Heavy element analysis by low energy accelerator mass spectrometry

Nuclear Instruments and Methods in Physics Research B40/41 North-Holland, Amsterdam

HEAW

ELEMENT

ANALYSIS

L.R. KILIUS, M.A. GARWAN, and X-L. ZHAO IsoTrace Luboratoty,

(1989) 745-749

745

BY LOW ENERGY ACCELERATOR A.E. LITHERLAND,

M-J. NADEAU,

MASS SPECI’ROMETRY J.C. RUCKLIDGE

University of Toronto, Toronto, Canada

Improvements have been made to the mass and energy analysis system at the IsoTrace Laboratory that have resulted in a factor of 30 increase in detection efficiency for some heavy ions and have reduced interference from the fragmentation of hydrides and other molecules. The implications of these improvements’for the detection of rare atomic negative ions and radioisotopes such as 1291 will be discussed. Evidence is also presented for the existence of a barium negative ion that is completely resolved from barium mono-, di- and trihydride interferences and which is clearly distinguished from any interference due to the fragmentation of BaH; to BaH- before the first analyzing magnet.

1. Introduction The primary emphasis in this paper will be on the detection of isotopes of elements that were not in the past expected to produce negative ions. Such a search could be useful in choosing elements for solar neutrino studies. One possible example was ‘37La (30000 yr half-life) which could be detected by accelerator mass spectrometry (AMS) if ‘37Ba did not form negative ions as lanthanum does. This search was also in part motivated by the recent calculations of Fischer et al. [l] that indicated the formation of stable negative ions with ns’np(‘P) configurations for the group IIa elements beyond magnesium. Previous experimental evidence [2] had shown the existence of a meta-stable state of Bein the 2~~p2(~P) configuration and a similar quartet P state for Mg negative ion was expected to have a lifetime of less than 10 ns [3]. Fischer et al. were able to demonstrate that correlation effects played a dominant role in determining the negative ion structure of the other group IIa elements. Their extensive multi-configuration Hartree-Fock and density functional theory calculations indicated that if the extra electron in Ca- is promoted to the 4p orbital, the resulting *P state would lie 45 meV below the ground state energy of the neutral atom. A search by Pegg [4] using a combined laser and electron spectrometer system determined the existence of a stable Ca negative ion with a binding energy of 43 meV. This has set the stage for the prediction [5] and the search of other potentially stable negative ions belonging to group IIa which include the isotopes of strontium, barium and radium. The detection of these negative ions was hampered in the past by the abundant hydride spectra that almost totally obscured the region of interest. The analysis of medium (20 < A < 50) and heavy A( > 50) elements also required a more detailed knowledge of the backgrounds

0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

from hydride fragmentation that are likely to be encountered. This is especially true for low energy accelerator mass spectrometry (AMS) systems which are dedicated to radiocarbon analytical services [6], as complete isotopic identification by rate of energy loss detectors is either very difficult or, as in the case of the heaviest ions, impossible. The complete elimination of backgrounds by appropriate energy and mass filters would be the preferred method [7]. The origins of the background ions injected into the accelerator differ from the backgrounds expected on the postaccelerator side of the spectrometer, although, as will be shown, the methods required to eliminate these undesirable ions are very similar. The subtlety of the backgrounds transmitted through an AMS system can be best illustrated for the case of 1291 detection and by a flow diagram of the ion processes through the spectrometer components, fig. 1, which also represents schematically the various components of the AMS system as configured for heavy element analysis at IsoTrace. For example, it is well known that the sputtering process will produce ions with an energy distribution that can interfere substantially with the mass analysis of negative ions. The complexity of the sputtered ion energy spectra was amply illustrated for the case of carbon [7] and would have a similar energy distribution for other elements. These higher energy components were shown to have an approximate l/E* distribution, as expected kinematically, that enabled 2 in every 10000 1271 ions to masquerade as ‘291. The addition of a spherical, 45O electric analyzer, ESAl, with an energy dispersion of E/AE = 400 reduced these backgrounds by at least three orders of magnitude for the case of 1291. Plans have been made to increase the total dispersion, by a factor of 2, by the addition of another 45” analyzer. This will counter the expected factor of 10 increase in background from the sputter tail around the VI. MATERIALS

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ESA2 and ESA3. This situation parallels the problems encountered at the ion source due to sputtering. However, the energy analysis (E/A E = 200) available immediately after acceleration is at best sufficient to select the required charge state and as such was not designed to remove a continuum of ions for heavier elements. It was precisely this type of background that limited the detection of “‘1 to approximately lo-l3 12gI/‘271. Obviously another electrostatic sector, ESA4, with the required energy resolution of E/A E > 200 is required. Some ions that pass the first electric analyzer, ESAZ, with the correct energy but an incorrect mass can still be transmitted past the magnet, MAGZ, after scattering by the residual gas in the vacuum system. These ions are a possible source of an E/q ambiguity and will pass through all subsequent electric analysis provided the charge has not changed during scattering. Finally, molecular fragments and isobars that also have the same E/q and M/q as the ion of interest will pass through any combination of electrostatic and magnetic analysis. The remaining E/q and M/q ambiguities that result from molecular fragmentation differ in energy by some integer fraction of the main isotope of interest and can be resolved by a solid state or gas ionization detector provided the counting rate is not excessive.

Fig. 1. A flow diagram of the AMS system used at IsoTrace is illustrated. All secondary paths indicate possible sources of

background which may enter the final detector. Triangles: electrostatic analyzers; squares: magnets; single circle: ion source; double circle: detector; polygon: background generators; ellipse: accelerator.

mass 200 region and further reduce the lz71 ambiguity at mass 129. The decay of metastable hydrides and, in particular, the hydride of heavier isotopes such as 13’Te could be injected at the same mass as i2’I as a result of a decay to a atomic ion of the same isotope. If this decay were to occur after ESAl, the resulting ion would be transmitted through MAGl as mass 129. This molecular decay process has the effect of bypassing the presence of the electric and magnetic analyzer (fig. 1). The negative molecular ions from sputter sources are also expected to be highly excited and so susceptible to spontaneous fragmentation [7]. Molecular fragmentation, charge changing and gas scattering also combine to complicate the postacceleration spectrum. A continuum of ions is generated within the accelerator, some of which will have the required mass-energy/charge2 to pass through the 90” magnet MAG2 and the lower resolution electrostatic analyzers,

2. Experimental procedure The experimental setup has already been described in some detail [7]. The primary change to that reported earlier was the replacement of a 90 o magnet that had a bending power of 12 MeV amu and a mass dispersion of 132 cm with a magnet that has a bending power of 120 MeV amu and a dispersion of 264 cm with improved ion optics. The larger magnet (designated as MAGZ) necessitated the addition of a lS” cylindrical analyzer, ESAZ, within the image drift space of the magnet to bend the ions away from the laboratory wall. The energy dispersion of this analyzer is equal to the 15” analyzer following the tandem accelerator and is also useful for removing ions that have changed their charge in the magnetic field. An additional 30 o electric analyzer will follow ESA2 at a later date. This new 30 o analyzer with a 250 cm radius of curvature will provide an additional dispersion of E/A E = 1000 to remove the remaining mass-energy/charge2 ambiguities before the final detector. A pair of cryopumps was also added before and after the magnet box to maintain a vacuum below 1 X 10m7 mbar. For the measurements reported in this paper an Ortec F series surface barrier detector was used for all measurements. Two samples of 99.95% pure barium were prepared by pressing the metal into a 2 mm diameter by 3 mm

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deep hole drilled into a standard 6 mm diameter aluminum sample holder. To reduce the formation of oxides the samples were prepared and transferred to the ion source under argon gas. Samples of natural iodine in the form of NaI pressed into tantalum were also prepared to setup the beam optics. Standard graphite samples were used for all beam diagnostic requirements. An accelerating potential of 1.75 MV was used because of the limitations of the present power supply for the large 90 o magnet. As has been our past experience with heavy ions, no additional argon stripper gas was required for maximum charge state 5 + production as the residual gas within the tube provides the needed gas when compressed with the terminal turbomolecular pump system. Based on the production of 12’1 charge state 5 + ions a transmission efficiency of 4.4% was achieved and would be similar for barium isotopes. This was an improvement by a factor of 30 over the previous experimental arrangement that required operation of the tandem accelerator at 0.75 MV. The mass resolution of the new magnet MAGZ, as determined by the shape of a “C spectrum, was over M/AM = 2000 as expected.

3. Experimental results Fig. 2 illustrates a mass spectrum of the negative ions and molecules from a barium sample that had been bombarded by cesiurn ions for more than one day immediately after an ion source pumpdown. The presence of barium hydrides and both the mono- and dioxide groups of barium are clearly visible. An analysis

of the BaH, ion currents from this sample measured after MAGl gave for n = 0, 1, 2 and 3, percentage relative abundances of 6.4, 16.5, 3.4 and 73.7 repectively. This is not proof of the existence of Ba- because hydride fragmentation, as discussed below, was ignored. A similar mass scan over a fresh barium sample did not reveal a distinct set of hydride or oxide peaks indicating the lack of a significant oxidation of the sample during preparation. Also, as barium and the subsequently deposited Cs layers are both very efficient getters, it was not surprising that the hydride and oxide components would increase with time due to the breakup of residual water vapour in the vacuum system under ion bombardment. It was also very fortuitous that these peaks were present during the initial stages of the experiment so that the magnetic field of MAGl could be calibrated for the barium isotope mass range. The electrostatic components on the injection side for negative ion selection and on the postaccelerator side for charge state + 5 selection were optimized using a beam of 12’1 ions. A simple calibration procedure was not readily available for MAG2. The magnet was therefore current-scaled up from mass 127 to mass 133 where ions were expected to be present from the Cs in the ion source. The final fine tuning, to reduce the effects of hysteresis, was achieved by adjusting the magnet for peak counting rates of 133Cs and later 13sBa ions in the solid state detector. Unfortunately, a Hall probe on MAG2 did not have the required resolution for accurate scaling of the field. This tuning procedure was not without risk as a lower magnetic field could be indicative of hydride fragmentation or the presence of other ME/q2 ambiquities. The magnet current changes required to compensate for hyster-

400

di-oxides

Negative

Ion Mur

Fig. 2. ‘Ihe bar graph represents a mass scan of the negative ion currents obtained around the barium mass region. Three groups of negative ions dominate the spectrum, hydrides, oxides, and dioxides. The locations of some 13’Ba molecular negative ions have been indicated. VI. MATERIALS

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L. R. Kilius et al. / Heavy element analysis by low energy Ah4S

L

4.4

4.6

4.5

Magnetic

Field

B,,,

(arb.

4.7

units)

Fig. 3. A two-dimensionalplot of the injection magnetic field (MAGl) vs the postaccelerator magnetic field (MAGZ) and charge state 5 + ion counting rate is illustrated. Points with counting rates in excess of 10000 cps are denoted by solid circles. The size of each symbol is also related to the logarithm of the counting rate at each point. Four bands of ions are evident. The lowest band correspondsto the sequence A- to A’+. The remainingbands are hydride fragmentations of the form AH; to A5+. esis were of the order of 0.02% and were found to be dependent on the direction of approach to the final mass. Hence these offsets could not be attributed to the fragmentation of the hydride from a lower mass isotope as this would require a factor of 5 larger change in the magnet current than was observed. Isotopes of the same mass could be injected by processes such as r3*BaH; which decay to 13*BaHafter the ion source electrostatic analyzer but before injection magnet. This molecule would appear as a mass 138 ion after MAGl and would also be treated by MAG2 as a mass 138 ion with a slightly lower (approx. 0.12%) energy due to the fragmentation process. It is difficult to distinguish this process from the injection of a 13’Ba- ion but this can be accomplished with the help of MAGZ. Two procedures were adopted to verify the presence of barium negative ions and to reduce the possibility of interference by some rare metastable molecular negative ion decay of this type. The first check was to look for “resonances” in the counting rate with changes in the terminal voltage. When each barium mass was tuned through the complete system the terminal voltage peaked sharply at the same potential with a resolution of approximately 1 kV. Peaks were also found at a slightly higher voltage (10 kV) which were indicative of a slice of the hydride continuum selected by the analyzing magnet. In the case of monohydride fragmentation, both 15 o electrostatic

analyzers, ESA2 and ESA3, would be insensitive to these minor energy changes and hence could be left at a constant value. The second requirement was a two-dimensional search of the positive ion counting rate by adjusting the injection and analyzing magnets over the barium mass range. To reduce the time taken for a complete mass scan, the injection magnet was incremented in steps of approximately 5 G (l/4 of a mass unit) and the analyzing magnet was scanned rapidly towards a region of peak counting rate. As the purpose of this mass scan was to provide the salient features in the spectrum, the details of the peak shapes and underlying backgrounds are somewhat artificial. The two-dimensional spectrum is illustrated in fig. 3 where points, representing counting rate of charge state 5 + ions, were plotted against the injection and analyzing magnetic field. Counting rates of less than 100 cps, which was typical of the background around the peak regions, were not plotted while points with counting rates in excess of 10000 cps were emphasized by solid circles. The relative size of each symbol was made proportional to the logarithm of the counting rate. Four bands of ions are clearly evident which obey the relationship B,,,, = \IE,/E,BMAGS, where BMVIAGI and BMAG1 are the injection and analyzer magnetic fields, El is the negative ion extraction energy and E2 is the postacceleration energy where E2 a (f + 5) and f

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is the ratio of the fragment mass to the injected mass. The isotopes indicated as barium lie on the lowest band labeled as atomic ions with f= 1 along with the 12’1, 12’Te 13’Te and 133Cs isotopes. These isotopes are known to form stable negative ions which strongly suggests that ions within this band originate from an atomic negative ion rather than from the decay of a molecular negative ion. The few counts beyond mass 138 also indicate that isobaric interferences, at mass 138 and mass 136 from the least abundant isotopes of lanthanum and cerium, were negligible. The mono-, di and trihydrides of barium isotopes are also present along bands of energy E2 = (1 - n/M + q)V, where M is the ion mass, n the number of hydrogen atoms associated with the hydride and V the accelerating potential. The lower relative abundance of BaH; fragmentation products was expected and fortified the hydride band identification. It is well known that calcium dihydrides are weak, being approximately 10% of the trihydride component [8]. One should expect a similar behaviour for all group IIa elements. This spectrum clearly demonstrates that the indicated barium isotopes cannot originate entirely from a molecular process such as the decay of 138BaH; to 13sBaH-. Had the barium ions originated from a molecule, the expected position of each isotope would coincide with the magnetic field B MAGZ of the associated parent hydride. In contrast, all barium isotopic positions have been shifted to the right of the associated hydride by an amount expected from the resolution of M/AM = 2000 for MAGZ. The binding energy of Ba- cannot be determined from these measurements as it is impossible to estimate the effect of the work function at the barium surface that has been covered with an unknown layer of cesium. However, compared to the 12’15+ current transmitted to the same location, the 13*Ba5+ counting rate was lower by 4 X 10V6 which indicates a much lower binding energy for Ba- than iodine.

for trace elements will require that a higher resolution electrostatic analyzer, ESA4, be commissioned. It was also clear that a reduction of ions injected into the accelerator by the addition of a higher resolution spherical electrostatic analyzer will be necessary to avoid similar background problems for the heavier masses of interest such as radium. 5. Conclusions The negative ion of barium has been detected as predicted by a recent theoretical calculation [5]. Any question regarding the magnitude of binding energy cannot be resolved in this first experiment. In principle the negative ion could have been produced in a metastable state with a lifetime sufficient to survive transport to the tandem accelerator stripping canal (approx. 50 ps). This is unlikely as these longer-lived metastable states are often a characteristic of low-Z negative ion isotopes for which the strong L-S coupling selection rules dominate. The possibility of metastability due to other causes cannot of course be ruled out from these measurements. The hydride pattern as deduced from the first barium target used was similar to that of calcium in that BaH; was weak compared with BaH; and in this particular case the Ba- was similar in yield to the dihydride. As a direct result of the barium negative ion detection, the AMS system at IsoTrace has been shown to be capable of detecting other rare negative ions such as strontium, and possibly radium for which the theoretical predictions need verification. The authors would like to thank the Chalk River National Laboratories for the loan of their MP analyzing magnet and the other members of the IsoTrace Laboratory for their support. The Natural Sciences and Engineering Council and the department of Energy, Mines and Resources are gratefully acknowledged for their financial support.

4. Future plans The convincing separation of the hydrides of barium was possible with the present state of development of heavy element analysis capabilities at IsoTrace because the contaminant beams were not significantly more intense than the ions of interest. This was not the case for “‘1 where the intensity of 12’1 negative ions injected along with 129I was greater by six orders of magnitude. Sufficient 12’1 ions were present and became part of the energy continuum after acceleration. The detection limit of lo-” 1291/1271achieved before the installation of the higher resolution magnet was expected to be and was found to be similar after installation, as each magnet would take a slice of the energy continuum. To enhance the detection limits for rare radioisotope and the search

References [l] C.F. Fischer, J.B. Lagowski and S.H. Vosko, Phys. Rev. Lett. 59 (1987) 2263. [2] T.J. KvaIe, G.D. Alton, R.N. Compton, D.J. Peg and J.S. Thompson, Phys. Rev. Lett. 55 (1985) 484. [3] D.R. Beck, Phys. Rev. A30 (1984) 3305. [4] D.J. Peg, J.S. Thompson, R.N. Compton and G.D. Alton, Phys. Rev. Lett. 59 (1987) 2267. [5] S.H. Vosko, private communication. [6] R.P. Beukens, D.M. Gurfinkel and H.W. Lee, Radiocarbon 28 (1986) 229. [7] L.R. Kilius, J.C. Rucklidge and A.E. Litherland, Nucl. Instr. and Meth. B31 (1988) 433. [8] P. Sharma and R. Middleton, Nucl. Instr. and Meth. B29 (1987) 63. VI. MATERIALS

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