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Sensors and Materials, Vol. 30, No. 7 (2018) 1591–1598
MYU Tokyo
S & M 1615

Thermoluminescence Properties for X-rayof Cr-doped Al2O3 Ceramics
Kiyomitsu Shinsho,1* Daiki Maruyama,1 Shin Yanagisawa,1 Yusuke Koba,2Mizuki Kakuta,1 Kazuki Matsumoto,3 Hiroaki Ushiba,3 and Takayuki Andoh4
1Tokyo Metropolitan University, 7-2-10 Higashi-Oku, Arakawa-ku, Tokyo 116-8551, Japan
2
QST NIRS, 4-9-1 Anagawa, Inage-ku, Chiba-shi, Chiba 263-8555, Japan
3Chiyoda Technol Corporation, 1-7-12 Yushima, Bunkyo-ku, Tokyo 113-8681, Japan4
Chibaceramic MFG Co. Ltd., 2-10-14 Inage-higashi, Inage-ku, Chiba-shi, Chiba 263-0031, Japan(Received January 28, 2018; accepted May 25, 2018)

Keywords: thermoluminescent phosphor, Al2O3, glow curve, two-dimensional dosimeter

We investigated the basic thermoluminescence (TL) characteristics of Cr-doped low-meltingpoint Al2O3. The concentration of Cr2O3 in this study ranged from 0.01 to 1.0 wt%. Owing tothe stabilization of the trapped state caused by Cr doping, we successfully improved the lowmelting-point Al2O3 fading property dramatically. Then, the sensitivity of 0.05 wt% Cr-doped
Al2O3 to X-rays was about 4.8 times higher than that of the nondoped Al2O3, allowing the Al2O3
to maintain its attractive properties to X-rays. Furthermore, the 0.05 wt% Cr-doped Al2O3
had good properties for dosimetry owing to its high TL sensitivity, high reproducibility, highradiation resistivity, and high thermal stability in two dimensions. Finally, we demonstrated
two-dimensional (2D) verifications of radiotherapy (RT) plans using Cr-doped Al2O3 ceramicplates. The results had good agreement with RT plans.

1.

Introduction

Modern radiotherapy (RT) can focus radiation into a tumor with high precision, which leads
to better therapeutic gains over other methods. Therefore, both the irradiation field and the
dosage require highly precise validation. Currently, a radiochromic film is typically used for thetwo-dimensional (2D) verification of RT plans. It has good tissue equivalence and a high spatial
resolution. However, the technique has some limitations, including high material costs in certain
countries and a narrow dynamic range, among others. There are some alternatives to the useof the radiochromic film. The thermoluminescence dosimeter (TLD) is one of the alternatives.
Recently, with the advent of the CCD camera, the high-resolution imaging of large areas has
become much faster, so the amount of research on X-ray imaging using 2D-TLD was increased.(1–6)
In particular, human-tissue-equivalent 2D-TLD has been paid more attention owing to focusingneeds in the medical field. However, it has low reproducibility, low signal-to-noise ratio, andlow solidity; thus, there is no suitable human-tissue-equivalent 2D-TLD.
*Corresponding author: e-mail: shinsho@tmu.ac.jp

This was published on July 27, 2018, and errors detected after publication were corrected on August 3, 2018.

http://dx.doi.org/10.18494/SAM.2018.1928

ISSN 0914-4935 © MYU K.K.
http://myukk.org/

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High-purity Al2O3 has been reported to maintain high sensitivity, but it is not a suitablematerial for 2D-TLD owing to its high melting point of 2,072 ℃.(7) Thus, the costs of boththe material and its production have become high. We focused on a thermoluminescence(TL) phosphor Al2O3 with a low melting point of approximately 1600 ℃ with some sintering
assistance. It is easy to produce a large plate using Al2O3, owing to the low costs of thematerials and production system, compared with that in the case of the high-purity Al2O3. We
reported that a commercially available Al2O3 plate with a low melting point of 1600 ℃ hasa low material cost and a wide dynamic range, and is reusable.(8) However, it is necessary
to improve this material’s fading property, owing to an unstable trap state at a glow peaktemperature of 148 ℃. In this study, we investigated the basic TL characteristics of Cr-doped
low-melting-point Al2O3 for improving the TL properties. The Cr2O3 concentration in thisstudy ranged from 0.01 to 1.0 wt%. The 0.05 wt% Cr-doped Al2O3 ceramic yielded the best
results. Finally, we demonstrated the verification of the RT plan with 0.05 wt% Cr-doped Al2O3
ceramic plates.

2.

Materials and Methods

2.1

Materials and glow curve measurement system

The low-melting-point Al2O3 of Chiba Ceramic MFG. Co,. Ltd., composed of Al2O3 > 99.5wt%, SiO2 < 0.10 wt%, Fe2O3 < 0.05 wt%, Na2O < 0.10 wt%, Cr < 2 ppm, Cd < 1 ppm, Pb < 1ppm, and Hg < 1 ppm, was used. The bulk density of the plates was 3.7 g·cm−3. The dimensions
used for the glow curve measurements were 10 × 10 × 1 mm3. The Cr2O3 concentration in thisstudy ranged from 0.01 to 1.0 wt%. In the demonstration of the verification of the RT plan, Crdoped Al2O3 ceramic plates of 80 × 80 × 0.7 mm3 size were used (Fig. 1).
Figure 2 shows our glow curve measurement system and schematic diagram. The glow
curves were recorded from room temperature up to 400 ℃ at a heating rate of 0.1 ℃ s−1 in

Fig. 1.

(Color online) Low-melting-point Cr-doped Al2O3 ceramic plate (80 × 80 × 0.7 mm3).

Sensors and Materials, Vol. 30, No. 7 (2018)(a)Fig. 2.

1593(b)(Color online) Glow curve measurement system and schematic diagram.(8)air. The sampling time was 10 s. The temperature was controlled using a Sakaguchi E.HVOC Corp. SCR-SHQ-A programmable thermoregulator. A Hamamatsu H11890-210 photoncounting head light detector was used to detect luminescence. The spectral sensitivity regionof this photon counting was 400–700 nm. A thermal-cut filter was the only spectroscopic
attachment used. The transmission band of the filter was 365–700 nm. The guaranteed spectral
value of 700 nm was 0.43. The X-ray irradiation was carried out using a Varian CLINAC21EX linear accelerator at 6 MV. The TL samples and a Scanditronix CC13 ion chamber weresandwiched between two tough-water phantoms (Kyoto Kagaku Co., Ltd) at a depth of 10 cm.
The distance from the X-ray source to the chamber was 100 cm. The X-ray exposure dose was5 Gy. In the dose-response curve measurements, the doses were 0.5–100 Gy.
The TL spectroscopic analysis was performed using a Hamamatsu PMA-11 multichannel
CCD detector. The measurement system converted the photon-counting head shown in Fig.
2 into the multichannel CCD detector. The measured temperatures in the region ranged fromroom temperature to 350 ℃. Rigaku Corp. Thermo plus EVO2 was used to measure thedifferential thermal analysis (DTA) and thermogravimetric analysis (TGA) curves.
2.2TL imaging (2D) measurements

The TL imaging measurement system (Seisei Manufactory Co., Ltd.) consists of acomplementary metal oxide semiconductor (CMOS) camera (ORCA®-Flash4.0 V2, C11440-22CU), an 80 × 80 mm2 heater (Sakaguchi E.H VOC Corp.), and a dark box (Fig. 3). After
exposure, the door of the dark box was closed once the ceramic plates were set on the threepins. Next, the in-house-developed plate heater was raised to the top of the pins and thenheated to 400 ℃ for 5 min. The heating rate was controlled using a programmable temperature
controller, and the heater provided a f lat temperature distribution and maintained thetemperature (<±3%). The TL images were captured using a CMOS camera equipped with athermal cut filter.

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Sensors and Materials, Vol. 30, No. 7 (2018)(a)Fig. 3.

(b)(Color online) (a) TL imaging (2D) measurement system and (b) schematic diagram.

3.

Results and Discussion

3.1

Basic TL characteristics

Figure 4 shows the TL intensity as a function of Cr2O3 concentration in Al2O3. The TL
intensity increased accordingly as Cr was doped up to 0.05 wt%, then decreased. The highest
sensitivity was about 4.8 times higher than that of nondoped Al2O3 at 0.05 wt% Cr doping.
Figure 5 shows TL glow curves of nondoped Al2O3 and 0–1.0 wt% Cr-doped Al2O3. The
main glow peaks were located at about 150 ℃ for nondoped Al2O3. The glow peak at 300℃ was produced by the Cr-doped Al2O3, and the glow peak intensity gradually increased inrelation to the Cr doping concentration until 0.05 wt%. From this point, it was suggested thatthe stabilization of a trap state was due to Cr doping. That is, we successfully improved its
fading property dramatically. From similar observations,(9) the TL wavelength was 693 nm andwas attributed to the 2E– 4A2 transition of the embedded Cr3+ (Fig. 6). In addition to the peak,a broad peak in the region of 600–800 nm was observed. We consider that the broad peak is
TL-scattered by thermal vibration. That is, the peak was caused by stoke and antistoke lines.
The glow peak temperature and TL intensity were changed by Cr doping; however, the TL
wavelength did not depend on it.
In the dose-response curve measurements (Fig. 7), the Cr-doped Al2O3 had a high TL
linearity from 0.5 to 10 Gy. Above 10 Gy, its TL efficiency showed supralinearity. Then, above
80 Gy, its measurement error increased.
Figure 8 shows the TL glow curve of 0.05 wt% Cr-doped Al2O3 phosphor after high-dose (150Gy) X-ray irradiation. The TL glow curve did not change. That is, the TL characteristics of theCr-doped Al2O3 phosphor have a high radiation resistivity.

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

Fig. 4. Relative TL intensity as a function of Cr2O3
concentration in Al2O3.

Fig. 6.

TL spectra of 0.05 wt% Cr-doped Al2O3.

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Fig. 5. (Color) TL glow curves of nondoped and Crdoped Al2O3.

Fig. 7.
Al2O3.

Dose-response curve of 0.05 wt% Cr-doped

Table 1
Reproducibility of relative TL intensity of 0.05 wt%
Cr-doped Al2O3 (annealed at 500 ℃).
1st2nd3rdCV (%)

Relative TL intensity
2.63 × 108
2.65 × 108
2.68 × 108
0.93

Fig. 8. (Color) TL glow curve of 0.05 wt% Crdoped Al2O3 phosphor after high-dose (150 Gy) X-ray
irradiation.

Table 1 shows the reproducibility of the relative TL intensity of the 0.05 wt% Cr-doped
Al2O3 (annealed at 500 ℃). It had a high reproducibility with a coefficient variation (CV) of<1.0%.
In the reproducibility of the DTA and TGA curves, the results did not depend on the 25
identical thermal cycles from room temperature to 1000 ℃. It was indicated that the 0.05 wt%
Cr-doped Al2O3 has high thermal stability.

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Demonstration of vilification of RT plan

RT planning is made using CT images. A 0.05 wt% Cr-doped Al2O3 plate stacked in a
tough-water phantom was CT-scanned. The CT images were forwarded to a treatment planningsystem (TPS) (Varian medical systems, Eclipse, Ver. 13.6) and two types of dose distribution
were made using a linear accelerator (Varian medical systems, Clinac iX) with a 10 MV photon
beam. The field sizes were 5 × 5 cm2 in static irradiation [Fig. 9(a)] and 4 × 5 cm2 in arc
irradiation [Fig. 9(b)]. The gantry was rotated counterclockwise from 140 to 240 degrees in thearc.
Figure 10 shows the dose distribution measured using a 0.05 wt% Cr-doped Al2O3 plate
subjected to static irradiation. The dose profile obtained using the Al2O3 plate (red line) iscompared with that obtained using TPS. Figure 11 shows the OCR curve of the 0.05 wt% Crdoped Al2O3 plate and TPS. The OCRs of the 0.05 wt% Cr-doped Al2O3 plate and TPS were inagreement within 1.5%.
Figure 12 shows the dose distribution measured using the 0.05 wt% Cr-doped Al2O3 platein arc irradiation. Figure 13 shows the OCR curve of the 0.05 wt% Cr-doped Al2O3 plate andTPS. The OCRs of the 0.05 wt% Cr-doped Al2O3 plate and TPS were in agreement within 1.0%.

100cm
90cm

10cm
20cm

Ceramic plate
Water equivalent phantom

20cm

100cm
X-ray source(a)Fig. 9.

(b)(Color) Measurement alignment: (a) static irradiation and (b) arc irradiation.

Fig. 10. (Color) Dose distribution of 0.05 wt% Crdoped Al2O3 plate subjected to static irradiation.

Fig. 11. (Color) OCR curve of static irradiation.

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

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Fig. 12. (Color) Dose distribution of 0.05 wt% Crdoped Al2O3 plate subjected to arc irradiation.

4.

Fig. 13. (Color) OCR curve of arc irradiation.

Conclusions

We investigated the basic TL characteristics of Cr-doped low-melting-point Al2O3. TheCr2O3 concentration in this study ranged from 0.01 to 1.0 wt%. Owing to the stabilization ofthe trapped state caused by Cr doping, we successfully improved the low-melting-point Al2O3
fading property dramatically. Then, the sensitivity of 0.05 wt% Cr-doped Al2O3 to X-rays
was about 4.8 times higher than that of nondoped Al2O3, allowing the Al2O3 to maintain its
attractive properties to X-rays. Furthermore, the 0.05 wt% Cr-doped Al2O3 had good properties
for dosimetry owing to its high TL sensitivity, high reproducibility, high radiation resistivity,and high thermal stability in two dimensions. Finally, we demonstrated 2D verifications of RTplans using Cr-doped Al2O3 ceramic plates. The results were in good agreement with RT plans.
We will attempt the 2D dose distribution of a large area of approximately 150 × 150 mm2 using
TL plates with this material soon.

Acknowledgments
This work was partially supported by Chiyoda Technol Co., Chibaceramic MFG Co., and theCooperative Research Project of the Research Institute of Electronics, Shizuoka University.

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