On the determination of carbon using charged particle accelerators

Nuclear Instruments and Methods in PhysicsResearch B66(1992) 139-145 North-Holland

Nuclew instruments & Methods Physics Research Section B

On the determination of carbon using charged particle accelerators R.D . Vis Faculty of Physics andAstronomy, Free University, Amsterdam, The Netherlands

In this paper, various aspects of the determination of carbon with beams of charged particles are discusset. Although initially analyses were performed with several forms of activation analysis, at present the majority of the work is done with in beam techniques, such as nuclear reaction analysis and prompt radiation analysis. Some methods for the determination of isotopic ratios will also be described. 1. Introduction Quantitative determination of carbon and its distribution are of importance in various areas of science. The quality of steel, weldings and resistance against corrosion depend for a great deal of the proper carbon concentration and its homogeneous distribution . In biology, carbon is of course a major element. Nevertheless, the determination of carbon in the hard tissues bone and teeth is of importance as the organic fraction plays an important role in these tissues. A nice example of the use of a stable tracer to follow biochemical processes is the use of t3C build in a suitable compound and analyzed after the metabolic process of interest with a suitable nuclear reaction . Especially if one can obtain the distribution of t3C in a biological section, one can study these metabolic processes in great detail . Sometimes, signals originating from carbon are used for the normalization of trace element determinations in biological samples. Nuclear backscattered protons are used to normalize the trace element signals to the amount of biological material irradiated. In geology, inclusions, and especially the fluid ones are analyzed for carbon compounds. Also in meteorites, the abundance of carbon is extensively studied to reconstruct the cooling history of carbonaceous and ordinary chondrites . In archeology, carbon of course is crucial for the determination of the age of a variety of subjects, for which purpose accelerators are being used as sophisticated mass spectrometers capable of determining ultra-traces of t4C. 2. Nuclear reactions used 2.1 . Charged particle activation analysis Initially, activation analysis with charged particle beams has been used to determine carbon . On 1ZC

reactions of the type (d, n), (3He, a) and (p, pn) were commonly used . The (p, pn) reaction has been studied in great detail, the excitation curve is known within an accuracy between 3 and 5% [1] and this reaction has been used as a standard for measuring other excitation functions. The disadvantage of these reactions is that the products are pure positron emitters, necessitating unfolding procedures to extract the isotope with the right half life . Standardization has been done with the numerical integration method [2] using excitation functions from literature or by usingappropriate standards . An attractive option for activation analysis is the use of the ('He, n) reaction on 1ZC, leading to 14 0 with a half-life of 71 s which emits 2.31 MeV -y-radiation during its decay. Activation analysis with 3He particles was described in detail by Ricci et al. [3] who reported detection limits down to 1 ppm for carbon . The 1ZC(3He, a)ttC reaction has been used by Misaelideset al . [4] for the determination of C in semiconductors. In this work, and more extensively in the work of Liebler et al. [5] the excitation function of this reaction is given. The (d, n) reaction, leading to t3N as final nucleus, was investigated by Michelmann et al . [6]. The cross section of the latter reaction is slightly smaller than the values for the 3He induced one. For these analyses, high beam currents are needed to introduce sufficient radioactivity. Real microanalysis is not conceivable since the currents in microbeams of charged particles are generally too low for activation analysis . Therefore, in beam reactions are commonly used for microanalysis and for the measurement of the distribution Jcarbon in given samples . 2.2. In beam techniques In beam techniques are characterised by the fact that measurement of radiation occurs during the irradiation . Rather arbitrarely, a subdivision is made in nuclear reaction analysis (NRA) in case particles are

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R.D. Ks /Determination ofcarbon usingcharged particle accelerators

chondrules was explained by the finer grain size of the matrix with an associated larger surface area accommodating more carbon. This increase of surface carbon after polishing could clearly be distinguished from the carbon build up mentioned before, because it is a much larger effect . 2.2.1. Proton induced reactions

TIME ( bOUrS)

Fig. 1. Carbon build up during deuteron irradiation on differentsurfaces . Measurements were done using the IZC(d, po)(3C reaction . 1 .4 MeV deuterons were used with a beam current of 100 pA. detected and prompt radiation analysis (PRA) in case y-radiation is detected. A further division is made into resonant and nonresonant reaction analysis, dependent of whether or not a resonance in the excitation function is used. Before going into the details of these reactions, two problems associated with the in beam analysis of carbon are mentioned. As most nuclear microprobes does not operate under ultrahigh vacuum conditions, inevitably carbon build up will occur caused by cracking of thin layers of hydrocarbons on the sample surface during the irradiation. Makjanic [7] studied this buildup in the Amsterdam nuclear microprobe during different conditions and on different surfaces . Cleaning the vacuum chamber and placing a cold trap near to the sample holder gave some improvement but, although the whole system was pumped with turbomolecular pumps down to 10 -5 Pa still a deposition rate of 5 ngcm - Z min -t was observed . For the results obtained on different surfaces, see fig. 1. It is most likely, that differences found for different materials are caused by the different emission rate of secondary electrons by these materials. These electrons are the main cause of cracking of adsorbed organic material. The build-up rate is also known to be temperature dependent; cooling of the target will reduce the deposition rate. A rather peculiar phenomenon has been observed by Freund et al. [8] and Van der Stap [9]. In an olivine matrix, carbon tends to occur in a concentration gradient with higher values near to the surface of the olivine crystals . The temperature dependence of these gradients has been studied by Freund et al. while Van der Stap measured that after polishing the fresh surface which is initially low in carbon becomes carbon richer just by waiting several hours. The higher matrix carbon concentration in the carbonaceous chondrite Allende as compared with the C-concentration in inclusions as

One of the first examples of the use of in beam analysis of C was the proton capture reaction on 1ZC, producing around 2.3 MeV -y-radiation [10]. This y)13N 12C(p, reaction was used to analyze steel samples . Rudolph et al. [11] used the same reaction for the simultaneous investigation of carbon layers on the surface and at the interfaces of solids, the so called buried layers . The large resonance at 0.457 MeV(T= 36 keV) and depth dependent proton energy within a thick target lead to the emission of -y-ray quanta whose energies are also depth dependent. In fig. 2 results of these measurements are shown. Note, that at beam energies well above the resonance the y-ray caused by the t3C(p, y)t° N reaction shows up clearly opening up possibilities for the measurement of isotopic ratios, especially since the cross section of the latter reaction is considerably higher. 2.2.2. Deuteron induced reactions

For NRA with particle detection, virtually only exothermic reactions have potential use for trace analysis . With a positive Q-value, the emitted particle has a higher energy than the incoming beam, which consequently facilitates detection of these emitted particles interference free from nuclear or Rutherford scattered particles. Options available are the use of deuteron beams (by far the most widespread method for C-analysis) and 3 He beams. Deuteron induced reactions on carbon has been studied in detail [12-14] for bombarding energies between 0.5 and 2.0 MeV. The (d, n) reaction was studied by Bennett at al. [15] but this reaction is due to its negative Q-value (Q = -0 .28) not very suitable for in beam analysis. Of much more interest are the I2 C(d, pu )t3 C (Q = +2.73) and (ZC(d, Ply) 13* C (Q = -0 .37), the latter being used for the PRA method using the -y-emission. In fig. 3 the excitation function for the reaction to the ground state is given for 4 different scattering angles . It is clear from the figure, that backward angles are favourite. This is because both the cross section reaches its highest values and the number of elastically scattered deuterons is decreasing towards backward angles. Kashy et al. [14] also give angular distributions of the ground state protons and the excitation function for the 12 C(á, pty)13* C reaction measured at (p = 80.5°. Pollard et al . [16] give proton spectra for the deuterium induced reaction on BaC0 3, both natural and

R.D. Vis / Determination ofcarbon using charged particle accelerators

enriched m 13 C. The bombarding energy was 1.3 MeV. Studied were also possible interferences which were quantified for the elements Li, N, B and F for bombarding energies between 1.2 and 1.6 MeV. The sensitivity forthe anAysis of 13C is, however, more than one order of magnitude lower than for 12C, which reduces the possibility of measuring small enrichements of the 13 C isotope while f.i . used as a stable tracer . A very selective reaction for the detection of 13C is 13C(a, n) 160. In forward direction neutrons are emitted of 3 .7 MeV while irradiating with 2.8 MeV 4He+ions . Only 'Be produces higher energetic neutrons but with a very small cross section. Measuring neutrons above a threshold of about 3.5 MeV enables to measure 13Cselectively. If no B is present in the sample one is able to lower the threshold to 2 MeV (to avoid

neutrons from 18 0 to be detected) which increases the sensitivity considerably opening possibilities to detect small variations in the 12.13C isotopic ratio . ,y-detection during deuteron bombardment increases the selectivity and can avoid possible interferences while detecting protons. y-detection, however, leads to a poorer sensitivity, partly as a consequence of the reaction cross sections involved and partly as a consequence of -y-ray detector efficiency. Pollard et al . [16] studied interferences and found spectral interference from deuteron induced reactions on Be, S and Mg . At the price of less sensitivity, one may choose other -y-transitions to avoid these overlaps. Lenglet et al . [17] used (d, py) reactions on C and O to measure mass loss during the irradiation of biological samples to be analyzed for trace elements.

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R.D. Vis /Determination ofcarbon usingcharged particle accelerators

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Fig. 3. The excitation function for the (d, po) reaction on I2 C at 4different angles. (Data taken from ref. [141) . 2.2.3. 3He-induced reactions Reactions of the type ( 3He, p) on both C isotopes have lower cross sections than the deuteron induced ones. The cross section for the reaction I3C( 3He, p)IS N increases slowly with increasing beam energy [18,19]. Spectral interference occurs between the ground state protons from I2C and the p2 and p3 groups from I3 C. Interferences from other elements under 3He bombardment are studied by Bromley et al. [20] and are most severe for the elements Be, Li, B and N. 2.2.4. Elastic scattering Cross sections for Rutherford scattering increase with increasing ; atomic number Z. Therefore, detection of light elements is not very sensitive, especially not if the matrix is from a higher Z material . To avoid the

C-peak riding on a huge background, specimens should be very thin . For RBS, 'He-beams from 1 or 2 MeV are commonly used. It should be noted, that, especially at large scattering angles, strong deviations can occur from Rutherford cross sections due to nuclear effects and forC theacronym NBS is more appropriate [21,22] . The situation for the light elements can be improved considerably by performing elastic recoil detection (ERD) during which beams are impinging at oblique angles and recoil atoms ejected from the sample are measured at forward angles . During this technique, it is possible by placing a thin foil in front of the detector to stop heavy recoil atoms and find an optimum in the sensitivity curve around carbon. This method works satisfactorily in case surface layers have to be analyzed ; due to the small angles of incoming and outgoing particles, the probed layer is very thin. To avoid particles from the beam reaching the detector, for light element analysis very often beams of heavy ions such as 28Si 6+ are used. These ions will also stop in the absorber foil placed in the detection path . 2.2.5. Position information Information about the position of carbon can be obtained by using a well focussed incoming beam. Beams below 1 wm with sufficient current for analysis are now available at a few places [23,24] . Scanning procedures in combination with advanc':d software packages enable to produce elemental maps with this lateral resolution . All techniques described above also give depth information. Using NRA, detailed knowledge of the excitation function combined with a known composition of the matrix and thus a known stopping power enable to reconstruct the concentration profile along the range of the projectiles. In general, these procedures have a limited depth resolution, due to straggling of the beam and the limited detector resolution. If, however, one is able to use a preferentially sharp resonance in the excitation function, one can obtain a much better depth resolution by probing in depth using beam energy changes [25]. The resolution is for a great deal determined by the width of the resonance; if this width is small, resolutions down to 10-20 nm are achievable in favourable cases, but these small resonances are not available in the excitation function of the commonly used reactions on carbon . 'He-induced reactions for depth profiling of C and O were used by Gossett [26]. The author improved the depth resolution using glancing incident beams. D'Agostino et al . [27] used a 1.6 MeV deuterium beam and obtained a depth resolution of 0.25 wm studying a weldingjoint. ERD features a very good depth resolution. The analysis goes along the same ways as RBS (which in itself also produces depth information) and the formalism for ERD has been well described by Doyle et al.

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R.D. Vis /Determination ofcarbon usingcharged particle accelerators MEZÖ MADARAS 2

(28] with a view on computer application . By selecting carefully the experimental parameters, a depth resolution for carbon of 10 nm can be reached. 3. Other accelerator based techniques For the determination of isotopic ratios of 12.13.í4C, accelerators are used. The technique is called AMS (accelerator mass spectrometry) and in cases a tandem accelerator is used (TAMS) . Basically, the sample is brought into the ion source where it is bombarded with Cs'-ions, which not only sputter away carbon atoms but also promote the formation of C--ions by means of a thin surface film of Cs on the sample. The C--ions are accelerated and stripped in the stripper canal of the tandem to C3+- or C4'-ions which again are accelerated, separated by means of a combination of electrostatic and magnetic fields and measured . The advantage of the accelerator is that isobaric molecular species do not survive the stripping avoiding mass spectral interferences and that 14C atoms can be counted individually giving very high sensitivity . Nitrogen does not interfere, as no negative ions are produced in the ion source . A good description of a modern TAMS facility can be found in ref. [29]. Cyclotrons also have sufficient mass separating power to be used for this purpose [30] . The method is so sensitive that one is able to determine a ratio of 14 C/ 12C of 10 -1 s which opens possibilities to backdate objects down to 50000 years. A completely different technique is bombardment of a sample with a low intensity heavy ion beam in order to sputter away molecular fragments from the surface which are detected with a time of flight mass spectrometer. Initially 252Cf spallation sources were used but the flexibility of an accelerator offers advantages . The technique is able to measure C-compounds on the sample surface up to very high molecular weights. The method is known as PDMS (particle desorption mass spectrometry) [31]. 4. Applications It is not the intention of this paper to review all sorts of applications of C analysis . Suffice it to give a few examples in which nuclear techniques have contributed to improve knowledge. Very extensively, steel has been analyzed for carbon . One of the earliest applications of in beam reactions was the determination of C in steel at Harwell (UK) by measuring the prompt y-radiation emitted during the reaction 12 C(p, y) 13N (E,, = 2.3 MeV) [10]. Shortly later, deuterons were used for the same purpose, also performing -y-ray spectroscopy [32]. Since then, a large

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Fig. 4. Results of line scans made on Mezö Madaras (see text). number of C-analyses have been done to analyze metals, alloys or welding joints [33-41]. Also in plasma fusion research and catalysis, there has been a notably increase in the exploitation of nuclear techniques [42] . At our institute, a large number of analyses have been performed on meteorites [43-47]. During a study of different classes and types of ordinary chondrites, it was found that in spite of large variations in the bulk C-concentrations reported in literature, the matrices of all these chondrites have a remarkably constant C-concentration . This has lead to an assumption that the matrix is common and was formed separately and presumably earlier than inclusions such as chondrules. In a later stage mixing occurred between these 2 phases [7] . During scanning, a peculiar C-distribution was observed . In fig. 4, results of line scans are displayed from Mezö Madaras. It is shown that C is present preferentially at the edges of Fe or FeS inclusions . The asymmetry in the distributions is caused by the different positions of the small inclusions with respect to the surface of the sample which, in combination with the different range of X-rays used to detect Fe and S and of protons used to detect C and O explain the results (fig . 5). It has been assumed that Fe acts as a catalyst for a Fischer-Tropsch type of reaction followed by pyrolysis to form elementary carbon as jackets around these inclusions . Neelmeyer et al. [48] used the d, pu reaction on í2 C in combination with RBS to improve the understanding of the high wear resistance of tool steel after carbon implantation . Depth profiling was performed within a 200 nm thick near surface layer structure, modified by carbon in beam assisted deposition of C together with carbon-substrate ion beam mixing. These 11 . REVIEW PAPERS

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R.D. VIs / Determination ofcarbon usingcharged particle accelerators Polished surface

Fig. 5. Possible positions of FeS grains in the sample with respect to its surface and the corresponding outcoming radiation.

modifications were done in order to obtain new types of wear resistant coatings . In biology, sometimes C-distributions are measured with RBS to normalize trace element concentrations present in these sections [49] . Other fields of applica-

tion include the semiconductor industry and other forms of materials science .

5. Conclusions Nuclear techniques for the determination of carbon are well developed . It is nowadays possible to detect trace concentrations of carbon and also to measure the C-distribution present in the sample in three dimensions. The resolution obtained is determined by the C-concentration and its chemical surrounding. With advanced techniques such as resonant nuclear reactions, elastic recoil detection or coincidence techniques to measure exactly the kinematics of the projectileatom collision, a depth resolution of 10 run is feasible.

Also for the determination of the isotopic ratio of C, accelerator based techniques have contributed considerably. For the most commonly used reactions on carbon, data are available from the very comprehensive compilation on nuclear cross section data from Jarjis [50].

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R.D. Vrs / Determination of carbon using charged particle accelerators [31] E.A. Schweikert, W.R . Summers, D.J. Keith, P.E. Filpus-Luyckx and M.U .D . Beug Deeb, J. Trace and Microprobe Techn. 5 (1987) 1. [32] T.B . Pierce, P.F. Peck and W.M. Henry, The Analyst 90 (1965) 339. [33] T.B . Pierce, J.W. McMillan, P.F . Peck and I.G. Jones, Nucl. Instr. and Meth . 118 (1974) 115. [34] D. Heck, Nucl. Instr . and Meth. 197 (1982) 91 . [35] F.C.W . Pummery and J.W. McMillan, Nucl. Instr. and Meth. 168 (1980) 181. [36] J.W . McMillan and F.C.W. Pummery, Corros. Sci. 20 (1980) 41. [37] T.B. Pierce, J. Radioanal. Chem . 17 (1973) 55 . L38]J.F. Singleton and N.E .W. Hartley, J. Radioanal. Chem . 48 (1979) 317. [39] J.W. McMillan, P.M . Hirst, F.C .W . Pummery, J. Huddleston and T.B. Pierce, Nucl . Instr. and Meth. 149 (1978) 83. [40] T.B. Pierce and J. Huddleston, Nucl. Instr . and Meth. 144 (1977) 231.

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