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Radiation detector based on piezoelectric lead zirconate titanate material
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 586 (2008) 309–313 www.elsevier.com/locate/nima
Radiation detector based on piezoelectric lead zirconate titanate material Seiji Takechia,, Takashi Miyachib, Masayuki Fujiib, Nobuyuki Hasebeb, Kunishiro Morib, Hiromi Shibatac, Takeshi Murakamid, Yukio Uchihorid, Nagaya Okadae a Graduate School of Engineering, Osaka City University, Osaka 558-8585, Japan Advanced Research Institute for Science and Engineering, Waseda University, Tokyo 169-8555, Japan c Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan d National Institute of Radiological Sciences, Chiba 263-8555, Japan e Honda Electronics Co., Ltd., Toyohashi, Aichi 441-3193, Japan
b
Received 23 August 2007; received in revised form 6 November 2007; accepted 26 November 2007
Abstract The feasibility of using piezoelectric lead zirconate titanate (PZT) elements as a radiation detector was examined. We tried to observe the pressure wave excited by irradiating liquid ethanol (C2H5OH) with a 400 MeV/n xenon (Xe) beam using an array of PZT elements, while changing the position of beam irradiation. The elements were designed to obtain two independent electric signals in order to estimate the beam position in one direction on a two-dimensional plane. The time at which the peak of the output signal appeared differed between the two signals obtained from one element under limited experimental conditions. This suggests that the trajectory of the Xe beam may be determined in three-dimensional space with an improved array of the PZT elements. r 2007 Elsevier B.V. All rights reserved. PACS: 29.40.n; 43.38.+n; 43.58.+z; 77.65.Ly Keywords: Acoustic radiation detector; Piezoelectricity; PZT element; Radiology
1. Introduction Radiation detectors based on acoustic principles have been investigated in a few studies [1–4]. We are interested in a radiation detector that is exclusively fabricated from piezoelectric lead zirconate titanate (PZT) material. PZT has some distinct features: it can easily be shaped arbitrarily and can be operated without a power supply because of its piezoelectricity. In addition, PZT can function not only as a sensor of acoustic signals but also as an absorber of radiation. Therefore, the PZT detector is considered to be a promising candidate as beam monitor for radiotherapy. The characteristics of the PZT detector have been investigated by the following two methods: one is a direct Corresponding author. Tel.: +81 6 6605 2677; fax: +81 6 6690 2745.
E-mail address: [email protected] (S. Takechi). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.11.048
method in which the sensor and absorber are unified in one element, and the other is an indirect method in which the sensor and absorber are separated. In previous work regarding the direct method [5–8], the characteristics of a PZT detector in the form of a disk were studied by directly irradiating the detector with a 400 MeV/n xenon (Xe) beam, while changing the beam pulse duration. It was found that the amplitude of the output signal obtained from the detector was almost independent of the beam pulse duration [7,8]. On the other hand, in the case of the indirect method [9,10], when liquid chloroform (CHCl3) was used as the absorber, a pressure wave was detected by an array composed of eight PZT elements in the form of a cylinder set in CHCl3 as the beam pulse duration was varied. The obtained amplitude explicitly depended on the beam pulse duration [10]. Therefore, these results imply that the mechanisms underlying the production of the acoustic signal by radiation differ between the direct and
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arrangement is indicated as B and each electrode is denoted as R or L in Fig. 1(b). The Xe beam was incident in the z direction as indicated in Fig. 1, and the excited pressure wave in the absorber was measured by monitoring the voltage across the inner and outer electrodes. Therefore, the two signals observed from the element with arrangement A were the output voltage between U and G and that between D and G. Hereafter, the voltage between U and G is referred to as U, and that between D and G is referred to as D. Similarly, for the element with arrangement B, the signals were obtained by monitoring the voltage between R and G and that between L and G. Hereafter, the voltage between R and G is referred to as R, and that between L and G is referred to as L. A frame made of epoxy resin supported each element. The frame was suspended by four springs to prevent noise arising from mechanical disturbances. The detector was composed of four elements with arrangement A and two elements with arrangement B. As shown in Fig. 1(c), these six elements, which were spaced 4 mm apart, were arranged from the side nearest to the front surface of the chamber in order A, A, B, A, B, A. The experimental configuration is schematically shown in Fig. 2. A 400 MeV/n Xe beam was supplied by the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National Institute of Radiological Sciences [11]. The beam was originally extracted for 0.3 s within a period of 3.3 s. In this experiment, the intensity of the extracted beam was 107 pps. A chopper was used to obtain a short pulsed beam. The chopper was a rotating stainless-steel disk with a diameter of 30 cm and a thickness of 10 mm, perforated by four equiangular slits with a length of 10 mm and a width of 1 mm. The disk was installed 73 cm downstream of the exit of the beam duct. By adjusting the rotation speed, the beam intensity could be varied in accordance with the change in the pulse duration from 50 to 200 ms [6]. A 1-mm-thick plastic scintillation counter was placed
indirect methods. In addition, it was also found that the time at which the peak of the signal in the first cycle appeared was dependent on the location at which each element was set in CHCl3 [10]. This result suggests that the position at which the Xe beam stops in CHCl3 may be accurately determined using the PZT detector. In this paper, we report on a test performed with the aim of enhancing the application of the PZT detector as a beam monitor. When liquid ethanol (C2H5OH) was used as the absorber of a 400 MeV/n Xe beam, we tried to observe the excited acoustic wave using a new PZT detector set in C2H5OH. The detector consisted of six PZT elements, where each element supplied two independent signals via two separate electrodes in order to estimate the position of the Xe beam in two-dimensional space (the details are described in Section 2). The results presented were considered to be useful for improving the acousticprinciple-based radiation detector at room temperature. 2. Experimental setup Fig. 1 shows schematic illustrations of the new PZT detector used in this study. The PZT element had a cylindrical shape 38 mm in inner diameter, 40 mm in outer diameter, and 2 mm in length as shown in Fig. 1(a). A silver (Ag) electrode with a thickness of a few mm was coated uniformly onto the outer side of the element. The electrode is denoted as G in Fig. 1(b). To coat the inner side of the element, two Ag electrodes with a thickness of a few mm were used with two different arrangements: In one, the two electrodes of the element were coated onto both the upper and lower sides at intervals of 2 mm; this arrangement is indicated as A and each electrode is denoted as U or D (for up or down) in Fig. 1(b). In the other arrangement, the two separate electrodes were coated onto both the right and left sides of the element at intervals of 2 mm; this
2 mm
x z 38 mm
40 mm
y
Ag Electrode
A
G U 2 mm
Chamber
G
PZT
PZT
x
x
Xe Beam
y
y D
4 mm
B
R
L 2 mm
3 mm 1
2
3
4
5
6
A
A
B
A
B
A
z
y
PZT Element
Fig. 1. Schematic views of detector consisting of six PZT elements. (a) Shape and dimensions of PZT element. (b) Two types of element with different configurations of Ag-coated electrodes. (c) Arrangement of six elements set in the chamber.
ARTICLE IN PRESS S. Takechi et al. / Nuclear Instruments and Methods in Physics Research A 586 (2008) 309–313
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SCALER PC Disc. Oscilloscopes PMT
Amplifiers
Xe Beam 1 2 3 4 5 6
Chopper
Scintillator
6 PZT Elements Chamber
Slit
Disk Fig. 2. Schematic view of experimental arrangement. Here, the photomultiplier is indicated as PMT, the discriminator as Disc., and the personal computer as PC.
36 cm downstream of the chopper. The beam intensity was monitored using the counter. A chamber, in which 200 cc of C2H5OH was enclosed, was placed 18 cm downstream of the scintillation counter. The detector was set 3 mm from the front surface of the chamber (see Fig. 1(c)). The signals obtained from the detector were processed using the respective amplifiers, and then fed to two digital oscilloscopes. The processed signal was averaged over 256 events to suppress the effects of random noise. Digitally processed data were stored in a personal computer. Hereafter, the signal observed from each element is denoted by a combination of the element number shown in Figs. 1(c) and 2, and the abovementioned voltage variable, e.g., 1U, 1D, 3R. 3. Results and discussion To evaluate the performance of the new PZT detector, a pressure wave excited in C2H5OH was observed while changing the beam position on the x–y plane (as indicated in Fig. 1) onto which a 400 MeV/n Xe beam was irradiated. Hereafter, the beam position on the x–y plane is denoted as (x coordinate, y coordinate), which are measured in units of mm. For example, the position of the center axis, that is, the origin, is denoted as (0, 0) and the position 5 mm below the origin is denoted as (5, 0). Fig. 3 shows the typical output signals obtained from the photomultiplier (PMT) and a PZT element when the chopper disk rotated at 50 rps and the Xe beam was incident to (0, 10). The output signals were reconstructed by inverse Fourier transformation to eliminate the noise signal. The output signal obtained from the PMT appeared when the beam passed through the slit. It can be seen from Fig. 3(a) that the pulse duration was 80 ms. Fig. 3(b) shows the output signal obtained from 4U. This waveform exactly followed the response of the PMT. Here, the
Fig. 3. Typical waveforms of output signal observed from (a) PMT and (b) 4U, which are reconstructed by inverse Fourier transformation, when chopper disk rotated at 50 rps and Xe beam was incident to (0, 10).
amplitude of the output signal is denoted by Vp and the time at which the peak appeared is denoted by Tp, as indicated in Fig. 3(b). First, the excited pressure wave was observed when the rotation speed of the disk was 50 rps and the Xe beam was incident to the origin (0, 0). Fig. 4 shows the values of both Vp and Tp obtained from each PZT element. Note that z ¼ 0 mm on the horizontal axis corresponds to the position of the front surface of the chamber, as indicated in Fig. 1(c). Each closed circle plotted in Fig. 4 represents the mean of the two values obtained from the split electrode that each element was equipped with. The horizontal error bars indicate the variation of 2 mm in the length of the elements. The vertical error bars indicate the difference between the two values. It can be deduced from the Bethe–Bloch formula that the range of the 400 MeV/n Xe ions in C2H5OH lies in the region of z ¼ 10–20 mm under these experimental conditions. It can be seen from Fig. 4(a) that the Vp obtained from elements located nearer to the range were larger. It can also be seen from Fig. 4(b) that the tendency of variation for Tp was dependent on whether the element was located within the range: Tp was almost constant regardless of the position of the element when the element was located within the range.
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S. Takechi et al. / Nuclear Instruments and Methods in Physics Research A 586 (2008) 309–313
Fig. 5. Peak time of Tp obtained from each element when chopper disk rotated at 50 rps and Xe beam was incident to (0, 10).
Fig. 4. (a) Wave amplitude of Vp and (b) peak time of Tp obtained from each element when chopper disk rotated at 50 rps and Xe beam was incident to (0, 0).
from one element. However, when the Xe beam was incident to (5, 0) and the chopper disk rotated at 50 rps, a result similar to the one above was not obtained. This may be due to the limited spatiotemporal resolution of the detector, which originates from the beam diameter of 4 mm at the position of the front surface of the chamber, and the sizes of the detector. In future work, the correlation of the capability of the detector with some factors including the ones cited must be elucidated to design a PZT detector with more accuracy. 4. Conclusions
On the other hand, Tp was longer when the element was located farther away from the range. These results were in agreement with that obtained when CHCl3 was used as the absorber of the Xe beam [10]. Moreover, it was confirmed that the difference for Tp was small; that is, the two output signals obtained from one element were observed at almost the same time when the Xe beam was irradiated on the center axis. Next, when the Xe beam was incident to (0, 10) and the chopper disk rotated at 50 rps, the excited pressure wave was observed. It was confirmed that the variation of Vp with the z position reflected the result expected from the Bethe–Bloch formula, similar to that shown in Fig. 4(a). Fig. 5 shows the values of Tp obtained from each element. It can be seen that the difference obtained from the element with arrangement B was larger than that from the element with arrangement A. When the speed of sound in C2H5OH was taken to be v1200 m/s and the distances from the beam position to R and L in the element with arrangement B were regarded as lR9 and lL22 mm, respectively, the difference between each Tp obtained from R and L was almost equal to (lLlR)/v. This result suggests that the beam position on the x–y plane may be determined using the difference between independent values of Tp obtained
To examine the capability of our new PZT detector when a 400 MeV/n Xe beam was irradiated onto C2H5OH, we tried to observe the excited pressure wave using the detector while changing the incident position of the beam on the x–y plane. The detector consisted of an array of two types of PZT element, which were used to measure the beam position on the two-dimensional plane. When the incident position of the beam was 10 mm from the center axis, the time at which the peak of the output signal appeared differed between the two signals obtained from the element that had two electrodes that were separated along the direction in which the beam was shifted. On the other hand, when the incident position of the beam was 5 mm from the center axis, the above result was not obtained. Further investigation is required to enhance the resolution of the acoustic-principle-based PZT detector at room temperature. Acknowledgments This study was carried out as part of the Research Project with Heavy Ions at NIRS-HIMAC. It was supported in part by grants-in-aid from JSPS and by the ‘‘Ground-based Research Program for Utilization’’ of JSF.
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References [1] L. Sulak, T. Armstrong, H. Baranger, M. Bergman, M. Levi, D. Mael, J. Strait, T. Bowen, A.E. Pifer, P.A. Polakos, H. Bradner, A. Parvulescu, W.V. Jones, J. Learned, Nucl. Instr. and Meth. A 161 (1979) 203. [2] J.G. Learned, Phys. Rev. D 19 (1979) 3293. [3] G.A. Askariyan, B.A. Dolgoshein, A.N. Kalinovsky, N.V. Mokhov, Nucl. Instr. and Meth. A 164 (1979) 267. [4] L.G. Dedenko, I.V. Denisov, A.A. Mironovich, I.M. Zheleznykh, in: PACON 99 Proceedings of the Symposium on Humanity and the World Ocean, 1999, p. 535. [5] T. Miyachi, N. Hasebe, H. Ito, T. Masumura, H. Okada, O. Okudaira, N. Yamashita, H. Yoshioka, M. Higuchi, T. Matsuyama, T. Murakami, Y. Uchihori, M. Kase, R. Kikuchi, M. Sato, T. Tou, Jpn. J. Appl. Phys. 42 (2003) 1456. [6] T. Miyachi, Y. Nakamura, G. Kuraza, M. Fujii, A. Nagashima, N. Hasebe, M.N. Kobayashi, S. Kobayashi, M. Miyajima, K. Mori, O. Okudaira, N. Yamashita, H. Shibata, T. Murakami, Y. Uchihori, N. Okada, Nucl. Instr. and Meth. A 568 (2006) 760.
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[7] S. Takechi, T. Onishi, S. Minami, T. Miyachi, M. Fujii, N. Hasebe, K. Mori, H. Shibata, T. Murakami, Y. Uchihori, N. Okada, Jpn. J. Appl. Phys. 46 (2007) 1704. [8] S. Takechi, T. Onishi, S. Minami, T. Miyachi, M. Fujii, N. Hasebe, K. Mori, H. Shibata, T. Murakami, Y. Uchihori, N. Okada, Nucl. Instr. and Meth. A 577 (2007) 741. [9] T. Miyachi, Y. Nakamura, G. Kuraza, M. Fujii, A. Nagashima, N. Hasebe, M.N. Kobayashi, S. Kobayashi, M. Miyajima, O. Okudaira, N. Yamashita, H. Shibata, T. Murakami, Y. Uchihori, N. Okada, T. Tou, Nucl. Instr. and Meth. A 560 (2006) 606. [10] S. Takechi, T. Onishi, S. Minami, T. Miyachi, M. Fujii, N. Hasebe, K. Mori, H. Shibata, T. Murakami, Y. Uchihori, N. Okada, Nucl. Instr. and Meth. A 577 (2007) 729. [11] Y. Hirao, H. Ogawa, S. Yamada, Y. Sato, T. Yamada, K. Sato, A. Itano, M. Kanazawa, K. Noda, K. Kawachi, M. Endo, T. Kanai, T. Kohno, M. Sudou, S. Minohara, A. Kitagawa, F. Soga, E. Takada, S. Watanabe, K. Endo, M. Kumada, S. Matsumoto, Nucl. Phys. A 538 (1992) 541.
Nuclear Instruments and Methods in Physics Research A 586 (2008) 309–313 www.elsevier.com/locate/nima
Radiation detector based on piezoelectric lead zirconate titanate material Seiji Takechia,, Takashi Miyachib, Masayuki Fujiib, Nobuyuki Hasebeb, Kunishiro Morib, Hiromi Shibatac, Takeshi Murakamid, Yukio Uchihorid, Nagaya Okadae a Graduate School of Engineering, Osaka City University, Osaka 558-8585, Japan Advanced Research Institute for Science and Engineering, Waseda University, Tokyo 169-8555, Japan c Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan d National Institute of Radiological Sciences, Chiba 263-8555, Japan e Honda Electronics Co., Ltd., Toyohashi, Aichi 441-3193, Japan
b
Received 23 August 2007; received in revised form 6 November 2007; accepted 26 November 2007
Abstract The feasibility of using piezoelectric lead zirconate titanate (PZT) elements as a radiation detector was examined. We tried to observe the pressure wave excited by irradiating liquid ethanol (C2H5OH) with a 400 MeV/n xenon (Xe) beam using an array of PZT elements, while changing the position of beam irradiation. The elements were designed to obtain two independent electric signals in order to estimate the beam position in one direction on a two-dimensional plane. The time at which the peak of the output signal appeared differed between the two signals obtained from one element under limited experimental conditions. This suggests that the trajectory of the Xe beam may be determined in three-dimensional space with an improved array of the PZT elements. r 2007 Elsevier B.V. All rights reserved. PACS: 29.40.n; 43.38.+n; 43.58.+z; 77.65.Ly Keywords: Acoustic radiation detector; Piezoelectricity; PZT element; Radiology
1. Introduction Radiation detectors based on acoustic principles have been investigated in a few studies [1–4]. We are interested in a radiation detector that is exclusively fabricated from piezoelectric lead zirconate titanate (PZT) material. PZT has some distinct features: it can easily be shaped arbitrarily and can be operated without a power supply because of its piezoelectricity. In addition, PZT can function not only as a sensor of acoustic signals but also as an absorber of radiation. Therefore, the PZT detector is considered to be a promising candidate as beam monitor for radiotherapy. The characteristics of the PZT detector have been investigated by the following two methods: one is a direct Corresponding author. Tel.: +81 6 6605 2677; fax: +81 6 6690 2745.
E-mail address: [email protected] (S. Takechi). 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.11.048
method in which the sensor and absorber are unified in one element, and the other is an indirect method in which the sensor and absorber are separated. In previous work regarding the direct method [5–8], the characteristics of a PZT detector in the form of a disk were studied by directly irradiating the detector with a 400 MeV/n xenon (Xe) beam, while changing the beam pulse duration. It was found that the amplitude of the output signal obtained from the detector was almost independent of the beam pulse duration [7,8]. On the other hand, in the case of the indirect method [9,10], when liquid chloroform (CHCl3) was used as the absorber, a pressure wave was detected by an array composed of eight PZT elements in the form of a cylinder set in CHCl3 as the beam pulse duration was varied. The obtained amplitude explicitly depended on the beam pulse duration [10]. Therefore, these results imply that the mechanisms underlying the production of the acoustic signal by radiation differ between the direct and
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arrangement is indicated as B and each electrode is denoted as R or L in Fig. 1(b). The Xe beam was incident in the z direction as indicated in Fig. 1, and the excited pressure wave in the absorber was measured by monitoring the voltage across the inner and outer electrodes. Therefore, the two signals observed from the element with arrangement A were the output voltage between U and G and that between D and G. Hereafter, the voltage between U and G is referred to as U, and that between D and G is referred to as D. Similarly, for the element with arrangement B, the signals were obtained by monitoring the voltage between R and G and that between L and G. Hereafter, the voltage between R and G is referred to as R, and that between L and G is referred to as L. A frame made of epoxy resin supported each element. The frame was suspended by four springs to prevent noise arising from mechanical disturbances. The detector was composed of four elements with arrangement A and two elements with arrangement B. As shown in Fig. 1(c), these six elements, which were spaced 4 mm apart, were arranged from the side nearest to the front surface of the chamber in order A, A, B, A, B, A. The experimental configuration is schematically shown in Fig. 2. A 400 MeV/n Xe beam was supplied by the Heavy Ion Medical Accelerator in Chiba (HIMAC) at the National Institute of Radiological Sciences [11]. The beam was originally extracted for 0.3 s within a period of 3.3 s. In this experiment, the intensity of the extracted beam was 107 pps. A chopper was used to obtain a short pulsed beam. The chopper was a rotating stainless-steel disk with a diameter of 30 cm and a thickness of 10 mm, perforated by four equiangular slits with a length of 10 mm and a width of 1 mm. The disk was installed 73 cm downstream of the exit of the beam duct. By adjusting the rotation speed, the beam intensity could be varied in accordance with the change in the pulse duration from 50 to 200 ms [6]. A 1-mm-thick plastic scintillation counter was placed
indirect methods. In addition, it was also found that the time at which the peak of the signal in the first cycle appeared was dependent on the location at which each element was set in CHCl3 [10]. This result suggests that the position at which the Xe beam stops in CHCl3 may be accurately determined using the PZT detector. In this paper, we report on a test performed with the aim of enhancing the application of the PZT detector as a beam monitor. When liquid ethanol (C2H5OH) was used as the absorber of a 400 MeV/n Xe beam, we tried to observe the excited acoustic wave using a new PZT detector set in C2H5OH. The detector consisted of six PZT elements, where each element supplied two independent signals via two separate electrodes in order to estimate the position of the Xe beam in two-dimensional space (the details are described in Section 2). The results presented were considered to be useful for improving the acousticprinciple-based radiation detector at room temperature. 2. Experimental setup Fig. 1 shows schematic illustrations of the new PZT detector used in this study. The PZT element had a cylindrical shape 38 mm in inner diameter, 40 mm in outer diameter, and 2 mm in length as shown in Fig. 1(a). A silver (Ag) electrode with a thickness of a few mm was coated uniformly onto the outer side of the element. The electrode is denoted as G in Fig. 1(b). To coat the inner side of the element, two Ag electrodes with a thickness of a few mm were used with two different arrangements: In one, the two electrodes of the element were coated onto both the upper and lower sides at intervals of 2 mm; this arrangement is indicated as A and each electrode is denoted as U or D (for up or down) in Fig. 1(b). In the other arrangement, the two separate electrodes were coated onto both the right and left sides of the element at intervals of 2 mm; this
2 mm
x z 38 mm
40 mm
y
Ag Electrode
A
G U 2 mm
Chamber
G
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PZT
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Xe Beam
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B
R
L 2 mm
3 mm 1
2
3
4
5
6
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A
B
A
B
A
z
y
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Fig. 1. Schematic views of detector consisting of six PZT elements. (a) Shape and dimensions of PZT element. (b) Two types of element with different configurations of Ag-coated electrodes. (c) Arrangement of six elements set in the chamber.
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SCALER PC Disc. Oscilloscopes PMT
Amplifiers
Xe Beam 1 2 3 4 5 6
Chopper
Scintillator
6 PZT Elements Chamber
Slit
Disk Fig. 2. Schematic view of experimental arrangement. Here, the photomultiplier is indicated as PMT, the discriminator as Disc., and the personal computer as PC.
36 cm downstream of the chopper. The beam intensity was monitored using the counter. A chamber, in which 200 cc of C2H5OH was enclosed, was placed 18 cm downstream of the scintillation counter. The detector was set 3 mm from the front surface of the chamber (see Fig. 1(c)). The signals obtained from the detector were processed using the respective amplifiers, and then fed to two digital oscilloscopes. The processed signal was averaged over 256 events to suppress the effects of random noise. Digitally processed data were stored in a personal computer. Hereafter, the signal observed from each element is denoted by a combination of the element number shown in Figs. 1(c) and 2, and the abovementioned voltage variable, e.g., 1U, 1D, 3R. 3. Results and discussion To evaluate the performance of the new PZT detector, a pressure wave excited in C2H5OH was observed while changing the beam position on the x–y plane (as indicated in Fig. 1) onto which a 400 MeV/n Xe beam was irradiated. Hereafter, the beam position on the x–y plane is denoted as (x coordinate, y coordinate), which are measured in units of mm. For example, the position of the center axis, that is, the origin, is denoted as (0, 0) and the position 5 mm below the origin is denoted as (5, 0). Fig. 3 shows the typical output signals obtained from the photomultiplier (PMT) and a PZT element when the chopper disk rotated at 50 rps and the Xe beam was incident to (0, 10). The output signals were reconstructed by inverse Fourier transformation to eliminate the noise signal. The output signal obtained from the PMT appeared when the beam passed through the slit. It can be seen from Fig. 3(a) that the pulse duration was 80 ms. Fig. 3(b) shows the output signal obtained from 4U. This waveform exactly followed the response of the PMT. Here, the
Fig. 3. Typical waveforms of output signal observed from (a) PMT and (b) 4U, which are reconstructed by inverse Fourier transformation, when chopper disk rotated at 50 rps and Xe beam was incident to (0, 10).
amplitude of the output signal is denoted by Vp and the time at which the peak appeared is denoted by Tp, as indicated in Fig. 3(b). First, the excited pressure wave was observed when the rotation speed of the disk was 50 rps and the Xe beam was incident to the origin (0, 0). Fig. 4 shows the values of both Vp and Tp obtained from each PZT element. Note that z ¼ 0 mm on the horizontal axis corresponds to the position of the front surface of the chamber, as indicated in Fig. 1(c). Each closed circle plotted in Fig. 4 represents the mean of the two values obtained from the split electrode that each element was equipped with. The horizontal error bars indicate the variation of 2 mm in the length of the elements. The vertical error bars indicate the difference between the two values. It can be deduced from the Bethe–Bloch formula that the range of the 400 MeV/n Xe ions in C2H5OH lies in the region of z ¼ 10–20 mm under these experimental conditions. It can be seen from Fig. 4(a) that the Vp obtained from elements located nearer to the range were larger. It can also be seen from Fig. 4(b) that the tendency of variation for Tp was dependent on whether the element was located within the range: Tp was almost constant regardless of the position of the element when the element was located within the range.
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Fig. 5. Peak time of Tp obtained from each element when chopper disk rotated at 50 rps and Xe beam was incident to (0, 10).
Fig. 4. (a) Wave amplitude of Vp and (b) peak time of Tp obtained from each element when chopper disk rotated at 50 rps and Xe beam was incident to (0, 0).
from one element. However, when the Xe beam was incident to (5, 0) and the chopper disk rotated at 50 rps, a result similar to the one above was not obtained. This may be due to the limited spatiotemporal resolution of the detector, which originates from the beam diameter of 4 mm at the position of the front surface of the chamber, and the sizes of the detector. In future work, the correlation of the capability of the detector with some factors including the ones cited must be elucidated to design a PZT detector with more accuracy. 4. Conclusions
On the other hand, Tp was longer when the element was located farther away from the range. These results were in agreement with that obtained when CHCl3 was used as the absorber of the Xe beam [10]. Moreover, it was confirmed that the difference for Tp was small; that is, the two output signals obtained from one element were observed at almost the same time when the Xe beam was irradiated on the center axis. Next, when the Xe beam was incident to (0, 10) and the chopper disk rotated at 50 rps, the excited pressure wave was observed. It was confirmed that the variation of Vp with the z position reflected the result expected from the Bethe–Bloch formula, similar to that shown in Fig. 4(a). Fig. 5 shows the values of Tp obtained from each element. It can be seen that the difference obtained from the element with arrangement B was larger than that from the element with arrangement A. When the speed of sound in C2H5OH was taken to be v1200 m/s and the distances from the beam position to R and L in the element with arrangement B were regarded as lR9 and lL22 mm, respectively, the difference between each Tp obtained from R and L was almost equal to (lLlR)/v. This result suggests that the beam position on the x–y plane may be determined using the difference between independent values of Tp obtained
To examine the capability of our new PZT detector when a 400 MeV/n Xe beam was irradiated onto C2H5OH, we tried to observe the excited pressure wave using the detector while changing the incident position of the beam on the x–y plane. The detector consisted of an array of two types of PZT element, which were used to measure the beam position on the two-dimensional plane. When the incident position of the beam was 10 mm from the center axis, the time at which the peak of the output signal appeared differed between the two signals obtained from the element that had two electrodes that were separated along the direction in which the beam was shifted. On the other hand, when the incident position of the beam was 5 mm from the center axis, the above result was not obtained. Further investigation is required to enhance the resolution of the acoustic-principle-based PZT detector at room temperature. Acknowledgments This study was carried out as part of the Research Project with Heavy Ions at NIRS-HIMAC. It was supported in part by grants-in-aid from JSPS and by the ‘‘Ground-based Research Program for Utilization’’ of JSF.
ARTICLE IN PRESS S. Takechi et al. / Nuclear Instruments and Methods in Physics Research A 586 (2008) 309–313
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