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Defect-oriented carbon stripper foil development
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 521 (2004) 183–186
Defect-oriented carbon stripper foil development D. Kabiraj*, S. Ojha, S.R. Abhilash, D.K. Avasthi, S.K. Datta Nuclear Science Centre, Aruna Asaf Ali Marg, P.O. Box 10502, New Delhi 110067, India
Abstract Carbon foils produced by electron beam evaporation of approximately 5 mg/cm2 thickness are regularly in use as stripper foils in the 15UD Pelletron Accelerator at Nuclear Science Centre. This report examines the role of defects produced by the ion beams in influencing usable life of these carbon foils. The life of the foils has been estimated by measuring the transmitted ion current at the high-energy end of the 15UD Pelletron. The lifetime of carbon stripper foils entirely depend on the defects produced by the ions. A relation between the onset of the degradation of current transmission and the accumulated defects has been established. An improved type of carbon stripper foils might be obtained by the introduction of thin C60 layers. r 2003 Published by Elsevier B.V. PACS: 81.15.Ef; 81.05. Tp; 29.27.Ac Keywords: Carbon; Self-supporting target; Stripper lifetime
1. Introduction Several years efforts were needed to improve the lifetime of carbon stripper foils used in tandem accelerators. The basic processes of stripper foil degradation are understood from the in-depth study by Dollinger et al. [1]. The degradation of standard stripper foils are due to thickening at the region of interaction with beam, and subsequent rupturing from the periphery due to radial strain. The methods adopted to improve the stripper foils lifetime are to retard the above-mentioned manifestation of the beam–foil interaction. Some of the techniques used are mechanical in nature, like increasing the surface area of foils thereby *Corresponding author. Tel.: +91-11-2689-2601; fax: +9111-2689-3666. E-mail address: [email protected] (D. Kabiraj). 0168-9002/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.nima.2003.11.149
providing more room to shrink. Some others, alter the structure of the carbon foils. Apart from the standard evaporation–condensation (EC) type of carbon foils, to our knowledge are three types of advanced stripper foil are in use. The Munich group was the first to prepare carbon foils by laser plasma ablation of pure carbon [2]. These foils have a nearly isotropic distribution of orientation of quasi-graphitic nanocrystals [3]. Then Sugai et al. [4] prepared foils by a controlled DC arcdischarge (CDAD) method. Sugai interprets that the longer lifetime of the CDAD foils is due to mixtures of carbon clusters of 0.5 mm particles and 0.003 mm fine particles. Finally, Liechtenstein et al. [5] developed thin self-supporting diamond-like carbon (DLC) stripper foils, which are reported to have long life. Here we present the results of a systematic study of the role of defects produced by energetic ion
ARTICLE IN PRESS 184
D. Kabiraj et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 183–186
beam in defining the life of carbon stripper foils. The applied carbon foils with thickness of about 5 mg/cm2 were prepared by electron beam evaporation of graphite on detergent-coated glass slides.
2. Experimental The usable life of the carbon foils has been estimated by measuring the beam current of the stripped ions in the Faraday cup (FC-CB) at the high-energy end of the 15UD Pelletron accelerator [6]. The injected ion current measurable in a Faraday cup (FC-CT) and the terminal potential were kept constant and the ion current at the highenergy end was recorded with proper averaging at a fixed time interval with the help of a computer program. The terminal potential was kept constant at 12 MV and Ni, Ag and Au ions were used for this experiment. The investigated foils were prepared by the evaporation–condensation method under highvacuum conditions by evaporating spectroscopically pure graphite from a water-cooled hearth of an electron beam gun and condensing it on detergent (Teepol)-coated glass slides. The thickness of the condensed carbon films was approximately 5 mg/cm2 as measured by a quartz crystal thickness monitor. A Hitachi UV-Vis spectrophotometer was applied for light absorption measurement for different types of carbon foils.
3. Result and discussion In Fig. 1 the transmitted ion current through the tandem is plotted, normalized by the injected ion current. This ratio was arbitrarily set to 1 at the beginning of each foil test. These transmission values were recorded for five foils per ion sort. Since there were no remarkable differences in the slopes, data of only one foil are presented in the figure. The injected ion currents were 50, 40 and 57 nA for Au, Ag and Ni, respectively. The plot Fig. 1a is for Au ions, Fig. 1b for Ag ions and Fig. 1c for Ni ions. The time of irradiation has been
Fig. 1. Normalized ion transmission as a function of number of irradiating ions through the Pelletron accelerator for different ion species.
converted to the number of irradiating ions N (xaxis) by the following equation: N ¼ qti; where q is the amount of injected negative ions for a beam current i of 1 nA during a time t of 1 s. The numerical value for q is 6.24 109. N is the number of ions passing through the irradiated region of the 4–5 mm diameter beam spot. The vacancies produced in a carbon foil by 12 MeV ions of each of the three ion kinds have been calculated using TRIM Monte Carlo simulation program. The vacancies per ion computed are 44, 13 and 4 for 12 MeV Au, Ag and Ni ions, respectively. The number of irradiating ions of Fig. 1 has been converted to the respective vacancies. The result is plotted in Fig. 2. These transmission curves almost merge together until the accumulation of some fraction of about 4 1015 vacancies in the irradiated area. The onset of the loss of transmission starts after accumulation of almost the same amount of vacancies. Vacancy–interstitial pairs might be annihilated due to the elevated temperature in the beam spot area caused by electronic energy loss. The beam spot
ARTICLE IN PRESS D. Kabiraj et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 183–186
185
current is almost constant. The duration of this constant current region varies for the foils prepared by different techniques. The loss in transmission is due to thickening of the carbon foils as discussed earlier.
4. A new type of carbon stripper foils by applying C60 molecules
Fig. 2. Normalized ion transmission as a function of number of created vacancies through the Pelletron accelerator for different ion species.
temperature can be estimated using Eq. (21) of Ref. [1]. The electronic energy loss for the three types of ions used being almost equal, the temperature rise is expected to be same in all three cases [1]. At elevated temperatures some of the defects get annihilated. The rest of the interstitial carbon atoms accumulate between the basal planes. This results in thickening and the gradual loss of ion transmission can be observed as shown in Fig. 2. The loss of ion transmission has been shown [7] to be due to multiple scattering by the increasing carbon foil thickness produced by irradiation damage. The relation between the thickening of the carbon foils and the vacancies and interstitial produced in the carbon foils under the energetic ions irradiation is described in details in Ref. [1]. From this study, along with many other studies related to the usable life of stripper foils as in Ref. [3], where transmission of Au ions in carbon foils, prepared by various techniques have been compared, one can observe that before the onset of the fall in transmission there is a time during which the
We have started some work to design a new type of carbon stripper foil by applying a thin film of C60 molecules. The size of each C60 molecule is 0.7 nm diameter, and it contains only 60 carbon atoms. So collapsing of such a molecule under ion bombardment may create a large open space in the structure and generate some space for the structure to shrink. There are reports of formation of onionand finally diamond-like structures under highenergy, high-current irradiation with heavy ions [8] and high-current electron beam irradiation [9] of fullerenes and carbon nano-tubes. In both cases, the temperature of the region of interaction has been measured to be 1000–1200 K. This is near to the temperature expected in a tiny beam spot at a stripper foil, leading to the possibility of formation of these structures. The formation of these structures may add to the strength of these foils. Thus the stripper foil lifetime might be enhanced. A 1 mg/cm2 layer of C60 was sandwiched between two-carbon layer of 2 mg/cm2 each. Since the vapor pressure of C60 material is quite high, two-carbon layers have been used to protect the C60 layer from evaporation due to temperature rise from ion beam interaction. To start with, 2 mg/cm2 of carbon were deposited on a detergent-coated glass substrate by electron beam evaporation. Next, 1 mg/cm2 of C60 were deposited by thermal evaporation from a Ta boat with a perforated Ta cover. The 99% pure C60 powder was purchased from Aldrich. The deposition rate was kept very low to avoid breaking of C60 molecules. Finally, another 2 mg/cm2 of carbon was deposited to cover the C60 layer. The foils were then floated in water and finally, fixed on our standard stripper foil frames. Fig. 3 shows the UV-vis absorption spectra of such a sandwich foil along with a normal carbon foil for comparison. The
ARTICLE IN PRESS 186
D. Kabiraj et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 183–186
Acknowledgements We acknowledge all the members of the group running the Pelletron at NSC and Dr. A. Sharma for providing us the UV-vis spectrophotometer.
References
Fig. 3. Absorption spectrum of a C60 sandwich foil in comparison to the one of a normal carbon foil.
absorption peaks seen are due to hexagonal planes in the C60 structure. Systematic studies of the lifetime of these foils are in progress.
[1] G. Dollinger, P. Maier-Komor, Nucl. Instr. and Meth. A 282 (1989) 223. [2] G. Dollinger, P. Maier-Komor, Nucl. Instr. and Meth. A 303 (1991) 50. [3] G. Dollinger, P. Maier-Komor, A. Mitwalsky, Nucl. Instr. and Meth. A 303 (1991) 79. [4] I. Sugai, Y. Takeda, M. Oyaizu, H. Kawakami, Y. Hattori, K. Kawasaki, N. Hyashizaki, Nucl. Instr. and Meth. A 480 (2002) 191. [5] V.Kh. Liechtenstein, T.M. Ivkova, E.D. Olshanski, A.M. Baranov, R. Repnow, R. Hellborg, R.A. Weller, H.L. Wirth, Nucl. Instr. and Meth. A 438 (1999) 79. [6] G.K. Mehta, A.P. Patro, Nucl. Instr. and Meth. A 268 (1988) 334. [7] G. Dollinger, P. Maier-Komor, Nucl. Instr. and Meth. A 282 (1989) 153. [8] P. Wesoloski, Y. Lyutovich, F. Banhart, H.D. Cartstanjen, H. Kronmuller, Appl. Phys. Lett. 71 (1997) 1948. [9] F. Banhart, P.M. Ajayan, Nature 382 (1996) 433.
Nuclear Instruments and Methods in Physics Research A 521 (2004) 183–186
Defect-oriented carbon stripper foil development D. Kabiraj*, S. Ojha, S.R. Abhilash, D.K. Avasthi, S.K. Datta Nuclear Science Centre, Aruna Asaf Ali Marg, P.O. Box 10502, New Delhi 110067, India
Abstract Carbon foils produced by electron beam evaporation of approximately 5 mg/cm2 thickness are regularly in use as stripper foils in the 15UD Pelletron Accelerator at Nuclear Science Centre. This report examines the role of defects produced by the ion beams in influencing usable life of these carbon foils. The life of the foils has been estimated by measuring the transmitted ion current at the high-energy end of the 15UD Pelletron. The lifetime of carbon stripper foils entirely depend on the defects produced by the ions. A relation between the onset of the degradation of current transmission and the accumulated defects has been established. An improved type of carbon stripper foils might be obtained by the introduction of thin C60 layers. r 2003 Published by Elsevier B.V. PACS: 81.15.Ef; 81.05. Tp; 29.27.Ac Keywords: Carbon; Self-supporting target; Stripper lifetime
1. Introduction Several years efforts were needed to improve the lifetime of carbon stripper foils used in tandem accelerators. The basic processes of stripper foil degradation are understood from the in-depth study by Dollinger et al. [1]. The degradation of standard stripper foils are due to thickening at the region of interaction with beam, and subsequent rupturing from the periphery due to radial strain. The methods adopted to improve the stripper foils lifetime are to retard the above-mentioned manifestation of the beam–foil interaction. Some of the techniques used are mechanical in nature, like increasing the surface area of foils thereby *Corresponding author. Tel.: +91-11-2689-2601; fax: +9111-2689-3666. E-mail address: [email protected] (D. Kabiraj). 0168-9002/$ - see front matter r 2003 Published by Elsevier B.V. doi:10.1016/j.nima.2003.11.149
providing more room to shrink. Some others, alter the structure of the carbon foils. Apart from the standard evaporation–condensation (EC) type of carbon foils, to our knowledge are three types of advanced stripper foil are in use. The Munich group was the first to prepare carbon foils by laser plasma ablation of pure carbon [2]. These foils have a nearly isotropic distribution of orientation of quasi-graphitic nanocrystals [3]. Then Sugai et al. [4] prepared foils by a controlled DC arcdischarge (CDAD) method. Sugai interprets that the longer lifetime of the CDAD foils is due to mixtures of carbon clusters of 0.5 mm particles and 0.003 mm fine particles. Finally, Liechtenstein et al. [5] developed thin self-supporting diamond-like carbon (DLC) stripper foils, which are reported to have long life. Here we present the results of a systematic study of the role of defects produced by energetic ion
ARTICLE IN PRESS 184
D. Kabiraj et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 183–186
beam in defining the life of carbon stripper foils. The applied carbon foils with thickness of about 5 mg/cm2 were prepared by electron beam evaporation of graphite on detergent-coated glass slides.
2. Experimental The usable life of the carbon foils has been estimated by measuring the beam current of the stripped ions in the Faraday cup (FC-CB) at the high-energy end of the 15UD Pelletron accelerator [6]. The injected ion current measurable in a Faraday cup (FC-CT) and the terminal potential were kept constant and the ion current at the highenergy end was recorded with proper averaging at a fixed time interval with the help of a computer program. The terminal potential was kept constant at 12 MV and Ni, Ag and Au ions were used for this experiment. The investigated foils were prepared by the evaporation–condensation method under highvacuum conditions by evaporating spectroscopically pure graphite from a water-cooled hearth of an electron beam gun and condensing it on detergent (Teepol)-coated glass slides. The thickness of the condensed carbon films was approximately 5 mg/cm2 as measured by a quartz crystal thickness monitor. A Hitachi UV-Vis spectrophotometer was applied for light absorption measurement for different types of carbon foils.
3. Result and discussion In Fig. 1 the transmitted ion current through the tandem is plotted, normalized by the injected ion current. This ratio was arbitrarily set to 1 at the beginning of each foil test. These transmission values were recorded for five foils per ion sort. Since there were no remarkable differences in the slopes, data of only one foil are presented in the figure. The injected ion currents were 50, 40 and 57 nA for Au, Ag and Ni, respectively. The plot Fig. 1a is for Au ions, Fig. 1b for Ag ions and Fig. 1c for Ni ions. The time of irradiation has been
Fig. 1. Normalized ion transmission as a function of number of irradiating ions through the Pelletron accelerator for different ion species.
converted to the number of irradiating ions N (xaxis) by the following equation: N ¼ qti; where q is the amount of injected negative ions for a beam current i of 1 nA during a time t of 1 s. The numerical value for q is 6.24 109. N is the number of ions passing through the irradiated region of the 4–5 mm diameter beam spot. The vacancies produced in a carbon foil by 12 MeV ions of each of the three ion kinds have been calculated using TRIM Monte Carlo simulation program. The vacancies per ion computed are 44, 13 and 4 for 12 MeV Au, Ag and Ni ions, respectively. The number of irradiating ions of Fig. 1 has been converted to the respective vacancies. The result is plotted in Fig. 2. These transmission curves almost merge together until the accumulation of some fraction of about 4 1015 vacancies in the irradiated area. The onset of the loss of transmission starts after accumulation of almost the same amount of vacancies. Vacancy–interstitial pairs might be annihilated due to the elevated temperature in the beam spot area caused by electronic energy loss. The beam spot
ARTICLE IN PRESS D. Kabiraj et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 183–186
185
current is almost constant. The duration of this constant current region varies for the foils prepared by different techniques. The loss in transmission is due to thickening of the carbon foils as discussed earlier.
4. A new type of carbon stripper foils by applying C60 molecules
Fig. 2. Normalized ion transmission as a function of number of created vacancies through the Pelletron accelerator for different ion species.
temperature can be estimated using Eq. (21) of Ref. [1]. The electronic energy loss for the three types of ions used being almost equal, the temperature rise is expected to be same in all three cases [1]. At elevated temperatures some of the defects get annihilated. The rest of the interstitial carbon atoms accumulate between the basal planes. This results in thickening and the gradual loss of ion transmission can be observed as shown in Fig. 2. The loss of ion transmission has been shown [7] to be due to multiple scattering by the increasing carbon foil thickness produced by irradiation damage. The relation between the thickening of the carbon foils and the vacancies and interstitial produced in the carbon foils under the energetic ions irradiation is described in details in Ref. [1]. From this study, along with many other studies related to the usable life of stripper foils as in Ref. [3], where transmission of Au ions in carbon foils, prepared by various techniques have been compared, one can observe that before the onset of the fall in transmission there is a time during which the
We have started some work to design a new type of carbon stripper foil by applying a thin film of C60 molecules. The size of each C60 molecule is 0.7 nm diameter, and it contains only 60 carbon atoms. So collapsing of such a molecule under ion bombardment may create a large open space in the structure and generate some space for the structure to shrink. There are reports of formation of onionand finally diamond-like structures under highenergy, high-current irradiation with heavy ions [8] and high-current electron beam irradiation [9] of fullerenes and carbon nano-tubes. In both cases, the temperature of the region of interaction has been measured to be 1000–1200 K. This is near to the temperature expected in a tiny beam spot at a stripper foil, leading to the possibility of formation of these structures. The formation of these structures may add to the strength of these foils. Thus the stripper foil lifetime might be enhanced. A 1 mg/cm2 layer of C60 was sandwiched between two-carbon layer of 2 mg/cm2 each. Since the vapor pressure of C60 material is quite high, two-carbon layers have been used to protect the C60 layer from evaporation due to temperature rise from ion beam interaction. To start with, 2 mg/cm2 of carbon were deposited on a detergent-coated glass substrate by electron beam evaporation. Next, 1 mg/cm2 of C60 were deposited by thermal evaporation from a Ta boat with a perforated Ta cover. The 99% pure C60 powder was purchased from Aldrich. The deposition rate was kept very low to avoid breaking of C60 molecules. Finally, another 2 mg/cm2 of carbon was deposited to cover the C60 layer. The foils were then floated in water and finally, fixed on our standard stripper foil frames. Fig. 3 shows the UV-vis absorption spectra of such a sandwich foil along with a normal carbon foil for comparison. The
ARTICLE IN PRESS 186
D. Kabiraj et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 183–186
Acknowledgements We acknowledge all the members of the group running the Pelletron at NSC and Dr. A. Sharma for providing us the UV-vis spectrophotometer.
References
Fig. 3. Absorption spectrum of a C60 sandwich foil in comparison to the one of a normal carbon foil.
absorption peaks seen are due to hexagonal planes in the C60 structure. Systematic studies of the lifetime of these foils are in progress.
[1] G. Dollinger, P. Maier-Komor, Nucl. Instr. and Meth. A 282 (1989) 223. [2] G. Dollinger, P. Maier-Komor, Nucl. Instr. and Meth. A 303 (1991) 50. [3] G. Dollinger, P. Maier-Komor, A. Mitwalsky, Nucl. Instr. and Meth. A 303 (1991) 79. [4] I. Sugai, Y. Takeda, M. Oyaizu, H. Kawakami, Y. Hattori, K. Kawasaki, N. Hyashizaki, Nucl. Instr. and Meth. A 480 (2002) 191. [5] V.Kh. Liechtenstein, T.M. Ivkova, E.D. Olshanski, A.M. Baranov, R. Repnow, R. Hellborg, R.A. Weller, H.L. Wirth, Nucl. Instr. and Meth. A 438 (1999) 79. [6] G.K. Mehta, A.P. Patro, Nucl. Instr. and Meth. A 268 (1988) 334. [7] G. Dollinger, P. Maier-Komor, Nucl. Instr. and Meth. A 282 (1989) 153. [8] P. Wesoloski, Y. Lyutovich, F. Banhart, H.D. Cartstanjen, H. Kronmuller, Appl. Phys. Lett. 71 (1997) 1948. [9] F. Banhart, P.M. Ajayan, Nature 382 (1996) 433.