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Carbon foils for nuclear accelerator experiments
NlKMl B
Nuclear Instruments and Methods in Physics Research B79 (1993) 841-844 North-Holland
Carbon foils for nuclear accelerator
Beam Interactions with Materials A Atoms
experiments
P. Maier-Komor Physik-Department,Techniwhe UniversiiiitMiinchen, D-W8044 Garching, Germany
Carbon foils are the most commonly used targets and backings in nearly all experiments in nuclear physics and related subjects performed at ion accelerators. Due to their high tensile strength carbon foils can be prepared even with a thickness below 1 pg/cm* therefore they are exclusivly used as backings whenever a low-Z element is tolerable. There are several methods available to prepare carbon foils with different properties. A review is presented about these methods and about the application of special techniques in order to achieve specific foil properties. The characterization and exact thickness determination of carbAn foils are described in detail.
2. Preparation
1. Introliuction Carbon holds a special status compared to all other elements. Without carbon the basis for life would be unimaginable. Organic chemistry or carbon chemistry is an independent part of chemistry. The element carbon occupies an exceptional position in target preparation for nuclear measurements due to its melting point, the highest of all elements, and also due to its very low vapor pressure and the good chemical stability which is comparable to that of the noble metals. Carbon has only two stable isotopes and one radioactive isotope, the half-life of which is long.enough to prepare targets in a standard way. The natural abundance of “C is nearly 99% which means that a natural carbon foil can be used in most cases as isotopic ‘*C target. The good tensile strength of self-supporting carbon foils can be estimated by their minimum obtainable thickness. Carbon foils can be prepared even below 1 p.g/cm* (= 5 nm), which is one order of magnitude thinner than those which can be produced from any other element. Most accelerator measurements which use thin targets which are less than 100 kg/cm* must have a thin backing material and this is in most cases carbon in the thickness range of 3-30 Fg/cm*. Carbon foils are indispensable in nearly all experiments in nuclear physics and related subjects performed at ion accelerators. The ion energy necessary to perform the wanted nuclear reaction is usually too high to be achieved by acceleration of a single charged particle. That is why mostly carbon stripper foils are installed in the beam line of the accelerator to increase the charge state of the accelerated ions. Due to the limited lifetime of these carbon foils, especially in heavy ion beams, the accelerators themselves are the major users of carbon foils. 0168-583X/93/$06.00
methods
2.1. Mechanical procedures Some of the mechanical procedures used to produce thick carbon foils are as follows: Hot pressed carbon or graphite material can be grinded and polished to a minimum thickness of 20 mg/cm*. Single crystals of graphite can be cleaved, the minimum self-supporting thickness is about 2 mg/cm*
111. Diamonds may be cut and polished down to a thickness of 50 mg/cm* [2]. Graphite powder can be settled and pressed for carbon foils of 1 mg/cm* minimum thickness [3]. - Carbon foils can also be prepared using a liquid in which graphite powder is disperged [4]. Such suspensions are utilized in painting [5] or spray painting methods. Carbon foils of such a kind can have a minimum thickness of 10 kg/cm*, but the homogeneity is doubtful. 2.2. Evaporation-condensation
of carbon in high vac-
uum The second method, the evaporation-condensation process in which the foils are produced in a high vacuum, is the most commonly used procedure for the preparation of self-supporting carbon foils. Thicknesses prepared in this way are from less than 1 pg/cm* up to greater than 1000 kg/cm*. The evaporation of carbon is carried out by three different methods: a) resistance heating of carbon filaments; b) arc evaporation; and c) electron bombardment heating.
8 1993 - Elsevier Science Publishers B.V. All rights reserved
XIV. ACCELERATOR
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842
P. Maier-Konwr / CarbonfoirSfor nuclear acceleratorexperiments
The resistance heating method is used most commonly, but the method published first by Bradley [6] has been modified by many laboratories. Bradley described this method as carbon-arc evaporation despite the fact that the two electrodes are always in physical contact. Two spectroscopically pure graphite rods. having pointed ends and being clamped horizontally in water cooled supports, are held in contact by springs while a current of more than 100 A passed through them. Sufficient temperature is generated to cause evaporation of carbon. Dearnaley [7], Sarma [8], and McCormick and McCormack [9] reported similar setups. Nobes [lo] modified this method by mounting the carbon rods vertically and by exchanging the spring loading by gravity using a weight of 0.8 N. The disadvantage of the methods described before is, that the carbon rods must be sharpened several times to evaporate enough carbon for thicker films, which requires breaking of the vacuum. Maier-Komor [ 1l] avoided this inconvenience by resistance heating of a 3 mm diameter carbon rod mounted in fitting boreholes of springloaded thick carbon rod electrodes. The real arc evaporation was published first by Pfeiffer [12] and Blue and Danielson [13], but, they did not notice a difference regarding the Bradleys method. The arrangement of the carbon evaporator is indeed the same as that of the Bradley method. In addition a push-pull feedthrough is needed, which is connected to one electrode to adjust the arc gap from outside the vacuum system. When carbon stripper foil -problems became serious Takeuchi et al. [14] and Sugai et al. [15,16] tried to improve the carbon arc procedure. Their reported improvement in stripper foil lifetime has not been confirmed by other accelerator laboratories. Electron bombardment heating, the third method, described first for carbon evaporation by Maxman [17] and Morgan [18], is still the state of the art when thermal evaporation of carbon is applied [19], because with this method the control of the evaporation rate is relatively simple and the carbon source is nearly unlimited. Either a thick, long carbon rod can be fed vertically into the electron gun crucible, or alternatively for isotopes the powdered carbon can be pressed into a suitable sized pellet for insertion in the crucible. 2.3. Carbon foils prepared by sputtering Sputtering is mainly used for industrial applications, when thin films having good adherence to the substrate are needed. Since the sputtering rates for carbon are very low, the deposition time is very long. In addition, the problem of separating the carbon foil from its substrate cannot easily be solved. Baumann and Wirth [20] tried to develop the heavy ion beam sputtering method for carbon in order to improve carbon stripper
foil qualities, but the very low efficiency of the floating process led them to give up this method. Sugai et al. [15,21] also tried this method but with more effort, but they also could not report to have solved all the problems mentioned above. 2.4. Chemical vapor decomposition Thermal cracking is only utilized in the preparation of 14C foils. The vap.or used is methyl-iodide. Setups are described in detail in refs. [22-261. An improvement of this old method was reported by Maier 1271. The obtainable carbon foil thickness range is 4-100 pg/cm’. 2.5. Additional methods utilized only for preparing carbon stripper foils The first method, the cracking of hydrocarbon gases in a dc glow discharge process was developed by Kiinig and Helwig in 1950 [28]. It was introduced for preparing carbon stripper foils by Tait et al. [29] in 1979 and modified or further developed by many laboratories (see e.g. ref. [30]). It was the only proved method for producing carbon foils having better stripper properties for nearly, ten years. A theoretical explanation for the good quality of the dc glow discharge foils was first given by Dollinger et al. [31,32] in 1988. They presented the theory of heavy ion irradiation damage in carbon foils on the basis of electron diffraction and electron transmission microscope investigations. The postulation for best stripper qualities was an isotropic orientation of the quasi-graphitic nanocrystals in the polycrystalline carbon foil. Carbon foils prepared by thermal evaporation are completely anisotropic, which explains their minor quality, but glow discharge foils show a partially isotropic distribution. The postulated isotropic structure could be realized when Dollinger and Maier-Komor [33] succeeded in preparing thin carbon stripper foils using laser plasma ablation-deposition of carbon in high vacuum.
3. Preparation and handling All described preparation methods deliver a film on a substrate. In order to get a self-supporting foil a parting agent is necessary which functions as an interface between the substrate and the deposited carbon film. In most cases it is a water-soluble compound [34] because the parting process is done in distilled water. But in some cases, especially when the deposition process is hot (thermal cracking) or ions are impinging on the substrate (e.g. glow discharge), thin metal foils mainly nickel or copper are used as a substrate. These metal foils are first dissolved in acids and the carbon
P. Maier-Komor
a43
/ Carbon foils for nuclear accelerator experiments
foil is then washed in distilled water. The floating process is described e.g. in ref. [34]. When preparing carbon foils using the standard so-called carbon arc method, various detergents are used as parting agents. Stoner [35], who seems to have the most experience with this method, prefers Creme-Cote. Detergent parting agents, however, are squeamish to heat; as a result they usually fail when electron bombardment evaporation is applied, because in this procedure the hot carbon source is larger and results in more substrate heating. That is one reason why betaine [ll] is preferred for this method. In addition, betaine results in a corrugated structure of the carbon foil, with the advantage of much better mechanical strength. Also for some applications salts [34] are used as parting agents. Depending on the parting agent, however, a different surface roughness of the carbon film is achieved, which means high or low homogeneity of the carbon foil. Brasky [36] and Abele et al. [37] investigated the homogeneity problems related to the parting agent. Carbon foils can be thickness calibrated very accurately if kg/cm* is the measuring unit [38,39]. Due to the low cost of the source material, which is spectroscopically pure graphite, quite large source-to-substrate distances can be used. In this way the deviation of the measured quartz thickness to the real substrate thickness caused by their slightly different locations is very small. A suitable setup could guarantee an error of < 2%. Most suppliers of carbon, foils measure the thickness by light transmittance and declare an error of < 10%. The light transmittance measurements [40], however, depend on crystal structure and impurity content of the carbon foil. The bulk impurities in standard carbon foils come from the residual gas in the vacuum process and are small when using a vacuum of better low4 Pa. Most impurities are on the side of the foil which was floated from the parting agent [41]. On the opposite side, mainly water is adsorbed, which remains even in vacuum, if the foil is not heated 1411. That is why even carbon foils seem to be thicker than quoted, when a-energy loss is used to control the thickness.
Armitage et al. 1431 and further developed by MaierKomor [44]. If further improvement is needed, then only the two techniques of dc glow discharge or laser plasma ablation deposition remain. The glow discharge carbon foils are produced in many laboratories and might be the first choice, if there would not be the poor reproducibility of properties from one deposition to the next one. Foils prepared by laser plasma ablation are the best. The preparation is well controlled which means that there is a reproducibly good quality of all foils [45]. Unfortunately these foils are so far not available in larger quantities.
5. Conclusion The variety of procedures used to prepare carbon foils meets all requirements for nuclear accelerator experiments as well as most other cases, where carbon foils are needed. Even the problem of distruction of carbon foils by heavy ion irradiation damage is solved, because a physical limit seems to be achieved for foils prepared by laser plasma ablation-deposition as irradiation damage calculations show for a foil penetrated by a 10 MeV ‘*‘I beam. In this foil each carbon atom was knocked 60 times from its place on average and about three times the amount of carbon atoms, which were placed in the original beam spot area, had left the foil by sputtering. This material loss was possible because the decrease of foil thickness was partly compensated by nearly the threefold material compared to the one originally present in the irradiated area, which was drawn into the beam spot during irradiation. Cheaper carbon deposition techniques, might be developed. If they show really the superior quality of foils prepared by laser plasma ablation-deposition can now be easily controlled by electron microscopy [31].
References [ll G.E. Myers and G.L. Montet, J. Appl. Phys. 37 (1966)
4. Carbon stripper foils If light ions up to oxygen are to be stripped, then standard carbon foils prepared by vacuum evaporation-condensation can be applied. They are commercially available if not prepared in ones own laboratory. But even for the light ions the foil thickness should be carefully chosen in order to achieve the optimum ion transmission for the wanted charge state [42]. For heavy ions even some improvement can be done when slackening of the foils is utilized as introduced by
4195. 121M. Rebak, J.P. F. Sellshop, T.E. Deny and R.W. Fearick, Nucl. Instr. and Meth. 167 (1979) 115. 131W.R. Lowzowski,Proc. 6th Ann. Conf. INTDS, Berkeley (LBG7950) (1977) p. 115. 141V.E. Viola Jr. and D.J. O’Connell, Nucl. Instr. and Meth. 32 (1965) 12.5. [S] R.D. McCormick and J.D. McCormack, Nucl. Instr. and Meth. 13 (1961) 151. [6] D.E. Bradley, Brit. J. Appl. Phys. 5 (1954) 65. 171 G. Deamaley, Rev. Sci. Instr. 31 (1960) 197. [81 N. Sarma, Nucl. Instr. and Meth. 2 (1958) 361.
XIV. ACCELERATOR
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P. Maier-Komor / Carbon foils for nuclear accelerator qwiments
191 R.D. McCormick and J.D. McCormack, Nucl. Instr. and Meth. 13 (1961) 147. ]lOl M. Nobes, J. Sci. Instr. 42 (196.5) 753. ill1 P. Maier-Komor, Nucl. Instr. and Meth. 102 (1972) 485. WI I. Pfeiffer, Naturwissenschaften 42 (1955) 508. D31 M.D. Blue and G.C. Danielson, J. Appl. Phys. 28 (1957) 583. 1141S. Takeuchi, C. Kobayashi, Y. Satoh, T. Yoshida, E. Takekoshi and M. Maruyama, Nuci. Instr. and Meth. 158 (1979) 333. M I. Sugai, T. Fujino, K Yamazaki, T. Hattori and M. Ogawa, Nucl. Instr. and Meth. A236 (1985) 576. H61 I. Sugai, T. Hatori, H. Muto, Y. Takahashi, H. Kato and K. Yamazaki, Nucl. Instr. and Meth. A282 (1989) 164. [171 S.H. Maxman, Nucl. Instr. and Meth. 50 (1967) 53. WI M. Morgan, Thin Solid Films 7 (1971) 313. I191 E. Ranzinger, P. Maier-Komor, H.J. Maier and H. Miinzer, Nucl. Instr. and Meth. 184 (1981) 211. DO1 H. Baumann and H.L. Wirth, Nucl. Instr. and Meth. 167 (1979) 71. Dll I. Sugai, M. Gyaizu, T. Hattori, K. Kawaski, T. Yano, H. Muto, Y. Takahashi and K. Yamazaki, Nucl. Instr. and Metli. A303 (1991) 59. WI G.C. Phillips and J.E. Richardson, Rev. Sci. Instr. 21 (1950) 885. [231 H.D. Holmgren, J.M. Blair, K.F. Famularo, T.F. Stratton and R.V. Stuart, Rev. Sci. Instr. 25 (1954) 1026. [241 E. I, R.R. Perry and J.R. Risser, Nucl. Instr. and Meth. 4 (1959) 167. r251 A.H.F. Muggleton, Proc. Seminar on The Preparation and Standardisation of Isotopic Targets and Foils, Harwell, Gxon AERER5097 (1965) p. 99. D61 R. Keller and H.H. Miiller, Nucl. Instr. and Meth. 119 (1974) 321.
[27] H.J. Maier, Proc. 8th World Conf. INTDS, Boston, 1979 (Plenum, New York, 1981) p. 151. [28] H. Kdniaand G. Helwig, Z. Phys. 129 (1951) 491. [29] N.R.S. Tait, B.H. Armitage and D.S. Whitmell, Nucl. Instr. and Meth. 167 (1979) 21. [30] B. Huck, W. Krltschmer, R. Repnow and H.L. Wirth, Nucl. Instr. and Meth. 184 (1981) 215. [31] G. Dollinger and P. Maier-Komor, Nucl. Instr. and Meth. A282 (1989) 223. [32] G. Dollinger, P. Maier-Komor and A. Mitwalsky, Nucl. Instr. and Meth. A303 (1991) 79. [33] G. Dollinger and P. Maier-Komor, Nucl. Instr. and Meth. A303 (1991) 50. [34] A.H.F. Muggleton, Vacuum 37 (1987) 785. [35] J.O. Stoner Jr., Nucl. Instr. and Meth. A236 (1985) 662. [36] D.N. Braski, Nucl. Instr. and Meth. 102 (1972) 553. [37] H.K. Abele, P. GIZssel, P. Maier-Komor, H.J. Scheerer, H. Rosier and H. Vonach, Nucl. Instr. and Meth. 137 (1976) 157. [38] P. Maier-Komor, Nucl. Instr. and Meth. 167 (1979) 125. [39] P. Maier-Komor, Nuci. Instr. and Meth. A236 (1985) 641. 1401 P. Maier-Komor, G. Dollinger and E. Hammann, Nucl. Instr. and Meth. A303 (1991) 88. [41] G. Dollinger, T. Faestermann and P. Maier-Komor, Nucl. Instr. and Meth. B64 (1992) 422. 1421 G. Dollinger and P. Maier-Komor, Nucl. Instr. and Meth. A282 (1989) 153. [43] B.H. Arm&age, J.D.H. Hughes, D.S. Whitmell and N.R.S. Tait, Nucl. Instr. and Meth. 167 (1979) 25. [44] P. Maier-Komor, Proc. 10th World Conf. of the INTDS, Rehovot, Israel (1981) p. 17. [45] G. Dollinger and P. Maier-Komor, Nucl. Instr. and Meth. B53 (19911352.
Nuclear Instruments and Methods in Physics Research B79 (1993) 841-844 North-Holland
Carbon foils for nuclear accelerator
Beam Interactions with Materials A Atoms
experiments
P. Maier-Komor Physik-Department,Techniwhe UniversiiiitMiinchen, D-W8044 Garching, Germany
Carbon foils are the most commonly used targets and backings in nearly all experiments in nuclear physics and related subjects performed at ion accelerators. Due to their high tensile strength carbon foils can be prepared even with a thickness below 1 pg/cm* therefore they are exclusivly used as backings whenever a low-Z element is tolerable. There are several methods available to prepare carbon foils with different properties. A review is presented about these methods and about the application of special techniques in order to achieve specific foil properties. The characterization and exact thickness determination of carbAn foils are described in detail.
2. Preparation
1. Introliuction Carbon holds a special status compared to all other elements. Without carbon the basis for life would be unimaginable. Organic chemistry or carbon chemistry is an independent part of chemistry. The element carbon occupies an exceptional position in target preparation for nuclear measurements due to its melting point, the highest of all elements, and also due to its very low vapor pressure and the good chemical stability which is comparable to that of the noble metals. Carbon has only two stable isotopes and one radioactive isotope, the half-life of which is long.enough to prepare targets in a standard way. The natural abundance of “C is nearly 99% which means that a natural carbon foil can be used in most cases as isotopic ‘*C target. The good tensile strength of self-supporting carbon foils can be estimated by their minimum obtainable thickness. Carbon foils can be prepared even below 1 p.g/cm* (= 5 nm), which is one order of magnitude thinner than those which can be produced from any other element. Most accelerator measurements which use thin targets which are less than 100 kg/cm* must have a thin backing material and this is in most cases carbon in the thickness range of 3-30 Fg/cm*. Carbon foils are indispensable in nearly all experiments in nuclear physics and related subjects performed at ion accelerators. The ion energy necessary to perform the wanted nuclear reaction is usually too high to be achieved by acceleration of a single charged particle. That is why mostly carbon stripper foils are installed in the beam line of the accelerator to increase the charge state of the accelerated ions. Due to the limited lifetime of these carbon foils, especially in heavy ion beams, the accelerators themselves are the major users of carbon foils. 0168-583X/93/$06.00
methods
2.1. Mechanical procedures Some of the mechanical procedures used to produce thick carbon foils are as follows: Hot pressed carbon or graphite material can be grinded and polished to a minimum thickness of 20 mg/cm*. Single crystals of graphite can be cleaved, the minimum self-supporting thickness is about 2 mg/cm*
111. Diamonds may be cut and polished down to a thickness of 50 mg/cm* [2]. Graphite powder can be settled and pressed for carbon foils of 1 mg/cm* minimum thickness [3]. - Carbon foils can also be prepared using a liquid in which graphite powder is disperged [4]. Such suspensions are utilized in painting [5] or spray painting methods. Carbon foils of such a kind can have a minimum thickness of 10 kg/cm*, but the homogeneity is doubtful. 2.2. Evaporation-condensation
of carbon in high vac-
uum The second method, the evaporation-condensation process in which the foils are produced in a high vacuum, is the most commonly used procedure for the preparation of self-supporting carbon foils. Thicknesses prepared in this way are from less than 1 pg/cm* up to greater than 1000 kg/cm*. The evaporation of carbon is carried out by three different methods: a) resistance heating of carbon filaments; b) arc evaporation; and c) electron bombardment heating.
8 1993 - Elsevier Science Publishers B.V. All rights reserved
XIV. ACCELERATOR
TARGETS
842
P. Maier-Konwr / CarbonfoirSfor nuclear acceleratorexperiments
The resistance heating method is used most commonly, but the method published first by Bradley [6] has been modified by many laboratories. Bradley described this method as carbon-arc evaporation despite the fact that the two electrodes are always in physical contact. Two spectroscopically pure graphite rods. having pointed ends and being clamped horizontally in water cooled supports, are held in contact by springs while a current of more than 100 A passed through them. Sufficient temperature is generated to cause evaporation of carbon. Dearnaley [7], Sarma [8], and McCormick and McCormack [9] reported similar setups. Nobes [lo] modified this method by mounting the carbon rods vertically and by exchanging the spring loading by gravity using a weight of 0.8 N. The disadvantage of the methods described before is, that the carbon rods must be sharpened several times to evaporate enough carbon for thicker films, which requires breaking of the vacuum. Maier-Komor [ 1l] avoided this inconvenience by resistance heating of a 3 mm diameter carbon rod mounted in fitting boreholes of springloaded thick carbon rod electrodes. The real arc evaporation was published first by Pfeiffer [12] and Blue and Danielson [13], but, they did not notice a difference regarding the Bradleys method. The arrangement of the carbon evaporator is indeed the same as that of the Bradley method. In addition a push-pull feedthrough is needed, which is connected to one electrode to adjust the arc gap from outside the vacuum system. When carbon stripper foil -problems became serious Takeuchi et al. [14] and Sugai et al. [15,16] tried to improve the carbon arc procedure. Their reported improvement in stripper foil lifetime has not been confirmed by other accelerator laboratories. Electron bombardment heating, the third method, described first for carbon evaporation by Maxman [17] and Morgan [18], is still the state of the art when thermal evaporation of carbon is applied [19], because with this method the control of the evaporation rate is relatively simple and the carbon source is nearly unlimited. Either a thick, long carbon rod can be fed vertically into the electron gun crucible, or alternatively for isotopes the powdered carbon can be pressed into a suitable sized pellet for insertion in the crucible. 2.3. Carbon foils prepared by sputtering Sputtering is mainly used for industrial applications, when thin films having good adherence to the substrate are needed. Since the sputtering rates for carbon are very low, the deposition time is very long. In addition, the problem of separating the carbon foil from its substrate cannot easily be solved. Baumann and Wirth [20] tried to develop the heavy ion beam sputtering method for carbon in order to improve carbon stripper
foil qualities, but the very low efficiency of the floating process led them to give up this method. Sugai et al. [15,21] also tried this method but with more effort, but they also could not report to have solved all the problems mentioned above. 2.4. Chemical vapor decomposition Thermal cracking is only utilized in the preparation of 14C foils. The vap.or used is methyl-iodide. Setups are described in detail in refs. [22-261. An improvement of this old method was reported by Maier 1271. The obtainable carbon foil thickness range is 4-100 pg/cm’. 2.5. Additional methods utilized only for preparing carbon stripper foils The first method, the cracking of hydrocarbon gases in a dc glow discharge process was developed by Kiinig and Helwig in 1950 [28]. It was introduced for preparing carbon stripper foils by Tait et al. [29] in 1979 and modified or further developed by many laboratories (see e.g. ref. [30]). It was the only proved method for producing carbon foils having better stripper properties for nearly, ten years. A theoretical explanation for the good quality of the dc glow discharge foils was first given by Dollinger et al. [31,32] in 1988. They presented the theory of heavy ion irradiation damage in carbon foils on the basis of electron diffraction and electron transmission microscope investigations. The postulation for best stripper qualities was an isotropic orientation of the quasi-graphitic nanocrystals in the polycrystalline carbon foil. Carbon foils prepared by thermal evaporation are completely anisotropic, which explains their minor quality, but glow discharge foils show a partially isotropic distribution. The postulated isotropic structure could be realized when Dollinger and Maier-Komor [33] succeeded in preparing thin carbon stripper foils using laser plasma ablation-deposition of carbon in high vacuum.
3. Preparation and handling All described preparation methods deliver a film on a substrate. In order to get a self-supporting foil a parting agent is necessary which functions as an interface between the substrate and the deposited carbon film. In most cases it is a water-soluble compound [34] because the parting process is done in distilled water. But in some cases, especially when the deposition process is hot (thermal cracking) or ions are impinging on the substrate (e.g. glow discharge), thin metal foils mainly nickel or copper are used as a substrate. These metal foils are first dissolved in acids and the carbon
P. Maier-Komor
a43
/ Carbon foils for nuclear accelerator experiments
foil is then washed in distilled water. The floating process is described e.g. in ref. [34]. When preparing carbon foils using the standard so-called carbon arc method, various detergents are used as parting agents. Stoner [35], who seems to have the most experience with this method, prefers Creme-Cote. Detergent parting agents, however, are squeamish to heat; as a result they usually fail when electron bombardment evaporation is applied, because in this procedure the hot carbon source is larger and results in more substrate heating. That is one reason why betaine [ll] is preferred for this method. In addition, betaine results in a corrugated structure of the carbon foil, with the advantage of much better mechanical strength. Also for some applications salts [34] are used as parting agents. Depending on the parting agent, however, a different surface roughness of the carbon film is achieved, which means high or low homogeneity of the carbon foil. Brasky [36] and Abele et al. [37] investigated the homogeneity problems related to the parting agent. Carbon foils can be thickness calibrated very accurately if kg/cm* is the measuring unit [38,39]. Due to the low cost of the source material, which is spectroscopically pure graphite, quite large source-to-substrate distances can be used. In this way the deviation of the measured quartz thickness to the real substrate thickness caused by their slightly different locations is very small. A suitable setup could guarantee an error of < 2%. Most suppliers of carbon, foils measure the thickness by light transmittance and declare an error of < 10%. The light transmittance measurements [40], however, depend on crystal structure and impurity content of the carbon foil. The bulk impurities in standard carbon foils come from the residual gas in the vacuum process and are small when using a vacuum of better low4 Pa. Most impurities are on the side of the foil which was floated from the parting agent [41]. On the opposite side, mainly water is adsorbed, which remains even in vacuum, if the foil is not heated 1411. That is why even carbon foils seem to be thicker than quoted, when a-energy loss is used to control the thickness.
Armitage et al. 1431 and further developed by MaierKomor [44]. If further improvement is needed, then only the two techniques of dc glow discharge or laser plasma ablation deposition remain. The glow discharge carbon foils are produced in many laboratories and might be the first choice, if there would not be the poor reproducibility of properties from one deposition to the next one. Foils prepared by laser plasma ablation are the best. The preparation is well controlled which means that there is a reproducibly good quality of all foils [45]. Unfortunately these foils are so far not available in larger quantities.
5. Conclusion The variety of procedures used to prepare carbon foils meets all requirements for nuclear accelerator experiments as well as most other cases, where carbon foils are needed. Even the problem of distruction of carbon foils by heavy ion irradiation damage is solved, because a physical limit seems to be achieved for foils prepared by laser plasma ablation-deposition as irradiation damage calculations show for a foil penetrated by a 10 MeV ‘*‘I beam. In this foil each carbon atom was knocked 60 times from its place on average and about three times the amount of carbon atoms, which were placed in the original beam spot area, had left the foil by sputtering. This material loss was possible because the decrease of foil thickness was partly compensated by nearly the threefold material compared to the one originally present in the irradiated area, which was drawn into the beam spot during irradiation. Cheaper carbon deposition techniques, might be developed. If they show really the superior quality of foils prepared by laser plasma ablation-deposition can now be easily controlled by electron microscopy [31].
References [ll G.E. Myers and G.L. Montet, J. Appl. Phys. 37 (1966)
4. Carbon stripper foils If light ions up to oxygen are to be stripped, then standard carbon foils prepared by vacuum evaporation-condensation can be applied. They are commercially available if not prepared in ones own laboratory. But even for the light ions the foil thickness should be carefully chosen in order to achieve the optimum ion transmission for the wanted charge state [42]. For heavy ions even some improvement can be done when slackening of the foils is utilized as introduced by
4195. 121M. Rebak, J.P. F. Sellshop, T.E. Deny and R.W. Fearick, Nucl. Instr. and Meth. 167 (1979) 115. 131W.R. Lowzowski,Proc. 6th Ann. Conf. INTDS, Berkeley (LBG7950) (1977) p. 115. 141V.E. Viola Jr. and D.J. O’Connell, Nucl. Instr. and Meth. 32 (1965) 12.5. [S] R.D. McCormick and J.D. McCormack, Nucl. Instr. and Meth. 13 (1961) 151. [6] D.E. Bradley, Brit. J. Appl. Phys. 5 (1954) 65. 171 G. Deamaley, Rev. Sci. Instr. 31 (1960) 197. [81 N. Sarma, Nucl. Instr. and Meth. 2 (1958) 361.
XIV. ACCELERATOR
TARGETS
844
P. Maier-Komor / Carbon foils for nuclear accelerator qwiments
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