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Study on the coloration response of a radiochromic film to MeV cluster ion beams
Nuclear Inst. and Methods in Physics Research, A 872 (2017) 126–130
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Study on the coloration response of a radiochromic film to MeV cluster ion beams Yosuke Yuri *, Kazumasa Narumi, Atsuya Chiba, Yoshimi Hirano, Yuichi Saitoh Takasaki Advanced Radiation Research Institute, National Institutes for Quantum and Radiological Science and Technology, 1233 Watanuki-machi, Takasaki, Gunma, 370-1292, Japan
a r t i c l e
i n f o
Keywords: Cluster ion beam C60 fullerene ion Radiochromic film Beam profile Electrostatic accelerator
a b s t r a c t A radiochromic film, Gafchromic HD-V2, is applied to a possible method of measuring a two-dimensional (2D) spatial profile of MeV cluster ion beams. The coloration responses of the HD-V2 film to MeV carbon and gold cluster ion beams are experimentally investigated since some cluster effect may appear. The degree of the film coloration is quantified as a change in optical density (OD) by reading the films with an image scanner for high-resolution measurement of the 2D beam profile. The OD response of HD-V2 is characterized as a function of the ion and atom fluence for comparison. The dependences of the OD response on the cluster size, kinetic energy, and ion species are discussed. It is found that the sensitivity of the OD change is reduced when the cluster size is large. The beam profile of MeV cluster ion beams delivered from the tandem accelerator in TIARA is characterized from the measurement result using HD-V2 films. The present results show that the use of the Gafchromic HD-V2 film is suitable for the detail beam profile measurement of MeV cluster ions, especially C60 ions, whose available intensity is rather low in comparison with that of monatomic ion beams. © 2017 Elsevier B.V. All rights reserved.
1. Introduction When a high-energy ion beam of atomic clusters like C60 fullerenes is injected into a solid, locally very high energy deposition that cannot be ever realized by monatomic ion irradiation can be achieved within the target material. Different irradiation effects induced by energetic cluster ion beams have been widely studied in various materials (for example, Refs. [1–6]) since the pioneering work that revealed the availability of high-energy C60 cluster ion beams at the tandem accelerator in IPN, Orsay [7,8]. Research and development studies on MeV C60 cluster ion beams are ongoing at the tandem accelerator in Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) [9]. A novel ion-source technique has been recently developed for producing highcurrent (∼100 nA) negative C60 ions [10]. In parallel, the transmission efficiency of intact cluster ions has been optimized with the chargeexchange gas pressure at a high-voltage terminal of the accelerator [11]. High-current (on the order of 100 pA) C60 cluster ion beams are now available in the MeV range routinely. These developments on the production of high-intensity MeV C60 ion beams will lead to further new applications with a large irradiation area and/or a high fluence. It is, therefore, more necessary to measure the spatial profile and irradiation field of MeV cluster ion beams precisely for the evaluation of * Corresponding author.
E-mail address: [email protected] (Y. Yuri). http://dx.doi.org/10.1016/j.nima.2017.07.054 Received 19 June 2017; Received in revised form 24 July 2017; Accepted 27 July 2017 Available online 5 August 2017 0168-9002/© 2017 Elsevier B.V. All rights reserved.
beam quality and irradiation effects of a sample. Among a wide range of choices, we here employ a Gafchromic radiochromic film (Ashland) as a possible tool for the two-dimensional (2D) beam profile measurement. The Gafchromic film is well-known in dosimetry of radiation therapy since it has several advantages as follows; high spatial resolution, low dose, and easy handling [12]. Several types of Gafchromic films are now utilized for the characterization of various ion beams including medical applications (for recent example, Refs. [13–15]). The availability of Gafchromic HD-V2 film to various monatomic ion beams has recently been shown experimentally in a wide kinetic-energy range from 10 MeV/u down to 10 keV/u [16,17]. In this paper, we investigate the coloration response, namely, the change in the optical density (OD) of HD-V2, to MeV carbon and gold cluster ion irradiation for the 2D profile measurement of the cluster ion beams using the films. A flatbed image scanner is employed to quantify the film coloration into OD at a high spatial resolution. Then, the OD change is characterized with respect to the ion and atom fluences. The dependences of the OD response on the cluster size, kinetic energy, and ion species are explored systematically. A ‘‘cluster effect’’ is observed in the OD response when the cluster size 𝑛 , i.e., the number of constituent atoms is large. Finally, the possibility of the 2D profile measurement
Y. Yuri et al.
Nuclear Inst. and Methods in Physics Research, A 872 (2017) 126–130
of a cluster ion beam is demonstrated on the basis of the OD response obtained. 2. Experimental setup 2.1. Gafchromic HD-V2 film Gafchromic films generally turn blue right after radiation exposure. This visible coloration results from the optical absorption at wavelengths of approximately 670 nm and 610 nm due to a sort of radiationinduced polymerization of radiochromic monomers contained in an active layer [12]. The Gafchromic HD-V2 film has an active layer with a nominal thickness and density of 12 μm and 1.2 g/cm3 , respectively, coated on a polyester base of about 100 μm in thickness [18]. A major characteristic of HD-V2 is that it has no surface-protection layer that covers the active layer, unlikely other models of Gafchromic films. This characteristic enables the use of the film in low-energy ion beams that have rather a short range in the active layer. We follow the same procedure as in Ref. [16] to analyze the degree of the film coloration quantitatively. Namely, HD-V2 film pieces were read in the transmission mode with an image scanner ES-10000G (Seiko Epson) and then digitized into 48-bit red–green–blue (RGB) values. A scanning resolution of 508 dpi (50 μm) was chosen as a sufficiently high spatial resolution for the beam-profile evaluation. The OD was obtained from the RGB color values according to the following equation: OD𝑋 = −log10 (𝐼𝑋 ∕65535), where 𝐼𝑋 is the 16-bit value of a color channel 𝑋 (red, green, or blue). In the following, we focus on the net OD behavior in the red color channel, namely, the difference between the ODs of the irradiated and unirradiated films since the OD response is the most sensitive in the red channel among three channels, similarly to previous studies [15–17].
Fig. 1. SRIM calculation result of the stopping powers when 0.15-MeV 12 C and 2.0-MeV Au monatomic ion beams are injected to the active layer of HD-V2. The projected ranges of the C and Au ions are 0.56 μm and 0.68 μm, respectively. 197
2.2. Ion irradiation Film irradiation was performed using a 400-kV ion implanter and a 3-MV tandem accelerator at TIARA, National Institutes for Quantum and Radiological Science and Technology (QST) [9]. To explore the dependence of the cluster size 𝑛 on the OD response, we adopted 12 C𝑛 monatomic and cluster ion beams with different 𝑛 (𝑛 = 1, 3, 4, 8, 42, and 60) at the same velocity of 0.15 MeV/atom (12.5 keV/u), namely, 0.15 MeV C+ , 0.45 MeV C+ , 0.60 MeV C+ , 1.2 MeV C+ , 6.3 MeV C2+ , and 9.0 1 3 4 8 42
Fig. 2. Coloration response of HD-V2 to carbon monatomic and cluster ion beams C𝑛 (𝑛 = 1, 3, 4, 8, 42, and 60) of 0.15 MeV/atom. The net OD in the red channel has been plotted as a function of the ion fluence.
3. Results and discussion
MeV C2+ . In addition, gold cluster ion beams 197 Au+ 𝑛 (𝑛 = 1, 2, and 3) of 60 2.0 MeV/atom (10.2 keV/u) were also employed for comparison of the difference in ion species. Only the 0.15-MeV C1 beam was delivered from the implanter and then raster-scanned to approximately 7 mm × 7 mm area on a target. On the other hand, all other beams, i.e., C cluster and Au ions, were provided by the tandem accelerator. The high-energy beam line followed by the tandem accelerator is deflected 5 degrees with a curvature radius of 6.5 m by a bending magnet, and has a 6.8m-long straight section to the target. The beam was restricted with an aperture of 3 or 5 mm in diameter, located about 50 cm before the target and then the spatial profile of the beam on the target was made approximately uniform using quadrupole magnets. Thus, various fragmented ions generated by the interaction with the charge-exchange gas at the tandem terminal can be well separated on the target. However, fragmented cluster ions with the identical magnetic rigidity to that of an intact object cluster ion, for example, 4.5 MeV C+ (fragment) in 9.0 30 MeV C2+ (object), are inevitably mixed and transported to the target. We 60 have confirmed, by the energy-spectrum measurement using a silicon detector, that the ratio of such mixing was only 3% with respect to the object cluster ions. Therefore, the effect of the mixing on the OD response is sufficiently small in the present study. Fig. 1 shows the stopping powers of the 0.15-MeV C and 2.0-MeV Au monatomic ion beams injected into the HD-V2 active layer, calculated using a software SRIM [19]. Both the beams stop within 1 μm in the active layer. The electronic stopping power is dominant for the C ion while the stopping power of the Au ion is dominated by the nuclear collision and about ten times larger than that of C.
3.1. C cluster ion irradiation The OD responses of HD-V2 to the C monatomic and cluster ions are shown as a function of the ion fluence in Fig. 2. As clearly seen in the Figure, the response curves are similar one another and move toward the lower-fluence side with the increase of the cluster size 𝑛 . The OD increases monotonically with fluence at a low-fluence region in all cases. Then, it starts decreasing, which corresponds to the color change from blue to orange [16]. The maximum OD (∼0.13 in the present case) is approximately the same regardless of the cluster size 𝑛. According to the previous study on monatomic ion irradiation, the maximum OD is well proportional to the projected range of the ion beam when the beam stops in the active layer [16]. Therefore, the present result implies that the projected range of these cluster beams is approximately equal to that of the monatomic ion beam. Note that the maximum OD is much smaller than those of high-energy ion beams and gamma rays because the projected range of these 0.15-MeV/atom beams in the active layer is much shorter than its thickness (12 μm). To estimate the degree of the cluster irradiation effect, the abscissa of Fig. 2 is transformed from the ion fluence to the atom fluence, namely, the ion fluence is multiplied by the cluster size 𝑛 . The result is depicted in Fig. 3. Now we can recognize that the response curves of C1 – C8 overlaps almost with one another and that response curves of C42 and C60 shift toward the higher-fluence side slightly with increasing 𝑛 . To see the difference of these responses quantitatively, the sensitivity of the OD change, which is here defined as a slope of OD in the linearresponse region at a low-fluence region for each ion species, is plotted 127
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Nuclear Inst. and Methods in Physics Research, A 872 (2017) 126–130
Fig. 5. Coloration response of HD-V2 to gold monatomic and cluster ion beams Au𝑛 (𝑛 = 1, 2, and 3) of 2.0 MeV/atom. The net OD in the red channel has been plotted as a function of the atom fluence.
Fig. 3. Coloration response of HD-V2 to carbon monatomic and cluster ion beams C𝑛 (𝑛 = 1, 3, 4, 8, 42, and 60) of 0.15 MeV/atom. The data are the same as those in Fig. 2, except that the abscissa has been converted from the ion fluence to the atom fluence, which corresponds to the ion fluence times the cluster size 𝑛.
irradiation (Fig. 2) is compared with that in 6.0 MeV C60 . We have confirmed that the OD response curves are relatively similar and the OD is always smaller at a given fluence for 6.0 MeV C60 . The ratio of the ODs is about 0.7, which is very close to the ratio of the ranges (0.39 μm∕0.56 μm ≈ 0.70) estimated as monatomic ions. This tendency agrees with the previous study of monatomic ion irradiation [16]. 3.3. Au cluster ion irradiation The results of Au𝑛 ion irradiation are shown in Fig. 5 where the ODs are plotted as a function of the atom fluence. The OD responses of the three beams (2.0-MeV Au, 4.0-MeV Au2 , and 6.0-MeV Au3 ) are almost the same, namely, no explicit cluster-size dependence has been observed for Au cluster ion irradiation up to 𝑛 = 3. This tendency of small 𝑛 is consistent with the case of C in Fig. 3. Note that the sensitivity and maximum OD in Au-ion irradiation are higher as compared to the case of C because of higher stopping power and longer range (estimated for the monatomic ion by SRIM, as shown in Fig. 1).
Fig. 4. Sensitivity of HD-V2 to C cluster ion beams of 0.15 MeV/atom. The slopes of the OD increase at a low-fluence region for each response curve in Fig. 3 are plotted as a function of the cluster size 𝑛.
3.4. Evaluation of the cluster-ion beam profile as a function of 𝑛 in Fig. 4. The dashed line in the Figure is a linear fitting result. The sensitivity obviously shows a decreasing tendency with 𝑛 . The sensitivity of C60 is reduced to about 40% of that of C1 . This experimental result indicates that some cluster irradiation effect occurs in the coloration of HD-V2 film when the cluster size is sufficiently large. This cluster effect, namely, the reduction of the sensitivity, is ascribed probably to the fact that the coloration of the film, or equivalently, the polymerization of the monomers is saturated locally due to the high-density energy deposition of large-𝑛 clusters in a small volume. According to the SRIM calculation, the magnitude of lateral struggling of monatomic C ions injected at 0.15 MeV is only a few hundred nm, which is much smaller than the size (several μm) of HD-V2 monomer microcrystals. The transverse extent of cluster constituent atoms due to multiple scattering should be similar to the above value after the fragmentation in the active layer. In other words, cluster constituent atoms are concentrated on a local part of the microcrystal. Moreover, strong sputtering due to fullerene cluster ions (see, e.g., Ref. [5]) might be another cause of the sensitivity reduction. In Appendix, we briefly report a spectrophotometric result of the film that is consistent with the above measurement results using an image scanner, shown in Fig. 3.
Finally, the spatial profile of a cluster ion beam is determined and its characteristics are discussed. According to the same procedure in previous studies [16,17], a response function that relates OD and fluence 𝐹 (Fig. 2) is determined by least squares fitting; namely, OD = −log10 [(𝑎 + 𝑏𝐹 )∕(𝑎 + 𝐹 )], where 𝑎 and 𝑏 are the fitting coefficients. Then, the OD distribution obtained from an ion-irradiated film was transformed to the fluence distribution. Analysis results of the profile measurement of C60 ion beams are shown in Fig. 6. The uniformity of the convex profile was around 10% in the central region and the beam diameter was 5.2 mm in the full width at half maximum, slightly larger than the diameter (5 mm) of the aperture. Moreover, we have found, from the result of higher fluence (Fig. 6(c)), that there is a tail around the beam core. The tail was produced by scattering at the edge of the aperture. The width of the tail was about 0.5 mm and the relative intensity of the tail was on the order of 0.1% with respect to the beam core. Such detail measurement of a low-intensity tail would be difficult using a screen monitor, which is a common monitoring tool, because the brightness degrades quickly due to the cluster-ion irradiation damage, while the use of the Gafchromic film is not suitable for the real-time measurement. In this way, we could handily evaluate the characteristics of various MeV cluster ion beams available at TIARA, QST. Although the coloration of common polymer films may be utilized for the evaluation of the beam profile, the fluence range in which the coloration of the polymers occurs is much higher than that of HDV2 [16,20].
3.2. Kinetic-energy dependence of C60 cluster ions We here mention the energy dependence of the C60 cluster ion beam on the OD change. The behavior of the OD in 9.0-MeV C60 ion 128
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was between 107 and 1011 ions/cm2 , depending on the cluster size, kinetic energy and ion species. We have observed a cluster effect in a change of the OD when the size 𝑛 of the carbon cluster is large (𝑛 = 42 and 60), while no clear cluster-size dependence has been seen for the Au cluster up to 𝑛 = 3. The sensitivity of the OD change was explicitly decreased for C42 and C60 ion irradiation. This 𝑛-dependence is caused probably by the local saturation of the polymerization event due to the high-density energy deposition along the tracks of the cluster ions. We have demonstrated that HD-V2 films can be applied to the precise measurement of MeV cluster ion beams (even with very low current). This measurement technique using Gafchromic films will be helpful for the beam characterization in cluster-ion applications. Acknowledgments The authors would like to thank the operators of the accelerators in TIARA, QST for fine beam tuning in our experiments. Appendix Discussions in this paper have been focused upon the responses of the OD obtained with an image scanner since the present purpose of use of HD-V2 film is the efficient measurement of the 2D beam profile. In this Appendix, we outline a measurement result of spectrophotometry to see the difference between monatomic and cluster ion irradiations in the visible range. Fig. 7 shows the net absorbance spectra of the films irradiated with 0.15-MeV C1 or 9.0-MeV C60 ions at a fluence of 2 × 1010 atoms/cm2 . These spectra were obtained with an ultraviolet– visible spectrophotometer LAMBDA45 (PerkinElmer). The absorbance for C60 is obviously lower than that for C1 over a wide wavelength range between 420 nm and 770 nm. The absorbance ratio of C60 to C1 is approximately 40% around 670 nm. This observation result is consistent with the red-channel OD response obtained with the image scanner, shown in Figs. 3 and 4. It might be worthy to note that the locations of main two peaks are slightly shifted toward the longer-wavelength side for C60 .
Fig. 6. (a) 2D spatial profile of the C60 cluster ion beam obtained from a response curve of HD-V2 fim. An HD-V2 film piece was irradiated with a 9.0-MeV C60 ion beam of 31 pA for 7 s. (b) 1D horizontal profile along the dashed line in (a). (c) 1D horizontal profile when the beam current and irradiation time were increased to 48 pA and 141 s, respectively. The ordinate is a logarithmic scale to emphasize the tail of the beam spot. The fluence beyond 1 × 1010 ions/cm2 cannot be determined using the response curve because the OD starts decreasing around 1010 ions/cm2 as shown in Fig. 2. The C60 fluence in the core part is estimated to be 1 × 1011 ions/cm2 .
References [1] D.D.N. Barlo Daya, A. Hallén, J. Eriksson, J. Kopniczky, R. Papaléo, C.T. Reimann, P. Håkansson, B.U.R. Sundqvist, A. Brunelle, S. Della-Negra, Y. Le Beyec, Nucl. Instrum. Methods Phys. Res. Sect. B 106 (1995) 38. [2] D. Fink, J. Vacik, R. Klett, L.T. Chadderton, V. Hnatowicz, Nucl. Instrum. Methods Phys. Res. Sect. B 119 (1996) 591. [3] A. Dunlop, G. Jaskierowicz, J. Jensen, S. Della-Negra, Nucl. Instrum. Methods Phys. Res. Sect. B 132 (1997) 93. [4] M. Döbeli, F. Ames, C.R. Musil, L. Scandella, M. Suter, H.A. Synal, Nucl. Instrum. Methods Phys. Res. Sect. B 143 (1998) 503. [5] K. Nakajima, K. Nagano, M. Suzuki, K. Narumi, Y. Saitoh, K. Hirata, K. Kimura, Appl. Phys. Lett. 104 (2014) 114103. [6] T. Kitayama, Y. Morita, K. Nakajima, K. Narumi, Y. Saitoh, M. Matsuda, M. Sataka, M. Tsujimoto, S. Isoda, M. Toulemonde, K. Kimura, Nucl. Instrum. Methods Phys. Res. Sect. B 356–357 (2015) 22. [7] H. Dammak, A. Dunlop, D. Lesueur, A. Brunelle, S. Della-Negra, Y. Le Beyec, Phys. Rev. Lett. 74 (1995) 1135. [8] B. Waast, S. Della-Negra, A. Lafoux, Nucl. Instrum. Methods Phys. Res. Sect. A 382 (1996) 348. [9] Y. Saitoh, S. Tajima, I. Takada, K. Mizuhashi, S. Uno, K. Ohkoshi, Y. Ishii, T. Kamiya, K. Yotsumoto, R. Tanaka, E. Iwamoto, Nucl. Instrum. Methods Phys. Res. Sect. B 89 (1994) 23. [10] A. Chiba, A. Usui, K. Yamada, JAEA Takasaki Annual Report 2013, JAEA-Review, 2014-050, 2015, p. 169. [11] K. Yamada, Y. Saitoh, Y. Ishii, S. Matoba, A. Chiba, A. Yokoyama, A. Usui, T. Satoh, T. Ohkubo, K. Narumi, S. Uno, JAEA Takasaki Annual Report 2013, JAEA-Review 2014-050, 2015, p. 167. [12] A. Niroomand-Rad, C.R. Blackwell, B.M. Coursey, K.P. Gall, J.M. Galvin, W.L. McLaughlin, A.S. Meigooni, R. Nath, J.E. Rodgers, C.G. Soares, Med. Phys. 25 (1998) 2093. [13] S. Reinhardt, M. Hillbrand, J.J. Wilkens, W. Assmann, Med. Phys. 39 (2012) 5257.
Fig. 7. Absorbance spectra of HD-V2 films irradiated with 0.15-MeV C1 monatomic and 9.0-MeV C60 cluster ion beams. The carbon atom fluence is 2 × 1010 atoms/cm2 in both cases.
4. Conclusion The coloration responses of Gafchromic HD-V2 have been experimentally studied using various C𝑛 (𝑛 = 1–60) and Au𝑛 (𝑛 = 1–3) cluster ion beams in the MeV range, i.e., in the 10-keV/u range for 2D profile measurement and evaluation of such cluster ion beams. The ODs were obtained with a flatbed image scanner and the available fluence range 129
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Nuclear Inst. and Methods in Physics Research, A 872 (2017) 126–130 [16] Y. Yuri, K. Narumi, T. Yuyama, Nucl. Instrum. Methods Phys. Res. Sect. A 828 (2016) 15. [17] Y. Yuri, T. Ishizaka, T. Agematsu, T. Yuyama, H. Seito, S. Okumura, Nucl. Instrum. Methods Phys. Res. Sect. B 406 (2017) 221. [18] http://gafchromic.com/. [19] http://www.srim.org/. [20] Z. Zhao, Y. Liu, J. Dai, Y. Chen, Nucl. Instrum. Methods. Phys. Res. Sect. B 262 (2007) 205.
[14] P.R. Bolton, M. Borghesi, C. Brenner, D.C. Carroll, C. De Martinis, F. Fiorini, A. Flacco, V. Floquet, J. Fuchs, P. Gallegos, D. Giove, J.S. Green, S. Green, B. Jones, D. Kirby, P. McKenna, D. Neely, F. Nuesslin, R. Prasad, S. Reinhardt, M. Roth, U. Schramm, G.G. Scott, S. Ter-Avetisyan, M. Tolley, G. Turchetti, J.J. Wilkens, Phys. Med. 30 (2014) 255. [15] M.C. Battaglia, D. Schardt, J.M. Espino, M.I. Gallardo, M.A. Cortés-Giraldo, J.M. Quesada, A.M. Lallena, H. Miras, D. Guirado, Phys. Rev. Accel. Beams 19 (2016) 064701.
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Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Study on the coloration response of a radiochromic film to MeV cluster ion beams Yosuke Yuri *, Kazumasa Narumi, Atsuya Chiba, Yoshimi Hirano, Yuichi Saitoh Takasaki Advanced Radiation Research Institute, National Institutes for Quantum and Radiological Science and Technology, 1233 Watanuki-machi, Takasaki, Gunma, 370-1292, Japan
a r t i c l e
i n f o
Keywords: Cluster ion beam C60 fullerene ion Radiochromic film Beam profile Electrostatic accelerator
a b s t r a c t A radiochromic film, Gafchromic HD-V2, is applied to a possible method of measuring a two-dimensional (2D) spatial profile of MeV cluster ion beams. The coloration responses of the HD-V2 film to MeV carbon and gold cluster ion beams are experimentally investigated since some cluster effect may appear. The degree of the film coloration is quantified as a change in optical density (OD) by reading the films with an image scanner for high-resolution measurement of the 2D beam profile. The OD response of HD-V2 is characterized as a function of the ion and atom fluence for comparison. The dependences of the OD response on the cluster size, kinetic energy, and ion species are discussed. It is found that the sensitivity of the OD change is reduced when the cluster size is large. The beam profile of MeV cluster ion beams delivered from the tandem accelerator in TIARA is characterized from the measurement result using HD-V2 films. The present results show that the use of the Gafchromic HD-V2 film is suitable for the detail beam profile measurement of MeV cluster ions, especially C60 ions, whose available intensity is rather low in comparison with that of monatomic ion beams. © 2017 Elsevier B.V. All rights reserved.
1. Introduction When a high-energy ion beam of atomic clusters like C60 fullerenes is injected into a solid, locally very high energy deposition that cannot be ever realized by monatomic ion irradiation can be achieved within the target material. Different irradiation effects induced by energetic cluster ion beams have been widely studied in various materials (for example, Refs. [1–6]) since the pioneering work that revealed the availability of high-energy C60 cluster ion beams at the tandem accelerator in IPN, Orsay [7,8]. Research and development studies on MeV C60 cluster ion beams are ongoing at the tandem accelerator in Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) [9]. A novel ion-source technique has been recently developed for producing highcurrent (∼100 nA) negative C60 ions [10]. In parallel, the transmission efficiency of intact cluster ions has been optimized with the chargeexchange gas pressure at a high-voltage terminal of the accelerator [11]. High-current (on the order of 100 pA) C60 cluster ion beams are now available in the MeV range routinely. These developments on the production of high-intensity MeV C60 ion beams will lead to further new applications with a large irradiation area and/or a high fluence. It is, therefore, more necessary to measure the spatial profile and irradiation field of MeV cluster ion beams precisely for the evaluation of * Corresponding author.
E-mail address: [email protected] (Y. Yuri). http://dx.doi.org/10.1016/j.nima.2017.07.054 Received 19 June 2017; Received in revised form 24 July 2017; Accepted 27 July 2017 Available online 5 August 2017 0168-9002/© 2017 Elsevier B.V. All rights reserved.
beam quality and irradiation effects of a sample. Among a wide range of choices, we here employ a Gafchromic radiochromic film (Ashland) as a possible tool for the two-dimensional (2D) beam profile measurement. The Gafchromic film is well-known in dosimetry of radiation therapy since it has several advantages as follows; high spatial resolution, low dose, and easy handling [12]. Several types of Gafchromic films are now utilized for the characterization of various ion beams including medical applications (for recent example, Refs. [13–15]). The availability of Gafchromic HD-V2 film to various monatomic ion beams has recently been shown experimentally in a wide kinetic-energy range from 10 MeV/u down to 10 keV/u [16,17]. In this paper, we investigate the coloration response, namely, the change in the optical density (OD) of HD-V2, to MeV carbon and gold cluster ion irradiation for the 2D profile measurement of the cluster ion beams using the films. A flatbed image scanner is employed to quantify the film coloration into OD at a high spatial resolution. Then, the OD change is characterized with respect to the ion and atom fluences. The dependences of the OD response on the cluster size, kinetic energy, and ion species are explored systematically. A ‘‘cluster effect’’ is observed in the OD response when the cluster size 𝑛 , i.e., the number of constituent atoms is large. Finally, the possibility of the 2D profile measurement
Y. Yuri et al.
Nuclear Inst. and Methods in Physics Research, A 872 (2017) 126–130
of a cluster ion beam is demonstrated on the basis of the OD response obtained. 2. Experimental setup 2.1. Gafchromic HD-V2 film Gafchromic films generally turn blue right after radiation exposure. This visible coloration results from the optical absorption at wavelengths of approximately 670 nm and 610 nm due to a sort of radiationinduced polymerization of radiochromic monomers contained in an active layer [12]. The Gafchromic HD-V2 film has an active layer with a nominal thickness and density of 12 μm and 1.2 g/cm3 , respectively, coated on a polyester base of about 100 μm in thickness [18]. A major characteristic of HD-V2 is that it has no surface-protection layer that covers the active layer, unlikely other models of Gafchromic films. This characteristic enables the use of the film in low-energy ion beams that have rather a short range in the active layer. We follow the same procedure as in Ref. [16] to analyze the degree of the film coloration quantitatively. Namely, HD-V2 film pieces were read in the transmission mode with an image scanner ES-10000G (Seiko Epson) and then digitized into 48-bit red–green–blue (RGB) values. A scanning resolution of 508 dpi (50 μm) was chosen as a sufficiently high spatial resolution for the beam-profile evaluation. The OD was obtained from the RGB color values according to the following equation: OD𝑋 = −log10 (𝐼𝑋 ∕65535), where 𝐼𝑋 is the 16-bit value of a color channel 𝑋 (red, green, or blue). In the following, we focus on the net OD behavior in the red color channel, namely, the difference between the ODs of the irradiated and unirradiated films since the OD response is the most sensitive in the red channel among three channels, similarly to previous studies [15–17].
Fig. 1. SRIM calculation result of the stopping powers when 0.15-MeV 12 C and 2.0-MeV Au monatomic ion beams are injected to the active layer of HD-V2. The projected ranges of the C and Au ions are 0.56 μm and 0.68 μm, respectively. 197
2.2. Ion irradiation Film irradiation was performed using a 400-kV ion implanter and a 3-MV tandem accelerator at TIARA, National Institutes for Quantum and Radiological Science and Technology (QST) [9]. To explore the dependence of the cluster size 𝑛 on the OD response, we adopted 12 C𝑛 monatomic and cluster ion beams with different 𝑛 (𝑛 = 1, 3, 4, 8, 42, and 60) at the same velocity of 0.15 MeV/atom (12.5 keV/u), namely, 0.15 MeV C+ , 0.45 MeV C+ , 0.60 MeV C+ , 1.2 MeV C+ , 6.3 MeV C2+ , and 9.0 1 3 4 8 42
Fig. 2. Coloration response of HD-V2 to carbon monatomic and cluster ion beams C𝑛 (𝑛 = 1, 3, 4, 8, 42, and 60) of 0.15 MeV/atom. The net OD in the red channel has been plotted as a function of the ion fluence.
3. Results and discussion
MeV C2+ . In addition, gold cluster ion beams 197 Au+ 𝑛 (𝑛 = 1, 2, and 3) of 60 2.0 MeV/atom (10.2 keV/u) were also employed for comparison of the difference in ion species. Only the 0.15-MeV C1 beam was delivered from the implanter and then raster-scanned to approximately 7 mm × 7 mm area on a target. On the other hand, all other beams, i.e., C cluster and Au ions, were provided by the tandem accelerator. The high-energy beam line followed by the tandem accelerator is deflected 5 degrees with a curvature radius of 6.5 m by a bending magnet, and has a 6.8m-long straight section to the target. The beam was restricted with an aperture of 3 or 5 mm in diameter, located about 50 cm before the target and then the spatial profile of the beam on the target was made approximately uniform using quadrupole magnets. Thus, various fragmented ions generated by the interaction with the charge-exchange gas at the tandem terminal can be well separated on the target. However, fragmented cluster ions with the identical magnetic rigidity to that of an intact object cluster ion, for example, 4.5 MeV C+ (fragment) in 9.0 30 MeV C2+ (object), are inevitably mixed and transported to the target. We 60 have confirmed, by the energy-spectrum measurement using a silicon detector, that the ratio of such mixing was only 3% with respect to the object cluster ions. Therefore, the effect of the mixing on the OD response is sufficiently small in the present study. Fig. 1 shows the stopping powers of the 0.15-MeV C and 2.0-MeV Au monatomic ion beams injected into the HD-V2 active layer, calculated using a software SRIM [19]. Both the beams stop within 1 μm in the active layer. The electronic stopping power is dominant for the C ion while the stopping power of the Au ion is dominated by the nuclear collision and about ten times larger than that of C.
3.1. C cluster ion irradiation The OD responses of HD-V2 to the C monatomic and cluster ions are shown as a function of the ion fluence in Fig. 2. As clearly seen in the Figure, the response curves are similar one another and move toward the lower-fluence side with the increase of the cluster size 𝑛 . The OD increases monotonically with fluence at a low-fluence region in all cases. Then, it starts decreasing, which corresponds to the color change from blue to orange [16]. The maximum OD (∼0.13 in the present case) is approximately the same regardless of the cluster size 𝑛. According to the previous study on monatomic ion irradiation, the maximum OD is well proportional to the projected range of the ion beam when the beam stops in the active layer [16]. Therefore, the present result implies that the projected range of these cluster beams is approximately equal to that of the monatomic ion beam. Note that the maximum OD is much smaller than those of high-energy ion beams and gamma rays because the projected range of these 0.15-MeV/atom beams in the active layer is much shorter than its thickness (12 μm). To estimate the degree of the cluster irradiation effect, the abscissa of Fig. 2 is transformed from the ion fluence to the atom fluence, namely, the ion fluence is multiplied by the cluster size 𝑛 . The result is depicted in Fig. 3. Now we can recognize that the response curves of C1 – C8 overlaps almost with one another and that response curves of C42 and C60 shift toward the higher-fluence side slightly with increasing 𝑛 . To see the difference of these responses quantitatively, the sensitivity of the OD change, which is here defined as a slope of OD in the linearresponse region at a low-fluence region for each ion species, is plotted 127
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Fig. 5. Coloration response of HD-V2 to gold monatomic and cluster ion beams Au𝑛 (𝑛 = 1, 2, and 3) of 2.0 MeV/atom. The net OD in the red channel has been plotted as a function of the atom fluence.
Fig. 3. Coloration response of HD-V2 to carbon monatomic and cluster ion beams C𝑛 (𝑛 = 1, 3, 4, 8, 42, and 60) of 0.15 MeV/atom. The data are the same as those in Fig. 2, except that the abscissa has been converted from the ion fluence to the atom fluence, which corresponds to the ion fluence times the cluster size 𝑛.
irradiation (Fig. 2) is compared with that in 6.0 MeV C60 . We have confirmed that the OD response curves are relatively similar and the OD is always smaller at a given fluence for 6.0 MeV C60 . The ratio of the ODs is about 0.7, which is very close to the ratio of the ranges (0.39 μm∕0.56 μm ≈ 0.70) estimated as monatomic ions. This tendency agrees with the previous study of monatomic ion irradiation [16]. 3.3. Au cluster ion irradiation The results of Au𝑛 ion irradiation are shown in Fig. 5 where the ODs are plotted as a function of the atom fluence. The OD responses of the three beams (2.0-MeV Au, 4.0-MeV Au2 , and 6.0-MeV Au3 ) are almost the same, namely, no explicit cluster-size dependence has been observed for Au cluster ion irradiation up to 𝑛 = 3. This tendency of small 𝑛 is consistent with the case of C in Fig. 3. Note that the sensitivity and maximum OD in Au-ion irradiation are higher as compared to the case of C because of higher stopping power and longer range (estimated for the monatomic ion by SRIM, as shown in Fig. 1).
Fig. 4. Sensitivity of HD-V2 to C cluster ion beams of 0.15 MeV/atom. The slopes of the OD increase at a low-fluence region for each response curve in Fig. 3 are plotted as a function of the cluster size 𝑛.
3.4. Evaluation of the cluster-ion beam profile as a function of 𝑛 in Fig. 4. The dashed line in the Figure is a linear fitting result. The sensitivity obviously shows a decreasing tendency with 𝑛 . The sensitivity of C60 is reduced to about 40% of that of C1 . This experimental result indicates that some cluster irradiation effect occurs in the coloration of HD-V2 film when the cluster size is sufficiently large. This cluster effect, namely, the reduction of the sensitivity, is ascribed probably to the fact that the coloration of the film, or equivalently, the polymerization of the monomers is saturated locally due to the high-density energy deposition of large-𝑛 clusters in a small volume. According to the SRIM calculation, the magnitude of lateral struggling of monatomic C ions injected at 0.15 MeV is only a few hundred nm, which is much smaller than the size (several μm) of HD-V2 monomer microcrystals. The transverse extent of cluster constituent atoms due to multiple scattering should be similar to the above value after the fragmentation in the active layer. In other words, cluster constituent atoms are concentrated on a local part of the microcrystal. Moreover, strong sputtering due to fullerene cluster ions (see, e.g., Ref. [5]) might be another cause of the sensitivity reduction. In Appendix, we briefly report a spectrophotometric result of the film that is consistent with the above measurement results using an image scanner, shown in Fig. 3.
Finally, the spatial profile of a cluster ion beam is determined and its characteristics are discussed. According to the same procedure in previous studies [16,17], a response function that relates OD and fluence 𝐹 (Fig. 2) is determined by least squares fitting; namely, OD = −log10 [(𝑎 + 𝑏𝐹 )∕(𝑎 + 𝐹 )], where 𝑎 and 𝑏 are the fitting coefficients. Then, the OD distribution obtained from an ion-irradiated film was transformed to the fluence distribution. Analysis results of the profile measurement of C60 ion beams are shown in Fig. 6. The uniformity of the convex profile was around 10% in the central region and the beam diameter was 5.2 mm in the full width at half maximum, slightly larger than the diameter (5 mm) of the aperture. Moreover, we have found, from the result of higher fluence (Fig. 6(c)), that there is a tail around the beam core. The tail was produced by scattering at the edge of the aperture. The width of the tail was about 0.5 mm and the relative intensity of the tail was on the order of 0.1% with respect to the beam core. Such detail measurement of a low-intensity tail would be difficult using a screen monitor, which is a common monitoring tool, because the brightness degrades quickly due to the cluster-ion irradiation damage, while the use of the Gafchromic film is not suitable for the real-time measurement. In this way, we could handily evaluate the characteristics of various MeV cluster ion beams available at TIARA, QST. Although the coloration of common polymer films may be utilized for the evaluation of the beam profile, the fluence range in which the coloration of the polymers occurs is much higher than that of HDV2 [16,20].
3.2. Kinetic-energy dependence of C60 cluster ions We here mention the energy dependence of the C60 cluster ion beam on the OD change. The behavior of the OD in 9.0-MeV C60 ion 128
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was between 107 and 1011 ions/cm2 , depending on the cluster size, kinetic energy and ion species. We have observed a cluster effect in a change of the OD when the size 𝑛 of the carbon cluster is large (𝑛 = 42 and 60), while no clear cluster-size dependence has been seen for the Au cluster up to 𝑛 = 3. The sensitivity of the OD change was explicitly decreased for C42 and C60 ion irradiation. This 𝑛-dependence is caused probably by the local saturation of the polymerization event due to the high-density energy deposition along the tracks of the cluster ions. We have demonstrated that HD-V2 films can be applied to the precise measurement of MeV cluster ion beams (even with very low current). This measurement technique using Gafchromic films will be helpful for the beam characterization in cluster-ion applications. Acknowledgments The authors would like to thank the operators of the accelerators in TIARA, QST for fine beam tuning in our experiments. Appendix Discussions in this paper have been focused upon the responses of the OD obtained with an image scanner since the present purpose of use of HD-V2 film is the efficient measurement of the 2D beam profile. In this Appendix, we outline a measurement result of spectrophotometry to see the difference between monatomic and cluster ion irradiations in the visible range. Fig. 7 shows the net absorbance spectra of the films irradiated with 0.15-MeV C1 or 9.0-MeV C60 ions at a fluence of 2 × 1010 atoms/cm2 . These spectra were obtained with an ultraviolet– visible spectrophotometer LAMBDA45 (PerkinElmer). The absorbance for C60 is obviously lower than that for C1 over a wide wavelength range between 420 nm and 770 nm. The absorbance ratio of C60 to C1 is approximately 40% around 670 nm. This observation result is consistent with the red-channel OD response obtained with the image scanner, shown in Figs. 3 and 4. It might be worthy to note that the locations of main two peaks are slightly shifted toward the longer-wavelength side for C60 .
Fig. 6. (a) 2D spatial profile of the C60 cluster ion beam obtained from a response curve of HD-V2 fim. An HD-V2 film piece was irradiated with a 9.0-MeV C60 ion beam of 31 pA for 7 s. (b) 1D horizontal profile along the dashed line in (a). (c) 1D horizontal profile when the beam current and irradiation time were increased to 48 pA and 141 s, respectively. The ordinate is a logarithmic scale to emphasize the tail of the beam spot. The fluence beyond 1 × 1010 ions/cm2 cannot be determined using the response curve because the OD starts decreasing around 1010 ions/cm2 as shown in Fig. 2. The C60 fluence in the core part is estimated to be 1 × 1011 ions/cm2 .
References [1] D.D.N. Barlo Daya, A. Hallén, J. Eriksson, J. Kopniczky, R. Papaléo, C.T. Reimann, P. Håkansson, B.U.R. Sundqvist, A. Brunelle, S. Della-Negra, Y. Le Beyec, Nucl. Instrum. Methods Phys. Res. Sect. B 106 (1995) 38. [2] D. Fink, J. Vacik, R. Klett, L.T. Chadderton, V. Hnatowicz, Nucl. Instrum. Methods Phys. Res. Sect. B 119 (1996) 591. [3] A. Dunlop, G. Jaskierowicz, J. Jensen, S. Della-Negra, Nucl. Instrum. Methods Phys. Res. Sect. B 132 (1997) 93. [4] M. Döbeli, F. Ames, C.R. Musil, L. Scandella, M. Suter, H.A. Synal, Nucl. Instrum. Methods Phys. Res. Sect. B 143 (1998) 503. [5] K. Nakajima, K. Nagano, M. Suzuki, K. Narumi, Y. Saitoh, K. Hirata, K. Kimura, Appl. Phys. Lett. 104 (2014) 114103. [6] T. Kitayama, Y. Morita, K. Nakajima, K. Narumi, Y. Saitoh, M. Matsuda, M. Sataka, M. Tsujimoto, S. Isoda, M. Toulemonde, K. Kimura, Nucl. Instrum. Methods Phys. Res. Sect. B 356–357 (2015) 22. [7] H. Dammak, A. Dunlop, D. Lesueur, A. Brunelle, S. Della-Negra, Y. Le Beyec, Phys. Rev. Lett. 74 (1995) 1135. [8] B. Waast, S. Della-Negra, A. Lafoux, Nucl. Instrum. Methods Phys. Res. Sect. A 382 (1996) 348. [9] Y. Saitoh, S. Tajima, I. Takada, K. Mizuhashi, S. Uno, K. Ohkoshi, Y. Ishii, T. Kamiya, K. Yotsumoto, R. Tanaka, E. Iwamoto, Nucl. Instrum. Methods Phys. Res. Sect. B 89 (1994) 23. [10] A. Chiba, A. Usui, K. Yamada, JAEA Takasaki Annual Report 2013, JAEA-Review, 2014-050, 2015, p. 169. [11] K. Yamada, Y. Saitoh, Y. Ishii, S. Matoba, A. Chiba, A. Yokoyama, A. Usui, T. Satoh, T. Ohkubo, K. Narumi, S. Uno, JAEA Takasaki Annual Report 2013, JAEA-Review 2014-050, 2015, p. 167. [12] A. Niroomand-Rad, C.R. Blackwell, B.M. Coursey, K.P. Gall, J.M. Galvin, W.L. McLaughlin, A.S. Meigooni, R. Nath, J.E. Rodgers, C.G. Soares, Med. Phys. 25 (1998) 2093. [13] S. Reinhardt, M. Hillbrand, J.J. Wilkens, W. Assmann, Med. Phys. 39 (2012) 5257.
Fig. 7. Absorbance spectra of HD-V2 films irradiated with 0.15-MeV C1 monatomic and 9.0-MeV C60 cluster ion beams. The carbon atom fluence is 2 × 1010 atoms/cm2 in both cases.
4. Conclusion The coloration responses of Gafchromic HD-V2 have been experimentally studied using various C𝑛 (𝑛 = 1–60) and Au𝑛 (𝑛 = 1–3) cluster ion beams in the MeV range, i.e., in the 10-keV/u range for 2D profile measurement and evaluation of such cluster ion beams. The ODs were obtained with a flatbed image scanner and the available fluence range 129
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Nuclear Inst. and Methods in Physics Research, A 872 (2017) 126–130 [16] Y. Yuri, K. Narumi, T. Yuyama, Nucl. Instrum. Methods Phys. Res. Sect. A 828 (2016) 15. [17] Y. Yuri, T. Ishizaka, T. Agematsu, T. Yuyama, H. Seito, S. Okumura, Nucl. Instrum. Methods Phys. Res. Sect. B 406 (2017) 221. [18] http://gafchromic.com/. [19] http://www.srim.org/. [20] Z. Zhao, Y. Liu, J. Dai, Y. Chen, Nucl. Instrum. Methods. Phys. Res. Sect. B 262 (2007) 205.
[14] P.R. Bolton, M. Borghesi, C. Brenner, D.C. Carroll, C. De Martinis, F. Fiorini, A. Flacco, V. Floquet, J. Fuchs, P. Gallegos, D. Giove, J.S. Green, S. Green, B. Jones, D. Kirby, P. McKenna, D. Neely, F. Nuesslin, R. Prasad, S. Reinhardt, M. Roth, U. Schramm, G.G. Scott, S. Ter-Avetisyan, M. Tolley, G. Turchetti, J.J. Wilkens, Phys. Med. 30 (2014) 255. [15] M.C. Battaglia, D. Schardt, J.M. Espino, M.I. Gallardo, M.A. Cortés-Giraldo, J.M. Quesada, A.M. Lallena, H. Miras, D. Guirado, Phys. Rev. Accel. Beams 19 (2016) 064701.
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