Nuclear Instruments and Methods North-Holland, Amsterdam
Section II. Accelerator ACCELERATOR
in Physics
Research
B35 (1988) 273-283
213
mass spectrometry
MASS SPECTROMETRY
(AMS) OF HEAVY ELEMENTS
K.W. ALLEN Nuclear Physics Laboratory, Keble Road, Oxford, UK
The fundamentals of accelerator mass spectrometry (AMS) will be discussed with particular reference to the detection of the heavier elements. The modifications to the Oxford EN tandem accelerator for AMS work will be outlined and some examples of the analysis of the platinum group metals in samples taken from the Cretaceous-Tertiary (K-T) boundary will be given. The use of smaller accelerators (terminal voltage < 3 MV) for the measurements of isotopic ratios of heavy elements (M > 180) will also be discussed.
1. Introduction The use of radioactive decay to provide an absolute time scale for the study of natural processes was recognized early in this century [l]. Forty years later, archaeological dating was revolutioned by the proposal to use a time scale based on the lifetime of 14C [2], and since carbon is a constituent of all living material, applications were widespread. In living organisms, the ratio of 14C/12C attains an equilibrium value which depends on the rate at which 14C is being produced in the atmosphere by cosmic radiation. Since this rate is approximately constant, so also is the t4C/‘*C ratio at about 1.2 X lo-‘*. When the organism dies, biological processes cease and no further 14C enters the system. The radiocarbon clock begins to tick and the amount of 14C remaining at the time of measurement enables the interval that has elapsed since the death of the organism to be calculated. Until about ten years ago, the 14C content of a sample was always determined by counting the emitted B particles. Since the rate is only - 15 counts min-’ less in older g -’ in modem carbon and correspondingly material, quite large samples are required and long counting times are needed to produce accurate dates. The problems are greater with longer lived radionuclides as the specific activity is correspondingly reduced. With AMS, much greater sensitivity can be achieved so that samples measured in mg rather than g can be used. The reasons for this will emerge later in this section. The advantages, at least in principle, of mass spectrographic methods have long been recognised [3] but it was not until 1977 [4,5] that they were realised by accelerating carbon ions to energies in the MeV region. Tandem electrostatic generators were used in the early 0168-583X/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
experiments and have proved to be the most successful type of accelerator for AMS. There are several reasons for this choice. It is an essential feature of the tandem accelerator that the first stage of acceleration takes place with negative ions. This is both an advantage and a disadvantage. The advantage is that isobars of the element to be detected, which might contribute seriously to the background, are automatically eliminated if they do not form negative ions e.g. i4N does not constitute a background when 14C is being accelerated. The disadvantage is that certain elements which we would like to accelerate and detect, e.g. the rare gases, do not readily form negative ions and therefore cannot be used. The second characteristic feature of the tandem is that negative ions are converted to positive ions by passage through a gas or foil in the central terminal and then further accelerated. Several electrons are removed by the stripper, depending on the atomic number and velocity of the ion, and in such collisions undesirable molecular complexes with the same mass as the negative ion are broken up and can subsequently be removed from the beam. Sometimes elements which do not readily form negative ions themselves, e.g. Be, Ca, can be accelerated as oxide or hydride ions. In the stripper the complex negative ions are converted to positive atomic ions which are further accelerated and selected in postacceleration analysers. Finally, the high sensitivity of AMS arises from the large yield of modem negative ion sources and the ability to detect single positive ions at very low background levels. These energetic positive ions may be identified by standard nuclear physics techniques such as simultaneous measurement of rate of energy loss and total energy. The higher energy of the positive ions, combined with careful choice of analysing systems, reII. AMS
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K. W. Allen
/Accelerator
mass specrrometv
of heavy elements
T2
Fig. 1. Schematic drawing of the accelerator mass spectrometer at Toronto (Kieser et al., 1986), showing the elements used for radiocarbon analysis. Labels: Al-A6, apertures; CG, the caesium gun (15 kV); Dl and D2, the ionization detector with AE, and AE, collection plates; DA, the data acquisition electronics; El, a 45 o spherical electric analyser; E2, a 15 ’ electric analyser; EE the sample ion-extraction electrode (0 v); Fl-F5, Faraday cups; FL, a Mylar foil window: GV, generating voltmeter, ID, gas ionization detector; IS, an insulated magnet box; Ll, split eizel lens (focusing and steering); L2 and L4, einzel lenses; L3, gridded lens; L5, the electric quadrupole; Ml, 90° magnet; M2 and M3, 45O magnets; PA, preacceleration electrode (60 kV); SC, stripping canal; SD, silicon particle detector; SH, sample holder (-20 kv); SM, stepping motor; Tl and T2, viewing and alignment telescopes; TP, stripper gas recycling turbopump; TR, 40 kHz 10 : 1 step-up transformer; VF, voltage multiplier parallel feed capacitor; VS, voltage multiplier diode stack.
duces measure
the
effect
isotopic
of ratios
scattering
and
it
as low as lo-l5
is
possible
to
in favourable
cases. In modem carbon, the i4C/12C ratio is 1.2 X 10-i’, so that the limit of measurement, currently - lo-l5 corresponds to ages in the region of 60000 years. A given statistical accuracy can be obtained in a much shorter time with AMS than by conventional g counting. For example, negative ion sources can produce 100 PA of C- ions which, at 14C/12C - 10-15, gives a 14C counting rate of 810 counts/h at a combined stripping and transmission efficiency of 30%. A S decay rate of this magnitude would require 1 kg of sample, whereas 100 PA of C- ions could be obtained from a few milligrams, or even less! This improvement in sensitivity is perhaps the most important advantage of AMS and it fully justifies the increased cost of the equipment. To be certain that positive 14C ions are not accompanied by molecular complexes such as “CH, which survive the stripping process, it is advisable to select triply charged positive ions after the stripper, since no triple charged complex have ever been observed, and would not be expected to survive. The maximum conversion from C- to C3+ ions is achieved by stripping at
about 2.5 MeV, and this has determined the size of tandem accelerators especially developed for 14C dating [6]. In practice, satisfactory dating has been carried out in many laboratories at between 1.8 and 2.5 MV terminal voltages, and even lower potentials have been used
[71.
A schematic diagram of the Tandetron accelerator mass spectrometer at the Isotrace Laboratory of the University of Toronto is shown in fig. 1 [8]. The components are identified in the figure caption. In essence, negative carbon ions are extracted from the source, and passed through an electrostatic analyser which removes the tails of the energy distribution. The negative ions then pass through a 90 o magnetic analyser which determines the mass (since energy and charge are known) and they are then accelerated and stripped in the Tandetron. The emerging positive ions are focussed and selected by a second electrostatic analyser which determines the energy/charge ratio i.e. it separates the charge states. The beam is then magnetically separated into the mass 12, 13 and 14 components and the individual 14C ions are counted with silicon detectors. Full details of this system and the method of measuring 14C : “C ratios have been published [9].
K. W. Allen /Accelerator mass spectrometry of heavy elements Several excellent reviews of the fundamentals of AMS have appeared in recent years 19,101 and conference reports [11,12] give an indication of the range of studies now being undertaken. The remainder of this paper will deal with some of the elements of accelerator mass spectrometers with emphasis on applications involving the heavier elements. We shall include a few specific applications and will end with some suggestions for future work.
2. Identification of ions
It is always desirable, though not always possible, to identify both the mass number A and the atomic number 2 of the ions being detected. Determination of A is usually straightforward; it involves only the use of analysers of sufficient resolution and maintenance of high vacuum throughout the ion path to minimise the effect of scattering and charge changing which can modify trajectories. This latter point is important in AMS because the ions it is desired to detect are often accompanied by an intense neighbouring beam of an abundant isotope. The scattering of even a tiny fraction of this beam can cause problems by contributing to the counting rate in the rare isotope channel. Determination of the atomic number, Z, of the ions is more difficult than determining A. In some cases, elements do not form stable negative ions at all, or it may happen that the electron affinity is so small that yields are very small (see section 3). It is then possible to separate isobars at the beginning of the mass spectrometer, i.e. at the ion source. This is possible for the pairs of isobars 14C i4N; 26Al, 26Mg and rz91, 12’Xe. In general, however, this separation is not possible and other methods must be employed, usually after acceleration. For ions with Z < 20, measurement of the rate of energy loss, dE/dx, together with the final energy E will often be adequate to determine 2. The resolution of the method, i.e. the degree of separation of adjacent isobars, increases with ion energy. The limit given above, Z 2 20, applies to energies obtained with tandem generators (Vr < 10 MV). At much higher energies, separation is possible at higher values of Z. If the isobar * it is required to select has Z < , then its range will be greater, for a given energy, than for the isobar with Z ’ . This can be a basis for Z determination, as proposed by Muller for the 14C, 14N pair [13). Often, however, we wish to select 2 ’ , e.g. 36Cl (T,,* = 3.01 x lo5 yr) from the stable isobar 36S (natural abundance 0.014%). Here the range method is not applicable
* Usually we are concerned with two isobars with adjacent atomic numbers. The larger will be designated by Z’ and the smaller by Z < .
275
and a combination of chemical separation (to remove as much sulphur as possible from the sample) and a specific ionization measurement sometimes involving several stages has been used [14]. As noted above, the efficiency of Z separation based on dE/dx measurement depends on the ion energy. At - 80 MeV, the 36C1-36S separation is - 10-6. This was measured at Rochester [lS] using the 7+ charge stage with 10 MV on the tandem terminal. At EN tandem energies (- 6 MV on terminal) the separation falls to 1O-4 and limits the 36Cl/Cl ratio to - lo-l4 which is barely sufficient [lo]. Recently, complete stripping of 36C1 and 36S ions has been achieved at the NSF at Daresbury [17]. Since chlorine has Z ’ , it can be uniquely identified, and some 29% total stripping (q = 17) has been observed at 18 MV on terminal. However, it is too expensive to use such large accelerators for routine AMS measurements, and other methods of separation applicable to lower energy ions are needed. An interesting application of AMS using a large accelerator, but not total stripping, to separate 4’Ca( Z ‘) from 41K( Z ‘) has been described [18]. Ions were accelerated to 200 MeV in the ATLAS tandem-linac combination at the Argonne National Laboratory, and then entered an Enge split-pole magnetic spectrometer. With a high vacuum in the magnet box, the usual fragmentation of the ion beam into many separately resolved charge states was observed. When gas was added at a pressure of 1.0-10.0 Tort, the charge states coalesced to give a single peak and it was possible to separate 4’Ca from 41K. Similarly, complete separation of 58Fe from 58Ni and of 60Fe from 60Ni was achieved [19]. In each case, the single peaks correspond to average charge states resulting from the many charging collisions taking place as the ion traverses the gas in the magnetic field region. This behaviour was simulated by a Monte Carlo calculation [19] with the very interesting result that the percentage separation in average charge states (100 Aq/i/a) for 4’Ca and 4’K varied slowly with energy from 3.4% at E = 20 MeV to 4.5% at E = 200 MeV. This suggests that the resolution would be adequate to separate 4’Ca from 41K at, say, SO MeV which is available from an FN tandem, or at even lower energies. For ions with A 2 40, and where adjacent isobars both form negative ions, there is no simple method of separation.
3. Ion sources The most frequently used negative ion source for AMS is the Cs sputter source described by Middleton [20]. In this source, a beam of Cs+ ions is focussed onto the surface of the sample. Inevitably some cesium is deposited on the surface. The neutral sputtered atoms from the sample have a high probability (up to - 5% II. AMS
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K. W. Allen / Accelerator mass spectromerry of heavy elemenis
for C) of attaching an electron as they traverse the Cs layer, which lowers the work function of the surface. The production of negative ion beams depends strongly on the electron affinity of the negative ion concerned. For the halogens, almost any type of source will produce good yields of negative ions. For Be, however, the negative ion yield is very small; therefore the most commonly used ion for “Be analysis is BeO-. This is entirely satisfactory for HVEC tandems (Vr > 5 MV) but is not so convenient for Tandetrons since the “Be ion energy at the stripper is only lo/25 of 2.5 MeV, i.e. 0.96 MeV. However BeO- has been used successfully at these energies [21]. The electron affinity of Al is small (0.46 MeV) and it is difficult to get good yields of 26Al from a Cs sputter source. A better approach might be to produce low energy positive ions and then convert some of them to negative ions by passage through gas at low pressure [22]. However, 26Al beams from the sputter source have proved adequate for many investigations [23]. The yield of negative Ca ions from a sputter source is very low. A more convenient and higher yielding ion is CaH; [24] which has the advantage that it improves the separation of 41Ca from 41K since 4’KH; is formed at a very low level, if at all. CaH;, like many hydride negative ions, can be conveniently formed by sputtering the metal in the presence of ammonia gas. As already noted, negative carbon ions at yields of 100 PA or more are readily produced in a sputter source [20]. Such sources, although in regular use in many laboratories, do require elemental carbon, and its chemical extraction from some samples can be a tedious process. A gas source based on the use of COZ would have many advantages and progress towards the realization of such a source has been made in recent years [25,26]. Yields of 20 PA of C- have been obtained at a gas consumption of - 0.25 atoms cm3 h-‘, and the “memory effect” when the gas is removed has been shown to be small [25] although not yet as small as in the standard sputter source. Further details of the gas source have been given in the review article by Middleton [25].
4. Accelerators for AMS A cyclotron was used in one of the earliest experiments using the technique of AMS which led to the discovery of 3He in 1939 [27]. It was not known at that time which of the two elements 3H and 3He was stable. Helium gas was fed into the ion source of the Berkeley 60 inch cyclotron, and the 3He ions were recognised by their energy and range after acceleration. Muller 1131 pointed out the advantages of using the cyclotron for 14C detection and he proposed the separation of 14C ions from 14Nionsbya r a n g e method. A small “desk-
top” negative ion cyclotron is currently being developed [ZS] at Berkeley for 14C dating applications. Raisbeck et al. [29] were able to detect “Be using a cyclotron, but in general tandem accelerators are to be preferred to cyclotrons for the reasons given in the Introduction. There are, however, some special factors for AMS applications using tandem generators and these will now be considered. Terminal voltage stability is of obvious importance when high resolution analysers are used in the later stages of the spectrometer. The separate high stability power supply of the Tandetron is ideal, and no external feedback loops are required. Tandem Van de Graaffs are frequently used for AMS, however, and here there can be a problem because the usual current feedback loop cannot be employed on account of the very small AMS ion currents. Stabilization using a signal from the generating voltmeter has been used successfully [30]. Either gas or foil stripping can be used to convert negative to positive ions in the central terminal. In general, gas stripping is preferred, as the thickness can be readily controlled by adjusting the gas pressure and multiple scattering can be kept to a minimum. A gas recirculating system based on a small turbo-molecular pump [31] is very convenient and minimises the gas flow into the accelerating tubes. It is usual to maintain the terminal voltage of the tandem constant during an isotope analysis, and separate masses and charge states by magnetic and electric analysers before and after the accelerator. Thus, for a given charge state, the energies of all isotopes are the same, or very nearly the same, and the velocities of the ions are different. Since atomic cross sections, such as charge changing, are a function of ion velocity, there is inevitably some isotopic fractionation. To determine accurate isotope ratios, comparison with known standards is therefore essential, as in conventional mass spectrometry. In most AMS studies, the very low abundance of a rare radioactive isotope has to be determined relative to its much more abundant stable neighbours, e.g. r4C with respect to “C and *‘C. Ideally, all isotopes should be passed simultaneously through the accelerator and then separated and measured. In practice, however, this is seldom possible because of the very wide disparity in currents. A logon approach is to pulse the beam before acceleration so that the transmitted intensities of the abundant isotopes are reduced [32]. This is done by insulating the magnet box and applying voltage pulses of known duration so that the energy of the ions reaching the magnetic analyser is modulated during the pulse (known as “bouncing”). In the absence of pulsing, the magnet is set to transmit the rare isotope. To obtain high accuracy in carbon dating, it is essential to measure the intensities of all three isotopes after accelerating [33].
K. W. Allen / Accelerator mass spectrometry
271
of heavy elements
5. Modifications to the Oxford EN tandem * for AMS with heavy ions
the heavy masses with relatively low charge diagram of the system is shown in fig. 2.
Apart from “‘1, which is easy to observe because the only stable isobar, ‘*‘Xe, does not form negative ions, rather little work has been reported for elements with A > 41. We have already seen that it is only possible to separate isobars by complete stripping with very high energy accelerators, when the centre of mass velocity is much larger than c/137 (c is the velocity of light). So with modest accelerators it is therefore possible to determine only the mass for heavy ions. However, since molecular complexes are broken up in the stripper, a mass measurement can often be associated with a particular element so that complete identification is possible. We shall return to this point in section 6.1. Although we have so far emphasized the value of AMS in the detection of long-lived radioactive nuclei, the technique does have an important role to play in the detection of stable elements at very low abundance levels. With this in mind, we modified the Oxford EN tandem to make it suitable for AMS of the heavy elements [34]. At the time, applications of particular interest were to study the long-lived radioactive products of nucleon decay [35], which we did not follow up, and to measure the changes in the abundance of the platinum group metals across the Cretaceous-Tertiary (K-T) boundary layer. To make the EN suitable for the acceleration and detection of heavy elements, we had to modify both the injector and the post-acceleration detection system. We needed a much larger magnet following the ion source to resolve completely individual masses up to A - 200, and we needed a new post-acceleration analysis system since our existing 90 o analysing magnet could not bend
5.1. The injector
The negative ion source is a commercial cesium sputter source [37] operated in the reflected mode [38] with a modified tungsten ionizer. Target material is pressed into 2 mm pellets, held in suitable holders, twelve of which are mounted on a rotatable wheel which can be operated by a remote link. Negative ions are extracted at 20 KV and further accelerated to a total energy of 80 keV by a short section of typical Van de Graaff accelerator tube before entry into the 180° analysing magnet. The analysing magnet was originally designed as a charged particle spectrometer [39]. The magnetic field is non-uniform with a field index n = l/2 and the mean radius of curvature, r, is 61 cm. The gap varies between 5.84 and 6.86 cm and the object and image distance in a symmetrical arrangement are each 43 cm (r/a). The resolution (M/AM) was originally measured to be 750 [39] and the mass-energy product is 28 MeV amu. The dispersion in the focal plane is large (12 mm per percent change in mass) so that an adjustable double slit could be used which enabled an auxilliary, usually intense, beam to be monitored in an offset Faraday cup while the selected negative ions could be transmitted to the accelerator. A fibre optic link allowed the remote operation of the ion source (80 KV below ground potential) and the double slit system. The coils of the 180 ’ magnet, which were water-cooled, were connected in series, and the current through them was computer-controlled. The magnetic field was measured by a radial compensated
* This accelerator is now at the University of Peking, Beijing, PR China.
* The new injector became known as OSIRIS (Oxford supersensitive injector for radioactive isotope separation).
Preocrelerator
Modlfled H~conex
Double-slit
box cup
A
*
TOFstop TOF start
834 EN tandem
ond Faraday
states.
Sltt box and Forodoy cup
Elertrostotac deflector
detector
detector \
Elertrostotlr quodrupole
Fig. 2. The Oxford EN tandem accelerator, as modified for AMS. II. AMS
L W. Alien /Accelerator mass spectrometry of heavy elemenis
278 30
120
Sn-
118 116 +
I
i
106
107
,108
109
110
I mag Fig.
NMR
which could be by adjusting the in Masses were selected by
Mass spectrum
112
111
the
The post-acceleration detection system The post-acceleration detection system consisted of an electrostatic quadrupole lens immediately following the tandem generator, a large electrostatic analyser (ESA) and a system for determining the time of flight (TOF) of the selected positive ions. Since the ion mass is determined by OSIRIS and the charge state (q) by
114
115
116
(Amp)
negative ions from
to an accuracy of systematic controlled
113
tin target
the 834
source.
ESA, the total energy [(m+r] is known and thus also the ion velocity. A second direct determination of this velocity from the TOF measurement was therefore not strictly necessary. However, since the measurement could be made without significant loss of intensity, and as it provided a useful check on the system as a whole, it was considered worth making. The electrostatic analyser has a radius of curvature of 3.20 m and the gap is 3.5 cm. One plate is run negative and the other positive; the analyser has been tested with up to 180 KV across the gap, corresponding to a maximum E/q of 8.2 MeV per charge. The angle of deflection is 20 *. Analysing slits were placed 3 m from the ESA and at this point the energy resolution (E/A\) was found to be 580 rt: 60. In normal operation the ESA was set to select E/q = 6.02 MeV per unit charge, i.e. 5+ ions with an energy of 30.10 MeV. The velocity of the ions was measured by timing their flight over a measured distance. The “start” detector [40] consisted of a pair of microchannel plates installed in a permanent magnetic field of 82 G. Secondary electrons liberated by the passage of a heavy ion through a 4 pg/cm2 carbon foil are accelerated, bent through 180 o on to the channel plates, thus giving rise to a fast pulse. A surface barrier detector 20 mm in diameter and 100 urn thick acted as the “stop” detector. This detector provided both a timing signal and an output pulse height which was proportional to the energy of the ion striking it. A timing resolution of 0.9 ns was obtained: this is to be compared with a me-of-act
219
K. W. Allen / Accelerator maw spectrometry of heavy elements
_
2001
difference of 1.45 ns for AM= 1 in the mass region between A = 188 and 198. The separation of the two detectors was 3.08 m, so the flight time for A = 195 at 30 MeV was 567 ns and the resolution (t/At) was 630. To improve transmission efficiency an electrostatic quadrupole doublet was placed at the centre of the flight path. 5.3. Measurements and analysis Isotopic ratios were normally measured in the following way. The 180” analysing magnet was tuned to select A = 197 from an Au pellet * and the rest of the system was optimised for maximum transmission as measured in a Faraday cup before the “start” detector (fig. 2). Then using an iron oxide standard (22.8 ppm Ir, 27.2 ppm Pt, 43.1 ppm Au and less than 1.4 ppm OS), the 180° magnet was set for the middle isotope of the triplet to be investigated, e.g. mass 195 was selected when measuring the ratio 194/195/196. Optimisation was carried out using the counting rate of the microchannel plate “start” detector. Data were accumulated for 5 s at each mass, with an allowance of 1 s settling time of the preaccelerator power supply between isotopes. Timing signals from the microchannel plate start detector and surface barrier stop detector were fed to a time-to-amplitude converter (TAC) whose output was then accepted by a PDP11/60 computer for on-line data acquisition and analysis. The PDP11/60 also simultaneously recorded the energy of the ions deposited in the stop detector. By gating on the fulI energy peak measured in the stop detector and on the appropriate preaccelerator voltage, the raw TAC spectrum was then sorted on-line into individual gated TAC spectra for different isotopes. Checks were made regularly to ascertain that the centroids of these TAC peaks were coincident with the calculated TOF. The intensity of each isotope per unit of time (chosen as multiples of 5 s) was determined from a suitable summation over the corresponding gated TAC peak and loaded into a data array in the PDP11/60 to enable standard deviation calculations to be carried out at a later stage. Figs. 4 and 5 show an energy spectrum measured in the stop detector and a raw TAC spectrum, respectively. Typical gated TAC spectra obtained simultaneously for three isotopes are also shown in fig. 6. To obtain the intensities of each isotope, the gated TAC peaks were analysed off-line by Gaussian fitting routines. The isotopic ratios obtained from the two methods of analysis were in agreement within the error quoted. In some
* For measuring 1S80s/‘890s, a tungsten pellet (M=186) used, with final optimisation on a - 10 ppm OS sample.
is
v)
5 IOO-
s
50 I
OL 0
200
400
600
1000
800
Channel number
Fig. 4. The pulse height spectrnm in the “stop” detector for ions with A = 194,195 and 196 selected by time of flight.
_
M=196
d)
loo-
0
I
/
500 -
600
700
600
700
600
700
600
700
M=195
loo2 =
2
0
I
500
100
0
I
500
500
-
I
Channel number
Fig. 5. Tie-of-flight spectra of masses A =194, 195 and 196. For (b), (c) and (d) single gates were set on the preaceelerator voltage and the fulI energy peak in the “stop” detector. For (a) three gates were set on the preaccelerator voltage. II. AMS
K. W. Alien / Accelerator mass spectromewy
280
_.___._“__.
Table 1 Isotopic abundances of some heavy elements [W]
M :I96
A 184 185 186 187 188 189 190 191 192 193 194 195 1% 197 198
01__,“__ 350
of heauy elements
Osmium
Tungsten
Rhenium
Platinum
Iridium
30.6
0.02
37.1 28.4
1.6 1.7 13.3 16.1 26.4
62.9
0.03 38.5
41.0
0.8 61.5 32.9 33.8 25.2 100 7.19
cm 400
450
500
550
Channel number
Fig. 6. The time-of-flight spectra for A = 196 selected, in the presence of contamination from A =197 from a geological sample.
cases, where a contaminant peak interfered with the isotopes of interest, e.g. Au (mass 197) leaking through for M = 196 selected, the Gaussian fit enabled the contaminant peak to be subtracted. The analysis of some samples containing Pt, Ir and OS using this technique will be given in section 6.
6, Some applications of AMS to heavy elements
+bC
56
Tertiary
+30
SS
l8
STKLR 4
+3
STKLR 3
0
STKLR 2 STKLR 1
-1
6.1. The K-T bou~da~ ammaZy The fossil record provides evidence for about five major and many minor biological crises during the last 500 million years. The most recent of these, 65 million years ago, marks the boundary between the Cretaceous and Tertiary geological periods and has been extensively studied. In 1980, it was shown 1361 by neutron activation analysis (NAA) that the iridium ~ncentration in samples of the boundary layer was greater than in the surrounding material by around two orders of magnitude and this iridium anomaly, as it came to be known, was subsequently observed worldwide and adduced as evidence for the collision of a massive extraterrestrial body with the earth. This ended the Cretaceous geological period and led to the annihilation of a wide range of plant and animal life, including the dinosaurs.
-3C
S4
Cretoceous
-6C
53
Fig. 7. Profile of the K-T boundary at Stevns Klint, showing the location of the samples analysed. Distances [cm] are measured from the centre of the boundary.
K. W. Allen / Accelerator mass spectrometty s3
I
1ttttt
IO 011
S4
STKLRI
STKLR
STKLR3
2
281
of heavy elements STKLR 4
SS
56
1 I
i 10
10
10
10
10
10
10
No of cvcles Fig. 8. Counting
rates for 1931r, 194Pt and 195Pt ions in the samples analysed. The shaded regions samples determined by neutron activation analysis.
It occurred to us [34] that it would be of interest to see whether the anomaly was present for all the platinum group metals, as would be expected, and not just for iridium. Since, apart from Ir, it is difficult to use NAA for the Pt group metals because the neutron capture cross sections are small, we decided to use AMS.
s3
S4
STKLR 1
show the Ir abundances
in similar
In table 1 the isotopic abundances for the chemical elements with mass numbers between 184 and 198 are shown. It can be seen that there is rather little overlap, so that, with sufficiently good resolution, mass selection identifies the elements. For Pt we used A = 194, 195 and 196; for Ir, A = 191 and 193 and for OS, A = 188
STKLR
2
STKLR 3
STKLR II
S5
56
No of cycles Fig. 9. Similar data to those shown in fig. 8, but for ‘s90s and 19’Ir ions. II. AMS
K. W. Allen /Accelerator mass spectrometty of heavy elements
282
Table 2 Half-lives of some heavy isotopes [years] A
Th
229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248
7.3 x 103 8.0 x lo4
Pa
U
Pu
Am
Cm
3.2x10” 1.4 x 10’0
7.1 x 108 2.4 x 10’ 4.5 x 109
and 189. The method of measurement and analysis has been described in section 5.1. Samples of the K-T boundary layer from three Danish sites at Stevns Klint, Dania and Nye Klov were used. The samples were ground up and heated to 1000°C before use in the sputter source. No chemical separations were carried out, and abundances were estimated from calibration samples. The negative ion output inevitably depends on the nature of the sample and separate studies suggested that the absolute accuracy of abundancy measurements could not be relied on to better than a factor of 2. As a typical example of the data we obtained, we show the results for samples from Stevns Klint. The positions of the various samples with respect of the K-T boundary are shown in fig. 7. Fig. 8 shows counting rates for 194Pt, ‘95Pt and 1931r ions in the various samples derived from our measurements and fig. 9 shows similar results for ‘s90s and 1911r. A sharp increase at the K-T boundary was found for samples from all three sites. The isotopic abundances were also determined for the isotopes mentioned above and shown to be consistent with normal terrestrial values. The shaded areas show the abundances of Ir determined from NAA [27]. 6.2. The uranium and thorium radioactive series The system developed at Oxford for AMS studies of the Pt group metals worked well for the abundance levels present in K-T boundary layer samples (0.01-l
2.4~10” 6.6 x 103
8X103 7.6 x 10’ 9 x10’ 5.5 x 103 4.7 x 104
ppb). It would be of interest to extent the technique to long-lived members of the naturally radioactive series at the highest level of sensitivity. Table 2 shows isotopes of thorium to curium with half-lives larger than 1000 years, where mass-spectroscopic measurements would compete effectively with direct counting. There is no overlap in mass number for the elements from thorium to uranium, so identification would not, in principle, be a problem. For many applications, e.g. the determination of bone ages, the highest sensitivity would be needed and background would have to be reduced to the lowest possible level. A high resolution analysing magnet such as OSIRIS, preceded by an electrostatic analyser would be essential at the injection state, If a Tandetron accelerator is used and 3+ charge states are selected, then a large postacceleration analysing magnet with good resolution and stability is required. The problems of AMS with heavy ions at Tandetron energies have recently been discussed
1411.
7. Future developments AMS has grown out of the application of nuclear physics techniques to mass spectrometry. Ion sources, accelerators and charged particle analysis and detection techniques have all played their part, and accelerators previously used for nuclear physics research have been taken over.
K. W. Allen / Accelerator mass spectrometty of heavy elements We are now seeing the growth of dedicated facilities suitable for AMS for all elements throughout the periodic system, although it is to be expected that applications involving 14C, lo Be, 36C1 and 12’1 will continue to dominate the field, at least for some years. The Tandetron accelerator had ideal characteristics to serve as the basis for an AMS system, except perhaps in terms of voltage capability, for the heaviest elements, where a terminal voltage of 5 MV, i.e. about twice that of the present version, would allow a wider range of charge states to be selected. This would be useful in eliminating parasitic beams, reducing background and in other ways.
It is a pleasure to acknowledge much valuable discussion with colleagues and former colleagues at Oxford, especially S.H. Chew, the late T.J.L. Greenway, H.R. Hyder (now at Yale University) and G. Doucas. We are grateful to H.J. Hansen and K. Rasmussen of the Geological Central Institute, University of Copenhagen, for the provision of K-T boundary samples and for helpful discussion of the results.
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II. AMS