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The first detection of naturally-occurring 236U with accelerator mass spectrometry
Nuclear Instruments and Methods in Physics Research B 92 (1994) 249-253 North-Holland
The first detection of naturally-occurring with accelerator mass spectrometry
Beam Intenotions with Materials %Atoms
236U
X-L. Zhao a**,M-J. Nadeau b, L.R. Kilius a and A.E. Li~herland a a IsoTrace Laboratory, Department of Physics, University of Toronto, 60 St. George St., Toronto,
Ontario, Canada M.5S IA7 b National Ocean Science Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
The Isol’race heavy element AMS system has been successfully used to detect naturally-occurring 236U (236LJ/ 238U= (5.6 + 1.5) X lo-“) in samples of uranium ore from Cigar Lake, Saskatchewan, Canada. This level of 236U agrees with that previously claimed for samples of a processed uranium ore (D.J. Rokop, D.N. Metta and CM. Stevens, Int. J. Mass Spectrom. Ion Phys. 8 (1972) 259 [I& and is consistent with the amount of u9Pu found in pitchblende (W.A. Myers and M. Lindner, J. Inorg. Nucl. Chem. 33 (1971) 3233 [Z]). This experiment illustrates the general capability of a small tandem-based AMS system for analyzing actinides, in particular 236U. It can be shown that the isotope-ratio detection limit of this system, is at present 5 x lo-’ for detecting a less abundant actinide isotope one mass unit above, and 5 X lo-” one mass unit below, a major isotope.
1. Introduction It has been known for half a century that both neutron-induced fission and the (n, y) reaction occur in uranium minerals and uranium ores. This is based on a number of observations, including the first isolation of the naturally-occurring 23gPu in pitchblende [3] and uranium ores 141,the observation of the variations of the fission yields of the stable isotopes of xenon and krypton [5], the measurement of the variability of the natural abundance of 235U [6], and more recently, the detection of the radiogenic “Be and 26Al in uranium and thorium ores [7]. The (n, 2n) reaction is also known to occur due to the isolation of the naturally-occurring 237Np in pitchblende 12,8]. The Oklo phenomenon [9] is obviously the most extraordinary example of naturally-occurring neutron-induced reactions. It is therefore rather obvious that 236U should be a naturally-occurring isotope due to the (n, r> reaction on 235U in uranium ores, in spite of the low abundance of 235U. If a uranium ore deposit is saturated with groundwater, as it was in the Oklo natural nuclear reactors, MeV neutrons from =‘U fission can be readily thermalized before interaction. Thus, the large cross section (_ 99 b) for 23sU thermal neutron absorption can be a major boost for 236U production. Since 236U is
* Corresponding author. Tel. + 1 416 978 4041, fax + 1 416 978 4711, e-mail [email protected].
a long-lived radioactive isotope (half-life 2.342 x 10’ a), a small amount of 236U should be present at an equilibrium level in natural uranium ores. By assuming a 100% neutron the~alization efficiency with 2.5 neutrons per 238U fission, it is estimated that the concentration of 236U over usU can be as high as 236U/ 238U u 7 X lo-” in 100% uranium ores (U,O,). 236U may potentially be a useful isotope for environmental studies, as well as an alternative to 2*Pu as a marker of supernova debris. However, this point has never been adequately investigated, probably due to the lack of any reliable analytical methods for detecting low levels of 236U at the presence of 238U and 235U. Naturally-~curring 236U was once reported to be detected in lunar samples [lo-131 and terrestrial minerals [ll using conventional tandem mass spectrometry. However, the validity of these results was severely affected by the later discovery of airborne 236U contamination in the laboratory 1141.Besides this contamination problem, it is also evident that conventional tandem mass spectrometry is not suitable for analyzing 236U at I lo-‘” levels due to the intense elastic scattering of the stronger beams, resulting in a nearly unit single-to-noise ratio at the ?J peak [l], and also due to the possible interferences from uranium monohydrides, as observed using the same mass spectrometer [15]. However, the measurement of 236U, and actinides in general, can benefit from the iower isotope-ratio detection limit made possible with accelerator mass spectrometry @MS) 1161, which completely removes
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III. NEW DIRECTIONS
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X-L. Zhao et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 249-253
the molecular interferences associated with conventional mass spectrometry. The ultra-high isotope-ratio sensitivity of AMS has so far been successful in measuring some important light- and medium-weighted isotopes, in particular “Be, “C, 26Al, 36C1 and 1291 [17]. The great potential of AMS as a sensitive method for actinides measurement, however, has not been exploited. It is generally assumed that for analyzing heavy elements by AMS, the use of high-energy (> 10 MV) large accelerators is required. However, as many of the long-lived actinide isotopes are not expected to be accompanied by atomic isobar interferences, it should also be possible to analyze these elements with low-energy (MV) AMS systems. It is therefore partly the aim of this paper to show, as demonstrated by the detection of naturally-occurring 236U in uranium ores, that a small tandembased AMS system such as that at IsoTrace [18,19], can be used to measure actinides with high isotope-ratio sensitivity.
2. Performance of the IsoTrace heavy element system for actinides measurement
AMS
There are some difficulties when using small tandem-based AMS systems for heavy element analysis. First, backgrounds of all kinds will be substantially higher due to larger cross sections of both elastic scattering and charge-changing collisions of low energy heavy ions, and possibly more complicated phenomena will occur in the heavy mass range due to the larger number of mass combinations. Second, the transmission efficiency will be fundamentally smaller because of the low yield of high charge states of heavy ions during charge-changing at l-2 MV terminal voltages. Third, it is technically a challenge to detect low energy (I 14 MeV) heavy ions with good energy resolution (< 2%). We have tested the capability of the IsoTrace heavy element AMS system for analyzing very heavy isotopes in terms of its performance in background elimination and transmission efficiency [20]. The design and evolution of this system can be found in a series of publications [18,19,21,22]. The major components of this system and their designed performance are (11 a milliprobe Cs sputter negative ion source; (2) an injection system consisting of a 4.5” electric analyzer (E/AE = 400) and a 90” magnet (M/AM I 400); (3) a tandem accelerator with terminal voltage usually I 2 MV; (41 a post-acceleration system consisting of a 15” electric analyzer (E/A,? = 3001, a 90” magnet (M/AM = 2600) and a 45” electric analyzer (E/AE I 900); (5) an ionfor deization chamber with N 5% energy resolution tecting 14 MeV actinide ions. When this system was used for detecting very heavy
ions, several types of background, as well as their causes, were identified. These background ions can be divided into two basic classes according to whether or not they can be distinguished from the desired species by the ionization chamber. Background ions from molecular fragmentation are distinguishable from the ions of interest, but the presence of adjacent charge-state ions due to the inadequate energy resolution of the post-acceleration analyzers can be particularly harmful, even though the performance of the ionization chamber is in many cases adequate to resolve the ions of interest. Fortunately, this type of interference can be readily reduced in intensity or avoided completely by using proper charge-changing processes for detecting the isotopes of interest. This strategy has been adopted in 1291/‘271 measurement by using the charge state +.5 for postacceleration analysis to avoid the 97M03f interference when charge state +4 is used [19]. For background ions which are not distinguishable from the ions of interest, the causes are mainly elastic scattering and charge-changing collisions with the residual gases in the post-acceleration magnet box, and/or, the charge-changing collisions in the acceleration tube. This problem is the result of the injection of high-intensity unwanted nearby major beams due to the insufficient mass resolution of the injection magnet. Sometimes it is also due to the high energy sputter tails and the decay of metastable hydride beams. These background ions have been the major problem for achieving a lower isotope-ratio detection level whenever mass-energy resolution is the limiting factor. If no intense interfering beam is present, or the isotopes of interest are far away from a major interference, it is still possible to achieve the background level which is only limited by the electronic noise of the detector system. For instance, the detection of 226Ra- and 231Pa- from samples made of geologically ancient uranium ore material [23] should be no problem from this point of view, because both 226Ra and 231Pa are several mass units away from the major isotopes 235U and 238U. However, to detect the naturally-occurring 236U in uranium ores, the present AMS system has to be operated in a scanning mode with the slits in the focal planes of all the critical analyzers being set to the same size as the beam. To determine the lowest isotope-ratio detection level for uranium with the present system, we performed a series of scans of the post-acceleration magnet while the rest of the system was respectively tuned for measuring the isotopes of 240Pu, 239Pu, 238U, 237Np and 236U through the charge-changing process Xi60-+ X5+ at terminal voltage 1.60 MV, using samples made of powdered Cigar Lake uranium ore material mixed with fine Nb powder. The results are presented in Fig. 1, where the beam intensities are normalized to that of 23aU5+. It can be seen that the
X-L. Zhao et al. /Nucl.
Instr. and Meth. in Phys. Rex B 92 (2994) 249-253
251
between 226 and 279 u. Consequently, the transmission efficiency from a 2 MV tandem accelerator is intrinsically low (_ 1%) for very heavy ions [24]. It was found [20] that even this already unsatisfactorily low theoretical transmission cannot be achieved in practice with the present stripping system and acceleration tube because of the beam loss due to multiple scattering and charge-changing collisions. In cases where the transmission efficiency is of primary importance, as in the case of detecting naturally-occurring 236U, charge states +4 or +5 have to be used with a lower terminal voltage of 1.25 or 1.60 MV, so that a transmission efficiency better than 1% can be obtained. The poor transmission efficiency of low energy very heavy ions is a serious limit to the total detection efficiency of actinides, which will not have a straightforward solution. However, the major limiting factor to the total detection efficiency, and ultimately the sensitivity, is at present not the transmission efficiency, but the low ionization efficiency of actinides by the Cs sputter ion source [23]. This problem cannot be addressed here. 1.0525
1.0550
Post -Acceleration
1.0575
1.0600
Magnetic
1.0625
Field
(T)
Fig. 1. A series of scans of the post-acceleration magnetic field to determine the isotope-ratio detection limit for measuring actinides with the present AMS system. The curves a to e were measured respectively while each of the following beams was injected: (a) 238U160-, (b) 239Pu’60-, (c) 240Pu’60-, (d) 237Np’60- and (e) 236U’60-. The discontinuity around the peaks of these curves &as probably due to the uncalibrated conversion between current measured on the
peaks and counting-rate measured below the peaks. The peaks A to D on curve e were caused by each of the following processes, respectively: (A) 235U’60- + 235U5‘, ~ 238~5+, and cD) 03) Z36U’60m,236D5+, (c) *3*u’60238U14N-
,238@+,
isotope-ratio detection limit is about 5 X lo-* for detecting a less abundant actinide isotope one mass unit above, and 5 X 10P1’ one mass unit below, a major isotope. The two orders of magnitude asymmetry in the isotope-ratio detection limit may be partly caused by the difference of the cross sections of charge-changing collisions which occurred inside the post-acceleration magnet box between situation q + q + 1 and q + q - 1 [20]. However, the possibility of this asymmetry as a direct result of an aberration of the magnet is also under investigation. Although these scans were performed under particular conditions, the indicated isotope-ratio detection limit is representative for measuring actinides in general with the present system. Due to the limited magnetic rigidity of the postacceleration magnet (u 1.5 T m), charge states 2 + 6 have to be selected after acceleration with 2 MV terminal voltage in order to detect isotopes of masses
3. Procedures and results of ?J
measurements
The isotope-ratio detection limit described above implies that a level of naturally-occurring 236U in uranium ores as low as 236U/ 238U _ 3.6 X lo-” can be detected. In fact, the 236U signals are already shown in Fig. 1. The measurement on the process 236U’60+ 236Us+ was performed several times at terminal voltage 1.60 MV. It was basically a scan of the postacceleration magnetic field over the 236U5+ peak. In order to avoid any possible molecular interferences charge state +5 was used instead of +4, so only u 0.5% transmission efficiency was obtained from UOat the entrance of the accelerator to IJ5+ in front of the final detector. The negative ion UO- was used because it has the highest yield from uranium ore targets [23]. Fig. 2 shows typical results of the scans of the post-acceleration magnetic field for the detection of 234,236,23Su’60 - --f 234,236,238@ + from a the processes target made of a compressed mixture of powdered Cigar Lake uranium ore material (0.6886 g/g) and Nb powder (0.3114 g/g). It shows that the detection of 236U5+ was accompanied by a significant 235U5+ peak. This was caused by the presence of a certain amount of 235U160- when 236U’60- was injected because of the inadequate mass resolution of the injection magnet. The magnetic field was calibrated precisely by the 234,235,23Su5 + peaks. During the experiment the 238U160- current was typically 650 pA with a 15% slow variation, which was measured frequently, and to correct for which all the data points in the scans have been normalized. The resulting 238U5+ current peaked III. NEW DIRECTIONS
X-L. Zhao et al. /Nucl.
252
Instr. and Meth. in Phys. Res. B 92 (1994) 249-2.53
235”+5 238”+5
\
,
1.0500
1.0525
II
II~~IIIIIII’~1~~~~I~I~’
1.0550
1.0575
1.0600
50,
I
04 1'.0530
1.0540
Post-Acceleration
1.056b
1.0550
Magnetic
Field
(T)
Fig. 2. A series of scans of the post-acceleration magnetic field for detecting the processes 234,236,235U160- --f 23432363238U5+,demonstrating the positive detection of a small amount of 236U in the Cigar Lake uranium ore.
at - 12 pA, which was not used to calculate the 236U/ 238U ratio because the Faraday cup was not designed to be quantitatively reliable. The 234U5+ was measured at a peak counting rate of - 690 s-l. The process 235u160-, 235U5+ was not measured because it produced either a too high counting rate or a too low current to measure. The peaks were fitted with Gaussian functions. The error bars represent a Poission distribution. The total area of the 236U5+ peak, after subtracting the 9.1 total background counts, was found to be 24.7 (k 26.5%) counts. The ratio u6U/234U was (1.02 f 0.27) X lo-‘, which, by assuming equilibrium of the Cigar Lake uranium ore, has been used to estimate the ratio 236U/ 238U to be (5.6 + 1.5) X 10p’o.
cant. The 236U51 peak, although very small, could not be an artifact caused by the strong u5U5+ peak, because no such effect was observed in the measurement of 239Pu from the same uranium ore targets #l. These facts insured that the detection of a small amount of 236U from the uranium ore targets was a positive detection. However, to undisputedly prove the very existence of naturally-occurring ?J requires many more measurements to be done on various uranium-bearing samples. These measurements will be carried out in the future. At present, our conclusion that this small amount of 236U measured was not due to contamination relies only on the following arguments. First, no count at the 236U5+ peak was detected within 1500 s. from either a target of compressed pure Nb powder or a graphite target. Second, the scan of the magnet was a process which lasted several hours, during which a significant layer of the target surface was removed. At the time when the 236U5+ peak was measured, the beam was generated from the interior of the target. Therefore, no surface contamination was possible. Third, although we used in our ion source during the very first run measuring the naturally-occurring 236U a very “hot” sample made of commercial uranium metal powder which was found to contain - 10 ppm 236U, the ion source has been cleaned many times since then and the 236U level found from the uranium ore samples in the later measurements has shown reproducibility. Finally, the measured 236U level in material from the Cigar Lake uranium ore body agrees very well with the theoretical expectation, considering the fact that the Cigar Lake uranium ore has indeed been saturated by groundwater since the early stage of its formation [25], and is the ore with the highest uranium concentration (40-60%) in the world. Furthermore, this z36U level also matches the following independent measurements: (1) the 1291concentration within the Cigar Lake uranium ore body ((4.63 * 0.52) X lo9 atm/g) [26], (2) the amount of 239Pu found in pitchblende (239Pu/ 238U = (3.10 & 0.14) X 10-12) [2], (3) the 236U level previously claimed for samples of processed uranium ore (236U/ 238U = (6.2 f 2.2) x lo- “1 [l].
5. Conclusion Although the present IsoTrace heavy element Ah4S system needs to be much improved to be more suitable for actinide analysis, we are still able to demonstrate
4. Discussion
Results obtained in several separate runs of the spectrometer over a period of five months were reproducible within experimental errors, and the signal-tonoise ratio of the 236U5+ peak was statistically signifi-
*‘The direct detection of naturally-occurring 239Pu from unprocessed uranium ore material requires a much lower isotope-ratio detection limit than our present system can achieve.
X-L. Zhao et al. /Nucl.
Instr. and Meth. in Phys. Rex B 92 (1994) 249-253
its capability in measuring at its natural level the trace amount of 236U produced by 235U neutron capture reactions within the highly enriched Cigar Lake uranium ore. This demonstration opens the possibility that natural~y~ccurring % at sub-ppb levels can be measured by AMS with technicaf performance superior to conventional mass spectrometry fl]. The isotope-ratio detection limit of our Ah4S system can be substantially improved with the installation of a better injection magnet with much higher mass resolving power. Of course, true improvement in isotope-ratio sensitivity for detecting actinides will also depend on the development of ion source techniques able ta increase the ionization efficiency of actinides significantly. These technical advances will allow us in the future to carry out a reliable wide-ranging survey of the naturally-occurring u6U in lunar materials and in various terrestriaf materials, in~Iuding material from the K-T boundary, to search for supernova debris. The perspectives here apply not o&y to the future application of 236U, but also to the detection of other important long-lived actinides such as ‘@Pu in nature.
Acknowledgements We would like to thank J.J. Cramer of Atomic Energy of Canada Ltd. for providing the Cigar Lake uranium ore material. Roelf Beukens of JsoTrace Laboratory is highly appreciated for his many helpful discussions and suggestions. This program was supported by the Natural Sciences and Engineering Research Council of Canada. The large high resolution magnet is on extended loan from Atomic Energy of Canada Ltd.
[I] D.J. R&on, D.N. Metta and CM. Stevens, lat. J, Mass Spectrom. Ion Phys. 8 (19721259. 121 W.A. Myers and M. Lindner, J. Inorg. Nucl, Chem. 33 (197113233. 131 G.T. Seaborg and M.L. Perlman, J. Am. Cbem. Sot. 70 (194% 1571. [4] C.S. Garner, N.A. Banner and G.T. Seaborg, J. Am. Chem. Sot. 70 (1948) 3453.
253
151W.H. Fleming and H.G. Thode, Phys. Rev. 92 (19531378. [6] G.A. Cowan and H.H. Adler, Geochim. Cosmochim. Acta 40 (19%) 1490. [7] P. Sharma and R. Middketon, Geochim. Cosmochim. Acta 53 (19g9) 716. ig] D.F. Peppard, G.W. Mason, P.R. Gray and J+F. Mech, 3. Am. Chem. Sot. 74 (1952) 6081. @I M. Maurette, Annu. Rev. Nucl. Sci. 26 f19761319. [IO] P.R. Fields, H. Diamond, D.N. Metta, CM. Stevens and DJ. Rokop, Proc. Lunar Sci. Conf. 2nd (MIT Press, Cambridge, MA, 1971) pp. 1571-1576. [ll] P.R. Fields, 11. Diamond, D.N. Metta, D.J. Rokup and C.M. Stevens, Proc. Lunar Sci. Conf. 3rd (MIT Press, Cambridge, MA, 1972) pp. 1637-1644, [12] P.R. Fields, H. Diamond, D.N. Metta and D.J. Rokop, Proc. Lunar Sci. Conf. 4th (Pergamon, Oxford, 1973) pp. 2123-2130, 1131P.R. Fields, H. Diamond. D.N. Metta and D.J. Rokop, Lunar Sci. V (1974) 227. ft4] P.R. Fields, H. Diamond, D.N. Mefta and D.J. Rokop, Lunar Sci. VII (1976) 251. 1151P.E. Moreland Jr., D.J. Rokop and CM. Stevens, Int. J. Mass Spectram. Ion Phys. 5 (19701 127. [16] A.E. Litherland, Annu. Rev. Nucl. Part. Sci. 30 (1980) 437. [17] D. Elmore and F.M. Phillips, Science 236 (1987) 543. [18] L.R. Kilius, M,A. Garwan, A.E. Litherland, M-J. Nadeau, J.C. Rucklidge and X-L. Zhao, Nucl. Instr. and Meth. B 40/41 (19891 745. 1191L.R. Kilius, N. Baba, M.A. Garwan, A.E. Litherland, M-J. Nadeau, J.C. Rucklidge, G.C. Wilson and X-L. Zhao, NucI, Instr. and Meth. B 52 (1990) 357. f20] X-L. Zhao, PhD thesis, Un~~e~i~ of Toronto, 1993. 1211 L.R. K&us, 3.C. Rucklidge and A.E. Litherland, NueI. In&r. and Meth. B 29 (1987) 72. f221 L.R. Kilius, J.C. Rucklidge and A.E. Litherland, Nucl. Instr. and Meth B 31 (1988) 433. [23] X-L. Zhau, M-J. Nadeau, M.A. Garwan, L.R. Kiiius and A.E. Litherland, these Proceedings (6th Int. Conf. on Accelerator Mass Spectrometry CAMS-61,Canberra-Sydney, Australia, 1993) Nucl. Instr. and Meth. B 92 (1994) 258. [24] A.B. Wittkower and H.D. Betz, At. Data 5 (1973) 113. 1251 J.J. Cramer, P. Vilks and J.P.A. Larocque, in: Proc. CEC Symp. on Natural Anaiogues in Radioactive Waste Disposal, Brussels, April 28-30, eds. B. C$me and N.A. Chapman (Graham & Trotman, 1987% 1267 L-R. Kilius, J.C. Rucktidge, X-L. Zhao, A.E. Litherland, G.M. Raisbeck, F. Yiou and Z.Q. Zhou, paper presented at the Workshop on Arctic Contamination, Archorage, Alaska, May 1993.
III. NEW DIRECTIGNS
The first detection of naturally-occurring with accelerator mass spectrometry
Beam Intenotions with Materials %Atoms
236U
X-L. Zhao a**,M-J. Nadeau b, L.R. Kilius a and A.E. Li~herland a a IsoTrace Laboratory, Department of Physics, University of Toronto, 60 St. George St., Toronto,
Ontario, Canada M.5S IA7 b National Ocean Science Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
The Isol’race heavy element AMS system has been successfully used to detect naturally-occurring 236U (236LJ/ 238U= (5.6 + 1.5) X lo-“) in samples of uranium ore from Cigar Lake, Saskatchewan, Canada. This level of 236U agrees with that previously claimed for samples of a processed uranium ore (D.J. Rokop, D.N. Metta and CM. Stevens, Int. J. Mass Spectrom. Ion Phys. 8 (1972) 259 [I& and is consistent with the amount of u9Pu found in pitchblende (W.A. Myers and M. Lindner, J. Inorg. Nucl. Chem. 33 (1971) 3233 [Z]). This experiment illustrates the general capability of a small tandem-based AMS system for analyzing actinides, in particular 236U. It can be shown that the isotope-ratio detection limit of this system, is at present 5 x lo-’ for detecting a less abundant actinide isotope one mass unit above, and 5 X lo-” one mass unit below, a major isotope.
1. Introduction It has been known for half a century that both neutron-induced fission and the (n, y) reaction occur in uranium minerals and uranium ores. This is based on a number of observations, including the first isolation of the naturally-occurring 23gPu in pitchblende [3] and uranium ores 141,the observation of the variations of the fission yields of the stable isotopes of xenon and krypton [5], the measurement of the variability of the natural abundance of 235U [6], and more recently, the detection of the radiogenic “Be and 26Al in uranium and thorium ores [7]. The (n, 2n) reaction is also known to occur due to the isolation of the naturally-occurring 237Np in pitchblende 12,8]. The Oklo phenomenon [9] is obviously the most extraordinary example of naturally-occurring neutron-induced reactions. It is therefore rather obvious that 236U should be a naturally-occurring isotope due to the (n, r> reaction on 235U in uranium ores, in spite of the low abundance of 235U. If a uranium ore deposit is saturated with groundwater, as it was in the Oklo natural nuclear reactors, MeV neutrons from =‘U fission can be readily thermalized before interaction. Thus, the large cross section (_ 99 b) for 23sU thermal neutron absorption can be a major boost for 236U production. Since 236U is
* Corresponding author. Tel. + 1 416 978 4041, fax + 1 416 978 4711, e-mail [email protected].
a long-lived radioactive isotope (half-life 2.342 x 10’ a), a small amount of 236U should be present at an equilibrium level in natural uranium ores. By assuming a 100% neutron the~alization efficiency with 2.5 neutrons per 238U fission, it is estimated that the concentration of 236U over usU can be as high as 236U/ 238U u 7 X lo-” in 100% uranium ores (U,O,). 236U may potentially be a useful isotope for environmental studies, as well as an alternative to 2*Pu as a marker of supernova debris. However, this point has never been adequately investigated, probably due to the lack of any reliable analytical methods for detecting low levels of 236U at the presence of 238U and 235U. Naturally-~curring 236U was once reported to be detected in lunar samples [lo-131 and terrestrial minerals [ll using conventional tandem mass spectrometry. However, the validity of these results was severely affected by the later discovery of airborne 236U contamination in the laboratory 1141.Besides this contamination problem, it is also evident that conventional tandem mass spectrometry is not suitable for analyzing 236U at I lo-‘” levels due to the intense elastic scattering of the stronger beams, resulting in a nearly unit single-to-noise ratio at the ?J peak [l], and also due to the possible interferences from uranium monohydrides, as observed using the same mass spectrometer [15]. However, the measurement of 236U, and actinides in general, can benefit from the iower isotope-ratio detection limit made possible with accelerator mass spectrometry @MS) 1161, which completely removes
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDZ 0168-583Xt93)ElOiO-J
III. NEW DIRECTIONS
250
X-L. Zhao et al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 249-253
the molecular interferences associated with conventional mass spectrometry. The ultra-high isotope-ratio sensitivity of AMS has so far been successful in measuring some important light- and medium-weighted isotopes, in particular “Be, “C, 26Al, 36C1 and 1291 [17]. The great potential of AMS as a sensitive method for actinides measurement, however, has not been exploited. It is generally assumed that for analyzing heavy elements by AMS, the use of high-energy (> 10 MV) large accelerators is required. However, as many of the long-lived actinide isotopes are not expected to be accompanied by atomic isobar interferences, it should also be possible to analyze these elements with low-energy (MV) AMS systems. It is therefore partly the aim of this paper to show, as demonstrated by the detection of naturally-occurring 236U in uranium ores, that a small tandembased AMS system such as that at IsoTrace [18,19], can be used to measure actinides with high isotope-ratio sensitivity.
2. Performance of the IsoTrace heavy element system for actinides measurement
AMS
There are some difficulties when using small tandem-based AMS systems for heavy element analysis. First, backgrounds of all kinds will be substantially higher due to larger cross sections of both elastic scattering and charge-changing collisions of low energy heavy ions, and possibly more complicated phenomena will occur in the heavy mass range due to the larger number of mass combinations. Second, the transmission efficiency will be fundamentally smaller because of the low yield of high charge states of heavy ions during charge-changing at l-2 MV terminal voltages. Third, it is technically a challenge to detect low energy (I 14 MeV) heavy ions with good energy resolution (< 2%). We have tested the capability of the IsoTrace heavy element AMS system for analyzing very heavy isotopes in terms of its performance in background elimination and transmission efficiency [20]. The design and evolution of this system can be found in a series of publications [18,19,21,22]. The major components of this system and their designed performance are (11 a milliprobe Cs sputter negative ion source; (2) an injection system consisting of a 4.5” electric analyzer (E/AE = 400) and a 90” magnet (M/AM I 400); (3) a tandem accelerator with terminal voltage usually I 2 MV; (41 a post-acceleration system consisting of a 15” electric analyzer (E/A,? = 3001, a 90” magnet (M/AM = 2600) and a 45” electric analyzer (E/AE I 900); (5) an ionfor deization chamber with N 5% energy resolution tecting 14 MeV actinide ions. When this system was used for detecting very heavy
ions, several types of background, as well as their causes, were identified. These background ions can be divided into two basic classes according to whether or not they can be distinguished from the desired species by the ionization chamber. Background ions from molecular fragmentation are distinguishable from the ions of interest, but the presence of adjacent charge-state ions due to the inadequate energy resolution of the post-acceleration analyzers can be particularly harmful, even though the performance of the ionization chamber is in many cases adequate to resolve the ions of interest. Fortunately, this type of interference can be readily reduced in intensity or avoided completely by using proper charge-changing processes for detecting the isotopes of interest. This strategy has been adopted in 1291/‘271 measurement by using the charge state +.5 for postacceleration analysis to avoid the 97M03f interference when charge state +4 is used [19]. For background ions which are not distinguishable from the ions of interest, the causes are mainly elastic scattering and charge-changing collisions with the residual gases in the post-acceleration magnet box, and/or, the charge-changing collisions in the acceleration tube. This problem is the result of the injection of high-intensity unwanted nearby major beams due to the insufficient mass resolution of the injection magnet. Sometimes it is also due to the high energy sputter tails and the decay of metastable hydride beams. These background ions have been the major problem for achieving a lower isotope-ratio detection level whenever mass-energy resolution is the limiting factor. If no intense interfering beam is present, or the isotopes of interest are far away from a major interference, it is still possible to achieve the background level which is only limited by the electronic noise of the detector system. For instance, the detection of 226Ra- and 231Pa- from samples made of geologically ancient uranium ore material [23] should be no problem from this point of view, because both 226Ra and 231Pa are several mass units away from the major isotopes 235U and 238U. However, to detect the naturally-occurring 236U in uranium ores, the present AMS system has to be operated in a scanning mode with the slits in the focal planes of all the critical analyzers being set to the same size as the beam. To determine the lowest isotope-ratio detection level for uranium with the present system, we performed a series of scans of the post-acceleration magnet while the rest of the system was respectively tuned for measuring the isotopes of 240Pu, 239Pu, 238U, 237Np and 236U through the charge-changing process Xi60-+ X5+ at terminal voltage 1.60 MV, using samples made of powdered Cigar Lake uranium ore material mixed with fine Nb powder. The results are presented in Fig. 1, where the beam intensities are normalized to that of 23aU5+. It can be seen that the
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Instr. and Meth. in Phys. Rex B 92 (2994) 249-253
251
between 226 and 279 u. Consequently, the transmission efficiency from a 2 MV tandem accelerator is intrinsically low (_ 1%) for very heavy ions [24]. It was found [20] that even this already unsatisfactorily low theoretical transmission cannot be achieved in practice with the present stripping system and acceleration tube because of the beam loss due to multiple scattering and charge-changing collisions. In cases where the transmission efficiency is of primary importance, as in the case of detecting naturally-occurring 236U, charge states +4 or +5 have to be used with a lower terminal voltage of 1.25 or 1.60 MV, so that a transmission efficiency better than 1% can be obtained. The poor transmission efficiency of low energy very heavy ions is a serious limit to the total detection efficiency of actinides, which will not have a straightforward solution. However, the major limiting factor to the total detection efficiency, and ultimately the sensitivity, is at present not the transmission efficiency, but the low ionization efficiency of actinides by the Cs sputter ion source [23]. This problem cannot be addressed here. 1.0525
1.0550
Post -Acceleration
1.0575
1.0600
Magnetic
1.0625
Field
(T)
Fig. 1. A series of scans of the post-acceleration magnetic field to determine the isotope-ratio detection limit for measuring actinides with the present AMS system. The curves a to e were measured respectively while each of the following beams was injected: (a) 238U160-, (b) 239Pu’60-, (c) 240Pu’60-, (d) 237Np’60- and (e) 236U’60-. The discontinuity around the peaks of these curves &as probably due to the uncalibrated conversion between current measured on the
peaks and counting-rate measured below the peaks. The peaks A to D on curve e were caused by each of the following processes, respectively: (A) 235U’60- + 235U5‘, ~ 238~5+, and cD) 03) Z36U’60m,236D5+, (c) *3*u’60238U14N-
,238@+,
isotope-ratio detection limit is about 5 X lo-* for detecting a less abundant actinide isotope one mass unit above, and 5 X 10P1’ one mass unit below, a major isotope. The two orders of magnitude asymmetry in the isotope-ratio detection limit may be partly caused by the difference of the cross sections of charge-changing collisions which occurred inside the post-acceleration magnet box between situation q + q + 1 and q + q - 1 [20]. However, the possibility of this asymmetry as a direct result of an aberration of the magnet is also under investigation. Although these scans were performed under particular conditions, the indicated isotope-ratio detection limit is representative for measuring actinides in general with the present system. Due to the limited magnetic rigidity of the postacceleration magnet (u 1.5 T m), charge states 2 + 6 have to be selected after acceleration with 2 MV terminal voltage in order to detect isotopes of masses
3. Procedures and results of ?J
measurements
The isotope-ratio detection limit described above implies that a level of naturally-occurring 236U in uranium ores as low as 236U/ 238U _ 3.6 X lo-” can be detected. In fact, the 236U signals are already shown in Fig. 1. The measurement on the process 236U’60+ 236Us+ was performed several times at terminal voltage 1.60 MV. It was basically a scan of the postacceleration magnetic field over the 236U5+ peak. In order to avoid any possible molecular interferences charge state +5 was used instead of +4, so only u 0.5% transmission efficiency was obtained from UOat the entrance of the accelerator to IJ5+ in front of the final detector. The negative ion UO- was used because it has the highest yield from uranium ore targets [23]. Fig. 2 shows typical results of the scans of the post-acceleration magnetic field for the detection of 234,236,23Su’60 - --f 234,236,238@ + from a the processes target made of a compressed mixture of powdered Cigar Lake uranium ore material (0.6886 g/g) and Nb powder (0.3114 g/g). It shows that the detection of 236U5+ was accompanied by a significant 235U5+ peak. This was caused by the presence of a certain amount of 235U160- when 236U’60- was injected because of the inadequate mass resolution of the injection magnet. The magnetic field was calibrated precisely by the 234,235,23Su5 + peaks. During the experiment the 238U160- current was typically 650 pA with a 15% slow variation, which was measured frequently, and to correct for which all the data points in the scans have been normalized. The resulting 238U5+ current peaked III. NEW DIRECTIONS
X-L. Zhao et al. /Nucl.
252
Instr. and Meth. in Phys. Res. B 92 (1994) 249-2.53
235”+5 238”+5
\
,
1.0500
1.0525
II
II~~IIIIIII’~1~~~~I~I~’
1.0550
1.0575
1.0600
50,
I
04 1'.0530
1.0540
Post-Acceleration
1.056b
1.0550
Magnetic
Field
(T)
Fig. 2. A series of scans of the post-acceleration magnetic field for detecting the processes 234,236,235U160- --f 23432363238U5+,demonstrating the positive detection of a small amount of 236U in the Cigar Lake uranium ore.
at - 12 pA, which was not used to calculate the 236U/ 238U ratio because the Faraday cup was not designed to be quantitatively reliable. The 234U5+ was measured at a peak counting rate of - 690 s-l. The process 235u160-, 235U5+ was not measured because it produced either a too high counting rate or a too low current to measure. The peaks were fitted with Gaussian functions. The error bars represent a Poission distribution. The total area of the 236U5+ peak, after subtracting the 9.1 total background counts, was found to be 24.7 (k 26.5%) counts. The ratio u6U/234U was (1.02 f 0.27) X lo-‘, which, by assuming equilibrium of the Cigar Lake uranium ore, has been used to estimate the ratio 236U/ 238U to be (5.6 + 1.5) X 10p’o.
cant. The 236U51 peak, although very small, could not be an artifact caused by the strong u5U5+ peak, because no such effect was observed in the measurement of 239Pu from the same uranium ore targets #l. These facts insured that the detection of a small amount of 236U from the uranium ore targets was a positive detection. However, to undisputedly prove the very existence of naturally-occurring ?J requires many more measurements to be done on various uranium-bearing samples. These measurements will be carried out in the future. At present, our conclusion that this small amount of 236U measured was not due to contamination relies only on the following arguments. First, no count at the 236U5+ peak was detected within 1500 s. from either a target of compressed pure Nb powder or a graphite target. Second, the scan of the magnet was a process which lasted several hours, during which a significant layer of the target surface was removed. At the time when the 236U5+ peak was measured, the beam was generated from the interior of the target. Therefore, no surface contamination was possible. Third, although we used in our ion source during the very first run measuring the naturally-occurring 236U a very “hot” sample made of commercial uranium metal powder which was found to contain - 10 ppm 236U, the ion source has been cleaned many times since then and the 236U level found from the uranium ore samples in the later measurements has shown reproducibility. Finally, the measured 236U level in material from the Cigar Lake uranium ore body agrees very well with the theoretical expectation, considering the fact that the Cigar Lake uranium ore has indeed been saturated by groundwater since the early stage of its formation [25], and is the ore with the highest uranium concentration (40-60%) in the world. Furthermore, this z36U level also matches the following independent measurements: (1) the 1291concentration within the Cigar Lake uranium ore body ((4.63 * 0.52) X lo9 atm/g) [26], (2) the amount of 239Pu found in pitchblende (239Pu/ 238U = (3.10 & 0.14) X 10-12) [2], (3) the 236U level previously claimed for samples of processed uranium ore (236U/ 238U = (6.2 f 2.2) x lo- “1 [l].
5. Conclusion Although the present IsoTrace heavy element Ah4S system needs to be much improved to be more suitable for actinide analysis, we are still able to demonstrate
4. Discussion
Results obtained in several separate runs of the spectrometer over a period of five months were reproducible within experimental errors, and the signal-tonoise ratio of the 236U5+ peak was statistically signifi-
*‘The direct detection of naturally-occurring 239Pu from unprocessed uranium ore material requires a much lower isotope-ratio detection limit than our present system can achieve.
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Instr. and Meth. in Phys. Rex B 92 (1994) 249-253
its capability in measuring at its natural level the trace amount of 236U produced by 235U neutron capture reactions within the highly enriched Cigar Lake uranium ore. This demonstration opens the possibility that natural~y~ccurring % at sub-ppb levels can be measured by AMS with technicaf performance superior to conventional mass spectrometry fl]. The isotope-ratio detection limit of our Ah4S system can be substantially improved with the installation of a better injection magnet with much higher mass resolving power. Of course, true improvement in isotope-ratio sensitivity for detecting actinides will also depend on the development of ion source techniques able ta increase the ionization efficiency of actinides significantly. These technical advances will allow us in the future to carry out a reliable wide-ranging survey of the naturally-occurring u6U in lunar materials and in various terrestriaf materials, in~Iuding material from the K-T boundary, to search for supernova debris. The perspectives here apply not o&y to the future application of 236U, but also to the detection of other important long-lived actinides such as ‘@Pu in nature.
Acknowledgements We would like to thank J.J. Cramer of Atomic Energy of Canada Ltd. for providing the Cigar Lake uranium ore material. Roelf Beukens of JsoTrace Laboratory is highly appreciated for his many helpful discussions and suggestions. This program was supported by the Natural Sciences and Engineering Research Council of Canada. The large high resolution magnet is on extended loan from Atomic Energy of Canada Ltd.
[I] D.J. R&on, D.N. Metta and CM. Stevens, lat. J, Mass Spectrom. Ion Phys. 8 (19721259. 121 W.A. Myers and M. Lindner, J. Inorg. Nucl, Chem. 33 (197113233. 131 G.T. Seaborg and M.L. Perlman, J. Am. Cbem. Sot. 70 (194% 1571. [4] C.S. Garner, N.A. Banner and G.T. Seaborg, J. Am. Chem. Sot. 70 (1948) 3453.
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III. NEW DIRECTIGNS