THE USE OF CdTe DETECTORS FOR DENTAL X-RAY SPECTROMETRY

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1 2007 International Nuclear Atlantic Conference - INAC 2007 Santos, SP, Brazil, September 30 to October 5, 2007 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: THE USE OF CdTe DETECTORS FOR DENTAL X-RAY SPECTROMETRY Marcus Aurélio P. dos Santos, Maria da Conceição F. Fragoso, Mércia L. Oliveira, Ricardo de A. Lima and Clovis A. Hazin Centro Regional de Ciências Nucleares (CRCN / CNEN - PE) Av. Professor Luiz Freire, Recife, PE masantos@cnen.gov.br mariacc05@yahoo.com.br mercial@cnen.gov.br ralima@cnen.gov.br chazin@cnen.gov.br ABSTRACT The cadmium telluride (CdTe) semiconductor detector provides high detection efficiency for use in the diagnostic x-rays energy range, because of the high atomic number and high density of the crystal. Moreover, it has the great advantage of working at room temperature, in contrast to the germanium detector, which operates in liquid nitrogen temperature. The CdTe detector has been utilized in diagnostic x-ray spectroscopy, but only scarce information about its use in dental X-ray beams has been published. In this way, a portable 3x3x1 mm 3 CdTe solid state detector (XR-100T CdTe by Amptek, Inc.) with tungsten pinhole collimators, alignment device and associated software was utilized in this work for measuring the photon spectra in the dental x-ray kvp range. A single-phase dental unit with adjustable kvp and ma was employed and the x-ray spectra were experimentally determined at 50, 60 and 70 kvp with 0.5 ma tube current. The pulse height distribution obtained with this detector, however, does not represent the true photon spectra. For this reason, a stripping procedure was implemented to correct the distribution in order to determine the real photon spectra. The x-ray spectra obtained with the CdTe detector were compared with the ones measured with a high-purity germanium detector (EGP TR by Eurisys Mesures). The reasonable agreement between the results obtained with both detectors for the 50 to 70 kev range show that CdTe detectors can be utilized for dental x-ray spectrometry. 1. INTRODUCTION In the diagnostic x-ray range, high-purity germanium (HPGe) detectors are known worldwide for measuring photon energy spectra, because of their excellent energy resolution and charge transport properties, due to the high purity of the crystal. On the other hand, these detectors require operations at cryogenic temperatures (a dewar is required) due to the small band gap of germanium. Since these spectroscopic systems are quite heavy and hard to move, their application in some experimental arrangements may not be feasible. For this reason, room temperature semiconductors with high atomic number, high density and wide band gap have been developed [1-5]. Cadmium zinc telluride (CdZnTe) and cadmium telluride (CdTe) are two of these solid state detectors which can be utilized for x-ray spectrometry, mainly under clinical conditions. These detectors have been manufactured by Amptek Inc., but since 2002 the production of the CdZnTe has been discontinued in favor of CdTe Schottky diode detector [6]. The CdTe detector has better charge transport properties than CdZnTe and his

2 Schottky contact has reduced the leakage current, improving the resolution. An electronic peltier cooler is utilized to improve the detector response. Due to the small size of their crystals, the main problem concerning the CdTe detector efficiency is the incomplete charge collection and the relatively high probability of K x-rays escape. Because of carrier trapping in the crystals, distortion towards the lower energy region (known as tailing) arises in the x-ray spectra. Spectral distortions can be corrected by means of an unfolding procedure (stripping method). There are several papers published about diagnostic x-ray spectroscopy with CdTe detectors. However, information about their use in dental X-ray beams is scarce. For this reason, the objective of this study was to show the feasibility of using CdTe detectors for dental x-ray spectrometry. The obtained X-ray spectra were corrected with a stripping method based on response functions [4]. The results were compared with the reference spectra obtained by means of an HPGe detector The X-ray Machine 2. MATERIAIS AND METHODS The x-ray generator utilized in this study was a single-phase unit (half wave rectification) developed in the Centro Regional de Ciências Nucleares (CRCN/CNEN). This device can be operated in the 20 to 75 kvp range with tube currents varying from 0.4 to 1 ma. The equipment has a voltage divider (1/10000) which is used for invasive waveform measurements. In order to investigate the behavior of this unit, the x-ray spectra obtained with the CdTe detector, without correction, was compared to the ones (determined with the same detector) generated by three commercial dental x-ray units (Figure 1). Figure 1. CdTe detector spectral measurements made with an adjustable x-ray unit and three commercial dental x-ray units.

3 2.2. The X-ray Spectrometers A CdTe Schottky diode detector (Amptek XR-100T CdTe) coupled an amplifier and bias supply (Amptek PX2T-CdTe) was utilized. The crystal size of this detector is 3x3 mm 2 with 1 mm thickness. The shaping time and the nominal bias voltage were 3 µs and 400 V, respectively. The rise time discrimination (RTD) circuit was switched off. A Multichannel Analyzer (Amptek MCA-8000A) with up to 16k data channel was utilized for pulses analysis. This MCA was operated with 1024 data channel in this study. Spectra data were collected by means of an acquisition software (Amptek ADMCA). A HPGe semiconductor detector for low energy x-rays (Eurisys Mesures EGP TR) was utilized to obtain the reference spectra data. The crystal size of this detector is 2 cm 2 with 13 mm thickness. A preamplifier (Eurisys Mesures PSC741-N-CMS) is built into the detector assembly so that the input field effect transistor (FET) is also cooled by a cryostat (Eurisys Mesures SBF 00-PA4) with 5 L liquid nitrogen capacity. The associated electronics included an amplifier, analogue-digital converter (ADC) and power supply (Eurisys Mesures 7244, 7601 and 7161). The shaping time and the nominal bias voltage were 3 µs and 1,600 V, respectively. A MCA (Eurisys Mesures PCA-RX) with 1024 data channel was utilized for pulse analysis. Spectra data were collected by means of an acquisition software (Eurisys Mesures INTERWINNER) The Experimental Setup and X-ray Spectroscopy These CdTe detectors have a high absorption efficiency for x-rays and, therefore, pulses pileup can become a problem at high counting rates [7]. In order to reduce this effect, the use of low tube currents, long focal spot to detector distances and collimators are required. In this way, the following experimental setup (Figure 2) for both detectors was utilized: (a) 1.8m focus-detector distance; (b) 0.5 ma tube current, and (c) two 2mm thick tungsten pinholes with 0.4 mm and 0.8 mm collimator aperture diameters (Amptek EXVC). In order to reduce the x-ray beam field and, therefore, the scattered x-rays, an additional lead collimator with 15mm aperture diameter was positioned at half distance between the focal spot and the detector. Alignment was carried out by means of a laser beam and radiographic films images. The x-ray spectroscopy was performed with both the CdTe and the HPGe detectors. The energy calibration, linearity checks and resolution of the spectroscopic systems were carried out with rays from the following calibration sources: 241 Am (59.54 kev), 109 Cd (22.1 kev and 88 kev) and 133 Ba (81 kev). The spectra (pulse height distribution) measurements were performed for x-ray beams attenuated by 2.5 mm thick aluminum filters and tube voltages set at 50, 60, and 70 kvp. Output spectra from de CdTe spectroscopic system were produced in channel intervals and each channel (1,024 channels in this study) was converted to energy according to the energy calibration curve. Corrected spectra were obtained from the measured spectra by the stripping methods described by Miyajima [4] and Seelentag and Panzer [8] for the CdTe and HPGe detectors, respectively. Output corrected spectra were produced in 1-keV intervals for the HPGe detector and 0.5-keV intervals for the CdTe detector.

4 Figure 2. Experimental setup for x-ray spectroscopy performed by HPGe and CdTe detectors. 3. RESULTS AND DISCUSSION The spectra measured with the CdTe detector and the reference corrected spectra obtained with the HPGe detector at 50, 60, and 70 kvp tube voltages are shown in Figure 3. In order to compare these spectra, each spectrum was normalized to relative counts from the channel that contains the maximum counts. The differences between the measured CdTe x-ray spectra and the reference corrected HPGe x-ray spectra are obvious. All the measured spectra have distortions towards the lower energy region and show dips around 27 kev and 32 kev. These dips are attributed to the escape of the K-fluorescent x-rays of the Cd and Te, respectively [4]. Due the number of detected photons per channel was low (about 10 3 maximum counts), statistical fluctuations in the spectra are visible. The corrected spectra obtained with the CdTe detector and the reference corrected spectra obtained with the HPGe detector at 50, 60 and 70 kvp tube voltages are shown in Fig. 4. Each spectrum is also normalized to relative counts. The corrected CdTe x-ray spectra have reasonable agreement with the reference HPGe spectra. Due to the distortion caused by carrier trapping, small discrepancies around the maximum energy are observed. The small dips at 27 kev and 32 kev still remain after stripping. This fact indicates that they are caused by the carrier trapping, since the escape k-fluorescence distortions were corrected [4, 9].

5 (a) (a) (b) (b) (c) (c) Figure 3. X-ray spectra measured with the CdTe detector. Tube voltages were: (a) 50 kvp, (b) 60 kvp and (c) 70 kvp. Figure 4. Corrected x-ray spectra obtained with the CdTe detector. Tube voltages were: (a) 50 kvp, (b) 60 kvp and (c) 70 kvp.

6 4. CONCLUSIONS The feasibility of performing dental x-ray spectrometry with a CdTe detector, for the 50 to 70 kvp range, was investigated. Measured x-ray spectra were corrected by means of the stripping method, taking into account the full-energy absorption efficiency, the escape of the Cd and Te K-fluorescent x-rays, and tailing. The results show a reasonably agreement with the reference x-ray spectra obtained with the HPGe detector. The results show that CdTe detectors can be useful for performing spectrometry in the dental x-ray energy range, with appropriate corrections. ACKNOWLEDGMENTS The authors are indebted to Clayton Benevides who shared some ideas for this study and to FINEP/CNPq for the financial support. REFERENCES 1. E. Di Castro, R.Pani, R. Pellegrini and C. Bacci, The use of cadmium telluride detectors for the qualitative analysis of diagnostic X-ray spectra, Phys. Med. Biol., 29, pp (1984). 2. C. Matsumoto, A.Yamamoto, I. Honda, A. Taniguchi and H. Kanamori, Performance of a new schottky CdTe detector for hard X-ray spectroscopy, IEEE Trans. Nucl. Sci., 45, pp (1998). 3. G. Sato, T. Takahashi, M. Sugiho, M. Kouda, T. Mitani, K. Nakazawa, Y. Okada and S. Watanabe, Characterization of CdTe/CdZnTe detectors, IEEE Trans. Nucl. Sci., 49, pp (2002). 4. S. Miyajima, Thin CdTe detector in diagnostic x-ray spectroscopy, Med. Phys., 30, pp (2003). 5. J.S. Iwanczyk, B.E. Patt, Y.J. Wang and A.Kh. Khusainov, Comparison of HgI 2, CdTe and Si (p-i-n) X-ray detectors, Nucl. Instr. and Meth. Phys. Res., A380, pp (1996). 6. Amptek Inc. The use of CdTe diode detectors vs. CZT, (2007) 7. K. Maeda, M. Matsumoto and A. Taniguchi, Compton-scattering measurement of diagnostic x-ray spectrum using high-resolution Schottky CdTe detector, Med. Phys., 32 (6), pp (2005). 8. W. W. Seelentag and W. Panzer, Stripping of x-ray Bremsstrahlung spectra up to 300kVp on a desk type computer, Phys. Med. Biol., 24, pp (1979). 9. S. Miyajima and K. Imagawa, CdZnTe detector in diagnostic x-ray spectroscopy, Me. Phys., 29 (7), pp (2002).

Improvement of the CdTe Diode Detectors using a Guard-ring Electrode

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