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Sensors and Materials, Vol. 30, No. 7 (2018)1599–1604
MYU Tokyo
S & M 1616

Dose Linearity and Linear Energy Transfer Dependenceof Cr-doped Al2O3 Ceramic Thermoluminescence Detector
Yusuke Koba,1* Risa Shimomura,2 Weishan Chang,2Kiyomitsu Shinsho,2 Shin Yanagisawa,2 Genichiro Wakabayashi,3Kazuki Matsumoto,4 Hiroaki Ushiba,4 and Takayuki Ando5
1Medical Exposure Research Project, National Institute of Radiological Sciences,4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan
2Division of Radiological Sciences, Faculty of Health Sciences, Tokyo Metropolitan University,
7-2-10 Higashi-Ogu, Arakawa-ku, Tokyo 116-8551, Japan
3
Atomic Energy Research Institute, Kindai University, 3-4-1 Kowakae, Higashiosaka-shi, Osaka 577-8502, Japan
4
Chiyoda Technol Co., 1-7-12 Yushima, Bunkyo-ku, Tokyo 113-8681, Japan
5
Chiba Ceramic Mfg. Co., Ltd., 2-10-14 Inage-higashi, Inage-ku, Chiba 263-0031, Japan(Received February 9, 2018; accepted March 30, 2018)

Keywords: Al2O3:Cr, alumina-based ceramics, TLD, dose linearity, LET dependence

A Cr-doped alumina (Al2O3:Cr) ceramic thermoluminescent detector (TLD) has the potential
to be applied as a two-dimensional (2D) imaging device in radiation therapy. In this work,
we investigated the linearity in the dose level of radiotherapy and the linear energy transfer(LET) dependence of Al2O3:Cr ceramic TLD using several species of heavy charged particles(HCPs). The Al2O3:Cr ceramic TLD had good linearity as a function of irradiation dose.
However, the large LET-dependent response made it difficult to apply the detector directly todose measurement in HCP therapy. The combination of two Al2O3 ceramic TLDs, one with Cr
doping and one without Cr doping, may be expected to be applied to the measurement of LET
distributions because of the different LET dependences.

1.

Introduction

Successful radiotherapy requires the correct dose to be delivered; this can be achieved by aroutine quality assurance (QA) program and dose verification before treatment. A radiochromic
film such as EBT3(1) (GAFCHROMICTM, EBT3 dosimetry film, Ashland Co.) is capable ofproviding a precise two-dimensional (2D) dose distribution and is one of the most widely used
dosimeters in conventional photon radiotherapy.(2–5) However, the linear energy transfer (LET)dependent response of EBT3 and the fact that it can be used only once make it unsuitablefor heavy charged particle (HCP) radiotherapy.(6,7) Consequently, ionization chambers (ICs)have been the most useable dosimeters for QA in HCP therapy. The scanning and multiarrayarrangement methods are the most common ways to obtain the dose distribution using ICs,
although these two methods have some drawbacks, such as requiring a large amount of time andproviding a poor spatial resolution.
*Corresponding author: e-mail: koba.yusuke@qst.go.jp
http://dx.doi.org/10.18494/SAM.2018.1930
ISSN 0914-4935 © MYU K.K.
http://myukk.org/

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Recently, it has been reported that Al2O3-based ceramic plates show good thermoluminescent(TL) characteristics and are available for use in 2D dosimetry.(8) In addition, it has beenreported that a nondoped Al2O3 ceramic thermoluminescent detector (TLD) has a smaller LETdependent response than commercially available TLDs such as LiF:Mg,Ti and LiF:Mg,Cu,P.(9)The application of a nondoped Al2O3 ceramic TLD to HCP therapy has been suggested. On theother hand, nondoped Al2O3 had a low-temperature glow peak that increases the fading effect.
Recent research shows that a Cr-doped alumina (Al2O3:Cr) ceramic TLD is suitable as a 2D
imaging device in conventional photon therapy because it has a linear relationship between doseand response, and a small fading property, while it is easy to handle and reusable.(10)In this work, with the aim of evaluating the potential of Al2O3:Cr ceramic TLDs in HCPtherapy, we investigated the dose linearity and LET dependence of an Al2O3:Cr ceramic TLD
using several HCP beams.

2.

Materials and Methods

2.1

Cr-doped alumina-based ceramic TLD

An Al2O3:Cr ceramic TLD was provided by Chiba Ceramic Mfg. Co., Ltd. Table 1 shows
the composition of this Al2O3:Cr ceramic TLD. The bulk density and effective atomic numberof the TL slab were 3.7 g/cm3 and 11.13, respectively. Al2O3:Cr ceramic TLDs were fabricatedwith a size of 11 × 11 × 0.7 mm3 (Fig. 1).
2.2

Irradiation

Irradiation experiments were performed at the BIO course of the heavy-ion medical
accelerator (HIMAC) in Chiba at the National Institute of Radiological Sciences. The beams
were laterally broadened by a wobbling and scattering system to produce a uniform field witha 10 cm diameter.(11) Table 2 lists the irradiation beams used in this study. The values of LET
in water were calculated using Geant4.(12) For each irradiation, four dose levels were given as
0.5, 1.0, 2.0, and 5.0 Gy in water based on the predicted dose in HCP therapy. In addition, all

Table 1
Nominal composition of Al2O3: Cr ceramic TLD(Chiba Ceramic Mfg. Co., Ltd. A10).
Ingredient
Wt.%
>99.5
Al2O3~0.05
Cr2O3Fig. 1. Sample of Al2O3:Cr ceramic TLD used inthis study.

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Table 2
Irradiation beams used in this study.
Particle
Energy (MeV/u) LET in water (keV/mm)
H
160
0.5He
150
2.0C
290
13C13524Ne40029Ar500
86
Fe500197

the Al2O3:Cr ceramic TLDs were irradiated using a 6 MV photon beam as a reference froma Varian 21Ex linear accelerator at Tokyo Metropolitan University. The irradiations wereperformed with a 100 cm source-to-axis distance and a field size of 10 × 10 cm2, and by placing
the film packages at a depth of 10 cm in water equivalent phantoms.
2.3

TL glow curve measurement

A readout system(9) developed in house was used to record TL glow curves of the Al2O3:Cr
ceramic TLDs from room temperature to 400 °C. The TLD was heated by a brass plateconnected to a programmable heat controller (Sakaguchi E.H VOC Corp. SCR-SHQ-A) at aheating rate of 0.2 °C/s in air. A thermal cut filter was set in the readout system to decrease
the effect of thermal radiation. After passing through the thermal cut filter, the TL signal
was collected with a pair of collecting lenses and then counted using a photon-counting unit
(Hamamatsu H11890-110). Generally, the TLD demonstrates a fading effect in which theamount of TL decreases as a function of the time elapsed from irradiation to reading. To
improve accuracy without correcting the fading effect, the timeline of the readout procedure
was made consistent for each experiment. The wavelength spectrum of the TL of the Al2O3:Crceramic TLD was detected with a multichannel spectroscope (Hamamatsu PMA-12).

3.

Results and Discussion

3.1

TL glow curve and dose linearity

Figure 2 shows the TL glow curve of the Al2O3:Cr ceramic TLD. The glow peaks werelocated at 310 °C. Figure 3 shows the wavelength spectrum of the TL; the wavelength of the TL
peak is located at 694 nm due to the embedded Cr3+.(13–16) As shown in Fig. 2, the TL intensity
was evaluated as an integrated value over a 10 °C range centered on the main peak. The TLintensity was normalized using the following equation,

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1.2

w/ BG
BG
w/o BG

100000∆T = 10[ﾟC]

Intensity (arb. unit)

TL Intensity (arb. unit)

120000

80000
60000
40000

10.8
0.6
0.4
0.2

20000
0100

150

200

250

300

350

400

0500

550

Temperature (ﾟC)

Fig. 2.

Glow curve of the Al2O3:Cr ceramic TLD.

nL( D) =Fig. 3.
TLD.

600

650 700 750 800
Wave Length (nm)850900

Wavelength spectrum of Al2O3:Cr ceramicL( D),LX − ray (1 Gy)(1)where D is the irradiation dose, L(D) is the TL intensity when the irradiation dose is D
Gy, LX-ray(1 Gy) is the TL intensity when an X-ray of 1 Gy is irradiated, and nL(D) is thenormalized TL intensity. In addition to nL(D), to evaluate the reproducibility, the TL efficiency
nL/D was defined as the TL intensity per irradiation dose. Figure 4 shows the dose response ofthis ceramic TLD to several species of charged particles. Dose linearity and repeatability werecomparatively good. On the other hand, the TL efficiency nL/D changed with the species ofHCP.
3.2

LET dependence

Table 3 and Fig. 5 show the relationship between the LET in water of the irradiating particle
and the TL efficiency of the Al2O3:Cr ceramic TLD for charged particles in this study. TheTL efficiency increased from 0.8 and reached a peak value of 1.2 when the LET was 13
keV/mm and decreased to 0.6 when the LET approached 200 keV/mm. The TL efficiency ofthe nondoped Al2O3 ceramic TLD showed a relatively small LET dependence over this LETrange,(9) while the Al2O3:Cr ceramic TLD had a large LET dependence. Because of its large
LET dependence, the Al2O3:Cr ceramic TLD is difficult to use directly in QA for HCP therapy.
However, the difference in the LET dependence of TL efficiency for these two TLDs withsimilar physical characteristics may be useful for the estimation of LET. Over a limited LETrange, the difference in the LET dependence of the two TLDs may be uniquely determined withrespect to the value of LET. In Refs. 17 and 18, a dose estimation method for HCP using twotypes of optically stimulated luminescence (OSL) elements with different LET dependences
has been reported. Using a similar estimation method and 2D imaging of ceramic TLDs,(8)it may be possible to measure the LET distribution in HCP therapy. Recently, research on
HCP therapy has focused on not only the physical dose but also the biological dose that makes
LET distribution information increasingly important. In HCP therapy with one nuclide
beam, the range of the LET of the irradiated beam is limited. Hence, it may be possible forSensors and Materials, Vol. 30, No. 7 (2018)1603

8H 160MeV

7He 150 MeV/u
C 290 MeV/u

6C 135 MeV/u
Ne 400 MeV/u

nL54Ar 500 MeV/u
Fe 500 MeV/u

3

1.5
TL efficiency (arb. unit)

2nL / D

10
1.4
1.2
1
0.8
0.6

C(290 MeV/u)C(135 MeV/u)

1HeHNeAr

0.5FeCalibration: 6 MV X-ray01234Irradiation Dose (Gy)56Fig. 4. (Color online) Dose responses to X-ray andcharged particles. The TL intensity was normalized
at 1 Gy.
Table 3
Relative TL efficiency of alumina-based ceramic TLD.
Irradiation LET in waternL/Dparticle(keV/mm)
0.5 Gy 1 Gy
2 Gy
5 Gy
H
  0.5
0.81
0.83
0.86
0.86
He
  2
0.98
1.03
1.10
1.09
C13
1.16
1.18
1.21
1.20
C24
1.01
0.99
1.03
1.04
Ne
29
0.94
0.93
0.96
0.93
Ar86
0.69
0.70
0.72
0.72
Fe197
0.590.59
0.60
0.61

00.1

Fig. 5.

110100LET in water (keV/μm)

1000

LET dependence of TL efficiency nL/D.

Average
0.84
1.05
1.19
1.02
0.94
0.71
0.60

the combination of one Al2O3 ceramic TLD with Cr doping and one without Cr doping to be
applied to the measurement of the LET distribution in HCP therapy.
In previous reports, the LET dependence of several TLDs based on Al2O3 has been reported.
Spurny and coworkers reported the LET dependence of Al2O3:Na TLD and Al2O3:C TLD.(19–21)
In this work, Al2O3:Cr ceramic TLDs showed characteristics similar to the TL efficiency of theAl2O3:Na TLD that increased up to a range of tens of keV/mm and then decreased in the higher
LET region. On the other hand, the TL efficiency of Al2O3:C TLD decreased monotonicallyin the range of several keV/mm to several hundred keV/mm. In Berger and Hajek’s review(22)of the LET dependence of many TLDs other than Al2O3, most TLDs showed a TL efficiency
peak in the LET region of tens of keV/mm or showed a monotonically decreasing TL efficiency
with increasing LET. In addition, they reported that the TL response to different HCPs with thesame LET is not a unique function of ionization density, but rather depends on the microscopic
pattern of energy deposition. Therefore, further study of the LET dependence of the Al2O3:Cr
ceramic TLD for each HCP is necessary to assess possible applications to HCP therapy.

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

Conclusions

In this study, we investigated the dose response in radiotherapy and the LET dependence ofAl2O3:Cr ceramic TLDs using several HCP beams. The main peak of the glow curve locatedat 300 °C showed good linearity with the irradiation dose. Even though it seems that a large
LET dependence may make the direct application of Al2O3:Cr ceramic TLDs in HCP therapy
difficult, the combination of two Al2O3 ceramic TLDs, one with Cr doping and one without
Cr doping, may possibly be applied to the measurement of LET distribution because of their different
LET dependences. In order to apply Al2O3:Cr ceramic TLDs to QA in HCP therapy, further
investigation of the LET dependence of Al2O3:Cr ceramic TLDs for each HCP is necessary.

Acknowledgments
This work was carried out as a research project with Heavy Ions at the NIRS-HIMAC. This
work was partially supported by the Cooperative Research Project of the Research Institute ofElectronics, Shizuoka University.

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