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Sensors and Materials, Vol. 30, No. 7 (2018) 1539–1546
MYU Tokyo
S & M 1609

Dosimetric Properties of Thulium-doped Strontium
Aluminoborate Glasses
Naoki Kawano,1* Noriaki Kawaguchi,2 Go Okada,2Yutaka Fujimoto,3 and Takayuki Yanagida2
1Graduate School of Engineering Science, Akita University,
1-1 Tegata-Gakuen-machi, Akita 010-8502, Japan
2
Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST),
8916-5 Takayama, Ikoma 630-0192, Japan
3Graduate School of Engineering, Tohoku University,
6-6-07 Aoba, Aramaki, Sendai 980-8579, Japan(Received January 16, 2018; accepted March 9, 2018)

Keywords: dosimeter, thermoluminescence, Tm, scintillator, luminescence

We investigated the photoluminescence (PL), scintillation, and dosimetric properties ofstrontium aluminoborate glasses with different thulium (Tm) concentrations of 0.1, 0.5, 1.0, and5.0%. The Tm-doped glasses were synthesized by the conventional melt-quenching technique.
In the PL and scintillation spectra, the Tm-doped samples showed a sharp peak at 460 nm due tothe 4f–4f (1D2→3F4) transitions of Tm3+. Among the samples tested, the 0.5% Tm-doped sample
exhibited the highest intensity in PL and scintillation. In thermally stimulated luminescence (TSL),
the Tm-doped samples showed a broad glow peak around 100 °C after X-ray irradiation of
10000 mGy. The 0.5% Tm-doped sample also showed the highest TSL intensity. Furthermore,
the 0.1–1.0% Tm-doped samples exhibited a linear response against X-ray doses over a range of
10−1–104 mGy.

1.

Introduction

Radiation dosimetric methods have attracted much attention for decades owing to theirapplication in many fields such as individual radiation monitoring,(1) medicine,(2) and space.(3)The fundamental requirements of dosimetric properties are high sensitivity, dose linearity,
energy response, and low fading.(4) Furthermore, in individual radiation monitoring, theeffective atomic number of materials used in dosimeter is expected to be close to that ofbiological tissue (Zeff = 7.35–7.65) from the viewpoint of bioequivalence.(5) In general,
radiation dosimetric methods utilize inorganic phosphors, which demonstrate radiation-inducedluminescence phenomena: thermally stimulated luminescence (TSL), optically stimulated
luminescence (OSL), and radio-photoluminescence (RPL). To date, various materials such as
LiF doped with Mg, Cu, P, Al2O3 doped with C, and NaPO3–Al(PO3)3 doped with Ag have beenused in practice.(4)*Corresponding author: e-mail: n-kawano@gipc.akita-u.ac.jp
http://dx.doi.org/10.18494/SAM.2018.1921
ISSN 0914-4935 © MYU K.K.
http://myukk.org/

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Recently, borate oxide glasses have been focused upon as TSL materials. Borate oxide is agood glass-network former owing to its high bond strength and low melting point, and borate
oxide glasses have high accommodation capability with rare-earth oxides and low effective
atomic number. For example, Li2B4O7 doped with Cu showed TSL sensitivity much higher thanthat of a commercial dosimeter (TLD-100),(6,7) and SrB4O7 exhibited a sensitivity equivalent
to that of TLD-700.(8) In addition, our research group has investigated aluminoborate glasses
with the inclusion of Mg, Ca, and Sr as host constituents.(9–11) Strontium aluminoborate glasses
doped with Ce exhibited high TSL sensitivity and covered an X-ray dose range of 10−1–104
mGy.(10) As these examples have shown, borate oxide glasses are one of the potential materials
for dosimeter applications.
In this study, we synthesized strontium aluminoborate glasses doped with various
concentrations of thulium (Tm) as luminescent centers by the conventional melt-quenching
technique. To the best of our knowledge, no studies have reported the dosimetric properties
of Tm-doped strontium aluminoborate glasses. Some Tm-doped materials such as Tm-doped
CaSO4 and Tm-doped CaF2 exhibited high TSL intensity and good dose response.(5,12) Thus,
the TSL sensitivity of the strontium aluminoborate glasses can be enhanced by the incorporationof Tm as a dopant. After the synthesis, we characterized the Tm-doped glasses for dosimeter
applications, and the characterization included optical in-line transmittance, PL, scintillation,and TSL. It is important to study both scintillation and TSL properties comprehensively
because some materials show a complementary relationship between these two properties.(13,14)

2.

Experimental Methods

Strontium aluminoborate (50B2O3–15Al2O3–35SrO–xTm2O3, x = 0–0.5) glasses doped withdifferent Tm concentrations were synthesized by the conventional melt-quenching technique.
B2O3 (99.99%, Furuuchi Chemical), Al2O3 (99.99%, Kojundo Chemical Laboratory), SrCO3(99.99%, Kojundo Chemical Laboratory), and Tm2O3 (99.99%, Rare Metallic) powders were
homogeneously mixed in a stoichiometric ratio. Then, the mixture was melted in an alumina
crucible inside an electric furnace heated at 1200 °C for 30 min under ambient atmosphere.
After that, the melt was quenched on a preheated stainless plate at 350 °C. The glass samples
obtained were mechanically polished before characterization.
The optical in-line transmittance spectrum was measured using a spectrophotometer (V670,JASCO), which covered the spectral region from 190 to 2700 nm. As a luminescence property,
the PL quantum yields (QY) were measured using Quantaurus QY (C11347, Hamamatsu). PL
decay curves were measured using Quantaurus τ (C11367, Hamamatsu). In this measurement,
the excitation wavelength was 360 nm, and the monitoring wavelength was 460 nm. As ascintillation property, X-ray-induced luminescence spectra were measured using our originalsetup.(15) For X-ray irradiation, the X-ray tube was operated by applying a bias voltage of 40
kV and a tube current of 5.2 mA. The scintillation decay time profile was measured using anafterglow characterization system equipped with a pulse X-ray source.(16) As a TSL property,TSL glow curves were measured using TL-2000 (Nanogray) at a heating rate of 1 °C/s over atemperature range of 50 to 490 °C.(17)Sensors and Materials, Vol. 30, No. 7 (2018)3.

Result and Discussion

3.1Samples

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Figure 1 shows a photograph of Tm-doped strontium aluminoborate glasses. The thicknessof the samples was about 0.9 mm. The check pattern on the back of the Tm-doped samples wasclearly observed. The sample color gradually varied from colorless to brown with increasing
Tm concentration.
Figure 2 shows in-line transmittance spectra of Tm-doped strontium aluminoborate glasses.
The transmittance of the 0.1–1.0% Tm-doped samples was about 60% over the 400–1500 nm
range while the transmittance of the 5.0% Tm-doped sample was about 40%. In addition, the1.0–5.0% Tm-doped samples exhibited sharp absorption peaks at 360, 470, 685, 793, and 1210
nm, which were attributed to the 4f–4f transitions of Tm3+ (3H6→1D2, 3H6→1G4, 3H6→3F3,
3H6→3H4, and 3H6→3H5, respectively).(18) The optical absorption edge of the Tm-doped
samples was almost equivalent to that of a nondoped sample from our previous study.(10)3.2

Photoluminescence properties

Figure 3 shows PL emission and excitation contour graphs of Tm-doped strontium
aluminoborate glasses. An emission peak at 460 nm was observed in the Tm-doped samples
under an excitation wavelength of 360 nm. The emission peak agreed well with that of theabsorption peak observed in the in-line transmission spectra in Fig. 2. Furthermore, theemission wavelength coincided with the reported value for other Tm-doped materials.(18,19)
Thus, the emission was ascribed to the 4f–4f (1D2→3F4) transitions of Tm3+. In addition, theQY values were 0.03 (0.1% Tm), 0.06 (0.5% Tm), and 0.01 (1.0% Tm) while a PL signal fromthe 0.5% Tm-doped sample could not be detected. The 0.5% Tm-doped sample exhibited theFig. 1. (Color online) Photograph of Tm-doped
strontium aluminoborate glasses.

Fig. 2. (Color online) In-line transmittance spectraof Tm-doped strontium aluminoborate glasses.

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Fig. 3.

(Color online) PL emission and excitation contour graphs of Tm-doped strontium aluminoborate glasses.

highest QY value among the samples tested. The maximum QY value of the 0.5% Tm-doped
sample was lower than that of the 0.3% Tm-doped NaPO3–Al(PO3)3 glass in a previous study.(19)
In addition, the QY value decreased with increasing Tm concentrations, perhaps because ofconcentration quenching.
Figure 4 shows the PL decay time profiles of Tm-doped strontium aluminoborate glasses.
In these measurements, the excitation wavelength was 360 nm, and the monitoring wavelength
was 460 nm. Each PL decay curve was approximated by an exponential decay function. In thisapproximation, the component of instrumental response was excluded to obtain the PL decaytime. The PL decay times for the Tm-doped samples were 14.0 μs (0.1% Tm), 12.0 μs (0.5%Tm), 10.3 μs (1.0% Tm), and 9.2 μs (5.0% Tm). The decay times were almost equivalent to thetypical decay time constant of the 4f–4f (1D2→3F4) transitions of Tm3+.(19) The decay timedecreased with increasing Tm concentration, perhaps because of concentration quenching.
3.3

Scintillation properties

Figure 5 shows the X-ray-induced scintillation spectra of Tm-doped strontium aluminoborate
glasses. The Tm-doped samples exhibited emissions at 360 and 460 nm. According to previous
studies,(18) the origin of the emissions at 360 and 460 nm was the 4f–4f transitions of Tm3+
(1D2→3H6 and 1D2→3F4, respectively). Among the samples tested, the 0.5% Tm-doped sample
exhibited the highest scintillation intensity, which was consistent with the result observed in PL.
Figure 6 shows the X-ray-induced scintillation decay time profiles of Tm-doped strontium
aluminoborate glasses. Each decay curve was approximated by the sum of two exponential
decay functions. The decay times were 15.8 and 216 μs (0.1% Tm), 10.5 and 123 μs (0.5%Tm), 8.9 and 125 μs (1.0% Tm), and 9.1 and 204 μs (5.0% Tm). From previous studies and an
effective sensitivity of the equipped photomultiplier tube (160–650 nm), the origin of the firstSensors and Materials, Vol. 30, No. 7 (2018)

Fig. 4.

(Color online) PL decay time profiles of Tm-doped strontium aluminoborate glasses.

Fig. 5.

(Color online) X-ray-induced scintillation spectra of Tm-doped strontium aluminoborate glasses.

Fig. 6.

(Color online) X-ray-induced decay time profiles of Tm-doped strontium aluminoborate glasses.

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Sensors and Materials, Vol. 30, No. 7 (2018)

component may be the 4f–4f (1D2→3H6, 1D2→3F4) transitions of Tm3+,(18–20) and the originof the second component may be the 4f–4f (1G4→3H6, 1G4→3F4) transitions of Tm3+.(18–20) In
addition, the decay times of the first component were almost comparable to those observed inother Tm-doped samples such as Tm-doped Lu3Al5O12(21) and Tm-doped NaPO3–Al(PO3)3.(19)
Furthermore, the decay time of the first component became faster with increasing Tm
concentrations, which was consistent with the results observed in PL. In addition, the decay
times in scintillation were almost equivalent to those in PL. In general, scintillation decay
time is longer than PL decay time owing to additional energy migration processes. This result
suggested that scintillation properties may primarily depend on PL processes in the Tm-dopedglasses.
3.4

Dosimetric properties

Figure 7 shows TSL glow curves of Tm-doped strontium aluminoborate glasses after X-rayirradiation at 10000 mGy. The Tm-doped samples exhibited a broad glow peak around 100 °C.
The TSL intensity of the 0.5% Tm-doped samples was the highest among all the samples, soscintillation intensity and TSL intensity exhibited a positive proportionality as with Ce-doped
LiCaAlF6.(14) Furthermore, glow peak temperature, activation energy, and frequency factors
were derived by numerical approximations assuming general-order kinetics to evaluate the traplevel.(22) The calculated results are summarized in Table 1. The TSL intensity of the 5.0% Tmdoped sample was too low to enable these factors to be evaluated accurately. The activationenergies and frequency factors were almost constant regardless of the Tm concentration andwere almost equivalent to those reported in Ce-doped strontium aluminoborate glasses.(10)Therefore, these results suggested that the origin of the dosimetric trapping centers responsible
for TSL may be the glass host. In addition, the low values of the frequency factors suggested
that the primary detrapping process of TSL was the tunneling effect.

Fig. 7. (Color online) TSL glow curves of Tm-doped strontium aluminoborate glasses after X-ray irradiation at10000 mGy.

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Table 1
Glow peak temperature, activation energies, and frequency factors.
Peak temperature Activation energy Frequency factors
(℃)
(eV)
(s−1)
93
0.46
1.12 × 105
0.1%Tm165
0.55
1.10 × 105100
0.47
1.36 × 105
0.5%Tm
166
0.61
4.31 × 105
102
0.47
1.26 × 105
1.0%Tm1700.56
1.22 × 105

Fig. 8.

(Color online) TSL glow curves of Tm-doped strontium aluminoborate glasses.

Figure 8 shows the TSL dose response curves of Tm-doped strontium aluminoborate glasses.
The integrated value of a TSL peak around 100 °C was defined as the TSL intensity. The lowest
detection limit of the 0.1–1.0% Tm-doped samples was confirmed to be 0.1 mGy while the limitof the 5.0% Tm-doped sample was confirmed to be 1 mGy. Furthermore, the 0.1–1.0% Tmdoped samples exhibited a linear response to X-ray dose over a dose range of 0.1–10000 mGy.
This dynamic range was comparable to that of Ce-doped strontium aluminoborate glasses
reported in our previous study.(10) A linear response with respect to X-ray dose is importantbecause it enables us to measure accurately and easily calibrate the X-ray dose. However, thelowest dose sensitivity (0.1 mGy) was not sufficient compared with TSL dosimeters such as
Al2O3:C and LiF:Mg, Cu, P.(4) The TSL sensitivity is expected to be improved by optimizing
the host composition and the dopant species.

4.

Conclusion

Strontium aluminoborate glasses with different concentrations of Tm (0.1, 0.5, 1.0, and 5.0%)
were synthesized by the conventional melt-quenching technique. The PL, scintillation, and TSL
properties of the glasses were investigated. The Tm-doped samples showed an emission at 460
nm due to the 4f–4f (1D2→3F4) transitions of Tm with decay time constants of approximately
9–16 μs in PL and scintillation. Among the samples investigated, the 0.5% Tm-doped sample
showed the highest intensity in PL and scintillation. Furthermore, the 0.5% Tm-doped sample1546

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also showed the highest TSL sensitivity after X-ray irradiation of 10000 mGy. The TSL
intensity of the 0.1–1.0% Tm-doped samples monotonically increased with an X-ray dose
irradiated over 0.1–10000 mGy.

Acknowledgments
This work was supported by Grant-in-Aid for Scientific Research (A) (17H01375), Grantin-Aid for Young Scientists (B) (17K14911), and Grant-in-Aid for Research Activity Start-up(16H06983) from the Ministry of Education, Culture, Sports, Science and Technology of theJapanese government (MEXT) as well as A-STEP from the Japan Science and Technology
Agency (JST). The Cooperative Research Project of the Research Institute of Electronics,
Shizuoka University, The Taisei Foundation, SEI Group CSR Foundation, Mazda Foundation,Konica Minolta Science and Technology Foundation, NAIST Foundation, and TEPCO
Memorial Foundation are also acknowledged.

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