Response of Alumina-Based Ceramic Thermoluminescence Detector to Charged Particles

Medical Exposure Research Project, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba 263-8555, Japan 1Division of Radiological Sciences, Faculty of Health Sciences, Tokyo Metropolitan University, 7-2-10 Higashi-Ogu, Arakawa-ku, Tokyo 116-8551, Japan 2Atomic Energy Research Institute, Kindai University, 3-4-1 Kowakae, Higashiosaka-shi, Osaka 577-8502, Japan 3Chiyoda Technol Co., 2-16-2 Sotokanda, Chiyoda-ku, Tokyo 101-0021, Japan 4Chiba Ceramic Mfg. Co., Ltd., 2-10-14 Inage-higashi, Inage-ku, Chiba 263-0031, Japan


Introduction
Thermoluminescence detectors (TLDs) have been widely used for dosimetry in various fields. In 1972, Broadhead and Newman reported 2D X-ray imaging using CdSO 4 . (1) Since then, there have been many reports on 2D dosimetry using TLD. Nariyama et al., (2) Shinsho et al., (3) and Marczewska et al. (4)(5)(6) measured the thermoluminescence (TL) distribution of TL sheets or TL pleats using charge-coupled device (CCD) cameras. These researchers predicted the use of TLDs as quality assurance (QA) / quality control (QC) tools in radiation therapy. In fact, 2D dosimetry using TL sheets in radiation therapy was reported in 1995. (6) Charged particle therapies, such as proton and carbon therapy, have beome popular internationally as radiation modalities for cancer treatment. Verification of the irradiation field or dose distribution in charged particle therapy requires measurements with high spatial resolution, because these distributions are precipitous and complex. Measurement of dose distributions in carbon therapy using plate TLD composed of Li 3 B 7 O 12 and synthetic resin has been carried out, but it was reported that the TL efficiency decreased crucially in high-LET regions. (7) Recently, it has been reported that commercial alumina-based ceramic plates show good TL characteristics and are available for use in 2D dosimetry. (8) This ceramic TLD is very easy to use, is low-cost, and has high spatial resolution. To use this ceramic TLD in charged particle therapy as a verification tool, we investigated the TL characteristics of this alumina-based ceramic TLD in response to charged particle beams.

Alumina-based ceramic TLDs
In this study, ceramics from Chiba Ceramic Mfg. Co., Ltd. were used as TLDs. Table 1 shows the composition of this ceramic TLD. Its size was 10 × 10 × 1 mm 3 . The glow curve of this TLD has a main peak at 694 nm due to the embedded Cr 3+ , which is paltry amount contained within the ceramic. (8) Figure 1 shows a sample of the wavelength and temperature property of the ceramic TLD recorded with a multichannel spectroscope (HAMAMATSU PMA-12).

Irradiation beams
Irradiation experiments with several charged particle beams were performed at the BIO course of the Heavy-Ion Medical Accelerator (HIMAC) at the National Institute of Radiological Sciences, Chiba, Japan. The beams were laterally broadened by a wobbling and scattering system to form a uniform field with a 10 cm diameter. (9) Table 2 shows the irradiation beams used in this study. The LET in water was calculated by Geant4. (10) The X-ray irradiation was carried out using a Varian CLINAC-21EX linear accelerator at 6 MV. For each irradiation, four dose levels were given: 0.5, 1.0, 2.0, and 5.0 Gy in water. This range simulates the predicted dose used in the charged particle therapy.

Glow curve measurement
The precise glow curve of the ceramic TLD was measured using a photon-counting unit (HAMAMATSU H11890-110) and a brass-plate-heater-connected programmable heat controller (Sakaguchi E.H VOC CORP. SCR-SHQ-A). Figure 2 shows the glow curves recorded from room temperature to 400 °C at a heating rate of 0.13 °C/s in air.  Figure 3 shows the precise glow curve of this alumina-based ceramic TLD. The peaks were located at 148 °C (main peak) and 285 °C (subpeak). The subpeak was affected by the thermal radiation of the heater. In this study, the main peak, which was not affected by the thermal radiation, was evaluated. The TL intensity was evaluated over a range of 10 °C centered on the main peak. Figure 4 shows the dose response of this ceramic TLD to several irradiations. The TL intensity was normalized using the following equation:

Glow curve and dose linearity
where D is the irradiation dose, L(D) is the TL intensity for an irradiation of D Gy, and nL(D) is the normalized TL intensity. For X-rays, dose linearity and repeatability were comparatively good. However, for charged particles, the TL efficiency nL/D decreased slightly with increasing irradiation dose. This dose response showed a greater tendency with particles of higher atomic number. Table 3 and Fig. 5 show the LET dependence of the TL efficiency nL(D) of the main peak. When the irradiation was lower than 1 Gy, the LET dependence was very small (less than 10%). In the range from 0.1 to 30 keV/mm, the LET dependence was less than 15% with 15 Gy irradiation. The TL efficiency of the peak located at 148 °C showed a small LET dependence as compared with common TLDs. (11) In Ar irradiation, the TL efficiency was >74% at 86 keV/mm. Berger and Hajek reported an overview of the TL efficiency of several common TLDs. (11) According to their report, the TL efficiency of most TLDs in the range of 10-100 keV/mm was less than 50%. For dosimetry in charged particle therapy, the use of solid-state detectors such as TLDs has been generally avoided, owing to the large dependence on LET. The ceramic TLD may be applicable as a verification tool in a limited LET range, such as that of proton therapy, because the measurement system using this ceramic TLD provides 2D dose distribution in a very simple manner. (8)

Conclusions
In this study, the basic characteristics of an alumina-based ceramic TLD (Chiba Ceramic Mfg. Co., Ltd. A8) in response to X-rays and charged particle beams were determined. The glow curve had two peaks, and the main peak located at 148 °C showed good linearity and repeatability in response to X-ray. However, the TL efficiency decreased slightly with increasing irradiation dose with charged particle irradiation. The TL efficiency of the main peak showed a very small dependence on LET compared with common TLDs. This ceramic TLD may be applicable as a verification tool in a limited LET range, such as that of proton therapy.