- Email: [email protected]
Materials research with swift heavy ions at the IMP accelerators
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 245 (2006) 189–193 www.elsevier.com/locate/nimb
Materials research with swift heavy ions at the IMP accelerators Z.G. Wang
q
*
Institute of Modern Physics, Chinese Academy of Sciences, No. 509 Nanchang Road, Lanzhou 730000, PR China Available online 27 December 2005
Abstract The main ion beams acceleration facilities and research fields of the Institute of Modern Physics (IMP) are briefly introduced. Some of the experimental instruments, typical works and the obtained results on the materials research with swift heavy ions at the IMP accelerators are presented. 2005 Elsevier B.V. All rights reserved. PACS: 61.72.y; 61.80.Jh; 61.82.d Keywords: Swift heavy ions; Accelerators; Materials; Damage production; Phase transition
1. Introduction The Institute of Modern Physics (IMP) affiliated with the Chinese Academy of Sciences (CAS) was founded in June 1957 in Lanzhou, China. It is now an institute making important contributions to basic research as well as to applications of heavy ion physics and nuclear technologies. The National Laboratory of Heavy Ion Accelerator, Lanzhou (NLHIAL) was established at IMP in 1991. IMP has a series of research facilities and its main research fields [1] are as the follows: (1) Heavy ion nuclear physics, including experimental and theoretical nuclear physics and nuclear chemistry, especially focusing on the study of the radioactive ion beam physics and exploration of the existing limits of nuclei; (2) Atomic physics and materials research with highly charged heavy ions, molecular as well as hot and dense plasmas;
q Supported by NSFC (Grant Nos. 10125522, 10475102) and Chinese Academy of Sciences. * Tel.: +86 931 496 9331; fax: +86 931 827 2100. E-mail address: [email protected]
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.11.099
(3) Biological effects by heavy ion irradiation and tumor therapy with ion beams; (4) Ion accelerator physics and technology as well as high-power electron accelerator. In the present work, the main attention was paid on the materials research with swift heavy ions at the IMP accelerators. First, the main ion beams acceleration facilities and experimental instruments for materials research are described, then the main research topics and some typical results on materials research with swift heavy ions at the IMP accelerators are reported. 2. Main facilities for ion beams acceleration and materials research IMP & NLHIAL had a series of ion beams acceleration facilities intensively used in the past two decades such as the Heavy Ion Research Facility (HIRFL), the Radioactive Ion Beam Line (RIBLL), a 2 · 2 MV tandem accelerator, a 600 kV Cockroft-Walton accelerator, a 200 kV heavy ion implanter as well as an ECR ion source station with the platform for experiments with highly charged ions. The HIRFL operates an ECR ion source, an injection cyclotron SFC (K = 69) and a main cyclotron SSC (K = 450). All these facilities have accelerated ions from H to Pb of energy ranging
190
Z.G. Wang / Nucl. Instr. and Meth. in Phys. Res. B 245 (2006) 189–193
from keV up to 100 MeV/u. Additionally there is also a 1.2 MeV/40 mA electron accelerator being used for materials modification studies and a 320 kV high-voltage ion accelerator which will start to run in the middle of this year. A new national project connected to the HIRFL approved by national scientific and technological authorities is the electron cooling storage ring (CSR) which is now under construction. The CSR consists of the main ring CSRm and the experimental ring CSRe with a circumference of 161.20 and 120.90 m, respectively. Typical ion parameters of the CSR are 2800 MeV H+, 900– 1100 MeV/u 12C6+ and 400–520 MeV/u 238U72+ ions. At present, the subassemblies of the two rings are installed on line and are under commissioning. First experiments are expected for next year. The IMP has various experimental setups for materials researches in combination with ion irradiation/implantation lines [1,2]. For examples, (i) a movable sample holder connected to a continuous-flow cryostat (CFC) allows irradiation experiments at low temperature (liquid He temperature region), and another movable sample holder provide laser control of sample position measurements; (ii) the irradiation setup used for the simulation of cosmic ray effects especially for single event effects in IC chips offers low ion flux detection, beam energy and homogeneity measurements, and controllable sample selection; (iii) high-temperature irradiation up to about 700 C and irradiations in air can be performed. Furthermore, the IMP has a series of apparatus for sample preparations and treatments, and several instruments for sample analyses. 3. Main topics and typical results of materials research with swift heavy ions The materials research carried out at the IMP accelerators mainly aim at a well understanding of the basic interaction processes of swift heavy ions with materials. Research projects include topics with potential uses in the fields of energy resource, electronics, material science, etc. The main topics of materials research are swift heavy ion-induced defect production and their evolution in pure metals and semiconductors, amorphization and ion track formation, chemical and physical modifications of polymers, phase transformation and recrystallization in materials, as well as the simulation of radiation effects in reactor materials and cosmic ray induced single event effects in large scale integrate circuits. Some typical experimental results on materials research with the swift heavy ions at the IMP accelerators are given in the following. 3.1. Defect production and evolution in Si Silicon is a typical semiconductor being used widely in the world. Damage production in Si induced by energetic ions is a hot topic over many years. We have studied defect production in single crystalline Si using Ar ions of several hundred MeV [3,4].
For single crystalline silicon irradiated with 112 MeV Ar ions at below 50 K [5], it was found that (i) two types of paramagnetic centers, Si–P3 and amorphous-centers, coexist in as-irradiated samples. The former is mainly due to electronic energy loss, the latter can be transformed from isolated amorphous regions to continued amorphous layer with increasing of ion fluence; (ii) divacancies are formed and the concentration increases with ion fluence and saturates at high fluence; (iii) the annealing behaviors of ion irradiation induced defects are fluence dependent, for example, isolated amorphous regions anneal at temperatures around 500 C, whereas the recrystallization temperature for the continued amorphous layer is higher than 600 C. For single crystalline silicon irradiated with 750 MeV argon ions at RT [4], it was found that divacancies of neutral charge state are the main vacancy clusters induced by the irradiations. No amorphous phase was detected up to the highest irradiation fluence. The divacancy concentration in the nuclear process dominated region increased dramatically with increasing irradiation fluence, whereas little change appeared in regions dominated by electronic processes. It is argued that the energy deposition by electronic processes can only induce limited damage in silicon and high electronic stopping power suppresses the formation of amorphous zones. These results are in agreement with that obtained by other research groups [5,6]. 3.2. Amorphization and ion track formation Intense electronic excitations play an important role in ion-induced damage processes such as amorphization and track formation in crystalline materials under swift ion irradiations. In yttrium iron garnet (YIG) a series irradiation effects induced by swift heavy ions have been extensively studied [7,8]. Our works were mainly aimed at the damage process. In our works [9,10], the transformation process from crystalline to amorphous state was shown to be a gradual process in which the electronic excitations play a dominant role. It was found that there is a critical fluence of 1.0 · 1014 ions/cm2 linked to a threshold value of electronic energy loss Set = 2.9 keV/nm for the appearance of phase transformation from crystalline to amorphous state. Above Set the lattice constant, the optical absorption coefficient, and the fraction of the paramagnetic phase Fp increase with increasing electronic energy loss, but saturation magnetization decreases. A nearly complete amorphous state was observed at Se = 8.3 keV/nm for an irradiation of 1.0 · 1014 ions/cm2 or above. For fullerite (C60) films under swift ion irradiations, phase transition from the crystalline to amorphous state was observed [11–13]. This amorphization process is mainly dominated by electronic excitations. Swift light ions result in an intermediate graphitization process before amorphization, i.e. the damage process is crystalline C60 ! intermediate graphitization process ! amorphous carbon. For swift heavy ions, the damage created in C60
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films could be partially recovered at certain Se and the damage cross-sections increased with increasing Se. The destructive action of the strong electronic excitations could exceed the annealing effect so that no intermediate graphitization process existed. Furthermore, the icosahedral symmetry of C60 molecules was reduced or destroyed by annealing effect via electronic excitations, and polymerized phases was formed that resulted in the final transformation of the molecules on the C60 lattice from crystalline into amorphous carbon. Highly oriented pyrolitic graphite (HOPG) irradiated with 6.5, 14 and 111.5 MeV Ar ions at normal incidence were investigated using STM for characterization of single ion impacts on surfaces [14]. Hillock defects were found on the surface. Each hillock is identified as the physical signature of the impact of one individual Ar ion on the HOPG surface, i.e. one track tail of one Ar ion. The size (diameter) of such tracks increased with increasing electronic energy loss. For the comparison of ion track formation at HOPG surface, experiments with some other swift heavy ions (Ne, Cr, Fe, Ni, Zn, Xe and U) in energy range MeV–GeV have been done [15]. Similarly, under Ar ion irradiation, when a large number of tracks protrudes from the HOPG surface, the ordered structure of the graphite lattice is destroyed.
imide is more sensitive to irradiation than para-substituted phenyl in the main chain. The measured enhanced electrical conductivity substantiated by the optical band gap shifts was attributed to the formation of condensed polycyclic aromatic ring compounds and clusters with greater degree of conjugation in the multiple overlapping ion tracks in the PI matrix.
3.3. Chemical and physical modifications of polymers
Selected phase transformation and recrystallization along or in the vicinity of the ion path are one of the most attractive phenomena in some solids induced by intense electronic excitations. The combination of ‘‘low energy ion implantation + swift heavy ion irradiations’’ was proposed for searching selected phase transformation and new crystalline phase formations [33,34]. It was used in atomic mixing materials such as C-doped SiO2 and Ndoped diamond or graphite-like carbon [33–42] for the synthesis of novel light-emitting materials and superhard carbon-nitrides. For the C-doped SiO2 [33–38], it was found that significant chemical bonds such as sp3C- and sp2C bonds and Si-related micro-structures were formed in the samples after swift heavy ion irradiations. The sp3C bonds increased when increasing the irradiation fluence and CO2 molecules were formed in the high-dose (>5 · 1017 ions/cm2) C-doped SiO2 films after large-fluence (>1 · 1012 ions/cm2) Pb–ion irradiations. Furthermore, PL results showed that two different mechanisms may exist for ion-induced luminescence centers in ion irradiated C:SiO2 samples: (1) point defects or bond scissions being a dominant in the Se < 4.85 keV/nm region; (2) formation of silicon carbide inclusions induced by ions with Se > 4.85 keV/nm. All these results imply that nano-sized Si clusters and/or SiC grains may form in the C-doped SiO2 films after swift heavy ion irradiations. Recently, nano-sized recrystallization in the vicinity of ion paths in C-doped region was observed [39]. In N-doped diamond films, it was found that specific chemical bonds such as N–sp2C and N–sp3C are formed [40,41]. Intense energy deposition from the incident swift
As being done by other research groups [16], we also have carried out experimental studies on swift heavy ion induced amorphization and chemical modification in polymer films such in polycarbonate (PC) [17–21], polyethylene terephthalate (PET) [21–24], polystyrene (PS) [21,25,26] and polyimide (PI) [27–29]. PC, PET and PS under swift heavy ions irradiations suffer from serious degradation but the radiation sensitivity varies for the different functional groups [17–26]. The phenyl ring, the most stable functional group inside polymers, is destroyed at Se > 8 keV/nm. The crystallinity of PET is reduced by a process of radiation promoted transformation of the ethylene glycol residue from the trans to the gauche configuration at absorbed doses higher than 4.0 MGy. Alkyne end groups can be produced above certain electronic energy loss threshold that varies with material. The threshold is about 0.8 keV/nm for PS and 0.4 keV/nm for PC. These polymers could be transformed gradually into hydrogenated amorphous carbon at high-energy deposition. A sigmoid relationship of the damage cross-section of the functional groups, the production cross-section of the alkyne end group and the number of chromophores induced per unit track length with electronic energy loss is found in the used electronic energy loss regime. For PI [27–29], the main chains of PI as well as the phenyl ring and imide are destroyed simultaneously and triple bonds (C„C and C„N) are formed. The formation radius of alkyne is lower than the chain disruption radii. The cyclic imide group has more radiation resistance than other functional groups. The tetra-substituted phenyl in cyclic
3.4. Simulation of radiation effects in reactor materials Radiation damage of materials is one of the major technological problems in design of fusion reactors. Swift heavy ion irradiation is a useful tool for the simulation of damage processes of structure materials in reactors. However, the relationship between swift heavy ions and reactor neutron irradiations is unknown. In our works, we found that 54 MeV C-ion irradiations could induce a c(fcc) ! a(bcc)-phase transformation in 316 L stainless steel [30], and the C-ion-induced electronic excitations enhanced the damage production such as serious pitting, flaking and crazing along grain boundaries of the irradiated surface [31,32]. 3.5. Phase transformation and recrystallization
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heavy ions induces the increase of sp3/sp2 bonding ratio and thus enhances the formation of N–sp3C bonds in the samples. Furthermore, X-ray diffraction analysis indicates the formation of new phases such as a- and b-C3N4. The larger the N content, or the larger the energy deposition, the more a- and b-C3N4 phases are formed. As a comparison, modification of N-doped graphite-like carbon samples induced by 30-MeV C60 ion irradiations (performed at INPO) has also been studied [42]. The results showed that C60 ion irradiation can induce C–N bond formation in three phases, namely, C3N4, CNx and tetrahedral carbon. 3.6. Simulation of single event effects Cosmic ray induced single event effects (SEE) on large scale integrate (LSI) circuits have become an important topic being investigated extensively by the scientists in spaceborne electronics. Swift heavy ion beams from accelerators are a useful tool to simulate the SEE simulation of electronic devices at ground. In IMP, an experimental equipment for SEE test at ground was installed at HIRFL and a series of on-line experiments [43–46] have been done using swift heavy ions provided by the IMP accelerators for studying the SEE such as single event upset (SEU), single event latch-up (SEL), single event burning (SEB), multiple-bit upset (MBU), total dose effect, and so on. The method to SEE characterization for various semiconductors under swift ion exposure was set up. Acknowledgement This work is attributed to all colleagues of the materials research group of IMP and supported by NSFC and CAS. I am very grateful to all of them especially to Profs. Y.F. Jin, Z.Y. Zhu, J. Liu, M.D. Hou, Drs. Y.M. Sun, C.H. Zhang and C.L. Liu. References [1] See IMP & NLHIAL Annual Report, 1993–2003, Atomic Energy Press, Beijing. [2] M.D. Hou, C.L. Li, Z.G. Wang, F. Ma, Y.M. Sun, J. Liu, J.M. Quan, Radiat. Eff. Def. Sol. 126 (1993) 355. [3] C.L. Liu, M.D. Hou, Z.Y. Zhu, Z.G. Wang, S. Cheng, Y.F. Jin, Y.M. Sun, C.L. Li, Nucl. Instr. and Meth. B 135 (1998) 219. [4] Z.Y. Zhu, M.D. Hou, Y.F. Jin, C.L. Liu, Y.S. Wang, J. Han, Nucl. Instr. and Meth. B 135 (1998) 260. [5] B. Canut, N. Bonardi, S.M.M. Ramos, S. Della-Negra, Nucl. Instr. and Meth. B 146 (1998) 296, and the references therein. [6] A. Dunlop, G. Jaskierowicz, S. Della-Negra, Nucl. Instr. and Meth. B 146 (1998) 302, and the references therein. [7] A. Meftah, F. Brisard, J.M. Costantini, M. Hage-Ali, J.P. Stoquert, F. Studer, M. Toulemonde, Phys. Rev. B 48 (1993) 920, and the references therein. [8] A. Dunlop, G. Jaskierowicz, J. Jensen, S. Della-Negra, Nucl. Instr. and Meth. B 132 (1997) 93, and the references therein. [9] Y. Jin, J. Han, Q. Meng, Y. Sun, C. Liu, Y. Ru, Y. Wang, C. Zhang, C. Li, M. Hou, Nucl. Instr. and Meth. B 135 (1998) 190. [10] Y.F. Jin, R.H. Xu, J.M. Quan, Z.G. Wang, Q.H. Meng, Y.M. Sun, F. Ma, J. Han, G. Liu, J. Liu, C.L. Li, Nucl. Instr. and Meth. B 107 (1996) 227.
[11] Y.F. Jin, R. Yang, Y.B. Wang, C.L. Liu, J. Liu, M.D. Hou, J.H. Yao, Nucl. Instr. and Meth. B 164–165 (2000) 391. [12] Y.F. Jin, H.X. Tian, E.Q. Xie, J. Liu, Z.G. Wang, Y. Sun, Z.Y. Zhu, Nucl. Instr. and Meth. B 193 (2002) 288. [13] Y.F. Jin, C.F. Yao, Z.G. Wang, E.Q. Xie, Y. Song, Y.M. Sun, C.H. Zhang, J. Liu, J.L. Duan, Nucl. Instr. and Meth. B 230 (2005) 565. [14] J. Liu, M.D. Hou, C.L. Liu, Z.G. Wang, Y.F. Jin, P.J. Zhai, S.L. Feng, Y. Zhang, Nucl. Instr. and Meth. B 146 (1998) 356. [15] J. Liu, M.D. Hou, C. Trautmann, et al., Nucl. Instr. and Meth. B 212 (2003) 303. [16] See SHIM proceedings published as special issues of Nucl. Instr. and Meth. B. For examples: F. Dehaye, E. Balanzat, E. Ferain, R. Legras, Nucl. Instr. and Meth. B 209 (2003) 103; M. Me´lot, Y. Ngono-Ravache, E. Balanzat, Nucl. Instr. and Meth. B 209 (2003) 205, and the references therein. [17] Y.B. Wang, Y.F. Jin, Z.Y. Zhu, et al., Nucl. Instr. and Meth. B 164– 165 (2000) 420. [18] Y.M. Sun, Z.Y. Zhu, Z.G. Wang, J. Liu, Y.F. Jin, M.D. Hou, Y. Wang, J.L. Duan, Nucl. Instr. and Meth. B 212 (2003) 211. [19] Y.M. Sun, Z.Y. Zhu, Z.G. Wang, Y.F. Jin, J. Liu, M.D. Hou, Q.X. Zhang, Nucl. Instr. and Meth. 209 (2003) 188. [20] H.X. Tian, Y.F. Jin, Z.Y. Zhu, C.L. Liu, Y.M. Sun, Z.G. Wang, J. Liu, X.X. Chen, Y.B. Wang, M.D. Hou, Nucl. Sci. Tech. B 15 (2003) 12. [21] Z.Y. Zhu, Y.M. Sun, C.L. Liu, J. Liu, Y.F. Jin, Nucl. Instr. and Meth. B 193 (2002) 271. [22] C.L. Liu, Z.Y. Zhu, Y.F. Jin, et al., Nucl. Instr. and Meth. B 166–167 (2000) 641. [23] C.L. Liu, Y.F. Jin, Z.Y. Zhu, et al., Nucl. Instr. and Meth. B 169 (2000) 72. [24] C.L. Liu, Y.F. Jin, Z.Y. Zhu, et al., Nucl. Instr. and Meth. B 169 (2000) 78. [25] Z.Y. Zhu, Y.F. Jin, C.L. Liu, et al., Nucl. Instr. and Meth. B 166–167 (2000) 621. [26] Z.Y. Zhu, Y.F. Jin, C.L. Liu, et al., Nucl. Instr. and Meth. B 169 (2000) 83. [27] C.L. Li, Y.M. Sun, Y.F. Jin, G. Liu, M.D. Hou, F. Ma, A. Patnaik, Nucl. Instr. and Meth. B 135 (1998) 234. [28] Y.M. Sun, Z.Y. Zhu, Y.F. Jin, C.L. Liu, Z.G. Wang, J. Liu, M.D. Hou, Q.X. Zhang, Nucl. Instr. and Meth. B 193 (2002) 214. [29] Y.M. Sun, C.H. Zhang, Z.Y. Zhu, Z.G. Wang, Y.F. Jin, J. Liu, Y. Wang, Nucl. Instr. and Meth. B 218 (2004) 318. [30] X.X. Chen, C.L. Li, Y.F. Jin, et al., Hyperfine Interact. 70 (1992) 931. [31] Z.G. Wang, K.Q. Chen, L.W. Li, et al., J. Nucl. Mater. 271–272 (1999) 306. [32] Z.G. Wang, K.Q. Chen, M.D. Hou, Y.F. Jin, Y.M. Sun, in: Z.G. Wang, Z.Y. Zhu, G.M. Jin (Eds.), Proceedings of 7th China–Japan Symposium on Materials for Advanced Energy Systems & Fission and Fusion Engineering, Word Scientific, Singapore, 2003, p. 285. [33] Z.G. Wang, Y.F. Jin, E.Q. Xie, et al., Nucl. Phys. Rev. 17 (2000) L189. [34] Z.G. Wang, E.Q. Xie, Y.F. Jin, X.X. Chen, C.L. Liu, Z.Y. Zhu, M.D. Hou, Y.M. Sun, J. Liu, Y.B. Wang, Nucl. Instr. and Meth. B 179 (2001) L289. [35] Z.G. Wang, J. Liu, Z.Y. Zhu, Y.F. Jin, E.Q. Xie, T.W. Zhu, X.X. Chen, Y.M. Sun, M.D. Hou, Nucl. Instr. and Meth. B 191 (2002) 396. [36] Z.G. Wang, Y.F. Jin, E.Q. Xie, Z.Y. Zhu, M.D. Hou, X.X. Chen, Y.M. Sun, Q.X. Zhang, Nucl. Instr. and Meth. B 193 (2002) 685. [37] Z.G. Wang, Y.F. Jin, E.Q. Xie, J. Liu, Z.Y. Zhu, Y.M. Sun, M.D. Hou, Q.X. Zhang, X.X. Chen, Nucl. Instr. and Meth. B 209 (2003) 200. [38] Z.M. Zhao, Z.G. Wang, Y. Song, et al., B-124 of this symposium. [39] Z.G. Wang, Z.M. Zhao, Y. Song, et al., IMP & HIRFL 2003 Annual Report, 2004, 56p. [40] E.Q. Xie, Y.F. Jin, Z.G. Wang, D.Y. He, Nucl. Instr. and Meth. B 135 (1998) 224.
Z.G. Wang / Nucl. Instr. and Meth. in Phys. Res. B 245 (2006) 189–193 [41] Z.G. Wang, Z.M. Zhao, Y. Song, J. Liu, Y.M. Sun, C.H. Zhang, J.L. Duan, Y.F. Jin, Nucl. Instr. and Meth. B 218 (2004) 472. [42] Z.G. Wang, A. Dunlop, Z.M. Zhao, Y. Song, Y.F. Jin, C.H. Zhang, Y.M. Sun, J. Liu, Nucl. Instr. and Meth. B 230 (2005) 262. [43] Q.X. Zhang, M.D. Hou, J. Liu, Z.G. Wang, Y.F. Jin, Z.Y. Zhu, Y.M. Sun, Nucl. Instr. and Meth. B 209 (2003) 367.
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Nuclear Instruments and Methods in Physics Research B 245 (2006) 189–193 www.elsevier.com/locate/nimb
Materials research with swift heavy ions at the IMP accelerators Z.G. Wang
q
*
Institute of Modern Physics, Chinese Academy of Sciences, No. 509 Nanchang Road, Lanzhou 730000, PR China Available online 27 December 2005
Abstract The main ion beams acceleration facilities and research fields of the Institute of Modern Physics (IMP) are briefly introduced. Some of the experimental instruments, typical works and the obtained results on the materials research with swift heavy ions at the IMP accelerators are presented. 2005 Elsevier B.V. All rights reserved. PACS: 61.72.y; 61.80.Jh; 61.82.d Keywords: Swift heavy ions; Accelerators; Materials; Damage production; Phase transition
1. Introduction The Institute of Modern Physics (IMP) affiliated with the Chinese Academy of Sciences (CAS) was founded in June 1957 in Lanzhou, China. It is now an institute making important contributions to basic research as well as to applications of heavy ion physics and nuclear technologies. The National Laboratory of Heavy Ion Accelerator, Lanzhou (NLHIAL) was established at IMP in 1991. IMP has a series of research facilities and its main research fields [1] are as the follows: (1) Heavy ion nuclear physics, including experimental and theoretical nuclear physics and nuclear chemistry, especially focusing on the study of the radioactive ion beam physics and exploration of the existing limits of nuclei; (2) Atomic physics and materials research with highly charged heavy ions, molecular as well as hot and dense plasmas;
q Supported by NSFC (Grant Nos. 10125522, 10475102) and Chinese Academy of Sciences. * Tel.: +86 931 496 9331; fax: +86 931 827 2100. E-mail address: [email protected]
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.11.099
(3) Biological effects by heavy ion irradiation and tumor therapy with ion beams; (4) Ion accelerator physics and technology as well as high-power electron accelerator. In the present work, the main attention was paid on the materials research with swift heavy ions at the IMP accelerators. First, the main ion beams acceleration facilities and experimental instruments for materials research are described, then the main research topics and some typical results on materials research with swift heavy ions at the IMP accelerators are reported. 2. Main facilities for ion beams acceleration and materials research IMP & NLHIAL had a series of ion beams acceleration facilities intensively used in the past two decades such as the Heavy Ion Research Facility (HIRFL), the Radioactive Ion Beam Line (RIBLL), a 2 · 2 MV tandem accelerator, a 600 kV Cockroft-Walton accelerator, a 200 kV heavy ion implanter as well as an ECR ion source station with the platform for experiments with highly charged ions. The HIRFL operates an ECR ion source, an injection cyclotron SFC (K = 69) and a main cyclotron SSC (K = 450). All these facilities have accelerated ions from H to Pb of energy ranging
190
Z.G. Wang / Nucl. Instr. and Meth. in Phys. Res. B 245 (2006) 189–193
from keV up to 100 MeV/u. Additionally there is also a 1.2 MeV/40 mA electron accelerator being used for materials modification studies and a 320 kV high-voltage ion accelerator which will start to run in the middle of this year. A new national project connected to the HIRFL approved by national scientific and technological authorities is the electron cooling storage ring (CSR) which is now under construction. The CSR consists of the main ring CSRm and the experimental ring CSRe with a circumference of 161.20 and 120.90 m, respectively. Typical ion parameters of the CSR are 2800 MeV H+, 900– 1100 MeV/u 12C6+ and 400–520 MeV/u 238U72+ ions. At present, the subassemblies of the two rings are installed on line and are under commissioning. First experiments are expected for next year. The IMP has various experimental setups for materials researches in combination with ion irradiation/implantation lines [1,2]. For examples, (i) a movable sample holder connected to a continuous-flow cryostat (CFC) allows irradiation experiments at low temperature (liquid He temperature region), and another movable sample holder provide laser control of sample position measurements; (ii) the irradiation setup used for the simulation of cosmic ray effects especially for single event effects in IC chips offers low ion flux detection, beam energy and homogeneity measurements, and controllable sample selection; (iii) high-temperature irradiation up to about 700 C and irradiations in air can be performed. Furthermore, the IMP has a series of apparatus for sample preparations and treatments, and several instruments for sample analyses. 3. Main topics and typical results of materials research with swift heavy ions The materials research carried out at the IMP accelerators mainly aim at a well understanding of the basic interaction processes of swift heavy ions with materials. Research projects include topics with potential uses in the fields of energy resource, electronics, material science, etc. The main topics of materials research are swift heavy ion-induced defect production and their evolution in pure metals and semiconductors, amorphization and ion track formation, chemical and physical modifications of polymers, phase transformation and recrystallization in materials, as well as the simulation of radiation effects in reactor materials and cosmic ray induced single event effects in large scale integrate circuits. Some typical experimental results on materials research with the swift heavy ions at the IMP accelerators are given in the following. 3.1. Defect production and evolution in Si Silicon is a typical semiconductor being used widely in the world. Damage production in Si induced by energetic ions is a hot topic over many years. We have studied defect production in single crystalline Si using Ar ions of several hundred MeV [3,4].
For single crystalline silicon irradiated with 112 MeV Ar ions at below 50 K [5], it was found that (i) two types of paramagnetic centers, Si–P3 and amorphous-centers, coexist in as-irradiated samples. The former is mainly due to electronic energy loss, the latter can be transformed from isolated amorphous regions to continued amorphous layer with increasing of ion fluence; (ii) divacancies are formed and the concentration increases with ion fluence and saturates at high fluence; (iii) the annealing behaviors of ion irradiation induced defects are fluence dependent, for example, isolated amorphous regions anneal at temperatures around 500 C, whereas the recrystallization temperature for the continued amorphous layer is higher than 600 C. For single crystalline silicon irradiated with 750 MeV argon ions at RT [4], it was found that divacancies of neutral charge state are the main vacancy clusters induced by the irradiations. No amorphous phase was detected up to the highest irradiation fluence. The divacancy concentration in the nuclear process dominated region increased dramatically with increasing irradiation fluence, whereas little change appeared in regions dominated by electronic processes. It is argued that the energy deposition by electronic processes can only induce limited damage in silicon and high electronic stopping power suppresses the formation of amorphous zones. These results are in agreement with that obtained by other research groups [5,6]. 3.2. Amorphization and ion track formation Intense electronic excitations play an important role in ion-induced damage processes such as amorphization and track formation in crystalline materials under swift ion irradiations. In yttrium iron garnet (YIG) a series irradiation effects induced by swift heavy ions have been extensively studied [7,8]. Our works were mainly aimed at the damage process. In our works [9,10], the transformation process from crystalline to amorphous state was shown to be a gradual process in which the electronic excitations play a dominant role. It was found that there is a critical fluence of 1.0 · 1014 ions/cm2 linked to a threshold value of electronic energy loss Set = 2.9 keV/nm for the appearance of phase transformation from crystalline to amorphous state. Above Set the lattice constant, the optical absorption coefficient, and the fraction of the paramagnetic phase Fp increase with increasing electronic energy loss, but saturation magnetization decreases. A nearly complete amorphous state was observed at Se = 8.3 keV/nm for an irradiation of 1.0 · 1014 ions/cm2 or above. For fullerite (C60) films under swift ion irradiations, phase transition from the crystalline to amorphous state was observed [11–13]. This amorphization process is mainly dominated by electronic excitations. Swift light ions result in an intermediate graphitization process before amorphization, i.e. the damage process is crystalline C60 ! intermediate graphitization process ! amorphous carbon. For swift heavy ions, the damage created in C60
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films could be partially recovered at certain Se and the damage cross-sections increased with increasing Se. The destructive action of the strong electronic excitations could exceed the annealing effect so that no intermediate graphitization process existed. Furthermore, the icosahedral symmetry of C60 molecules was reduced or destroyed by annealing effect via electronic excitations, and polymerized phases was formed that resulted in the final transformation of the molecules on the C60 lattice from crystalline into amorphous carbon. Highly oriented pyrolitic graphite (HOPG) irradiated with 6.5, 14 and 111.5 MeV Ar ions at normal incidence were investigated using STM for characterization of single ion impacts on surfaces [14]. Hillock defects were found on the surface. Each hillock is identified as the physical signature of the impact of one individual Ar ion on the HOPG surface, i.e. one track tail of one Ar ion. The size (diameter) of such tracks increased with increasing electronic energy loss. For the comparison of ion track formation at HOPG surface, experiments with some other swift heavy ions (Ne, Cr, Fe, Ni, Zn, Xe and U) in energy range MeV–GeV have been done [15]. Similarly, under Ar ion irradiation, when a large number of tracks protrudes from the HOPG surface, the ordered structure of the graphite lattice is destroyed.
imide is more sensitive to irradiation than para-substituted phenyl in the main chain. The measured enhanced electrical conductivity substantiated by the optical band gap shifts was attributed to the formation of condensed polycyclic aromatic ring compounds and clusters with greater degree of conjugation in the multiple overlapping ion tracks in the PI matrix.
3.3. Chemical and physical modifications of polymers
Selected phase transformation and recrystallization along or in the vicinity of the ion path are one of the most attractive phenomena in some solids induced by intense electronic excitations. The combination of ‘‘low energy ion implantation + swift heavy ion irradiations’’ was proposed for searching selected phase transformation and new crystalline phase formations [33,34]. It was used in atomic mixing materials such as C-doped SiO2 and Ndoped diamond or graphite-like carbon [33–42] for the synthesis of novel light-emitting materials and superhard carbon-nitrides. For the C-doped SiO2 [33–38], it was found that significant chemical bonds such as sp3C- and sp2C bonds and Si-related micro-structures were formed in the samples after swift heavy ion irradiations. The sp3C bonds increased when increasing the irradiation fluence and CO2 molecules were formed in the high-dose (>5 · 1017 ions/cm2) C-doped SiO2 films after large-fluence (>1 · 1012 ions/cm2) Pb–ion irradiations. Furthermore, PL results showed that two different mechanisms may exist for ion-induced luminescence centers in ion irradiated C:SiO2 samples: (1) point defects or bond scissions being a dominant in the Se < 4.85 keV/nm region; (2) formation of silicon carbide inclusions induced by ions with Se > 4.85 keV/nm. All these results imply that nano-sized Si clusters and/or SiC grains may form in the C-doped SiO2 films after swift heavy ion irradiations. Recently, nano-sized recrystallization in the vicinity of ion paths in C-doped region was observed [39]. In N-doped diamond films, it was found that specific chemical bonds such as N–sp2C and N–sp3C are formed [40,41]. Intense energy deposition from the incident swift
As being done by other research groups [16], we also have carried out experimental studies on swift heavy ion induced amorphization and chemical modification in polymer films such in polycarbonate (PC) [17–21], polyethylene terephthalate (PET) [21–24], polystyrene (PS) [21,25,26] and polyimide (PI) [27–29]. PC, PET and PS under swift heavy ions irradiations suffer from serious degradation but the radiation sensitivity varies for the different functional groups [17–26]. The phenyl ring, the most stable functional group inside polymers, is destroyed at Se > 8 keV/nm. The crystallinity of PET is reduced by a process of radiation promoted transformation of the ethylene glycol residue from the trans to the gauche configuration at absorbed doses higher than 4.0 MGy. Alkyne end groups can be produced above certain electronic energy loss threshold that varies with material. The threshold is about 0.8 keV/nm for PS and 0.4 keV/nm for PC. These polymers could be transformed gradually into hydrogenated amorphous carbon at high-energy deposition. A sigmoid relationship of the damage cross-section of the functional groups, the production cross-section of the alkyne end group and the number of chromophores induced per unit track length with electronic energy loss is found in the used electronic energy loss regime. For PI [27–29], the main chains of PI as well as the phenyl ring and imide are destroyed simultaneously and triple bonds (C„C and C„N) are formed. The formation radius of alkyne is lower than the chain disruption radii. The cyclic imide group has more radiation resistance than other functional groups. The tetra-substituted phenyl in cyclic
3.4. Simulation of radiation effects in reactor materials Radiation damage of materials is one of the major technological problems in design of fusion reactors. Swift heavy ion irradiation is a useful tool for the simulation of damage processes of structure materials in reactors. However, the relationship between swift heavy ions and reactor neutron irradiations is unknown. In our works, we found that 54 MeV C-ion irradiations could induce a c(fcc) ! a(bcc)-phase transformation in 316 L stainless steel [30], and the C-ion-induced electronic excitations enhanced the damage production such as serious pitting, flaking and crazing along grain boundaries of the irradiated surface [31,32]. 3.5. Phase transformation and recrystallization
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heavy ions induces the increase of sp3/sp2 bonding ratio and thus enhances the formation of N–sp3C bonds in the samples. Furthermore, X-ray diffraction analysis indicates the formation of new phases such as a- and b-C3N4. The larger the N content, or the larger the energy deposition, the more a- and b-C3N4 phases are formed. As a comparison, modification of N-doped graphite-like carbon samples induced by 30-MeV C60 ion irradiations (performed at INPO) has also been studied [42]. The results showed that C60 ion irradiation can induce C–N bond formation in three phases, namely, C3N4, CNx and tetrahedral carbon. 3.6. Simulation of single event effects Cosmic ray induced single event effects (SEE) on large scale integrate (LSI) circuits have become an important topic being investigated extensively by the scientists in spaceborne electronics. Swift heavy ion beams from accelerators are a useful tool to simulate the SEE simulation of electronic devices at ground. In IMP, an experimental equipment for SEE test at ground was installed at HIRFL and a series of on-line experiments [43–46] have been done using swift heavy ions provided by the IMP accelerators for studying the SEE such as single event upset (SEU), single event latch-up (SEL), single event burning (SEB), multiple-bit upset (MBU), total dose effect, and so on. The method to SEE characterization for various semiconductors under swift ion exposure was set up. Acknowledgement This work is attributed to all colleagues of the materials research group of IMP and supported by NSFC and CAS. I am very grateful to all of them especially to Profs. Y.F. Jin, Z.Y. Zhu, J. Liu, M.D. Hou, Drs. Y.M. Sun, C.H. Zhang and C.L. Liu. References [1] See IMP & NLHIAL Annual Report, 1993–2003, Atomic Energy Press, Beijing. [2] M.D. Hou, C.L. Li, Z.G. Wang, F. Ma, Y.M. Sun, J. Liu, J.M. Quan, Radiat. Eff. Def. Sol. 126 (1993) 355. [3] C.L. Liu, M.D. Hou, Z.Y. Zhu, Z.G. Wang, S. Cheng, Y.F. Jin, Y.M. Sun, C.L. Li, Nucl. Instr. and Meth. B 135 (1998) 219. [4] Z.Y. Zhu, M.D. Hou, Y.F. Jin, C.L. Liu, Y.S. Wang, J. Han, Nucl. Instr. and Meth. B 135 (1998) 260. [5] B. Canut, N. Bonardi, S.M.M. Ramos, S. Della-Negra, Nucl. Instr. and Meth. B 146 (1998) 296, and the references therein. [6] A. Dunlop, G. Jaskierowicz, S. Della-Negra, Nucl. Instr. and Meth. B 146 (1998) 302, and the references therein. [7] A. Meftah, F. Brisard, J.M. Costantini, M. Hage-Ali, J.P. Stoquert, F. Studer, M. Toulemonde, Phys. Rev. B 48 (1993) 920, and the references therein. [8] A. Dunlop, G. Jaskierowicz, J. Jensen, S. Della-Negra, Nucl. Instr. and Meth. B 132 (1997) 93, and the references therein. [9] Y. Jin, J. Han, Q. Meng, Y. Sun, C. Liu, Y. Ru, Y. Wang, C. Zhang, C. Li, M. Hou, Nucl. Instr. and Meth. B 135 (1998) 190. [10] Y.F. Jin, R.H. Xu, J.M. Quan, Z.G. Wang, Q.H. Meng, Y.M. Sun, F. Ma, J. Han, G. Liu, J. Liu, C.L. Li, Nucl. Instr. and Meth. B 107 (1996) 227.
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