heater. As starting

materials Tb4O7 and Ga2O3 were used for TGG. Tb4O7, Sc2O3 and Al2O3 were used for TSAG. Purities

of these powders were 99.99%. They were mixed to garnet composition and loaded into an Ir

crucible. Single crystals were grown on <111> seed crystal with a pulling rate of 0.4-2 mm/h and a

speed of crystal rotation of 2-10 rpm. As-grown crystals were annealed at 1500-1550 °C in air

before cutting.

Optical properties were characterized as follows. In-line transmittance spectra were collected by

JASCO V670 spectrometer from 190 to 2500 nm. A photoluminescence (PL) emission map was

evaluated by using Quantaurus-QY (Hamamatsu), and PL internal quantum yields (QY) were

obtained at the same time. The internal QY was calculated by the following equation, QY =

Nemit/Nabsorb where Nemit and Nabsorb were numbers of emission and absorption photons, respectively.

In this evaluation, the Nabsorb was an integration of photons from 250 to 270 nm and Nemit was from

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450 to 650 nm. The PL decay time profiles were evaluated by Quantaurus-t, and the excitation and

the monitoring wavelengths were 265 and 470 nm, respectively.

Scintillation properties were characterized as follows. X-ray induced radioluminescence spectra

were observed by using our original setup [20]. The excitation source was an X-ray tube equipped

with W anode. The supplied bias voltage and tube current were 40 kV and 5.2 mA, respectively.

When X-ray is absorbed by the scintillator, scintillation photons are immediately emitted, and these

photons were detected by CCD detector (Andor DU420) via monochromator (Andor SR163) to

obtain a spectrum. The scintillation decay time profiles were also evaluated by our original

instrument [21]. The root of the excitation source was a timing and pulse width controlled LED, and

photons from LED was converted to photoelectrons at the photocathode of pulse X-ray tube. These

generated electrons were accelerated by 30 kV bias voltage, and they emit X-rays under the

interaction with W target via Bremsstrahlung. Thus the pulse timing was generated by the root LED,

and the endpoint energy was 30 keV. By using the same equipment, we also measured X-ray

induced afterglow profiles of two samples. In order to confirm the observed afterglow profiles, we

measured thermally stimulated luminescence (TSL) glow curves by using TL-2000 (Nanogray) [22]

with the heating rate of 1 °C/s.

Last, we demonstrated X-ray imaging using the TAG and TSAG samples as scintillator screens.

The setup is illustrated in Figure 1. The object sample was placed between the X-ray generator and

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scintillator. Scintillation light emitted was guided to the CCD camera (BU-54DUV, Bitran Inc.) by the

first reflection mirror. The objective lens equipped on the camera was UV-105mmF4.5, Nikon. This

configuration prevents from X-ray photons directly striking to the CCD camera.

3. Results and discussion

Figure 2 shows a picture of the samples under room right and UV irradiation by the UV-lamp.

The sample size was 10 × 10 × 2 mm3. The wide surfaces were polished and these samples were

visually transparent. When 365 nm UV photons were irradiated to the samples, yellow-green

emissions were observed by naked eyes. As clearly demonstrated in the figure, TSAG showed higher

emission intensity.

Figure 3 demonstrates in-line transmittance of TGG and TSAG. Except for some sharp

absorption lines due to 4f-4f transitions of Tb3+

, typical transmittance values of TGG and TSAG were

75 and 80% from 400 to 1200 nm, respectively. In the visible wavelength, the sharp absorption line

at 484 nm was due to the 7F6 ->

5D4 transition of Tb

3+. In the shorter wavelength, two strong

absorption bands were observed around 280 and 325 nm due to the transitions from 7F6 to 5d

levels (7E and

9E) of Tb

3+, respectively. No broad absorption bands due to the charge transfer of Tb

4+

were observed in the UV and visible ranges so Tb4+

was not generated in the crystal growth

processes.

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PL emission maps of TGG and TSAG are shown in Figure 4. Some sharp emission lines due to

Tb3+

4f-4f transition were observed especially in TSAG. In the case of TSAG, four intense emission

lines appeared around 470, 550, 580 and 625 nm, and the corresponding electron transitions were

from 5D4 to

7F6,

7F5,

7F4 and

7F3, respectively. The weak lines around 670 nm would be

5D4 ->

7F2,1

transitions. In TGG, three emission lines around 550, 580 and 625 nm were detected. The internal

QY of TGG and TSAG were 6.5 ± 2 and 50.9 ± 2%, respectively. The PL QY of TSAG was largely higher

than that of TGG, and this result was consistent with Figure 1.

PL decay time profiles of TGG and TSAG are displayed in Figure 5. In order to avoid to detect

the diffraction light, we selected to observe 470 nm emission line. Although the emission intensity

of TGG was very weak, the coincidence measurement enabled us to observe the decay curve of

TGG clearly. The decay curve of TGG was reproduced by the triple exponential approximation, and

the deduced decay times were 0.1, 1.1 and 12.2 ms. On the other hand, TSAG could be

approximated by the single exponential function, and the decay time resulted 1.1 ms. The shortest

component in the TGG is ascribed to the excitation pulse of the equipment, and the longest one

would be due to some kinds of defects since the typical decay times of emission due to Tb3+

4f-4f

transition are from several hundred ms to a few ms [23, 24].

Figure 6 represents X-ray induced radioluminescence spectra of TGG and TSAG. Although the

radioluminescence intensity is a qualitative value, the intensities were compared between these

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two samples since the sample size is the same and the effective atomic numbers are equivalent. As

a result, TSAG showed higher emission intensity than that of TGG. The emission wavelengths of

TSAG were the same as those in PL, and the wavelengths were 470, 550, 580, 625 and 670 nm. The

electron transitions of these emission lines were also the same as those in PL, 5D4 ->

7Fi (i = 1-6)

transitions. Among these emission lines, the 550 nm peak due to 5D4 ->

7F5 transition was the

strongest. The intensities of the latter emission in TGG and TSAG differed approximately by one

order of magnitude. In fact, this difference corresponds to the difference of PL QYs. Scintillation

light yields are understood to be a product of the energy migration efficiency from the host and QY

of emission centers [25-28]. In TGG and TSAG, the former term is on a similar level since the

observed ratios of radioluminescence intensities to PL QYs are the almost same. From the

viewpoint of detector applications, since the main emission wavelengths are green-yellow

wavelengths, the matching with Si-based detectors such as Si-photodiode or CCD will be adequate

for TGG and TSAG.

Scintillation decay time profiles are presented in Figure 7. The delta function like component in

this measurement was due to the instrumental response of the detector system. The main

scintillation decay components of TGG and TSAG were 94 and 678 ms, respectively. These decay

times are faster than those in PL decay, and the possible reasons are as follows. One is the

difference of detected emissions. In PL, we only observed the 470 nm emission while, in

9

scintillation, all the emitted photons from 160 to 650 nm were integrated. Since the time correlated

single photon counting was conducted in these measurements, some emissions of garnet host

which was not observed in the spectra may be also detected in these decay curves. Generally, the

garnet host emissions are faster than those from Tb3+

4f-4f transitions [29], and if the emissions

from the host and Tb3+

4f-4f transitions are merged, the observed decay curves look faster. The

other is the energy migration path. In PL, we only observe the excitation and relaxation of localized

emission centers while some additional processes are involved in scintillation. In the case that

scintillation decay is faster than PL, it is typically interpreted that some competitions between the

energy transfer and quenching due to interactions among energetic secondary electrons might

occur. Although the observations of Tb3+

emissions in ms range are limited due to the difficulty of

the measurement, some reports can be found. For example, main decay time of Tb-doped

12CaO-7Al2O3 crystal was around 2 ms [30] and Tb-doped NaPO3-Al(PO3)3 glass was around 800 ms

[31]. Compared with these previous works, decay times of TGG and TSAG were significantly fast,

and it was blamed for the concentration quenching of scintillation since Tb was a main element in

these compounds.

Figure 8 shows afterglow time profiles of the samples with 2 ms X-ray irradiation. The

afterglow level of TSAG was higher than that of TGG by more than one-order of magnitude. In order

to understand the afterglow profiles, TSL glow curves of these two samples are compared in the

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inset of Figure 8. In the TSL glow curves, TGG showed a glow peak around 400 °C while TSAG

showed a glow peak around 200 °C. From these results, we understand that the high afterglow level

of TSAG is originated from a larger concentration of shallow traps included in TSAG than TGG.

Compared with the conventional scintillators such as BGO and CWO [21], the afterglow levels of

TGG and TSAG were higher. Although these materials were not suited for the application of X-ray

inspections systems in airports by such a high afterglow levels, they can be applied for other

integrated type radiation detectors.

Figure 9 demonstrates X-ray imaging using TGG and TSAG samples. The sample and OP Amp.

are firmly held by a Kapton tape during the experiments. Under X-ray irradiation, strong

luminescence was observed and the internal circuit patterns of OP. amp chip were clearly visualized.

Therefore, we confirmed that TGG and TSAG crystal scintillators are applicable for scintillator screen

applications. The image qualities by TGG and TSAG were similar but an image contrast using TSAG

seems to be slightly better due to higher scintillation intensity. The spatial resolution strongly

depends on the S/N [32], thus the contrast of the Kapton tape of TSAG screen was higher than that

of TGG screen.

4. Conclusion

Optical, scintillation and scintillation screen detector properties of TGG and TSAG were

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investigated. In PL and radioluminescence spectra, intense emission lines appeared 470, 550, 580,

625 and 670 nm due to 5D4 ->

7Fi (i = 1-6) transitions in both samples. The radioluminescence

intensity of TSAG was one digit higher than that of TGG. The scintillation decay times of TGG and

TSAG were 94 and 678 ms, respectively. In scintillation screen experiments, we confirmed that TGG

and TSAG worked as a scintillation screen, and the contrast of TSAG screen was better than that of

TGG. Throughout this work, the capability of TGG and TSAG for scintillator screen is confirmed.

Acknowledgement

This work was supported by a Grant in Aid for Scientific Research (A)-26249147 from the Ministry of

Education, Culture, Sports, Science and Technology of the Japanese government (MEXT) and

partially by JST A-step. The Cooperative Research Project of Research Institute of Electronics,

Shizuoka University, KRF foundation, Hitachi Metals Materials Science foundation, and Inamori

foundation are also acknowledged.

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Figure captions

Figure 1 Schematic drawing of the experimental setup for scintillator screen measurement.

Figure 2 Pictures of TGG and TSAG under room light (left) and UV irradiation by UV-lamp (right).

Figure 3 Optical in-line transmittance of TGG and TSAG.

17

Figure 4 PL emission map of TGG (top) and TSAG (bottom). The horizontal axis is the emission

wavelength, and the vertical axis is the excitation wavelength.

Figure 5 PL decay time profiles of TGG and TSAG monitoring at 470 nm under 265 nm excitation.

Figure 6 X-ray induced radioluminescence spectra of TGG and TSAG.

Figure 7 Scintillation decay time profiles of TGG and TSAG under pulse X-ray excitation.

Figure 8 Afterglow profiles of TGG and TSAG, and inset shows TSL glow curves of these two

samples after 1 Gy X-ray irradiation.

Figure 9 Sample object of the scintillator screen experiment (left) and observed images by using

TGG and TSAG (right).

Figure 1

Figure

Figure 2

TSAG TGG TSAG TGG

UV

irradiation

Figure 3

Figure 4 250

300

350

400

450

Ex

cita

tio

n W

avel

eng

th, n

m

250

300

350

400

450

400 600 500 800 700

Emission Wavelength, nm

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9