- Email: [email protected]
14C and 90Sr measurements at the Erlangen AMS facility
NOMB
Nuclear Instruments and Methods in Physics Research B 92 (1994) 39-42 North-Holland
Beam Interactions
with Materials&Atoms
14C and 90Sr measurements
at the Erlangen AMS facility
F. Arslan, M. Behrendt, W. Ernst, E. Finckh, C. Greb, F. G~mbmann, M. HalIer, S, Hofmann, R. Karschnick, M. Klein, W. Kretschmer *, J. Mackiol, G. Morgenroth, C. Pagels and M. Schleicher Physikalisches Institut, Vniuersitii’t Erlangen-Niirnberg,
D-91058 Erlangen, Germany
The AMS beamline at the Erlangen EN tandem accelerator has been completed and improved. The high-current Cs sputter ion source has been finished and is being used mainly with carbon and SrF, samples. Sample exchange is done by remote control, under high voltage and without breaking the vacuum. Fast isotope switching is performed by applying high voltage pulses of adjustable length at the chamber of the 90” injection magnet, which can be used for different kinds of ions and beam currents. Using a new 120” magnetic split-pole spectrograph, as% was measured at isotopic ratias seSr,JssSr between 10v9 and 10s for environmental applications. Radiocarbon dating of samples along the cross section of tropical trees yielded information about the growth rhythms of the tropical wood, which are difficult to determine because of missing tree ring patterns.
1. Developments
and imp~vements
at the A&IS facility
The Erlangen AMS facility [1,21 consists of a Cs sputter ion source, a 90” injection magnet, an EN tandem accelerator, a 15” electrostatic deflector, a 55” anteing magnet (for 14C), a new 120” magnetic splitpole spectrograph (for the heavier ions), and a A.&E gas detector. The ion source 131was optimized for high negative ion currents to improve counting statistics, and for a small beam emittance to achieve maximum transmission from the source to the final detector. Thus a spherical shape 141 of the Cs ionizer was chosen. The geometries of the ionizer, the target region and the electrostatic lenses were optimized by extensive ion optical computer simulations [5]. The ion source has been operated so far with various ions, e.g. C-, F-, Al-, S-, Fe-, Cu-, SrF;. Typical ion currents are 60 PA for graphite and between 10 and 30 @A for the tropical wood samples as well as for the ANU cahbration samples. For the SrF; ions, 200 nA was obtained. The measured beam emittance is 2.5~ mmmrad MeVtj2, and does not depend on the kind of sputter target being used. To avoid cross-talk between different samples, the samples are stored in a separately pumped magazine chamber. Target exchange is performed by computer control via four stepper motors, under vacuum and high voltage. Two of these motors are also responsible for wobbling the target during the measurements, thus avoiding cratering of the target surface.
* Corres~nding author. Tel. +49 9131 857075, fax i-49 9131 15249, e-mail pj4kret~p~~.physik.uni-erIangen.de. 0168.583X/94/$07.~
Isotopic ratios (e,g. 14C/‘3C and 14C/‘2C) are measured by fast cycling between isotopes, which is effected by applying short high voltage pulses to the insulated vacuum chamber of the 90” injection magnet [6]. The switching is performed by fast insulated-gate bipolar transistors, up to a voltage of 10 kV. During r4C measurements, i3C, “C and 14C are sequentially injected for 1.5 ms, 250 ps and 28.25 ms, respectively. One of two “C pulses is used to stabilize the tandem voltage by measuring the beam position with a position sensitive Faraday cup at the “‘C off-axis location after the 55” analyzing magnet. The second of each two 12C pulses is used to measure the ‘*C current after the analyzing magnet. In the case of ?Sr measurements, “SrF; is injected every 30 ms with a pulse length of 5 ms. Inside the tandem accelerator, new tantalum spiral inclined beam tubes and a new terminal have been installed. The improvements in transmission and vacuum yield better statistics and less background. With the 4 + charge state, which is now routinely used, the transmission from ion source to final Faraday cup is 20% for r3C at 4.5 MV terminal voltage. A special microcomputer system has been developed to control most elements of the AMS setup, including the ion source. It consists of several 6502based single-card microcomputers (EMUFs), which are connected to two Amiga computers via a 375 kbit/s serial bus. Each of these EMUFs is connected to up to 14 control cards via a 64 pin backplane. Different kinds of control cards have been developed: motor cards are used to drive stepper motors and de motors for the positioning of slit units and Faraday cups, for the target exchange mechanism, for the wobbling of the
0 1994 - Elsevier Science B.V. All rights reserved
SSDI 0168-583X(94)00034-S
I. FACILITY REPORTS
F. Arslan et al. /Nucl.
40
Instr. and Meth. in Phys. Res. B 92 (1994) 39-42
target, and to adjust the gap between target and ionizer. ADC/DAC cards (12 bit and 16 bit versions) are used to set and control the high voltage power supplies. Since the control units of the ion source are on high voltage potential, serial optical links are provided to connect the EMUFs with the serial bus. On the Amiga computers, a graphical user interface software has been developed, which allows manual control of each element as well as automatic setups and procedures, e.g. target exchange. In this way, a quick and reproducible setup of the whole facility can be achieved.
2. %r
measurements
The radionuclide 90Sr, which can be emitted in nuclear accidents, is especially dangerous for man because of its long half-life (28.5 years), and because it is deposited in bone. Since the P-energy is very small (Ea = 0.5 MeV), only the radiation of the daughter nucleus “‘Y can be used for detection, which requires several days of counting. Alternatively, the isotopic ratio 90Sr/88Sr can be measured within one day via AMS, including the chemical pretreatment of the dust samples. In thecase of a nuclear accident, dust will be collected via airplane and chemically pretreated to reduce the other fission products. After mixing the 90Sr samples with sufficient %rF, material for the sputter target, the expected isotopic ratio g”Sr/s8Sr is in the order of 10d8 to lo- ‘” . Since Sr- ions are not formed in the sputter source, SrFq molecular ions are injected with a source current of 200 nA, using a SrF, sample. SrF- cannot be used, because traces of Ag- interfere. Even with SrF;, high background rates arise from interfering molecules, if materials such as copper or aluminum are used for the sample holder. Tantalum, which is too heavy to form interfering molecules, produced much less background. The separation of 90Sr from the 88Sr beam requires a high resolution analyzing magnet. For this purpose, we use a newly constructed 120” magnetic split-pole spectrometer with a
bending radius of 1.8 m and a maximum magnetic rigidity of 189 MeV amu q2. The 90Sr and *sSr ions are separated by 8 cm in the focal plane, which is 3 m behind the exit of the magnetic field. In order to minimize the contribution from the 90Zr isobar which ‘90 at our energies cannot be distinguished from Sr, a special high-quality chemical pretreatment of the samples is being developed in collaboration with Prof. Knochel, Hamburg. Since this chemical method of Zr reduction is not yet finished, our first 90Sr measurements have been performed using calibration samples made from commercial standards. Two blanks and three calibration samples have been investigated: commercial Optipur, a high purity SrF, material; blank SrF, chemically processed (without special Zr reduction) from SrCO, using high purity HF; and calibration SrF, samples, made from SrCO, with addition of !?Sr standard of known activity. The 90Sr/8sSr ratio of the three calibration samples was 1.4 x 10-9, 7 x 10-9, and 14 x 10e9, respectively. The SrF, materials were mixed with ultra pure gold powder in the ratio of 1: 2 by mass, to increase the electrical and thermal conductivity of the sputter target. The material was then ground and pressed into the tantalum sample holders with 15 kbar pressure. The life-time of these targets, containing 10 mg SrF,, in the ion source was 2-3 h. In these measurements, a source voltage of 35 kV and a terminal voltage of 5 MV have been used, which corresponds to 43 MeV beam energy for the 8 + charge state. The %r current measured in a Faraday cup at the focal plane of the 120” analyzing magnet was about 1.5 nA. Due to the small current, the tandem accelerator could be stabilized by the generating voltmeter only, which is inferior to stabilization with the position sensitive Faraday cup used in the radiocarbon measurements. Typical spectra for the blank sample and for one of the calibration samples are shown in Fig. 1. The results for each sample are summarized in Table 1, which shows a reasonable agreement between the known concentrations and the measured values for the two
Fig. 1. AE-E REs spectra of ‘“Sr measurements with samples of different %r content: (a) blank and (b) calibration 7 X lo-“.
41
F. A&an et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 39-42 Table 1 Summary of the 90Sr results for samples of known activities. The blank samples can be taken as a background measurement, which gives the results with background correction. The given errors were deduced from the variation of the single
data points Sample
Optipur Blank 1.4x 10-9 7 x10-9 14 x10-9
Measured g”Sr/88Sr isotope ratio (X 10m9) uncorrected
background subtracted
1.15+0.15 1.28 k 0.28 2.25 rf-0.13 7.09 f 0.5 14.4 kO.9
0.97 + 0.31 5.81+ 0.57 13.12 f 0.94
highest calibration targets. Each of the results is deduced from 6 to 10 independent measurements (10 min run time) of the corresponding sample, and the given errors were calculated from the variation of the single data points. From these data a detection limit of 10e9 can be deduced, which corresponds to an activity of 39 Bq, if 10 mg of SrF, is used. To further reduce this background, which is mostly due to 90Zr, the purity of the samples must be improved. This is especially important for the measurement of environmental samples from a nuclear accident, which contain a lot of fission products with concentrations similar to 9”Sr.
3 . 14C measurements
means that the growth zones of tropical trees may be dated by measuring their 14C contents. Small samples of the tree, belonging to a certain growth zone, were cut into pieces of about 80 mg. These were pretreated [lo] in the following way: lixiviating in NaOH at 80°C (6 h), rinsing with H,O, simmering in HCl at 80°C (2 hl, again rinsing with H,O. Afterwards, the samples are dried at 150°C for 2 h. By this procedure, most of the mobile components are removed: the hemicellulose, the resins and the tannins are dissolved in the NaOH, and the carbonates are dissolved in the HCl; the weight of a wood sample is diminished by about 50%. The resulting material is reduced to carbon by pyrolysis [ll] at about 800°C in flowing N, for 20 min. The resulting charcoal is ground and heated together with iron powder. This additional carbidisation step leads to improved ion currents. The resulting iron carbide is ground again and mixed with silver powder (1 to 5 parts by weight). This mixture is then pressed into the target holder with 5 kbar, using a diamond-polished pressing tool to achieve a smooth target surface. In each tropical tree sputter target, the amount of carbon was about 10 mg. In Fig. 2, the reproducibility of the 14C/13C ratio for repeated measurements of the same ANU calibration sample, with some other measurements in between, can be seen. The statistical errors of the individual measurements are compared with the standard deviation of their distribution and with the error of the mean value. For the normalization of the results, measurements of wood samples of known age have also been used. The machine background, which is mea-
Age determinations in tropical trees are of special interest due to world-wide deforestation, but were long considered impossible due to the difficulty in interpreting growth zones. In fact, many species in tropical regions with a definite dry season (2-3 months) or with extended periods of flooding form more or less distinct structures [7]. The great variations in the distinctiveness of growth zones, depending on varying site conditions and specific wood structure, require a careful investigation of the growth rhythm of trees. Growth rhythm can be detected by independent age determinations of individual growth zones using radiocarbon dating based on the 14C nuclear testing bomb pulse. One effect of the more than 400 above ground atomic bomb explosions in the early 60s is that the 14C level nearly doubled in the northern hemisphere, with a slightly smaller effect in the southern hemisphere. So the 14C/‘*C ratio in trees increases drastically from 1962 to 1966. A close correlation between the atmoin tree spheric t4C variation and 14C concentrations rings has already been found in some investigations of trees in the northern hemisphere [7-91. We expect this correlation in the southern hemisphere also, which
Fig. 2. Reproducibility of repeated measurements of the same ANU calibration sample with other measurements in between. The individual data and their statistical errors (from the number of 14C events) are compared with the mean value (solid line), the 1 (r standard deviation (1% of mean value) of the individual data (short dashed lines) and with the error (0.3%) of the mean value (long dashed lines). I. FACILITY
REPORTS
F. Arslan et al. /Nucl.
42
Leguminosae
Instr. and Meth. in Phys. Res. B 92 (1994) 39-42
gas-filled magnet, which should allow environmental 90Sr samples to be measured also. For the heavier ions a new injection spectrometer consisting of an electric and a magnetic sector field will be installed, and a recirculating gas stripper will be mounted in the terminal to improve the vacuum and the transmission. In the case of 14C, the possibility of radiocarbon dating of tropical trees has been established. The results are reproducible, and the performance of the whole facility has been much improved. Other applications, e.g. the investigation of 14C in the organic fraction of sediments for geological research, are in progress.
(Amazonas)
150 11
Year3
Acknowledgements
Fig. 3. Measured 14C concentrations of samples from Macrolobium acaciaefolium, Leguminosae (Amazonas), compared with 14C concentrations in the atmosphere of the southern hemisphere (taken from ref. [7]). The dates of the data points are from predating by a structure analysis of the wood (accuracy about two years), which must be corrected by two or three years to match the atmospheric curve.
sured with the graphite of about 40000 years.
samples,
corresponds
to an age
The results for one tropical tree, Macrolobium acaLeguminosae, from the inundation forest of the Amazonas region, are plotted in Fig. 3 and compared with the atmospheric 14C variations [7] of the southern hemisphere. The samples were provided by Prof. Worbes (MPI fiir Limnologie, P16n, Germany). In this special case, a rough predating of the tree by structure analysis was possible with an accuracy of about two years before 1967. By the measured 14C data, this predating must be corrected by two or three years before 1967; then the 14C dates correspond well to the known atmospheric 14Cvariation in the southern hemisphere. Measurements of trees with very little information about the age profile were also made. The 14C data allow unique dating of the individual growth zones. ciaefolium,
4. Conclusions
and outlook
The Erlangen AMS facility is now complete and has been successfully used for 14C and %jr measurements. In the case of 90Sr, a detection limit of lop9 was achieved. This can probably be reduced in the future by better purity of the samples or by the use of a
We thank Prof. Worbes (MPI fiir Limnologie, P16n, Germany) for providing the tropical tree samples and for fruitful discussions, and R. Sen Gupta (University of Hamburg, Germany) for preparing the %r calibration samples. Financial support by the Deutsche Forschungsgemeinschaft and the Bundesminister fiir Umwelt, Naturschutz und Reaktorsicherheit is gratefully acknowledged.
References
[ll M. Baumglrtner, U. Emmerling, W. Ernst, E. Finckh, G. Fuchs, F. Gumbmann, M. Hailer, R. Hiipfl, R. Karschnick, W. Kretschmer, A. Rauscher, M. Schleicher and W. Schuster, Nucl. Instr. and Meth. B 50 (1990) 286. 121 M. Baumggrtner, U. Emmerling, W. Ernst, E. Finckh, G. Fuchs, F. Gumbmann, M. Haller, R. Hapfl, R. Karschnick, W. Kretschmer, A. Rauscher, M. Schleicher and W. Schuster, Nucl. Instr. and Meth. B 52 (1990) 273. [31 R. Hiipfl, T. Bretschneider, A. Buchler, Y. Charasse, W. Ernst, E. Finckh, R. Goblirsch, F. Gumbmann, M. Hailer, R. Karschnick, W. Kretschmer, A. Rauscher, H. Schielein and M. Schleicher, Rev. Sci. Instr. 63 (1992) 2469. [41 N.R. White, Nucl. Instr. and Meth. 206 (1983) 15. 151R. Hiipfl, Ph.D. thesis, Erlangen, Germany, 1991. [61 R. Balzer, Nucl. Instr. and Meth. B 5 (1984) 247. [71 M. Worbes and W.J. Junk, Ecology 70(2) (1989) 503. [81 W.F. Cain and H.E. Suess, J. Geophys. Res. 81 (1976) 3688. 191 P.M. Grootes, G.W. Farwell, F.H. Schmidt, D.D. Leach and Minze Stuiver, Radiocarbon 31 (1989) 475. [lOI J.W. Green, in: Methods of Carbohydrite Chemistry, ed. R.L. Whistler (Academic Press, New York, 1963) p. 9. [ill W.A. Keller, J.R. Giinter and R. Erne, Nucl. Instr. and Meth. B 5 (1984) 280.
Nuclear Instruments and Methods in Physics Research B 92 (1994) 39-42 North-Holland
Beam Interactions
with Materials&Atoms
14C and 90Sr measurements
at the Erlangen AMS facility
F. Arslan, M. Behrendt, W. Ernst, E. Finckh, C. Greb, F. G~mbmann, M. HalIer, S, Hofmann, R. Karschnick, M. Klein, W. Kretschmer *, J. Mackiol, G. Morgenroth, C. Pagels and M. Schleicher Physikalisches Institut, Vniuersitii’t Erlangen-Niirnberg,
D-91058 Erlangen, Germany
The AMS beamline at the Erlangen EN tandem accelerator has been completed and improved. The high-current Cs sputter ion source has been finished and is being used mainly with carbon and SrF, samples. Sample exchange is done by remote control, under high voltage and without breaking the vacuum. Fast isotope switching is performed by applying high voltage pulses of adjustable length at the chamber of the 90” injection magnet, which can be used for different kinds of ions and beam currents. Using a new 120” magnetic split-pole spectrograph, as% was measured at isotopic ratias seSr,JssSr between 10v9 and 10s for environmental applications. Radiocarbon dating of samples along the cross section of tropical trees yielded information about the growth rhythms of the tropical wood, which are difficult to determine because of missing tree ring patterns.
1. Developments
and imp~vements
at the A&IS facility
The Erlangen AMS facility [1,21 consists of a Cs sputter ion source, a 90” injection magnet, an EN tandem accelerator, a 15” electrostatic deflector, a 55” anteing magnet (for 14C), a new 120” magnetic splitpole spectrograph (for the heavier ions), and a A.&E gas detector. The ion source 131was optimized for high negative ion currents to improve counting statistics, and for a small beam emittance to achieve maximum transmission from the source to the final detector. Thus a spherical shape 141 of the Cs ionizer was chosen. The geometries of the ionizer, the target region and the electrostatic lenses were optimized by extensive ion optical computer simulations [5]. The ion source has been operated so far with various ions, e.g. C-, F-, Al-, S-, Fe-, Cu-, SrF;. Typical ion currents are 60 PA for graphite and between 10 and 30 @A for the tropical wood samples as well as for the ANU cahbration samples. For the SrF; ions, 200 nA was obtained. The measured beam emittance is 2.5~ mmmrad MeVtj2, and does not depend on the kind of sputter target being used. To avoid cross-talk between different samples, the samples are stored in a separately pumped magazine chamber. Target exchange is performed by computer control via four stepper motors, under vacuum and high voltage. Two of these motors are also responsible for wobbling the target during the measurements, thus avoiding cratering of the target surface.
* Corres~nding author. Tel. +49 9131 857075, fax i-49 9131 15249, e-mail pj4kret~p~~.physik.uni-erIangen.de. 0168.583X/94/$07.~
Isotopic ratios (e,g. 14C/‘3C and 14C/‘2C) are measured by fast cycling between isotopes, which is effected by applying short high voltage pulses to the insulated vacuum chamber of the 90” injection magnet [6]. The switching is performed by fast insulated-gate bipolar transistors, up to a voltage of 10 kV. During r4C measurements, i3C, “C and 14C are sequentially injected for 1.5 ms, 250 ps and 28.25 ms, respectively. One of two “C pulses is used to stabilize the tandem voltage by measuring the beam position with a position sensitive Faraday cup at the “‘C off-axis location after the 55” analyzing magnet. The second of each two 12C pulses is used to measure the ‘*C current after the analyzing magnet. In the case of ?Sr measurements, “SrF; is injected every 30 ms with a pulse length of 5 ms. Inside the tandem accelerator, new tantalum spiral inclined beam tubes and a new terminal have been installed. The improvements in transmission and vacuum yield better statistics and less background. With the 4 + charge state, which is now routinely used, the transmission from ion source to final Faraday cup is 20% for r3C at 4.5 MV terminal voltage. A special microcomputer system has been developed to control most elements of the AMS setup, including the ion source. It consists of several 6502based single-card microcomputers (EMUFs), which are connected to two Amiga computers via a 375 kbit/s serial bus. Each of these EMUFs is connected to up to 14 control cards via a 64 pin backplane. Different kinds of control cards have been developed: motor cards are used to drive stepper motors and de motors for the positioning of slit units and Faraday cups, for the target exchange mechanism, for the wobbling of the
0 1994 - Elsevier Science B.V. All rights reserved
SSDI 0168-583X(94)00034-S
I. FACILITY REPORTS
F. Arslan et al. /Nucl.
40
Instr. and Meth. in Phys. Res. B 92 (1994) 39-42
target, and to adjust the gap between target and ionizer. ADC/DAC cards (12 bit and 16 bit versions) are used to set and control the high voltage power supplies. Since the control units of the ion source are on high voltage potential, serial optical links are provided to connect the EMUFs with the serial bus. On the Amiga computers, a graphical user interface software has been developed, which allows manual control of each element as well as automatic setups and procedures, e.g. target exchange. In this way, a quick and reproducible setup of the whole facility can be achieved.
2. %r
measurements
The radionuclide 90Sr, which can be emitted in nuclear accidents, is especially dangerous for man because of its long half-life (28.5 years), and because it is deposited in bone. Since the P-energy is very small (Ea = 0.5 MeV), only the radiation of the daughter nucleus “‘Y can be used for detection, which requires several days of counting. Alternatively, the isotopic ratio 90Sr/88Sr can be measured within one day via AMS, including the chemical pretreatment of the dust samples. In thecase of a nuclear accident, dust will be collected via airplane and chemically pretreated to reduce the other fission products. After mixing the 90Sr samples with sufficient %rF, material for the sputter target, the expected isotopic ratio g”Sr/s8Sr is in the order of 10d8 to lo- ‘” . Since Sr- ions are not formed in the sputter source, SrFq molecular ions are injected with a source current of 200 nA, using a SrF, sample. SrF- cannot be used, because traces of Ag- interfere. Even with SrF;, high background rates arise from interfering molecules, if materials such as copper or aluminum are used for the sample holder. Tantalum, which is too heavy to form interfering molecules, produced much less background. The separation of 90Sr from the 88Sr beam requires a high resolution analyzing magnet. For this purpose, we use a newly constructed 120” magnetic split-pole spectrometer with a
bending radius of 1.8 m and a maximum magnetic rigidity of 189 MeV amu q2. The 90Sr and *sSr ions are separated by 8 cm in the focal plane, which is 3 m behind the exit of the magnetic field. In order to minimize the contribution from the 90Zr isobar which ‘90 at our energies cannot be distinguished from Sr, a special high-quality chemical pretreatment of the samples is being developed in collaboration with Prof. Knochel, Hamburg. Since this chemical method of Zr reduction is not yet finished, our first 90Sr measurements have been performed using calibration samples made from commercial standards. Two blanks and three calibration samples have been investigated: commercial Optipur, a high purity SrF, material; blank SrF, chemically processed (without special Zr reduction) from SrCO, using high purity HF; and calibration SrF, samples, made from SrCO, with addition of !?Sr standard of known activity. The 90Sr/8sSr ratio of the three calibration samples was 1.4 x 10-9, 7 x 10-9, and 14 x 10e9, respectively. The SrF, materials were mixed with ultra pure gold powder in the ratio of 1: 2 by mass, to increase the electrical and thermal conductivity of the sputter target. The material was then ground and pressed into the tantalum sample holders with 15 kbar pressure. The life-time of these targets, containing 10 mg SrF,, in the ion source was 2-3 h. In these measurements, a source voltage of 35 kV and a terminal voltage of 5 MV have been used, which corresponds to 43 MeV beam energy for the 8 + charge state. The %r current measured in a Faraday cup at the focal plane of the 120” analyzing magnet was about 1.5 nA. Due to the small current, the tandem accelerator could be stabilized by the generating voltmeter only, which is inferior to stabilization with the position sensitive Faraday cup used in the radiocarbon measurements. Typical spectra for the blank sample and for one of the calibration samples are shown in Fig. 1. The results for each sample are summarized in Table 1, which shows a reasonable agreement between the known concentrations and the measured values for the two
Fig. 1. AE-E REs spectra of ‘“Sr measurements with samples of different %r content: (a) blank and (b) calibration 7 X lo-“.
41
F. A&an et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 39-42 Table 1 Summary of the 90Sr results for samples of known activities. The blank samples can be taken as a background measurement, which gives the results with background correction. The given errors were deduced from the variation of the single
data points Sample
Optipur Blank 1.4x 10-9 7 x10-9 14 x10-9
Measured g”Sr/88Sr isotope ratio (X 10m9) uncorrected
background subtracted
1.15+0.15 1.28 k 0.28 2.25 rf-0.13 7.09 f 0.5 14.4 kO.9
0.97 + 0.31 5.81+ 0.57 13.12 f 0.94
highest calibration targets. Each of the results is deduced from 6 to 10 independent measurements (10 min run time) of the corresponding sample, and the given errors were calculated from the variation of the single data points. From these data a detection limit of 10e9 can be deduced, which corresponds to an activity of 39 Bq, if 10 mg of SrF, is used. To further reduce this background, which is mostly due to 90Zr, the purity of the samples must be improved. This is especially important for the measurement of environmental samples from a nuclear accident, which contain a lot of fission products with concentrations similar to 9”Sr.
3 . 14C measurements
means that the growth zones of tropical trees may be dated by measuring their 14C contents. Small samples of the tree, belonging to a certain growth zone, were cut into pieces of about 80 mg. These were pretreated [lo] in the following way: lixiviating in NaOH at 80°C (6 h), rinsing with H,O, simmering in HCl at 80°C (2 hl, again rinsing with H,O. Afterwards, the samples are dried at 150°C for 2 h. By this procedure, most of the mobile components are removed: the hemicellulose, the resins and the tannins are dissolved in the NaOH, and the carbonates are dissolved in the HCl; the weight of a wood sample is diminished by about 50%. The resulting material is reduced to carbon by pyrolysis [ll] at about 800°C in flowing N, for 20 min. The resulting charcoal is ground and heated together with iron powder. This additional carbidisation step leads to improved ion currents. The resulting iron carbide is ground again and mixed with silver powder (1 to 5 parts by weight). This mixture is then pressed into the target holder with 5 kbar, using a diamond-polished pressing tool to achieve a smooth target surface. In each tropical tree sputter target, the amount of carbon was about 10 mg. In Fig. 2, the reproducibility of the 14C/13C ratio for repeated measurements of the same ANU calibration sample, with some other measurements in between, can be seen. The statistical errors of the individual measurements are compared with the standard deviation of their distribution and with the error of the mean value. For the normalization of the results, measurements of wood samples of known age have also been used. The machine background, which is mea-
Age determinations in tropical trees are of special interest due to world-wide deforestation, but were long considered impossible due to the difficulty in interpreting growth zones. In fact, many species in tropical regions with a definite dry season (2-3 months) or with extended periods of flooding form more or less distinct structures [7]. The great variations in the distinctiveness of growth zones, depending on varying site conditions and specific wood structure, require a careful investigation of the growth rhythm of trees. Growth rhythm can be detected by independent age determinations of individual growth zones using radiocarbon dating based on the 14C nuclear testing bomb pulse. One effect of the more than 400 above ground atomic bomb explosions in the early 60s is that the 14C level nearly doubled in the northern hemisphere, with a slightly smaller effect in the southern hemisphere. So the 14C/‘*C ratio in trees increases drastically from 1962 to 1966. A close correlation between the atmoin tree spheric t4C variation and 14C concentrations rings has already been found in some investigations of trees in the northern hemisphere [7-91. We expect this correlation in the southern hemisphere also, which
Fig. 2. Reproducibility of repeated measurements of the same ANU calibration sample with other measurements in between. The individual data and their statistical errors (from the number of 14C events) are compared with the mean value (solid line), the 1 (r standard deviation (1% of mean value) of the individual data (short dashed lines) and with the error (0.3%) of the mean value (long dashed lines). I. FACILITY
REPORTS
F. Arslan et al. /Nucl.
42
Leguminosae
Instr. and Meth. in Phys. Res. B 92 (1994) 39-42
gas-filled magnet, which should allow environmental 90Sr samples to be measured also. For the heavier ions a new injection spectrometer consisting of an electric and a magnetic sector field will be installed, and a recirculating gas stripper will be mounted in the terminal to improve the vacuum and the transmission. In the case of 14C, the possibility of radiocarbon dating of tropical trees has been established. The results are reproducible, and the performance of the whole facility has been much improved. Other applications, e.g. the investigation of 14C in the organic fraction of sediments for geological research, are in progress.
(Amazonas)
150 11
Year3
Acknowledgements
Fig. 3. Measured 14C concentrations of samples from Macrolobium acaciaefolium, Leguminosae (Amazonas), compared with 14C concentrations in the atmosphere of the southern hemisphere (taken from ref. [7]). The dates of the data points are from predating by a structure analysis of the wood (accuracy about two years), which must be corrected by two or three years to match the atmospheric curve.
sured with the graphite of about 40000 years.
samples,
corresponds
to an age
The results for one tropical tree, Macrolobium acaLeguminosae, from the inundation forest of the Amazonas region, are plotted in Fig. 3 and compared with the atmospheric 14C variations [7] of the southern hemisphere. The samples were provided by Prof. Worbes (MPI fiir Limnologie, P16n, Germany). In this special case, a rough predating of the tree by structure analysis was possible with an accuracy of about two years before 1967. By the measured 14C data, this predating must be corrected by two or three years before 1967; then the 14C dates correspond well to the known atmospheric 14Cvariation in the southern hemisphere. Measurements of trees with very little information about the age profile were also made. The 14C data allow unique dating of the individual growth zones. ciaefolium,
4. Conclusions
and outlook
The Erlangen AMS facility is now complete and has been successfully used for 14C and %jr measurements. In the case of 90Sr, a detection limit of lop9 was achieved. This can probably be reduced in the future by better purity of the samples or by the use of a
We thank Prof. Worbes (MPI fiir Limnologie, P16n, Germany) for providing the tropical tree samples and for fruitful discussions, and R. Sen Gupta (University of Hamburg, Germany) for preparing the %r calibration samples. Financial support by the Deutsche Forschungsgemeinschaft and the Bundesminister fiir Umwelt, Naturschutz und Reaktorsicherheit is gratefully acknowledged.
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