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Manganese-53: Development of the AMS technique for exposure-age dating applications
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 259 (2007) 236–240 www.elsevier.com/locate/nimb
Manganese-53: Development of the AMS technique for exposure-age dating applications L.G. Gladkis, L.K. Fifield, C.R. Morton *, T.T. Barrows, S.G. Tims Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia Available online 12 February 2007
Abstract The cosmogenic isotope 53Mn is produced by spallation of iron in surface rocks. The long half life of this isotope makes it attractive for use in erosion rate studies in slowly eroding landscapes such as Australia. We describe the development of AMS methods for detection of 53Mn using the 14UD accelerator at the Australian National University. The first step of this development involved the production of 53Mn using a heavy-ion fusion–evaporation reaction to make test standards. Then, the chemistry protocol for isolating 53Mn and reducing the Cr levels, of which 53Cr is a serious interfering isobar, was developed. Lastly we employed a gas-filled magnet which was used to discriminate 53Mn from the intense 53Cr background. 2007 Elsevier B.V. All rights reserved. PACS: 07.75.+h; 28.41.Rc; 29.40.Cs; 92.40.Iv; 93.85.Np Keywords: Accelerator mass spectrometry; Manganese-53; Gas-filled magnet; Chromium suppression; Erosion rate studies
1. Introduction Ancient iron-rich landscapes are a common feature around the world, especially in arid Australia. Such surfaces are often heavily weathered, with highly-resistant iron minerals being all that remains after other minerals have been leached away. It follows that erosion rates can be extremely low, below 1 m/Ma. In this context, a long-lived isotope such as 53Mn that is produced in situ in surface rocks by cosmic rays is potentially a useful tool for studying the evolution of the landscape. It is produced via spallation of the iron by the removal of a proton and two neutrons. Hence, its production rate is expected to be substantially higher than the widely used 10Be and comparable with the production of 36Cl from potassium, of the order of 100 atoms g 1 a 1 [1]. Its long half life of 3.7 Ma is well suited to the measurement of erosion rates of less than 1 m/
*
Corresponding author. Tel.: +61 2 6125 2082; fax: +61 2 6125 0748. E-mail address: [email protected] (C.R. Morton).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.233
Ma or exposure times of millions of years. Paired with other commonly used cosmogenic isotopes, either radioactive 36Cl, 26Al and 10Be or stable 21Ne, it could be used to explore more complex exposure histories. Most measurements of 53Mn have been from meteorites, in which typical 53Mn/55Mn ratios are 10 9 or higher. In contrast, geochronological applications such as exposure dating require sensitivities at the 10 12 level. Whereas measurements in meteorites can be made using decay counting or neutron-activation techniques, only accelerator mass spectrometry (AMS) has the requisite sensitivity for geochronological applications. The chief technical problem confronting AMS of 53Mn is the presence of the stable 53 Cr isobar, which is difficult to strip by chemical means to concentrations below 1 ppm in the final purified MnO2 sample. In other words, typical 53Cr/55Mn ratios in these samples will be 10 6, or six orders of magnitude higher than the required 53Mn/55Mn sensitivity. This has two consequences. First, the counting rate of 53Cr ions after acceleration and analysis is very high, in the range of 100 kHz to a few MHz, which is well beyond the count rate capability
L.G. Gladkis et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 236–240
of gas-ionization detectors. Secondly, even if the 53Cr flux at the detector can be reduced to acceptable levels, the detector must be able to distinguish a few 53Mn ions from the much more abundant 53Cr ions of the same energy. A gas-filled magnet (GFM) instrumented with a multielement gas-ionization detector is the means of solving both problems, and the required sensitivity for exposure dating has been recently achieved by the Munich group [2]. A gas-filled magnet enables separation of Cr and Mn isobars because their differing Z means their average charge states are different in the region of the gas, and hence they follow different trajectories through the magnetic field. Further details on the operation and design of GFMs can be found in the literature [3,4]. At the Australian National University (ANU), a splitpole magnetic spectrograph (Scanditronix ESP90) has been used successfully as a GFM for AMS measurements of 32Si [5,6]. The goal of the present work was to extend this capability to the technically more challenging case of 53 Mn with geochronological applications in mind. This paper describes the GFM and the development of the gas-ionization detector with which it is instrumented. In addition, the production of picogram quantities of 53Mn with a nuclear reaction for testing purposes and to make standards is described. 2. Experimental methods 2.1. Production of
53
Mn
Since 53Mn is not available commercially, it was necessary to produce it, both for testing the technique and for the preparation of a standard. A heavy-ion fusion–evaporation reaction, 13C + Ti, was chosen because sufficiently intense 13C beams are readily obtainable from the 14UD accelerator and the cross-section for 53Mn production is large enough to produce picogram quantities of the isotope in a few hours [7]. Note that the cleanest reaction for producing 53Mn, which has a well-known cross-section, is 53 Cr(p,n)53Mn. This suffers from the major drawback, however, that the target material, from which the 53Mn must be extracted, is the very isobar that we are trying to exclude. Initially, a natural Ti foil, rolled to a thickness of 10 mg cm 2, was used. At a bombarding energy of 56 MeV, a 9 h irradiation produced 6.5 pg of 53Mn. Most of the 53Mn yield derives, however, from the 46Ti isotope [7], and hence 46Ti foils, at 86% enrichment and 9 mg cm 2, were purchased from Oak Ridge National Laboratory for subsequent work. Using one of these enriched foils, a second irradiation, also performed with a 56 MeV 13C beam, produced 21.5 pg of 53Mn in 6 h. These estimated yields are based on the cross-sections reported by d’Onofrio et al. [7] for 53Mn, 52Mn, 58Co, 56 Co and 51Cr. Yields of the latter four isotopes were determined by measuring the c-rays from their decay with a germanium detector, and provided four independent
237
estimates of the 53Mn yield that were in reasonable agreement. In principle, 54Mn could also be used, but in practice the estimate of the 53Mn yield based on the yield of this isotope is a factor of three less than the others. We do not have a good explanation for this discrepancy, but suspect that the cross-section for 54Mn may have been overestimated in Ref. [7]. Cross-sections were based on measurements of in-beam c-rays. In the case of 54Mn, the decay scheme is complex, and the cross-section was based on a 156 keV c-ray that would have been on the tail of the presumably intense 159 keV line from inelastic excitation of 47 Ti. Following irradiation, the total activity of the targets was reduced by allowing the short-lived isotopes to decay for a period of 3 or 4 weeks. The targets were then dissolved in 36% HCl. Manganese-55, as MnCl2, was added to obtain the desired 53Mn/55Mn ratio and to act as an isotopic carrier. To separate the Mn from the bulk Ti material, titanic acid was precipitated at pH 4 using NaOH, leaving Mn in solution. After drying down and taking up in 10.2 M HCl, this manganese fraction was put through an anion exchange column to reduce its Cr content, as described below in Section 2.2. Using the above procedure, two ‘‘standards’’ were produced, with 53Mn/55Mn ratios of 1.8 · 10 9 and 4.8 · 10 10. We conservatively estimate uncertainties of 25% on these ratios. An additional benefit of the above fusion reaction is that the shorter-lived 54Mn isotope, with a half life of 312 d, is produced at the same time as 53Mn. Hence, it can be used as a chemical yield tracer to follow manganese through the chemistry via the detection of the 835 keV c-ray following its decay. This proved to be very advantageous since it allowed the chemical procedure to be optimised for manganese yield. 2.2. Chemical suppression of chromium Compared to 32Si, the detection of 53Mn with a GFM is much less favourable. In the 32Si case, the stable isobar 32S differs in charge by 2 units and DZ=Z is 13.3%, which leads to a large spatial separation of the two species at the exit to the gas-filled magnet. The intense 32S ions also have the higher Z, which means that any lower-energy scattered ions are deflected even further from the 32Si position than the bulk of unscattered 32S ions. In contrast, the 53Cr isobar differs by only 1unit of charge from 53Mn, DZ/Z is only 4.1%, and the stable isobar has the lower Z. As a result, the spatial separation is more than a factor of three less than for the 32Si–32S system, and the 53Cr scattering tail extends under the 53Mn peak. Consequently, it is important to ensure that Cr levels in each sample are reduced as far as possible and kept low throughout any sample processing. Following Merchel et al. [8,9], Mn was separated from Cr on an anion exchange column using AG 1 · 8, 100– 200 mesh resin. Because Mn(II) only weakly adheres to the resin, however, the Mn elution curve overlaps with the Cr(III) elution curve and the cutoff point is a compro-
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mise between the amount of Cr one is prepared to accept and the amount of Mn material one is prepared to lose. Preliminary tests showed that the analytical-grade HCl acid used initially in this procedure was itself a significant source of chromium. Subsequently, high purity acid prepared by sub-boiling distillation has been employed. In addition, considerable care is taken after the column to use only high purity reagents and minimum quantities in order to minimise post-column Cr contamination of the samples. After the column separation, manganese is precipitated as MnO(OH)2 from solution with NaClO3 following the procedure of Merchel et al. [8,9], washed three times, dried on a hot plate at 80 C for at least 24 h, and then baked in a furnace at 350 C for 2 h. Samples are mixed with 99.999% Ag powder, in a ratio of 1 part MnO2 to 2 parts Ag to improve beam output, and pressed into Cu sample holders. In principle, multiple passes through the ion exchange column should progressively lower the Cr content, although at the expense of increasing loss of Mn. In order to test whether there was any benefit to be had from multiple passes, two sets of samples were prepared. One set had only a single pass, and the cutoff was chosen so that only 1% of the Cr was in the Mn fraction. In this case, 8% of the Mn remains in the Cr fraction. The second set of samples were passed through the column twice, with the cutoff each time chosen to accept 5% of the Cr and lose 5% of the Mn. In two passes, the Cr content should therefore be reduced to 0.25% and 10% of the Mn will be lost. Although it appears that the second case should be more favourable, the 53Cr rates from the two sample sets, as measured in the detector after the GFM, were not discernibly different. This suggests that other sources of Cr, probably in the ion source, dominate over the Cr remaining in the sample itself after the chemical preparation. 2.3. AMS method, gas-filled magnet and gas-ionization detector MnO ions were selected for injection into the 14UD Pelletron accelerator. Typical currents of 55MnO beam were 0.5 lA. A sequence of gas and foil stripping was employed to break up the molecular ions and produce high charge states respectively. The 11+ charge state was selected, and accelerating voltages of 14.4 and 13.8 MV were used for 53Mn and 55Mn, respectively, in order to produce ions of the same magnetic rigidity that could be transmitted to the GFM. A velocity filter eliminated ions with m/q different from the ions of interest. The 55Mn11+ beam current was measured in a Faraday cup immediately before the magnet. The 53Mn11+ (and 53Cr11+) ions with an energy of 170 MeV passed into the gas-filled region through a 1.5 lm mylar foil placed just before the entrance aperture into the magnet. Details of the magnet, including a diagram, are given in another paper in these proceedings [10]. After traversing the GFM, the 53Mn and 53Cr distri-
Position 1 Position 2 ΔE1 ΔE2 ΔE3 Grid 1:
Eres
Anode: 500 V Grid 2: 200V
External window support grid 100 Ω 50 Ω
Mylar foil Cathode: 600V Internal grid
Fig. 1. Schematic illustration (side view) of the gas-ionization detector in its modified arrangement with a front-window support grid and a voltagegraded grid, designed for improved energy resolution.
butions are separated by 3.4 cm and each have a width (FWHM) of 2 cm. By suitable choice of magnetic field, 99% of the 53Cr ions can be absorbed on a metal plate at the high-radius end of the detector, while 90% of the 53 Mn ions pass through into the detector. Ions are detected in a gas-ionization chamber situated approximately 34 cm from the exit of the magnetic field region. The detector [11] is position sensitive and provides a measurement of the total energy, and four measurements of the rate of energy loss from the segmented anode (Fig. 1). Propane was used as the detector gas, at a typical pressure of 75 Torr. The detector was previously used for nuclear reaction studies and was extensively modified for gas-filled magnet use as follows: • The vertical height of the entrance gap was increased to 40 mm in order to increase the acceptance of ions that were scattered vertically by the gas. • One consequence of such a large vertical gap is that the electric field lines are not straight in the vicinity of the window, which leads to a vertical height dependence of the signal from the first DE electrode. Hence, a voltage-graded grid was installed close behind the entrance window to straighten the field lines. • A second consequence of the large aperture is that the thin mylar window bows out under the pressure of the gas in the detector, leading to degradation in the total energy resolution due to differences in energy loss between the window and voltage-graded grid. This bow was reduced substantially by supporting the window with four external wires strung horizontally. An additional benefit of these supporting wires was that the thickness of the mylar window could be reduced by a factor of two to a final thickness of 2.5 lm, which improved particle discrimination because of the additional energy deposited in the active region of the detector and the reduction in straggling in the window. A schematic cross-section of the detector is shown in Fig. 1.
L.G. Gladkis et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 236–240
239
Table 1 Typical energy resolutions obtained after the detector modifications described in the text Signal
Resolution (%)
Etot DE1 DE2 DE3 Eres
2.8 7.9 5.7 3.2 16.1
Typical values of the resolution achieved for the various signals are given in Table 1 for a mass 55 beam after passing through 5.6 mbar of N2 gas in the GFM and for a detector pressure of 66 Torr.
3. Measurements and results Fig. 2 shows a plot of the residual energy, Eres, versus DE2 for the ANU standard with a 53Mn/55Mn ratio of 1.8 · 10 9. The 53Mn group is clearly visible, and the number of 53Mn counts can be reliably extracted with appropriate gates on the various signals from the detector. A position spectrum of the 53Mn events is shown in Fig. 3 and indicates that 10% of the 53Mn ions are lost at the magnetic field setting which excludes 99% of the 53Cr ions. Counting rates of 53Cr ions in the detector were 1000 s 1, which correspond to fluxes of 100 kHz of 53Cr ions at the entrance to the GFM. Evidently, the identification and sensitivity for 53Mn would be enhanced if the 53Cr rates in the detector could be reduced, and this issue is discussed further below. Preliminary results for the two ANU ‘‘standards’’ and for another standard prepared from the Grant USNM 836 Fe meteorite at Rutgers University are presented in Table 2. These are absolute measurements, based on the currents of 55Mn11+ in an electron-suppressed Faraday cup upstream of the GFM, and the count rate of 53Mn in the detector. The latter rate was corrected for the loss noted
Fig. 3. The position spectrum (first wire) for 53Mn events only. The Gaussian fit (solid line) was used to estimate the loss of 53Mn events when 99% of the Cr rate was excluded from the detector.
Table 2 Results for the AMS of two ANU standards and the standard material from Rutgers University 53
Mn/55Mn
Sample
Nominal ratio
ANU std 1 ANU std 2 Rutgers std Blank
1.8 · 10
9
1.3 · 10
9
4.8 · 10
10
2.0 · 10 1.7 · 10
2.59 · 10 –
10
8 · 10
12
Uncertainty (%)
Times measured
±5
1
10
±10
8
10
±10
2
±17
1
Also shown is an estimate of the background as obtained from a blank MnO2 sample.
above. ANU standard 1 and the Rutgers standard are 28% and 34%, respectively, below their nominal ratios. ANU standard 2 is 58% below its nominal ratio. These discrepancies could be due to a number of factors: • Incorrect cross-sections for the production of 53Mn relative to other activities produced in the 13C + Ti reactions. Note that the two ANU standards are not strictly comparable, since they were produced from different isotopic mixtures of titanium isotopes and hence the various activities were produced in different proportions. • Unidentified losses of 53Mn ions, either in the GFM or in the beam transport. • Uncertainties in stripping yields.
Fig. 2. A plot of Eres versus DE2 obtained from the gas-ionization detector for a ANU standard at 1.8 · 10 9. The 53Mn events, lower right, have a higher DE2 and lower Eres compared to 53Cr, middle of plot (z-axis is a log scale).
Future work will concentrate on a better understanding of these factors. Nevertheless, considering the notorious difficulty of making absolute measurements of isotope ratios by AMS, the results are considered to be encouraging.
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4. Summary and outlook Initial measurements of 53Mn standards using the gasfilled magnet at the ANU have been performed and encouraging results obtained. Our standards made from picogram quantities of 53Mn were produced by nuclear reactions between a 13C beam and titanium targets. Chemical procedures for separating manganese from chromium were employed using ion exchange columns. This work has highlighted two areas where improvements can be made: • A split-pole spectrometer is not ideal for gas-filled magnet operation. As it is presently configured, only 55% of the path length of the ions in the gas is within the full magnetic field, which is where the separation between 53 Mn and 53Cr occurs. No separation occurs in the remaining 45% where the ions are simply scattered and lose energy to the detriment of both position resolution and energy of the ions at the detector. Better position resolution allows rejection of a larger fraction of the 53 Cr ions, while more energy at the detector allows better ion identification. Modifications are presently underway that will increase the fraction of the path length in the full magnetic field to 78%. These are detailed in another paper in these proceedings [10]. • A reduction in the total flux of 53Cr ions would clearly be beneficial. Chromium can be present in the sample, in the sample holders or from contamination in the ion source. Measurements are planned to explore the contributions from these sources with a view to reducing rates below the 100 kHz levels presently observed.
Finally, for exposure dating applications, it will be necessary to calibrate the production rate of 53Mn in iron-rich rocks at the earth’s surface. We plan to use haematite (70% Fe) collected from surfaces in Brazil and the Pilbara region of Western Australia, which are known to have very low rates of erosion and low concentrations of stable manganese, for this purpose. Acknowledgements The authors are very grateful to Peixue Ma, Feride Serefiddin and Greg Herzog, Rutgers University, for the production and supply of the meteoritic standard used in this work. References [1] B. Heisinger, D. Lal, A.J.T. Jull, P. Kubik, S. Ivy-Ochs, K. Knie, E. Nolte, Earth Planet. Sci. Lett. 200 (2002) 357. [2] K. Knie, T. Faestermann, G. Korschinek, G. Rugel, W. Ru¨hm, C. Wallner, Nucl. Instr. and Meth. B 172 (2000) 717. [3] M. Paul, Nucl. Instr. and Meth. B 52 (1990) 315. [4] U. Zoppi, Ph.D. Thesis, ETH No. 10373, 1993. [5] J. Popplewell, S.J. King, J.P. Day, P. Ackrill, L.K. Fifield, R.G. Cresswell, M.L. di Tada, K. Liu, Inorg. Biochem. 69 (1997) 177. [6] U. Morgenstern, L.K. Fifield, A. Zondervan, Nucl. Instr. and Meth. B 172 (2000) 605. [7] A. D’Onofrio, H. Dumont, M.-G. Saint Laurent, B. Delaunay, F. Terrasi, J. Delaunay, Nucl. Phys. A 378 (1982) 111. [8] S. Merchel, U. Herpers, Radiochim. Acta 84 (1999) 215. [9] S. Merchel, T. Faestermann, U. Herpers, K. Knie, G. Korschinek, I. Leya, R. Michel, G. Rugel, C. Wallner, Nucl. Instr. and Meth. B 172 (2000) 806. [10] L.K. Fifield, S.G. Tims, L.G. Gladkis, C.R. Morton, Nucl. Instr. and Meth. B, these Proceedings, doi:10.1016/j.nimb.2007.01.156. [11] T.R. Ophel, A. Johnston, Nucl. Instr. and Meth. B 157 (1978) 461.
Nuclear Instruments and Methods in Physics Research B 259 (2007) 236–240 www.elsevier.com/locate/nimb
Manganese-53: Development of the AMS technique for exposure-age dating applications L.G. Gladkis, L.K. Fifield, C.R. Morton *, T.T. Barrows, S.G. Tims Department of Nuclear Physics, Research School of Physical Sciences and Engineering, Australian National University, Canberra ACT 0200, Australia Available online 12 February 2007
Abstract The cosmogenic isotope 53Mn is produced by spallation of iron in surface rocks. The long half life of this isotope makes it attractive for use in erosion rate studies in slowly eroding landscapes such as Australia. We describe the development of AMS methods for detection of 53Mn using the 14UD accelerator at the Australian National University. The first step of this development involved the production of 53Mn using a heavy-ion fusion–evaporation reaction to make test standards. Then, the chemistry protocol for isolating 53Mn and reducing the Cr levels, of which 53Cr is a serious interfering isobar, was developed. Lastly we employed a gas-filled magnet which was used to discriminate 53Mn from the intense 53Cr background. 2007 Elsevier B.V. All rights reserved. PACS: 07.75.+h; 28.41.Rc; 29.40.Cs; 92.40.Iv; 93.85.Np Keywords: Accelerator mass spectrometry; Manganese-53; Gas-filled magnet; Chromium suppression; Erosion rate studies
1. Introduction Ancient iron-rich landscapes are a common feature around the world, especially in arid Australia. Such surfaces are often heavily weathered, with highly-resistant iron minerals being all that remains after other minerals have been leached away. It follows that erosion rates can be extremely low, below 1 m/Ma. In this context, a long-lived isotope such as 53Mn that is produced in situ in surface rocks by cosmic rays is potentially a useful tool for studying the evolution of the landscape. It is produced via spallation of the iron by the removal of a proton and two neutrons. Hence, its production rate is expected to be substantially higher than the widely used 10Be and comparable with the production of 36Cl from potassium, of the order of 100 atoms g 1 a 1 [1]. Its long half life of 3.7 Ma is well suited to the measurement of erosion rates of less than 1 m/
*
Corresponding author. Tel.: +61 2 6125 2082; fax: +61 2 6125 0748. E-mail address: [email protected] (C.R. Morton).
0168-583X/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.01.233
Ma or exposure times of millions of years. Paired with other commonly used cosmogenic isotopes, either radioactive 36Cl, 26Al and 10Be or stable 21Ne, it could be used to explore more complex exposure histories. Most measurements of 53Mn have been from meteorites, in which typical 53Mn/55Mn ratios are 10 9 or higher. In contrast, geochronological applications such as exposure dating require sensitivities at the 10 12 level. Whereas measurements in meteorites can be made using decay counting or neutron-activation techniques, only accelerator mass spectrometry (AMS) has the requisite sensitivity for geochronological applications. The chief technical problem confronting AMS of 53Mn is the presence of the stable 53 Cr isobar, which is difficult to strip by chemical means to concentrations below 1 ppm in the final purified MnO2 sample. In other words, typical 53Cr/55Mn ratios in these samples will be 10 6, or six orders of magnitude higher than the required 53Mn/55Mn sensitivity. This has two consequences. First, the counting rate of 53Cr ions after acceleration and analysis is very high, in the range of 100 kHz to a few MHz, which is well beyond the count rate capability
L.G. Gladkis et al. / Nucl. Instr. and Meth. in Phys. Res. B 259 (2007) 236–240
of gas-ionization detectors. Secondly, even if the 53Cr flux at the detector can be reduced to acceptable levels, the detector must be able to distinguish a few 53Mn ions from the much more abundant 53Cr ions of the same energy. A gas-filled magnet (GFM) instrumented with a multielement gas-ionization detector is the means of solving both problems, and the required sensitivity for exposure dating has been recently achieved by the Munich group [2]. A gas-filled magnet enables separation of Cr and Mn isobars because their differing Z means their average charge states are different in the region of the gas, and hence they follow different trajectories through the magnetic field. Further details on the operation and design of GFMs can be found in the literature [3,4]. At the Australian National University (ANU), a splitpole magnetic spectrograph (Scanditronix ESP90) has been used successfully as a GFM for AMS measurements of 32Si [5,6]. The goal of the present work was to extend this capability to the technically more challenging case of 53 Mn with geochronological applications in mind. This paper describes the GFM and the development of the gas-ionization detector with which it is instrumented. In addition, the production of picogram quantities of 53Mn with a nuclear reaction for testing purposes and to make standards is described. 2. Experimental methods 2.1. Production of
53
Mn
Since 53Mn is not available commercially, it was necessary to produce it, both for testing the technique and for the preparation of a standard. A heavy-ion fusion–evaporation reaction, 13C + Ti, was chosen because sufficiently intense 13C beams are readily obtainable from the 14UD accelerator and the cross-section for 53Mn production is large enough to produce picogram quantities of the isotope in a few hours [7]. Note that the cleanest reaction for producing 53Mn, which has a well-known cross-section, is 53 Cr(p,n)53Mn. This suffers from the major drawback, however, that the target material, from which the 53Mn must be extracted, is the very isobar that we are trying to exclude. Initially, a natural Ti foil, rolled to a thickness of 10 mg cm 2, was used. At a bombarding energy of 56 MeV, a 9 h irradiation produced 6.5 pg of 53Mn. Most of the 53Mn yield derives, however, from the 46Ti isotope [7], and hence 46Ti foils, at 86% enrichment and 9 mg cm 2, were purchased from Oak Ridge National Laboratory for subsequent work. Using one of these enriched foils, a second irradiation, also performed with a 56 MeV 13C beam, produced 21.5 pg of 53Mn in 6 h. These estimated yields are based on the cross-sections reported by d’Onofrio et al. [7] for 53Mn, 52Mn, 58Co, 56 Co and 51Cr. Yields of the latter four isotopes were determined by measuring the c-rays from their decay with a germanium detector, and provided four independent
237
estimates of the 53Mn yield that were in reasonable agreement. In principle, 54Mn could also be used, but in practice the estimate of the 53Mn yield based on the yield of this isotope is a factor of three less than the others. We do not have a good explanation for this discrepancy, but suspect that the cross-section for 54Mn may have been overestimated in Ref. [7]. Cross-sections were based on measurements of in-beam c-rays. In the case of 54Mn, the decay scheme is complex, and the cross-section was based on a 156 keV c-ray that would have been on the tail of the presumably intense 159 keV line from inelastic excitation of 47 Ti. Following irradiation, the total activity of the targets was reduced by allowing the short-lived isotopes to decay for a period of 3 or 4 weeks. The targets were then dissolved in 36% HCl. Manganese-55, as MnCl2, was added to obtain the desired 53Mn/55Mn ratio and to act as an isotopic carrier. To separate the Mn from the bulk Ti material, titanic acid was precipitated at pH 4 using NaOH, leaving Mn in solution. After drying down and taking up in 10.2 M HCl, this manganese fraction was put through an anion exchange column to reduce its Cr content, as described below in Section 2.2. Using the above procedure, two ‘‘standards’’ were produced, with 53Mn/55Mn ratios of 1.8 · 10 9 and 4.8 · 10 10. We conservatively estimate uncertainties of 25% on these ratios. An additional benefit of the above fusion reaction is that the shorter-lived 54Mn isotope, with a half life of 312 d, is produced at the same time as 53Mn. Hence, it can be used as a chemical yield tracer to follow manganese through the chemistry via the detection of the 835 keV c-ray following its decay. This proved to be very advantageous since it allowed the chemical procedure to be optimised for manganese yield. 2.2. Chemical suppression of chromium Compared to 32Si, the detection of 53Mn with a GFM is much less favourable. In the 32Si case, the stable isobar 32S differs in charge by 2 units and DZ=Z is 13.3%, which leads to a large spatial separation of the two species at the exit to the gas-filled magnet. The intense 32S ions also have the higher Z, which means that any lower-energy scattered ions are deflected even further from the 32Si position than the bulk of unscattered 32S ions. In contrast, the 53Cr isobar differs by only 1unit of charge from 53Mn, DZ/Z is only 4.1%, and the stable isobar has the lower Z. As a result, the spatial separation is more than a factor of three less than for the 32Si–32S system, and the 53Cr scattering tail extends under the 53Mn peak. Consequently, it is important to ensure that Cr levels in each sample are reduced as far as possible and kept low throughout any sample processing. Following Merchel et al. [8,9], Mn was separated from Cr on an anion exchange column using AG 1 · 8, 100– 200 mesh resin. Because Mn(II) only weakly adheres to the resin, however, the Mn elution curve overlaps with the Cr(III) elution curve and the cutoff point is a compro-
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mise between the amount of Cr one is prepared to accept and the amount of Mn material one is prepared to lose. Preliminary tests showed that the analytical-grade HCl acid used initially in this procedure was itself a significant source of chromium. Subsequently, high purity acid prepared by sub-boiling distillation has been employed. In addition, considerable care is taken after the column to use only high purity reagents and minimum quantities in order to minimise post-column Cr contamination of the samples. After the column separation, manganese is precipitated as MnO(OH)2 from solution with NaClO3 following the procedure of Merchel et al. [8,9], washed three times, dried on a hot plate at 80 C for at least 24 h, and then baked in a furnace at 350 C for 2 h. Samples are mixed with 99.999% Ag powder, in a ratio of 1 part MnO2 to 2 parts Ag to improve beam output, and pressed into Cu sample holders. In principle, multiple passes through the ion exchange column should progressively lower the Cr content, although at the expense of increasing loss of Mn. In order to test whether there was any benefit to be had from multiple passes, two sets of samples were prepared. One set had only a single pass, and the cutoff was chosen so that only 1% of the Cr was in the Mn fraction. In this case, 8% of the Mn remains in the Cr fraction. The second set of samples were passed through the column twice, with the cutoff each time chosen to accept 5% of the Cr and lose 5% of the Mn. In two passes, the Cr content should therefore be reduced to 0.25% and 10% of the Mn will be lost. Although it appears that the second case should be more favourable, the 53Cr rates from the two sample sets, as measured in the detector after the GFM, were not discernibly different. This suggests that other sources of Cr, probably in the ion source, dominate over the Cr remaining in the sample itself after the chemical preparation. 2.3. AMS method, gas-filled magnet and gas-ionization detector MnO ions were selected for injection into the 14UD Pelletron accelerator. Typical currents of 55MnO beam were 0.5 lA. A sequence of gas and foil stripping was employed to break up the molecular ions and produce high charge states respectively. The 11+ charge state was selected, and accelerating voltages of 14.4 and 13.8 MV were used for 53Mn and 55Mn, respectively, in order to produce ions of the same magnetic rigidity that could be transmitted to the GFM. A velocity filter eliminated ions with m/q different from the ions of interest. The 55Mn11+ beam current was measured in a Faraday cup immediately before the magnet. The 53Mn11+ (and 53Cr11+) ions with an energy of 170 MeV passed into the gas-filled region through a 1.5 lm mylar foil placed just before the entrance aperture into the magnet. Details of the magnet, including a diagram, are given in another paper in these proceedings [10]. After traversing the GFM, the 53Mn and 53Cr distri-
Position 1 Position 2 ΔE1 ΔE2 ΔE3 Grid 1:
Eres
Anode: 500 V Grid 2: 200V
External window support grid 100 Ω 50 Ω
Mylar foil Cathode: 600V Internal grid
Fig. 1. Schematic illustration (side view) of the gas-ionization detector in its modified arrangement with a front-window support grid and a voltagegraded grid, designed for improved energy resolution.
butions are separated by 3.4 cm and each have a width (FWHM) of 2 cm. By suitable choice of magnetic field, 99% of the 53Cr ions can be absorbed on a metal plate at the high-radius end of the detector, while 90% of the 53 Mn ions pass through into the detector. Ions are detected in a gas-ionization chamber situated approximately 34 cm from the exit of the magnetic field region. The detector [11] is position sensitive and provides a measurement of the total energy, and four measurements of the rate of energy loss from the segmented anode (Fig. 1). Propane was used as the detector gas, at a typical pressure of 75 Torr. The detector was previously used for nuclear reaction studies and was extensively modified for gas-filled magnet use as follows: • The vertical height of the entrance gap was increased to 40 mm in order to increase the acceptance of ions that were scattered vertically by the gas. • One consequence of such a large vertical gap is that the electric field lines are not straight in the vicinity of the window, which leads to a vertical height dependence of the signal from the first DE electrode. Hence, a voltage-graded grid was installed close behind the entrance window to straighten the field lines. • A second consequence of the large aperture is that the thin mylar window bows out under the pressure of the gas in the detector, leading to degradation in the total energy resolution due to differences in energy loss between the window and voltage-graded grid. This bow was reduced substantially by supporting the window with four external wires strung horizontally. An additional benefit of these supporting wires was that the thickness of the mylar window could be reduced by a factor of two to a final thickness of 2.5 lm, which improved particle discrimination because of the additional energy deposited in the active region of the detector and the reduction in straggling in the window. A schematic cross-section of the detector is shown in Fig. 1.
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Table 1 Typical energy resolutions obtained after the detector modifications described in the text Signal
Resolution (%)
Etot DE1 DE2 DE3 Eres
2.8 7.9 5.7 3.2 16.1
Typical values of the resolution achieved for the various signals are given in Table 1 for a mass 55 beam after passing through 5.6 mbar of N2 gas in the GFM and for a detector pressure of 66 Torr.
3. Measurements and results Fig. 2 shows a plot of the residual energy, Eres, versus DE2 for the ANU standard with a 53Mn/55Mn ratio of 1.8 · 10 9. The 53Mn group is clearly visible, and the number of 53Mn counts can be reliably extracted with appropriate gates on the various signals from the detector. A position spectrum of the 53Mn events is shown in Fig. 3 and indicates that 10% of the 53Mn ions are lost at the magnetic field setting which excludes 99% of the 53Cr ions. Counting rates of 53Cr ions in the detector were 1000 s 1, which correspond to fluxes of 100 kHz of 53Cr ions at the entrance to the GFM. Evidently, the identification and sensitivity for 53Mn would be enhanced if the 53Cr rates in the detector could be reduced, and this issue is discussed further below. Preliminary results for the two ANU ‘‘standards’’ and for another standard prepared from the Grant USNM 836 Fe meteorite at Rutgers University are presented in Table 2. These are absolute measurements, based on the currents of 55Mn11+ in an electron-suppressed Faraday cup upstream of the GFM, and the count rate of 53Mn in the detector. The latter rate was corrected for the loss noted
Fig. 3. The position spectrum (first wire) for 53Mn events only. The Gaussian fit (solid line) was used to estimate the loss of 53Mn events when 99% of the Cr rate was excluded from the detector.
Table 2 Results for the AMS of two ANU standards and the standard material from Rutgers University 53
Mn/55Mn
Sample
Nominal ratio
ANU std 1 ANU std 2 Rutgers std Blank
1.8 · 10
9
1.3 · 10
9
4.8 · 10
10
2.0 · 10 1.7 · 10
2.59 · 10 –
10
8 · 10
12
Uncertainty (%)
Times measured
±5
1
10
±10
8
10
±10
2
±17
1
Also shown is an estimate of the background as obtained from a blank MnO2 sample.
above. ANU standard 1 and the Rutgers standard are 28% and 34%, respectively, below their nominal ratios. ANU standard 2 is 58% below its nominal ratio. These discrepancies could be due to a number of factors: • Incorrect cross-sections for the production of 53Mn relative to other activities produced in the 13C + Ti reactions. Note that the two ANU standards are not strictly comparable, since they were produced from different isotopic mixtures of titanium isotopes and hence the various activities were produced in different proportions. • Unidentified losses of 53Mn ions, either in the GFM or in the beam transport. • Uncertainties in stripping yields.
Fig. 2. A plot of Eres versus DE2 obtained from the gas-ionization detector for a ANU standard at 1.8 · 10 9. The 53Mn events, lower right, have a higher DE2 and lower Eres compared to 53Cr, middle of plot (z-axis is a log scale).
Future work will concentrate on a better understanding of these factors. Nevertheless, considering the notorious difficulty of making absolute measurements of isotope ratios by AMS, the results are considered to be encouraging.
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4. Summary and outlook Initial measurements of 53Mn standards using the gasfilled magnet at the ANU have been performed and encouraging results obtained. Our standards made from picogram quantities of 53Mn were produced by nuclear reactions between a 13C beam and titanium targets. Chemical procedures for separating manganese from chromium were employed using ion exchange columns. This work has highlighted two areas where improvements can be made: • A split-pole spectrometer is not ideal for gas-filled magnet operation. As it is presently configured, only 55% of the path length of the ions in the gas is within the full magnetic field, which is where the separation between 53 Mn and 53Cr occurs. No separation occurs in the remaining 45% where the ions are simply scattered and lose energy to the detriment of both position resolution and energy of the ions at the detector. Better position resolution allows rejection of a larger fraction of the 53 Cr ions, while more energy at the detector allows better ion identification. Modifications are presently underway that will increase the fraction of the path length in the full magnetic field to 78%. These are detailed in another paper in these proceedings [10]. • A reduction in the total flux of 53Cr ions would clearly be beneficial. Chromium can be present in the sample, in the sample holders or from contamination in the ion source. Measurements are planned to explore the contributions from these sources with a view to reducing rates below the 100 kHz levels presently observed.
Finally, for exposure dating applications, it will be necessary to calibrate the production rate of 53Mn in iron-rich rocks at the earth’s surface. We plan to use haematite (70% Fe) collected from surfaces in Brazil and the Pilbara region of Western Australia, which are known to have very low rates of erosion and low concentrations of stable manganese, for this purpose. Acknowledgements The authors are very grateful to Peixue Ma, Feride Serefiddin and Greg Herzog, Rutgers University, for the production and supply of the meteoritic standard used in this work. References [1] B. Heisinger, D. Lal, A.J.T. Jull, P. Kubik, S. Ivy-Ochs, K. Knie, E. Nolte, Earth Planet. Sci. Lett. 200 (2002) 357. [2] K. Knie, T. Faestermann, G. Korschinek, G. Rugel, W. Ru¨hm, C. Wallner, Nucl. Instr. and Meth. B 172 (2000) 717. [3] M. Paul, Nucl. Instr. and Meth. B 52 (1990) 315. [4] U. Zoppi, Ph.D. Thesis, ETH No. 10373, 1993. [5] J. Popplewell, S.J. King, J.P. Day, P. Ackrill, L.K. Fifield, R.G. Cresswell, M.L. di Tada, K. Liu, Inorg. Biochem. 69 (1997) 177. [6] U. Morgenstern, L.K. Fifield, A. Zondervan, Nucl. Instr. and Meth. B 172 (2000) 605. [7] A. D’Onofrio, H. Dumont, M.-G. Saint Laurent, B. Delaunay, F. Terrasi, J. Delaunay, Nucl. Phys. A 378 (1982) 111. [8] S. Merchel, U. Herpers, Radiochim. Acta 84 (1999) 215. [9] S. Merchel, T. Faestermann, U. Herpers, K. Knie, G. Korschinek, I. Leya, R. Michel, G. Rugel, C. Wallner, Nucl. Instr. and Meth. B 172 (2000) 806. [10] L.K. Fifield, S.G. Tims, L.G. Gladkis, C.R. Morton, Nucl. Instr. and Meth. B, these Proceedings, doi:10.1016/j.nimb.2007.01.156. [11] T.R. Ophel, A. Johnston, Nucl. Instr. and Meth. B 157 (1978) 461.