Surface characterization of diamond-like carbon for ultracold neutron storage

ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 587 (2008) 82–88 www.elsevier.com/locate/nima

Surface characterization of diamond-like carbon for ultracold neutron storage F. Atchisona, A. Bergmaierb, M. Dauma, M. Do¨belia,c, G. Dollingerb, P. Fierlingera,1, A. Foelskea, R. Hennecka,, S. Heulea,d, M. Kasprzaka,e, K. Kircha, A. Knechta,d, M. Kuz´niaka,f, A. Pichlmaiera, R. Schelldorfera, G. Zsigmonda a Paul Scherrer Institute, CH-3052 Villigen, Switzerland Universita¨t der Bundeswehr Mu¨nchen, D-85577 Neubiberg, Germany c ETH Zu¨rich, CH-8093 Zu¨rich, Switzerland d University Zurich, CH-8057 Zurich, Switzerland e Stefan Meyer Institute, Vienna, Austria f Marian Smoluchowski Institute of Physics, Jagiellonian University, Cracow, Poland b

Received 4 October 2007; received in revised form 21 December 2007; accepted 21 December 2007 Available online 5 January 2008

Abstract We report the characterization of diamond-like carbon (DLC) surfaces to be used for the storage of ultracold neutrons (UCN). The samples investigated were 100–300-nm-thick tetragonal amorphous carbon (ta-C) coatings produced by vacuum-arc technology on thin foils (0.1–0.2 mm aluminum, stainless steel, PET). The diamond sp3 fraction was determined by X-ray photoelectron spectroscopy (XPS) to be in the range 45–65%. Secondary-ion mass spectroscopy (SIMS) and elastic recoil detection analysis (ERDA) yielded consistent results for the hydrogen contribution (about 1  1016 cm 2 within the top 20 nm), strongly concentrated within a surface layer of 1 nm thickness. The boron contamination was found to be around 50 at. ppm. The fractional hole area of the coatings is on a level of about 1  10 4. Temperature cycling of mechanically pre-stressed samples between 77 and 380 K revealed no detrimental effect. r 2008 Elsevier B.V. All rights reserved. PACS: 03.75.Be; 28.20.Gd; 61.10.Ht Keywords: Diamond-like carbon (DLC); sp3 content; X-ray-induced photoelectron spectroscopy (XPS); Secondary-ion mass spectroscopy (SIMS); Elastic recoil detection analysis (ERDA); Storage of ultracold neutrons

1. Introduction Storage of ultracold neutrons (UCN) in material bottles is one of the key elements in several experiments to observe fundamental properties of the neutron, e.g. the neutron lifetime or a hypothetical electric dipole moment (for a review on UCN physics see Refs. [1,2]). The storage involves mainly two aspects: (a) the critical velocity, up to which the neutrons can be stored, determined by the material’s Fermi potential and (b) the loss probability per wall collision, depending on Corresponding author. Tel.: +41 56 310 5157; fax: +41 56 310 5230. 1

E-mail address: [email protected] (R. Henneck). Now at Stanford University, Stanford, CA, USA.

0168-9002/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.12.037

the loss cross-sections of the materials at the surface. Since UCN probe the surface to a depth of typically 10 nm, the surface quality with respect to contamination with strong absorbers, defects, cracks, coating artifacts, etc. becomes highly important. Particularly important for UCN applications is the requirement for low hydrogen contamination since hydrogen has a large incoherent cross-section (about 80 b at thermal energy), resulting in temperature-dependent up-scattering to velocities above the critical velocity and subsequent loss from the system. The relevance of this issue has been recognized in the 1980s and a series of investigations has been devoted to measuring and removing hydrogen for then commonly used good UCN reflector materials like beryllium, copper, steel, graphite and glass [3–8].

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Recently, diamond-like carbon (DLC)-coated materials have been shown to be a viable alternative to the toxic beryllium coatings, in terms of the critical velocity as well as for the losses [9–14]. The DLC density determines the critical velocity vC for neutron reflection (which corresponds to the maximum velocity reflected under any angle of incidence) and should thus be as high as possible. For carbonic substances, vC varies between 6.1 m/s (zero porosity graphite of density 2.25 g/cm3, 100% sp2 bonding) and 7.65 m/s (diamond, density 3.5 g/cm3, 100% sp3 bonding). The ratio sp3/(sp2+sp3) is called the sp3 fraction and is directly related to the density and thus to the critical velocity. Another important point is the preparation of DLC, essentially free of neutron absorbing elements (including hydrogen) within a top layer corresponding to the neutron penetration depth upon reflection. DLC with high sp3 fraction and very low hydrogen contribution (usually referred to as ta-C) can be produced by physical vapor deposition methods such as pulsed laser deposition (PLD) [15] or vacuum-arc technology [16]. With respect to the different losses measured for DLC on two types of substrates (Al and polyethyleneterephtalate (PET) [10,14,17]), the interesting question is to study the hydrogen contamination with good depth resolution within the UCN penetration depth. In view of these questions, we have conducted a systematic investigation applied to good-quality ta-C coatings (sp3X0.5), which were prepared by vacuum-arc deposition. We have made use of the experience gained in previous studies of beryllium coatings [18] and of ta-C (with special emphasis on the sp3 determination [19]). In this study, we concentrate mostly on the sp3 determination, the fractional hole area, the surface roughness and the top layer elemental composition for the large-area samples used in Refs. [10–14,17]. 2. Sample preparation The following sample types were tested: (a) DLC(Al); 0.2-mm-thick aluminum foils, coated by a layer of approximately 120-nm-thick DLC on a thin interface layer of Ti/ TiO2, (b) DLC(PET); 0.2-mm-thick PET foils, coated by a layer of approximately 120-nm-thick DLC, (c) DLC(SS); 0.1-mm-thick stainless steel foils, first coated electrochemically by several micrometers of Ni and then by 200–300 nm of DLC. The samples were cut from larger sheets (up to 80 cm  300 cm) of DLC-coated foils, commercially available from Fraunhofer Institut fu¨r

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Werkstoff- und Strahltechnik (IWS), Dresden, Germany. The coatings were produced by laser-controlled vacuumarc deposition [16] in a large vacuum chamber for industrial coating applications. Homogeneity of the coating thickness is of the order of 5% over length scales around 20 cm and 10% over length scales of meters [20]. The graphitic precursor material was high-purity graphite. Before coating, the substrates were cleaned in situ in an argon discharge. The characteristics of the different coatings and the most relevant results are summarized in Table 1. By eye the DLC coatings on PET have a yellowish/ brownish tinge; DLC on Al appears between clearly transparent and slightly brownish, while DLC on SS has a transparent, brownish tinge. 3. X-ray-induced photoelectron spectroscopy (XPS) Based on an initial study, where we compared various methods to determine the sp3 fraction (more exactly the ratio sp3/(sp2+sp3+C–O contamination)) [19,21], we selected X-ray photoelectron spectroscopy (XPS) as the method most relevant to UCN applications because the typical UCN penetration depth (10 nm) is only slightly larger than the average electron escape length (2 nm). Small specimens were cut from the large foils and analyzed without further cleaning; the procedures used follow closely the ones described in Ref. [19]. The results for the sp3 fraction are included in Table 1. A typical spectrum for DLC(Al) with a low sp3 fraction is shown in Fig. 1, while a spectrum for a high sp3 fraction can be seen in Ref. [19]. A small impurity component (corresponding to various carbon–oxygen bindings) on a level of about 5–10 at% was included in the analysis of the carbon 1s peak (see also Ref. [22]). Apart from carbon, we also found small admixtures of oxygen (5 at%) and nitrogen (1 at%) within the first nanometer of the coating, which could be easily removed by argon sputtering. 4. Surface defects, surface roughness and thermal cycling The samples were inspected by optical microscopy in order to search for surface defects, scratches, pinholes etc. Figs. 2–5 show examples of typical surfaces. The inspection was done in a clean room without surface-cleaning except for a blow-off with dry nitrogen. All surfaces show a certain amount of dust/dirt particles; no effort was made to determine whether these were incorporated during coating

Table 1 The diamond sp3 fraction, the fractional hole area FH and the surface-hydrogen areal density NH for the different coating types Coating

Foil thickness (mm)

Coating thickness (nm)

sp3 fraction

FH (  10 5)

NH (  1016 at/cm2)

DLC(Al) DLC(PET) DLC(SS)

0.2 0.2 0.1

120–150 120–150 200–300

0.4570.05 0.6570.06 0.4570.05

1575 772

1.070.2 0.970.2

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Fig. 1. XPS spectrum of a typical DLC(Al) sample, with its decomposition in background (dashed line) and the peaks corresponding to sp2 (at 284.4 eV, 46% relative contribution), sp3 (at 285.2 eV, 43% relative contribution) and C–O (at 286.5 eV, 11% relative contribution).

Fig. 3. Microscope picture of a typical DLC(PET) surface; vertical extent 90 mm. The light-colored patches are presumably holes; the many smaller specks are thought to be coating residues and/or dirt particles. The surface texture resulting in the increased roughness of Rrms 100 nm is clearly visible.

Fig. 2. Microscope picture of a typical DLC(Al) surface; vertical extent 100 mm. The light-colored patches are interpreted as holes and the dark club-like structures as graphitic residues from the coating process. The many smaller specks are thought to be coating residues and/or dirt particles. Clearly visible is also the vertical substrate structure due to the sheet-rolling process.

Fig. 4. Microscope picture of a typical DLC(Si) surface, prepared by magnetically filtered high-current arc deposition [23]; vertical extent 180 mm. Compared to the DLC surfaces prepared by the industrial vacuum-arc process, the amount of dirt particles is considerably reduced (cf. Figs. 2 and 3).

or afterwards. By contrast, surfaces prepared on silicon by magnetically filtered high-current arc deposition [23] in a small research-type facility (see Fig. 4) are considerably cleaner due to the much smoother surface and to the cleaner environment during coating. In order to obtain quantitative results for the ‘fractional hole area’ FH (i.e. the ratio ‘open substrate area’ to ‘DLCcoated area’), we used microscopy and X-ray-induced photoelectron spectroscopy (XPS). We have shown recently [18] that the fractional hole area measured in this way for a small surface area is consistent with the transmission of UCN through thin foils of much larger

area and can therefore be regarded as a reliable measure for FH. For DLC(PET), we took advantage of the fact that PET is luminescent with emission in the UV regime. Using a microscope where the light reflected from the sample was passing a UV filter facilitated the hole search and made the determination more reliable as compared to using visible light. Fig. 5 shows a DLC(PET) sample with a typical hole of diameter 20 mm. Scanning over two samples (about 5 cm2 area each) with a resolution to detect holes larger than about 10 mm2, we extracted FHE7  10 5 with an estimated uncertainty of about 20%. The measured value is mainly

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Fig. 6. DLC(SS) sample mounted in bending test device.

radius without the development of cracks, delamination, etc. arises. This was investigated by inspecting bent samples under the microscope. As expected, the number of cracks which develop during bending increases with decreasing radius, such that a bending radius rX10 mm is considered safe, for e.g. DLC on SS substrates. Fig. 6 shows the mechanical setup of a DLC(SS) sample subjected to a bending radius of about 15 mm. Two such samples were subjected to slow temperature cycling between 77 and 380 K, with transition times of the order of 2–3 h. Inspection by microscope after 12 such cycles revealed no visible cracks. Fig. 5. Microscope picture of a typical DLC(PET) surface, observed through a UV filter; vertical extent 100 mm.

due to large holes, on a level of up to several 1000 mm2, which are rather homogeneously distributed. Holes in DLC(Al) could not be identified reliably by optical microscopy; therefore, we used XPS with a Mg Ka-line anode which allows to probe over an area of about 0.5 cm2 (for details see Ref. [18]). Since the sensitivity for Ti is about five times larger than that for carbon, observation times around 10 h were sufficient to detect the Ti- and/or Ti–Ox 2p3/2 lines (between 254 and 259 eV) on a level below 10 4. Altogether three samples were measured, with roughly consistent results, leading to FHE15  10 5, with an estimated uncertainty of about 30%. The present results for the fractional hole area are comparable to those obtained for beryllium coatings [18]. The roughness of the surface was evaluated by atomic force microscopy (AFM). For DLC(PET), we deduced an average roughness Rrms 100 nm and a correlation length of about 1 mm. For DLC(Al), the residues from the sheetrolling process along one direction are clearly visible (see Fig. 2). We obtained an average roughness Rrms 20 nm and a correlation length of about 4 mm along the roll direction and 0.3 mm against the roll direction. Concerning the potential presence of ‘loose’ nanoparticles in the size range around 10 nm, which has been invoked by Ref. [24] as a source of inelastic up-scattering with small energy transfer, we cannot make a definite statement given the roughness level of our samples. Based on our experience with AFM, we consider detecting such particles to be a challenging task even on much smoother surfaces. For applications where one wants to construct three-dimensional storage vessels by bending, rolling etc. (see e.g. Refs. [10,12]), the question of minimum bending

5. Elastic recoil detection analysis (ERDA) Elastic recoil detection analysis (ERDA) with heavy ions at grazing incidence is a very sensitive method to probe for low-level light element admixtures. Utilizing a magnetic spectrograph depth resolution better than nanometers can be achieved, in special cases even single atomic layer resolution [25]. Details of the experimental arrangement can be found in Ref. [26]; for the present experiment, 40 MeV 197Au ions were used at 41 incidence (with respect to the sample surface) and a scattering angle of 151. The depth resolution of the system varied from about 0.5 nm FWHM at the surface to about 2 nm at a depth of 20 nm (limited by energy loss straggling and small angle scattering effects). The irradiated area was about 1 cm2. The integral fluence and fluence rate (about 5  1010 and 5  107 cm 2 s 1, respectively) were low enough to avoid local heat up, in addition it was verified that the hydrogen content did not vary over the irradiation time. The normalization was obtained by monitoring the incident beam intensity. Fig. 7 shows the hydrogen distribution near the surface for DLC(Al) and DLC(PET) as a function of the depth. The sub-surface hydrogen tail as well as the difference in the hydrogen distribution between DLC(Al) and DLC(PET) may be due to surface roughness and its variation between the aluminum and PET substrate: for 41 incidence and depending on the roughness, a considerable fraction of the incident ion beam can lose energy by passing through surface elevations before knocking out a top-layer hydrogen nucleus; such events are then erroneously recorded as due to sub-surface hydrogen location and would produce the sub-surface tail. The hydrogen density relevant for a mean neutron energy of 66 neV (as in Ref. [10]) was estimated by integrating over the corresponding

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depth [nm] 0

2

4

6

8

10

12

16

18

20 concentration [at-%]

40

hydrogen concentration [at-%]

H

1.E+02 14

DLC(Al) 30

20

DLC / Ni / SS

1.E+01

B11 C

1.E+00 Cr

1.E-01

Fe

1.E-02

Ni

1.E-03 1.E-04 0

DLC(PET)

10

50

100 150 depth [nm]

200

250

Fig. 8. Elemental composition of a DLC(SS) sample as a function of depth, measured by SIMS depth profiling.

0 0

1

2

3

depth [1017at/cm2]

penetration depth of 18 nm (or 2.6  1017 at/cm2 in DLC with 50% sp3). We obtained NH 1  1016 at/cm2 for both cases, DLC(Al) and DLC(PET), with an estimated uncertainty of about 20%. These findings are in good agreement (to within 30%) with the results of another ERDA investigation performed at ETH Zurich [27] obtained with lower depth resolution and using mica with a bulk hydrogen concentration of 9.5 at% for normalization. By contrast, the depth resolution of SIMS is much lower. Still, an indication of the surface peak is visible for DLC(Al) (see Section 6 and Fig. 9). At larger depths, the concentration measured by SIMS is in reasonable agreement (within a factor 2) with the ERDA results. 6. Secondary-ion mass spectroscopy (SIMS) Secondary-ion mass spectroscopy (SIMS) is a quantitative method to determine the elemental composition on and within the material surface, once the efficiencies for removing a certain element from the host matrix and for its detection are known. In practice, this is done by using calibration samples with known concentrations of the element in question within the same host matrix. The SIMS depth-profiling measurements of DLC(Al) and DLC(SS) samples (DLC(PET) could not be done, PET being an insulator) were performed at the Fraunhofer Institut fu¨r Schicht-und Oberfla¨chentechnik, IST, Braunschweig, Germany with an FEI-Atomika instrument, using Cs+ ions between 0.5 and 3 keV as primaries [28]. Our main objective was to look for boron and hydrogen. The boron calibration was achieved by evaluating various boron

DLC / Ti / Al

concentration [at-%]

Fig. 7. The relative hydrogen concentration (i.e. cH/(cH+cC)) versus depth at the surface of the DLC coatings, measured by ERDA [26]. The depth values in nm (upper axis) have been obtained by assuming a sp3 fraction of 50%. Integrating over the mean penetration depth of 18 nm, one obtains a hydrogen areal density of 1016 at/cm2 in both cases; however, much more surface concentrated for DLC(Al).

H B C O Al Ti

1.E+02 1.E+01 1.E+00 1.E-01 1.E-02 1.E-03 0

50

100 150 depth [nm]

200

250

Fig. 9. Elemental composition of a DLC(Al) sample as a function of depth, measured by SIMS depth profiling.

samples doped at levels between 350 and 10,000 at-ppm. The hydrogen calibration employed DLC coatings produced by reactive rf sputtering with a hydrogen atomic concentration of 15 at-%, measured by ERDA [29]. Fig. 8 shows a depth profile obtained for a DLC(SS) sample. Depth calibration was achieved by measuring the depth profiles of various sputter craters produced within specified times. This allows relating sputter time to depth. The Ni layer (correlated with a peak in Fe and Cr admixture) is clearly visible at the transition from the DLC coating to the Ni/SS substrate at a depth of about 210 nm. The boron content is constant on a level of about 50 atppm throughout the coating and decreases in the substrate. Hydrogen reveals a similar behavior, with about 1 at-% throughout the coating and much less below. The results for DLC(Al) (see Fig. 9) within the coating are qualitatively similar; interestingly enough, the Ti/TiOx interface layer at a depth of 130 nm is connected to a peak in the boron and hydrogen content. 7. Discussion The hydrogen areal density within the top layer of the DLC was measured independently at two different ERDA

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facilities, for different samples and at different times after the sample production, yielding consistent results. From this, we conclude that the hydrogen admixture was stable over the time between the two measurements (1 year) and that it was also the same during the neutron experiment [10], which happened 1 year before (the foils were coated another 4 months before). Our results are at the lower end of the range of areal density values reported in the literature for untreated UCN storage surfaces: Lanford and Golub [3] reported minimum hydrogen coverages between about 1016 and 1017 at/cm2 (integrated over the top 10 nm) for polished copper, graphite and glass bulk samples. Heating at temperatures as high as 265 1C for up to 60 h in vacuum resulted in a factor of 2 improvement. Ref. [4] reported 2–3  1016 at/cm2 within a depth of 20 nm from the surfaces of copper, stainless steel and nickel evaporated on glass, the reduction to about one-tenth of these values by ion bombardment in good vacuum being only momentary. Argon glow discharge was used to reduce the hydrogen density from an initial 2  1016 at/cm2 (integrated over the top 20 nm) by a factor of 6. Ref. [8] found densities from 2 to 4  1016 at/ cm2 on commercial beryllium plates and around 1  1016 at/cm2 for Ni surfaces evaporated on polished Al, Cu and Si substrates at pressures below 10 4 Pa. Deuteration or heating to 100 1C in vacuum gave only moderate improvement. Ref. [7] reported on several hydrogen reduction procedures (deuterium replacement, continuous in situ evaporation, gas discharge cleaning and heating to 1000 1C) and found that values below 1014 at/ cm2 after high-temperature baking and around a few times 1015 at/cm2 after oxygen discharge cleaning were obtained, starting from values around 1016 at/cm2. After baking, the hydrogen recontamination in ultrahigh vacuum appeared to saturate around 5  1015 at/cm2 over a period of about 10 h. 8. Conclusions In the study presented, we have characterized DLC surfaces which had been shown to display excellent UCN storage behavior. The good storage characteristics are supported by the present results: (i) the DLC sp3 fraction, measured by XPS, ranges from 0.45 to 0.65, corresponding to Fermi potentials between about 240 and 270 neV; (ii) the fractional hole area is on a level of 1  10 4, comparable to what has been observed e.g. for beryllium coatings [18]; (iii) the boron content is negligibly low and cannot explain the observed temperature-independent losses [14]. The hydrogen profiles presented were obtained with a depth resolution of 1 nm, a factor of at least 10 better than those measured with narrow-width nuclear resonance reactions like H(11B,a)2a [4] or H(15N,ag)12C [3,7], allowing to extract more detail about the depth distribution. Since the amplitude of the neutron wave function is exponentially decreasing in the material, it can be expected that the hydrogen depth profile will strongly influence the loss

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behavior: a given amount of hydrogen within the first 1 nm will produce more loss than the same amount of hydrogen spread over the full penetration depth. The quantitative verification of this behavior would require a suitable and sufficiently accurate treatment of the problem of UCN reflection from a contaminated surface. Unfortunately, this to our knowledge at the moment is not available. Acknowledgments This work was performed at the Paul Scherrer Institute, Villigen, Switzerland. We appreciate the help and advice provided at PSI by K. Ballmer, T. Lippert, R. Koetz, B. Theiler, J. Wambach as well as the support at Fraunhofer Institut fuer Werkstoff-und Strahltechnik, IWS, Dresden, Germany, by C.-F. Meyer, B. Schultrich, D. Schneider, T. Stucky and at Fraunhofer Institut fuer Schicht- und Oberflaechentechnik, IST, Braunschweig, Germany by P. Willich. References [1] R. Golub, D.J. Richardson, S.K. Lamoureaux, Ultra-Cold Neutrons, Adam Hilger, Bristol, Philadelphia and New York, ISBN 0-75030115-5, 1991. [2] V.K. Ignatovich, The Physics of Ultracold Neutrons, Clarendon Press, Oxford, ISBN 0-19-851015-2, 1990. [3] W.A. Lanford, R. Golub, Phys. Rev. Lett. 39 (1977) 1509. [4] J.P. Bugeat, W. Mampe, Z. Physik B 35 (1979) 273. [5] W. Mampe, J.P. Bugeat, Phys. Lett. 78A (1980) 293. [6] W. Mampe, P. Ageron, R. Gaehler, Z. Physik 66 (1981) 3052. [7] P.H. LaMarche, W.A. Lanford, R. Golub, Nucl. Instr. and Meth. 189 (1981) 533. [8] Y. Kawabata, M. Utsuro, S. Hayashi, H. Yoshiki, Nucl. Instr. and Meth. B 30 (1988) 557. [9] M.G.D. van der Grinten, J.M. Pendlebury, D. Shiers, C.A. Baker, Nucl. Instr. and Meth. A 423 (1999) 421. [10] F. Atchison, T. Brys, M. Daum, P. Fierlinger, A. Foelske, M. Gupta, R. Henneck, S. Heule, M. Kasprzak, K. Kirch, R. Koetz, M. Kuzniak, T. Lippert, C.F. Meyer, F. Nolting, A. Pichlmaier, D. Schneider, P. Siemroth, U. Straumann, Phys. Lett. B 625 (2005) 19. [11] F. Atchison, T. Blau, M. Daum, P. Fierlinger, A. Foelske, P. Geltenbort, M. Gupta, R. Henneck, S. Heule, M. Kasprzak, K. Kirch, M. Kuzniak, M. Meier, A. Pichlmaier, C. Plonka, R. Reiser, B. Theiler, O. Zimmer, G. Zsigmond, Phys. Lett. B 642 (2006) 24. [12] F. Atchison, B. Blau, M. Daum, P. Fierlinger, P. Geltenbort, R. Henneck, S. Heule, M. Kasprzak, K. Kirch, K. Kohlik, M. Kuzniak, M. Meier, C.F. Meyer, A. Pichlmaier, C. Plonka, P. Schmidt-Wellenburg, B. Schultrich, T. Stucky, V. Weihnacht, O. Zimmer, Phys. Rev. C 74 (2006) 055501. [13] F. Atchison, B. Blau, M. Daum, P. Fierlinger, P. Geltenbort, M. Gupta, R. Henneck, S. Heule, M. Kasprzak, A. Knecht, M. Kuzniak, K. Kirch, M. Meier, A. Pichlmaier, R. Reiser, B. Theiler, O. Zimmer, G. Zsigmond, Nucl. Instr. and Meth. B 260 (2007) 647. [14] F. Atchison, T. Brys, M. Daum, P. Fierlinger, P. Geltenbort, R. Henneck, S. Heule, M. Kasprzak, K. Kirch, A. Pichlmaier, C. Plonka, U. Straumann, C. Wermelinger, G. Zsigmond, Phys. Rev. C 76 (2007) 044001. [15] A.A. Voevodin, M.S. Donley, Surf. Coat. Technol. 82 (1996) 199. [16] H.J. Scheibe, B. Schultrich, Thin Solid Films 246 (1994) 92. [17] T. Brys, M. Daum, P. Fierlinger, P. Geltenbort, D. George, M. Gupta, R. Henneck, S. Heule, M. Horvat, M. Kasprzak, K. Kirch, K. Kohlik, M. Negrazius, A. Pichlmaier, U. Straumann,

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