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Accelerator mass spectrometry with 26Al
430
Nuclear Instruments and Methods in Physics Research 218 (1983) 430 438 North-Holland. Amsterdam
A C C E L E R A T O R M A S S S P E C T R O M E T R Y W I T H 26Ai * R. M I D D L E T O N
a n d J. K L E I N
Physics Department, Unit,ersi(v of Penn~[vh~ania, Philadelphia. PA. 191043859. USA G . M . R A I S B E C K a n d F. Y I O U Laboratoire Rend Bernas, 91406 Orsay, France
The cosmogenic radioisotopes 1°Be, 14(i and 36C1 are routinely being measured in several laboratories but the potentially important 26A1 ('1-,- 7.2×105 y) has received scant attention - largely because it is difficult to produce negative-ion beams of aluminum (particularly from the oxide) and the high abundance of 27AI results in small 2{'AI: 27A1ratios (typically < 10 t4 terrestrial and 10 to extraterrestrial). A new ion source is described which typically generates l to 2 ,ttA of 27AI ions from small samples ( - 4 rag) of AI203 with an ionization efficiency of about 0.25%. Modifications have been made to the University of Pennsylvania's FN-tandem accelerator for quantitative measurement of 2e~AI:27Al ratios as low as 5 × 10 15 and some of the problems encountered are contrasted with those met during 1°Be measurements. Although measurement of 2~'AI: 27A1 ratios in terrestrial samples are often close to the limit of our sensitivity, we have made measurements on a variety of terrestrial samples including stratospheric air filters, manganese nodules, tektites, and various impact glasses and on extraterrestrial samples including cosmic spherules, a meteorite and several lunar samples.
1. Introduction A l u m i n u m - 2 6 like beryllium-10, carbon-14 and chlorine-36 is p r e d o m i n a n t l y produced, on earth, through the interaction of cosmic rays with the atmosphere. Like 36C1, the only target nucleus in the atmosphere is At, so that its estimated rate of production ( - 1 . 4 × 10 -4 a t o m s cm 2 s 1) [1] is several orders of magnitude smaller than that of 14C and 1°Be [2]. But in spite of the realization that a m e a s u r e m e n t of the ratio of 26A1 to 1°Be in a sample might provide a useful c h r o n o m e t e r in m a n y geophysical systems (due to the similar origins a n d the supposed similarity in the chemistries of cosmogenie beryllium and aluminum), only a very few measurements were m a d e on terrestrial samples using the decay counting technique [3] in the more than twenty years that elapsed since the suggestion was first made [4]. Only in extraterrestrial samples has the measurem e n t of 26A1 been in any sense routine [5]. Accelerator mass spectrometry has changed this situation dramatically, most significantly because of the greater efficiency of " a t o m c o u n t i n g " ( - 0 . 0 3 % ) comp a r e d to " d e c a y c o u n t i n g " ( - 1 0 - 5 % for a m o n t h of counting) which allows smaller samples, often with orders of m a g n i t u d e fewer atoms of 26A1 per gram. For extraterrestrial samples, m e a s u r e m e n t s on samples as small as 100 # g or less are possible. But even accelerator mass spectrometry is limited by the high natural a b u n * Work supported by the National Science Foundation. 0 1 6 7 - 5 0 8 7 / 8 3 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
d a n c e of a l u m i n u m which, for terrestrial samples, means 26A1/27AI c o n c e n t r a t i o n s rarely exceed 10 -14. This restriction places severe d e m a n d s on the ion source, since a microampere current of AI would produce at the detector only 30 counts per hour for a sample with an 2°A1/ZVAl concentration of 10 14. As even these currents are difficult to obtain, especially from the oxide, most early accelerator experiments were performed either on extraterrestrial samples [6] or on samples m a d e by irradiation [7] where the 26A1/27AI ratios are higher. We report here the developments that have enabled us to make measurements on naturally occurring terrestrial 26A1. They are represented principally by' imp r o v e m e n t s in ion-source design a n d sample preparation which allow us to produce AI beams of 2.5 /*A from reagent grade A I 2 0 3, and 1.4 # A from A I 2 0 3 p r e p a r e d from natural samples. Even with these currents, measurements on terrestrial samples are exceedingly difficult, so that m a n y of our early measurements were aimed at exploring alternative problems which, while beyond the reach of decay counting, are accessible to the accelerator method.
2. Instrumentation 2.1. Introduction A l t h o u g h m a n y existing t a n d e m accelerators such as H V E C model EN, F N and M P tandems hold consider-
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R. Middleton et al. / Accelerator mass spectromett~v with A/
able potential for accelerator mass spectrometry, none are ideally suited. All were designed for nuclear research with scant attention paid to optimizing the ion optics, particularly as regards reproducibility of transmission. Most are equipped with low resolution injector systems, poorly regulated steerer and lens supplies, and with the terminal voltage usually controlled by a feedback system which senses the deflection of an intense b e a m by a magnet field. As nuclear research instruments they performed admirably, but as mass spectrometers they left much to be desired - indeed isotopic ratios could rarely be measured with a precision of better than 15%. A b o u t five years ago, when the field of accelerator mass spectrometry began to bloom, we at the University of Pennsylvania decided to concentrate all our efforts on improving the i n s t r u m e n t a t i o n of our F N accelerator for the m e a s u r e m e n t of l ° B e / g B e ratios. Several factors influenced our decision to concentrate on beryllium, not the least being a knowledge that most isotopes present unique difficulties a n d problems. Most of the major changes that we m a d e are described in ref. 8 and coupled with some recent minor improvements, we currently are able to measure l ° B e / g B e ratios with a precision of a b o u t 4% and at a sensitivity level of about 10 15 A b o u t two years ago, we decided to expand our efforts to include 26A1. This isotope, which hitherto had barely been studied at all, presents several new and challenging problems. (1) By preference, one would like to accelerate AIO ions since A120 3 is convenient to
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m a n u f a c t u r e and extremely difficult to reduce to elemental form, and A 1 0 - is a prolific negative ion. However, 26MgO also readily forms a negative ion, and as it is lower in Z than AI, it is impossible to eliminate with an absorber foil. (2) Fortunately, the Mg negative ion appears to be unstable - at least all of the present work is consistent with this conclusion - so one can accelerate A1- without fear of interference from its only stable isobar. Unfortunately, it is not a prolific negative ion a n d is particularly difficult to form from A1203 in a sputter source. (3) Unlike Be, stable A1 is a major constituent of most samples of interest. This coupled with an extremely low production rate (at least from the atmosphere), results in terrestrial ratios of 26AI/27AI of < 10 t4. (4) Whereas l°B can be used as a pilot b e a m to tune the accelerator for ]°Be in the case of 26AL there is no convenient pilot beam. A n d (5) as a consequence of accelerating negative elemental ions as opposed to molecules, a second dispersive element is necessary between the detector and the high energy 90 ° analyzing magnet to eliminate 27A1 ions that charge exchange within the magnet and are indistinguishable from the 26A1 of interest. 2.2. Beam-transport 6:vstem
This is shown schematically in fig. I. The low-energy b e a m transport system closely resembles that described in ref. 8 with the exceptions of a vastly improved negative-ion source, and the addition of a variable-post-
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432
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tion auxiliary F a r a d a y cup. When 26A1 is being injected into the accelerator this is positioned to intercept the 2VAI- b e a m and the current is integrated. More substantial changes have been made in the high-energy transport system. For beryllium, the detector is m o u n t e d approximately in the plane of the image slit, but for a l u m i n u m we are compelled to m o u n t the detector behind the switching magnet, Fig. 2 illustrates why this is necessary. In spite of the good mass resolution of our 90 ° negative-ion magnet ( A m / m - 1 in 60), some 27A1- ions are injected into the accelerator along with the mass 26 beam. These enter the 90 ° positive-ion magnet with precisely the same energy as the 2~AI particles, and a small fraction charge exchange from 7 + to 8 + . If the detector were located in the image p l a n e of the 90 ° magnet, some of these would be able to enter it, and having the same energy as the 26A1 and the same Z , they would be totally indistinguishable. Fig. 2 clearly illustrates that the second magnet completely eradicates these u n w a n t e d particles. 2.3 Negative-ion source
A sectional drawing of the negative-ion source that is now routinely used to make m e a s u r e m e n t s on both
beryllium and a l u m i n u m is shown in fig. 3. Since this source is fully described elsewhere [9] it will not be described here. However, we have made extensive studies of the generation of 27A1 - ions from small samples of A1203 powder both on the accelerator and on our ion-source test facility and these will be reviewed. Fig. 4 shows the results of two tests made on our ion-source test facility. The broken-line curve shows the 27A1 current as a function of time from a cathode containing reagent AI203 powder. The cathode was made from pure copper, and the A I 2 0 ~ powder was pressed (and h a m m e r e d ) into a 1.6 m m diameter by 1.6 m m long hole (see insert sketch). The total weight of 27#,1 was 3.7 mg, corresponding to an ionization efficiency of - 0.25% over the five-hour duration of the run. Interestingly, in spite of vigorous pressing and hammering, the sample's density was only 2.6, substantially less than the theoretical value of 4.0. The full-line curve in fig. 4 was obtained under identical conditions, but with the AI zO~ powder mixed with Ag powder (1:4.1 by weight). Note that unlike the pure sample, the current immediately climbed to a m a x i m u m and thereafter steadily declined. It is also noticeable that not only was the current substantially less, but also much less stable. Usually, the a l u m i n u m oxide samples that we prepare produce substantially less current, and until recently 0.6 to 0.7 /,A was typical, c o m p a r e d with - 2.4 ffA from commercial oxide. Recently we discovered that this was due to the presence of water and, after firing at 1200°C, we are now routinely able to obtain 1.4 # A from our samples. The remaining disparity is presently not understood, but it is not believed to be due to the presence of impurities. The available negative-ion currents on the accelerator are usually a factor of two less than from the test facility. A 10% loss occurs in a second gridded einzel lens, but we believe that the main loss is due to vertical blow-up of the b e a m caused by astigmatism in the 90 ° negative-ion magnet. This p r o b l e m was recently eliminated on the test facility by the addition of field clamps, and similar clamps are being constructed for the magnet on the accelerator. 2.4. Veloci O, selector
The velocity selector used in the present work is the same as that described in ref. 8 with the exception that new _+60 kV high-stability power supplies have been added. The role of the selector, as is discussed' later, is much more critical in 26AI than in I°Be detection, and the resolution of the present instrument is considered marginal. It is intended to build a new one, at least 50% longer, and capable of withstanding the full o u t p u t of the power supplies (the present i n s t r u m e n t arcs at voltages > 35 kV).
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R. Middleton et al. / Accelerator mass spe
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2.5. A E E telescope
A new J E - E telescope was designed and built (fig. 5) primarily for 1°Be detection. The detector has three principal elements, an ion chamber in which H)B is stopped (and the amplified current measured), an axialfield A E ionization chamber, and a silicon surface barrier detector (300 m m 2). Since the second Havar window must be at ground potential, the electric field in the A E c h a m b e r is provided by a centrally located grid. The grid consists of parallel strands of 0.02 m m wire with a separation of 0.79 m m and results in a 2.6% loss of particles. Since a relatively thick absorber is needed to stop the ]°B particles, multiple scattering of roBe is serious and the design philosophy was to minimize the length of the A E c h a m b e r to reduce scattering losses. The overall efficiency for 1°Be detection is - 90%. The detector is mounted behind a fixed 1 cm diameter aperture and can be retracted to allow measurement of the 9Be current. The same detector is used for 26A1 but, as shown in the inset of fig. 5, the mB ion chamber is replaced by a thin aluminized Mylar window.
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R. Middleton et al. / Accelerator ma,ss spectrometi3" with AI
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3. Experimental procedure
3.1. :~AI/-'7AI measurement procedure
All measurements were made at a terminal voltage of 7.5 MV, with carbon-foil stripping ( - 5 # g / c m 2 foils) a n d with charge state 7 + a l u m i n u m ions (60 MeV). Transmission through the accelerator was determined by measuring the ratio of the current in the Faraday cup following the analyzing magnet, to that measured in the low-energy cup before the accelerator. After adjusting all electrostatic and magnetic elements, it is typically - 13%.
The rest of the beam transport system between the high-energy cup and the 2~'AI detector (i.e. the switching magnet a n d following magnetic quadrupole) were adjusted using the 27A1 beam with the same magnetic rigidity as the 60 MeV 26AI of interest. This was done by setting the analyzing magnet to the field a p p r o p r i a t e for b e n d i n g 60 MeV 2e'al (using its N M R ) , and d r o p p i n g the terminal voltage to 7.22 MV to produce a beam of 57.78 MeV 27A17+ ions. After appropriate a d j u s t m e n t was made to the velocity selector, the switching magnet
a n d quadrupole were optimized to obtain m a x i m u m current in the Faraday cup located behind the withdrawn detector. This current was usually 2 to 4% less than that measured in the high-energy cup. Once set up, it was unnecessary to repeat this procedure as the stability of the switching magnet ,,,,,as monitored with an N M R . After setting the switching magnet, the analyzing magnet and velocity selector were returned to the appropriate settings for 60 MeV 27A17", and the terminal voltage was returned to 7,5 MV and adjusted to maximize the analyzed beam in the high-energy cup. It was not found necessary to make adjustments to the highenergy quadrupole when switching between the isotopes of aluminum. Only the E x B setting for >AI was not optimized by this procedure. An approximate value for the electric field (the magnetic field was maintained constant) was calculated from the settings obtained for 27A1 at 60 and 57.78 MeV, but the final value was obtained by maximizing the 26A1 count rate with a standard with an 2¢'AI/:VAI ratio of - 10 i] With the b e a m - t r a n s p o r t system thus set up, a sampie is introduced into the ion source and after 10 to 30 min have elapsed to allow the 27A1 current to stabilize, the eVAI transmission to the high energy cup is measured. The negative-ion magnet, the velocity selector a n d the analyzing magnet are then reset for mass 26 and the :~AI is counted for 3 periods of 5 to 10 min duration each - during which the 27A1 current entering the auxiliary Faraday cup is integrated. At the end of the c o u n t i n g period, the 27A1 transmission is re-measured. This procedure is repeated two or three times. Initially, because of our experience with roBe. we were apprehensive of this procedure and it was only after some twenty measurements on standards had been made with better than 10% reproducibility that we became confident. Our concern arose because after tuning the accelerator for 9Be and after resetting the velocity selector and magnetic elements for roBe, conditions were usually not optimal for roBe transmission. Indeed, by adjusting either the low-energy y-steerers or the terminal steerer, the mB pilot beam could often be increased by as much as 50% with a corresponding increase in the IBBe count rate. We presently have no evidence for similar behavior with 2%1 and 27A1 despite tests made with sources containing sufficient ~-~AI that the accelerator could be tuned using a rate meter. It is thought that the a n o m a l o u s tuning condition observed with '~Be and l°Be is associated with the fact that these are injected as molecular ions rather than elemental. 3.2. Sensitivi O, and precision
Sample requirements for terrestrial and extraterrestrial materials are very different due to the four orders of magnitude which separate their 26AI/ZTAI
R. Middleton et al. / Accelerator mass spectromet O' with AI
ratios. For terrestrial samples, limitations are imposed b o t h by the A120 3 concentration a n d by the 26A1 content of the sample. If we take as typical a sputter rate of - 2 m g / h and a m e a s u r e m e n t time of - 1 h, then only 2 mg of material are c o n s u m e d by the sputter source d u r i n g a measurement. Terrestrial samples have A1203 c o n c e n t r a t i o n s that range from a b o u t 1 to 30%. Consequently, the m a x i m u m sample weight prior to processing that can be effectively used in a measurement ranges from 200 mg to as few as 6 mg. Three million atoms of 26A1 must be loaded into the ion source to make a m e a s u r e m e n t where the counting uncertainty is equal to the transmission variability. This n u m b e r is based on an overall detection efficiency of 0.03% (0.25% ionization efficiency and 13% transmission) a n d a requirement of 100 particles reaching the detector. Thus, the 26A1 concentration is required to be greater than 1.5 × 10 7 a t o m s / g for samples with 1% A120 3, and greater than 5 × l 0 s a t o m s / g for samples with 30% A I 2 0 3. To get a count rate of 100 p a r t i c l e s / h for a sample with an 26AI/27A1 concentration of 10 -a4 requires a negative-ion current (again assuming 13% transmission) of 3.4 tzA. This is a b o u t 3 times the current we are currently able to achieve and is more than ten times the current obtained from most of the terrestrial samples we have measured. (They were measured before we discovered the effectiveness of high-temperature elimination of water.) Consequently for terrestrial samples, m e a s u r e m e n t precision is limited by the small n u m b e r of 26A1 particles detected, and m i n i m u m 26A1 concentrat i o n s / g are limited by the A1203 contents. For extraterrestrial samples, lower limits for sample size are determined solely by the requirement that a sufficient n u m b e r of 26A1 atoms be loaded into the ion source (3 × 106 atoms), and m e a s u r e m e n t precision is determined by transmission variability (_< 10%). The saturated chondritic value for 26A1 is - 3 × 10 l° a t o m s / g a n d typical A120 3 concentrations are - 1 % . For samples where there are substantial solar cosmic ray contributions (SCR) to the 26A1 production, 26A1 concentrations may be greater than 1011 a t o m s / g . Even for the typical chondritic value, however, the stable a l u m i n u m c o n c e n t r a t i o n is of little concern, a n d samples as small as 100 ~tg could be measured while still m a i n t a i n i n g one h u n d r e d counts in the detector. For samples with significant SCR contributions, size requirements may be reduced in some cases by as much as a factor of 10.
cathode was replaced with a blank containing reagent A120 3. This gave a count rate, during the first ten minutes, of 80 c o u n t s / m i n corresponding to a cross talk of 4 × 10 4. After ninety minutes of source operation, the count rate had fallen to twenty c o u n t s / m i n , corres p o n d i n g to a cross talk of 10 4
4. Some
An excellent o p p o r t u n i t y to measure the extent of ion-source cross talk presented itself while making measurements with a s t a n d a r d containing an extremely high c o n c e n t r a t i o n of 26A1. This particular sample gave an 26A1 count rate of 2 × l 0 s c o u n t s / m i n and was in the ion source for four hours. At the end of the run, the
typical
results
We have made measurements on several samples of b o t h terrestrial and extraterrestrial origin, but as the main purpose of this p a p e r is a description of the experimental technique, only two will be presented here as illustrative examples. 4.1. Stratospheric' air filter
In a recent publication [2] we reported measurements of the 26Al/l°Be ratios in two stratospheric air filters in which we determined the relative 26A1/1°Be p r o d u c t i o n ratio to be 3.8 × 10 -3. Fig. 6 shows a typical A E / E T spectrum obtained for filter 12412. During this particular run of forty minutes duration, 113 26A1 particles were counted and as is evident from the figure, were well resolved from the intense 27A1 group which arises from 2VAI of the same magnetic rigidity as the 26A1 and is produced by charge exchange in the high-energy tube of the accelerator. The measured 26AI/27A1 ratio was 6.88 × 10 13. Before this m e a s u r e m e n t was made a s t a n d a r d containing 26AI/27AI ratio of 4.36 x 10 11 was
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436
R. Middleton el al. / Accelerator mass spectromet(v with A I
run followed by a cathode containing reagent A1203 the latter to verify that cross contamination was negligible. (Zero counts were observed in twenty minutes corresponding to a contamination level of < 10 ~4.) The run on the filter was immediately followed by a second measurement of the standard. Three similar, but slightly shorter duration, runs were made on the filter yielding a total of 211 26A1 events and an average 26AI/27Al ratio of 6.8 x 10 13. From the known amount of 27A1 spike added to the filter during processing (see ref. 2) the total number of 26A] atoms in the filter was calculated to be (1.52 + 0.23) × 107. This corresponds to a decay rate of approximately twenty per year and clearly the 26AI content could only have been determined by direct atom counting. 4.2. Lunar grain
The selection of the lunar-grain spectrum, shown in fig. 7, was made to illustrate the ease with which extraterrestrial samples can be measured and to show that measurements can be made on extremely small samples. This particular grain had a total mass of 221 fig and, during processing, 1 mg of A1 carrier was added. In spite of this, the final sample was extremely small and after mixing with 10 mg of Ag powder produced an 27A1 current of about 190 hA. During a twenty-four minute run, 193 26A1 events were accumulated, corresponding to an 26A1/27AI ratio of (1.36 _+ 0.16) × 10- ]2. The number of 26A1 a t o m s / g in the grain was 1.42 × 10 II + 12%. It is
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5. Measurement of the half-life of 26AI 5.1. Introduction
Normally measurements on accelerator mass spectrometers, and indeed even on conventional mass spectrometers, are made relative to standards. It is possible however, to make " a b s o l u t e " measurements of isotopic ratios, though it is less desirable to do so as a routine practice because of the necessity of removing, or determining the extent of, systematic differences in the efficiency with which different isotopes are measured. In the case of radioactive isotopes, an absolute measuremerit of the number of radioactive atoms present in a sample of known activity allows a calculation of the radioactive element's half-life. Since there was reason to suspect the accepted value for the half-life of e6AI (primarily as a result of discrepancies in exposure ages determined for several meteorites based on measurements of their 26A1 contents and on their noble gas contents) [10], and since accelerator mass spectrometers have already been used to determine the half-life of at least one radioisotope [11] we decided to make a half-life determination for ~'6AI. 5.2. Sample preparation
Samples of certified activity were obtained from the National Bureau of Standards in Washington and Laboratoire de M&rologie des R a y o n n e m e n t s lonisants in Saclay, France. The NBS solution was diluted to produce a sample with an 26AI/27AI ratio of 4.82 X 10 s, weak enough to be counted without substantial dead time in our detector system, but capable of providing an intense enough Z6AI beam that it was possible to tune the accelerator using a rate meter. A sample with an 2~'Al/ZVA1 ratio of 4.36 x 10 -ix was produced from the L M R I standard. Both samples were made by careful dilution of the activity-standard with a certified-standard ZTAI solution and converted to the hydroxide with N H 4 O H . For the NBS sample, the dilution was - 1000 times, for the L M R I about t 0 0 0 0 times. The AI(OH)~ precipitate was dried and baked at - 500°C for several hours after centrifugation.
MEASUREMENT TIME : 24m,n 26AL : 193 COUNTS I
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Fig. 7. A particle identification spectrum similar to that shown in fig. 6 obtained from a 221 ~g lunar grain. It is thought that similar measurements might be possible with grains as small as 25 fig.
5.3. Possible sources o f 6Tstematic error
A series of preliminary measurements were performed to eliminate systematic errors that might affect an absolute measurement. In particular, attention was paid to the proper calibration of meters, assessment of dead time, and the possibility of multiple scattering
R. Middleton et al. / Accelerator mass ~pectrometrr with AI
losses in the detector. N o n e of these factors proved to be of serious concern. In order to minimize the velocity d e p e n d e n c e of the stripping efficiency at the terminal, a voltage of 7.5 MV was chosen since it is near the maximum in the probability distribution for the 7 + charge state in AI. In order to assess the extent of the velocity dependence of the stripping probability, we measured 27A1 transmission for the 6 + , 7 + and 8 + charge states with 7.5 MV and 7.79 MV on terminal (at the latter voltage the velocity of 27A1 is the same as >AI at 7.5 MV). Less than a 2% difference in transmission was observed at these two velocities, with a slight improvement at the higher velocity. As expected, the 6 + charge state decreased slightly ( - 3 . 6 % ) at the higher velocity, and the 8 + charge state improved ( - 8.7%). Ion-source fractionation and general mass dependence of the beam-transport system were assessed by measuring isotopic ratios for 29Si and 3°Si through the accelerator. This was undertaken both with and without tuning when switching from one beam to another. Retuning between isotopes does appear to improve transmission slightly ( - 2 % effect) and this was observed also for 26A1 and 2VAI using the NBS standard ( - 5%). Ion-source fractionation appears to be small under these conditions ( < 3%). 5.4. Measurement proeedure
The procedure adopted for the half-life measurement differed somewhat from the one we use for measuring isotopic ratios relative to a standard. After very carefully tuning the beam-transport system, a series of runs were initiated, alternating integration of the 27A1 current at the high-energy cup with the collection of 26A1 at the detector. For the NBS standard, the integration time of the 27A1 current was equal to the counting time for 2°Al. one minute. For the L M R I standard, the 26A1 was counted for two minutes. During the time that the >A1 was collected, the 27A1- current was integrated in the auxiliary cup. This procedure was repeated at least four times for each sample. Each standard was loaded into two separate cathodes, and a total of nineteen measurements were made on the two standards. 5.5. Results
The 26A1/27A1 ratio was calculated from the integral of the auxiliary cup and the average of the transmissions measured before and after each counting period. The NBS standard was measured with the beam-transport system " t u n e d " on 27A1 for one run, and on 26A1 for the second. When " t u n e d " for 2VA1, the measured ratio of 26A1 to 2VAI (taking into account the 2% transmission loss between the high-energy Faraday cup and the Faraday cup located behind the detector, and the 2.6% loss
437
of 26A1 due to the grid in the A E-chamber) was 4.54 × 1 0 - s . When " t u n e d " for 26A1, the ratio was 4.87 × 10 s. As one tuning method probably overestimates the ratio, while the other underestimates it, the average of 4.71 × 10 - s was adopted. This is 97.7% of the expected value assuming a half-life of 7.15 X 10 5 y, or a value for the half-life of 6.99 x 10 5 y. * The LMRI standard was also measured twice, but as its 26AI/27AI ratio was three orders of magnitude less than that of the NBS standard, it was impossible to tune on the 26A1 beam. Nonetheless, the two measurements give an average of 98.6% of the value expected if the half-life of 26A1 were 7.15 × 10 5 y or a half-life of 7.05 x l0 s y. * The average for both standards is 7.02 × 10 5 y. * Within the accuracy of these measurements, which considering all the known sources of systematic and random errors is believed to be - 8 % , the value we measure is consistent with the accepted value. A measurement of the half-life of >A1 is now in progress at Los Alamos, using a strong sample of 26A1 produced in a Si beam stop at L A M P F which has experienced a fluence of 10 23 protons. Their preliminary value for the half-life is 7.0 × 10 5 y ± 5% [12]. The authors would like to express their gratitude to Robert Zurmi]hle for pointing out the virtues of an axial field ( A E ) ionization chamber and for much help with its design and construction.
References
[1] D. Lal and B. Peters, Handbuch der Physik XLVI/2, ed., K. Sitte, (Springer-Verlag, Berlin, 1967) 551. [2] G.M. Raisbeck, F. Yiou, J. Klein and R. Middleton, Nature 301 (1983) 690. [3] R. McCorkell. E.L. Fireman and C.C. Langway, Jr, Science 158 (1967) 1690: S. Tanaka, K. Sakamoto, J. Takagi and M. Tsuchimoto, Science 160 (1968) 1348; J.-L. Reyss and Y. Yokoyama, Nature 262 (1976) 203; J.-L. Reyss, Y. Yokoyama and S. Tanaka, Science 193 (1976) 1119; F. Guichard, J.-L. Reyss and Y. Yokoyama, Nature 272 (1978) 155. [4] D. Lal, J. Ocean. Soc. Japan 18 (1962) 600. [5] W.D. Ehmann and T.P. Kohman, Geochim. Cosmochim. Acta 14 (1958) 340; M.A. Van Dilla, J.R. Arnold and E.C. Anderson, Geochim. Cosmochim. Acta 20 (1960) 115; D. Heymann and E. Anders, Geochim. Cosmochim. Acta 31 (1967) 1793: E.L. Fireman, Geochim. Cosmochim. Acta 31 (1967) 1691; R.W. Perkins, Nucl. Instr. and Meth. 33 (1965) 71: J.C. Evans, LA. Rancitelli and J.H. Reeves, Proc. 10th Lunar Planet. Sci. Conf. (1979) p. 1061. [6] J.H. Thomas, P. Parker, G. Herzog and D. Pal, Nucl. Instr. and Meth., 211 (1983) 511. * These values differ slightly from those in ref. 2 and are a result of a more detailed evaluation of the data.
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[7] G.M. Raisbeck, F. Yiou and C.J. Stephan, J. Phys. Eett. 40 (1979) L241; L.R. Kilius, R.P. Beukens, K.H. Chang, H.W. Lee, A.E. Litherland, D. Elmore, R. Ferraro and H.E. Gove, Nature 282 (1979) 488; M. Paul, W. Henning, W. Kutschera, E.J. Stephenson and J.E. Yntema, Phys. Lett. 94B (1980) 303. [8] J. Klein, R. Middleton and H.-Q. Tang, Nucl. Instr. and Meth. 193 (1982) 601. [9] R. Middleton, Nucl. Instr. and Meth., 214 (1983) 139. [10] K. Nishiizumi, S. Regnier and K. Marti, Earth Planet. Sci.
Lett. 50 (1980) 156; O. Mfiller, W. Hampel, T~ Kirsten and G.F. Herzog, Geochim. Cosmochim. Acta 45 (1981) 447. [11] D. Elmore, N. A n a n t a n a m a n , H.W. Fulbright, H.E. Gove~ H.S. Hans, K. Nishiizumi, M.T. Murrell and H. Honda, Phys. Rev. Lett. 45 (1980) 589; W. Kutschera, W. Henning, M. Paul, R.K. Smither, E.J. Stephenson, J.L. Yntema, D.E. Alburger, J.B. C u m m i n g and G. Harbottle, Phys. Rev. Lett. 45 (1980) 592. [12] T.E. Norris, A.J. Gancarz, D.J. Rokop and K.W. Thomas, Abst. 14th Lunar Planet, Sci. Conf. (1983) p. 568.
Nuclear Instruments and Methods in Physics Research 218 (1983) 430 438 North-Holland. Amsterdam
A C C E L E R A T O R M A S S S P E C T R O M E T R Y W I T H 26Ai * R. M I D D L E T O N
a n d J. K L E I N
Physics Department, Unit,ersi(v of Penn~[vh~ania, Philadelphia. PA. 191043859. USA G . M . R A I S B E C K a n d F. Y I O U Laboratoire Rend Bernas, 91406 Orsay, France
The cosmogenic radioisotopes 1°Be, 14(i and 36C1 are routinely being measured in several laboratories but the potentially important 26A1 ('1-,- 7.2×105 y) has received scant attention - largely because it is difficult to produce negative-ion beams of aluminum (particularly from the oxide) and the high abundance of 27AI results in small 2{'AI: 27A1ratios (typically < 10 t4 terrestrial and 10 to extraterrestrial). A new ion source is described which typically generates l to 2 ,ttA of 27AI ions from small samples ( - 4 rag) of AI203 with an ionization efficiency of about 0.25%. Modifications have been made to the University of Pennsylvania's FN-tandem accelerator for quantitative measurement of 2e~AI:27Al ratios as low as 5 × 10 15 and some of the problems encountered are contrasted with those met during 1°Be measurements. Although measurement of 2~'AI: 27A1 ratios in terrestrial samples are often close to the limit of our sensitivity, we have made measurements on a variety of terrestrial samples including stratospheric air filters, manganese nodules, tektites, and various impact glasses and on extraterrestrial samples including cosmic spherules, a meteorite and several lunar samples.
1. Introduction A l u m i n u m - 2 6 like beryllium-10, carbon-14 and chlorine-36 is p r e d o m i n a n t l y produced, on earth, through the interaction of cosmic rays with the atmosphere. Like 36C1, the only target nucleus in the atmosphere is At, so that its estimated rate of production ( - 1 . 4 × 10 -4 a t o m s cm 2 s 1) [1] is several orders of magnitude smaller than that of 14C and 1°Be [2]. But in spite of the realization that a m e a s u r e m e n t of the ratio of 26A1 to 1°Be in a sample might provide a useful c h r o n o m e t e r in m a n y geophysical systems (due to the similar origins a n d the supposed similarity in the chemistries of cosmogenie beryllium and aluminum), only a very few measurements were m a d e on terrestrial samples using the decay counting technique [3] in the more than twenty years that elapsed since the suggestion was first made [4]. Only in extraterrestrial samples has the measurem e n t of 26A1 been in any sense routine [5]. Accelerator mass spectrometry has changed this situation dramatically, most significantly because of the greater efficiency of " a t o m c o u n t i n g " ( - 0 . 0 3 % ) comp a r e d to " d e c a y c o u n t i n g " ( - 1 0 - 5 % for a m o n t h of counting) which allows smaller samples, often with orders of m a g n i t u d e fewer atoms of 26A1 per gram. For extraterrestrial samples, m e a s u r e m e n t s on samples as small as 100 # g or less are possible. But even accelerator mass spectrometry is limited by the high natural a b u n * Work supported by the National Science Foundation. 0 1 6 7 - 5 0 8 7 / 8 3 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
d a n c e of a l u m i n u m which, for terrestrial samples, means 26A1/27AI c o n c e n t r a t i o n s rarely exceed 10 -14. This restriction places severe d e m a n d s on the ion source, since a microampere current of AI would produce at the detector only 30 counts per hour for a sample with an 2°A1/ZVAl concentration of 10 14. As even these currents are difficult to obtain, especially from the oxide, most early accelerator experiments were performed either on extraterrestrial samples [6] or on samples m a d e by irradiation [7] where the 26A1/27AI ratios are higher. We report here the developments that have enabled us to make measurements on naturally occurring terrestrial 26A1. They are represented principally by' imp r o v e m e n t s in ion-source design a n d sample preparation which allow us to produce AI beams of 2.5 /*A from reagent grade A I 2 0 3, and 1.4 # A from A I 2 0 3 p r e p a r e d from natural samples. Even with these currents, measurements on terrestrial samples are exceedingly difficult, so that m a n y of our early measurements were aimed at exploring alternative problems which, while beyond the reach of decay counting, are accessible to the accelerator method.
2. Instrumentation 2.1. Introduction A l t h o u g h m a n y existing t a n d e m accelerators such as H V E C model EN, F N and M P tandems hold consider-
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able potential for accelerator mass spectrometry, none are ideally suited. All were designed for nuclear research with scant attention paid to optimizing the ion optics, particularly as regards reproducibility of transmission. Most are equipped with low resolution injector systems, poorly regulated steerer and lens supplies, and with the terminal voltage usually controlled by a feedback system which senses the deflection of an intense b e a m by a magnet field. As nuclear research instruments they performed admirably, but as mass spectrometers they left much to be desired - indeed isotopic ratios could rarely be measured with a precision of better than 15%. A b o u t five years ago, when the field of accelerator mass spectrometry began to bloom, we at the University of Pennsylvania decided to concentrate all our efforts on improving the i n s t r u m e n t a t i o n of our F N accelerator for the m e a s u r e m e n t of l ° B e / g B e ratios. Several factors influenced our decision to concentrate on beryllium, not the least being a knowledge that most isotopes present unique difficulties a n d problems. Most of the major changes that we m a d e are described in ref. 8 and coupled with some recent minor improvements, we currently are able to measure l ° B e / g B e ratios with a precision of a b o u t 4% and at a sensitivity level of about 10 15 A b o u t two years ago, we decided to expand our efforts to include 26A1. This isotope, which hitherto had barely been studied at all, presents several new and challenging problems. (1) By preference, one would like to accelerate AIO ions since A120 3 is convenient to
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m a n u f a c t u r e and extremely difficult to reduce to elemental form, and A 1 0 - is a prolific negative ion. However, 26MgO also readily forms a negative ion, and as it is lower in Z than AI, it is impossible to eliminate with an absorber foil. (2) Fortunately, the Mg negative ion appears to be unstable - at least all of the present work is consistent with this conclusion - so one can accelerate A1- without fear of interference from its only stable isobar. Unfortunately, it is not a prolific negative ion a n d is particularly difficult to form from A1203 in a sputter source. (3) Unlike Be, stable A1 is a major constituent of most samples of interest. This coupled with an extremely low production rate (at least from the atmosphere), results in terrestrial ratios of 26AI/27AI of < 10 t4. (4) Whereas l°B can be used as a pilot b e a m to tune the accelerator for ]°Be in the case of 26AL there is no convenient pilot beam. A n d (5) as a consequence of accelerating negative elemental ions as opposed to molecules, a second dispersive element is necessary between the detector and the high energy 90 ° analyzing magnet to eliminate 27A1 ions that charge exchange within the magnet and are indistinguishable from the 26A1 of interest. 2.2. Beam-transport 6:vstem
This is shown schematically in fig. I. The low-energy b e a m transport system closely resembles that described in ref. 8 with the exceptions of a vastly improved negative-ion source, and the addition of a variable-post-
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432
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tion auxiliary F a r a d a y cup. When 26A1 is being injected into the accelerator this is positioned to intercept the 2VAI- b e a m and the current is integrated. More substantial changes have been made in the high-energy transport system. For beryllium, the detector is m o u n t e d approximately in the plane of the image slit, but for a l u m i n u m we are compelled to m o u n t the detector behind the switching magnet, Fig. 2 illustrates why this is necessary. In spite of the good mass resolution of our 90 ° negative-ion magnet ( A m / m - 1 in 60), some 27A1- ions are injected into the accelerator along with the mass 26 beam. These enter the 90 ° positive-ion magnet with precisely the same energy as the 2~AI particles, and a small fraction charge exchange from 7 + to 8 + . If the detector were located in the image p l a n e of the 90 ° magnet, some of these would be able to enter it, and having the same energy as the 26A1 and the same Z , they would be totally indistinguishable. Fig. 2 clearly illustrates that the second magnet completely eradicates these u n w a n t e d particles. 2.3 Negative-ion source
A sectional drawing of the negative-ion source that is now routinely used to make m e a s u r e m e n t s on both
beryllium and a l u m i n u m is shown in fig. 3. Since this source is fully described elsewhere [9] it will not be described here. However, we have made extensive studies of the generation of 27A1 - ions from small samples of A1203 powder both on the accelerator and on our ion-source test facility and these will be reviewed. Fig. 4 shows the results of two tests made on our ion-source test facility. The broken-line curve shows the 27A1 current as a function of time from a cathode containing reagent AI203 powder. The cathode was made from pure copper, and the A I 2 0 ~ powder was pressed (and h a m m e r e d ) into a 1.6 m m diameter by 1.6 m m long hole (see insert sketch). The total weight of 27#,1 was 3.7 mg, corresponding to an ionization efficiency of - 0.25% over the five-hour duration of the run. Interestingly, in spite of vigorous pressing and hammering, the sample's density was only 2.6, substantially less than the theoretical value of 4.0. The full-line curve in fig. 4 was obtained under identical conditions, but with the AI zO~ powder mixed with Ag powder (1:4.1 by weight). Note that unlike the pure sample, the current immediately climbed to a m a x i m u m and thereafter steadily declined. It is also noticeable that not only was the current substantially less, but also much less stable. Usually, the a l u m i n u m oxide samples that we prepare produce substantially less current, and until recently 0.6 to 0.7 /,A was typical, c o m p a r e d with - 2.4 ffA from commercial oxide. Recently we discovered that this was due to the presence of water and, after firing at 1200°C, we are now routinely able to obtain 1.4 # A from our samples. The remaining disparity is presently not understood, but it is not believed to be due to the presence of impurities. The available negative-ion currents on the accelerator are usually a factor of two less than from the test facility. A 10% loss occurs in a second gridded einzel lens, but we believe that the main loss is due to vertical blow-up of the b e a m caused by astigmatism in the 90 ° negative-ion magnet. This p r o b l e m was recently eliminated on the test facility by the addition of field clamps, and similar clamps are being constructed for the magnet on the accelerator. 2.4. Veloci O, selector
The velocity selector used in the present work is the same as that described in ref. 8 with the exception that new _+60 kV high-stability power supplies have been added. The role of the selector, as is discussed' later, is much more critical in 26AI than in I°Be detection, and the resolution of the present instrument is considered marginal. It is intended to build a new one, at least 50% longer, and capable of withstanding the full o u t p u t of the power supplies (the present i n s t r u m e n t arcs at voltages > 35 kV).
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2.5. A E E telescope
A new J E - E telescope was designed and built (fig. 5) primarily for 1°Be detection. The detector has three principal elements, an ion chamber in which H)B is stopped (and the amplified current measured), an axialfield A E ionization chamber, and a silicon surface barrier detector (300 m m 2). Since the second Havar window must be at ground potential, the electric field in the A E c h a m b e r is provided by a centrally located grid. The grid consists of parallel strands of 0.02 m m wire with a separation of 0.79 m m and results in a 2.6% loss of particles. Since a relatively thick absorber is needed to stop the ]°B particles, multiple scattering of roBe is serious and the design philosophy was to minimize the length of the A E c h a m b e r to reduce scattering losses. The overall efficiency for 1°Be detection is - 90%. The detector is mounted behind a fixed 1 cm diameter aperture and can be retracted to allow measurement of the 9Be current. The same detector is used for 26A1 but, as shown in the inset of fig. 5, the mB ion chamber is replaced by a thin aluminized Mylar window.
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R. Middleton et al. / Accelerator ma,ss spectrometi3" with AI
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3. Experimental procedure
3.1. :~AI/-'7AI measurement procedure
All measurements were made at a terminal voltage of 7.5 MV, with carbon-foil stripping ( - 5 # g / c m 2 foils) a n d with charge state 7 + a l u m i n u m ions (60 MeV). Transmission through the accelerator was determined by measuring the ratio of the current in the Faraday cup following the analyzing magnet, to that measured in the low-energy cup before the accelerator. After adjusting all electrostatic and magnetic elements, it is typically - 13%.
The rest of the beam transport system between the high-energy cup and the 2~'AI detector (i.e. the switching magnet a n d following magnetic quadrupole) were adjusted using the 27A1 beam with the same magnetic rigidity as the 60 MeV 26AI of interest. This was done by setting the analyzing magnet to the field a p p r o p r i a t e for b e n d i n g 60 MeV 2e'al (using its N M R ) , and d r o p p i n g the terminal voltage to 7.22 MV to produce a beam of 57.78 MeV 27A17+ ions. After appropriate a d j u s t m e n t was made to the velocity selector, the switching magnet
a n d quadrupole were optimized to obtain m a x i m u m current in the Faraday cup located behind the withdrawn detector. This current was usually 2 to 4% less than that measured in the high-energy cup. Once set up, it was unnecessary to repeat this procedure as the stability of the switching magnet ,,,,,as monitored with an N M R . After setting the switching magnet, the analyzing magnet and velocity selector were returned to the appropriate settings for 60 MeV 27A17", and the terminal voltage was returned to 7,5 MV and adjusted to maximize the analyzed beam in the high-energy cup. It was not found necessary to make adjustments to the highenergy quadrupole when switching between the isotopes of aluminum. Only the E x B setting for >AI was not optimized by this procedure. An approximate value for the electric field (the magnetic field was maintained constant) was calculated from the settings obtained for 27A1 at 60 and 57.78 MeV, but the final value was obtained by maximizing the 26A1 count rate with a standard with an 2¢'AI/:VAI ratio of - 10 i] With the b e a m - t r a n s p o r t system thus set up, a sampie is introduced into the ion source and after 10 to 30 min have elapsed to allow the 27A1 current to stabilize, the eVAI transmission to the high energy cup is measured. The negative-ion magnet, the velocity selector a n d the analyzing magnet are then reset for mass 26 and the :~AI is counted for 3 periods of 5 to 10 min duration each - during which the 27A1 current entering the auxiliary Faraday cup is integrated. At the end of the c o u n t i n g period, the 27A1 transmission is re-measured. This procedure is repeated two or three times. Initially, because of our experience with roBe. we were apprehensive of this procedure and it was only after some twenty measurements on standards had been made with better than 10% reproducibility that we became confident. Our concern arose because after tuning the accelerator for 9Be and after resetting the velocity selector and magnetic elements for roBe, conditions were usually not optimal for roBe transmission. Indeed, by adjusting either the low-energy y-steerers or the terminal steerer, the mB pilot beam could often be increased by as much as 50% with a corresponding increase in the IBBe count rate. We presently have no evidence for similar behavior with 2%1 and 27A1 despite tests made with sources containing sufficient ~-~AI that the accelerator could be tuned using a rate meter. It is thought that the a n o m a l o u s tuning condition observed with '~Be and l°Be is associated with the fact that these are injected as molecular ions rather than elemental. 3.2. Sensitivi O, and precision
Sample requirements for terrestrial and extraterrestrial materials are very different due to the four orders of magnitude which separate their 26AI/ZTAI
R. Middleton et al. / Accelerator mass spectromet O' with AI
ratios. For terrestrial samples, limitations are imposed b o t h by the A120 3 concentration a n d by the 26A1 content of the sample. If we take as typical a sputter rate of - 2 m g / h and a m e a s u r e m e n t time of - 1 h, then only 2 mg of material are c o n s u m e d by the sputter source d u r i n g a measurement. Terrestrial samples have A1203 c o n c e n t r a t i o n s that range from a b o u t 1 to 30%. Consequently, the m a x i m u m sample weight prior to processing that can be effectively used in a measurement ranges from 200 mg to as few as 6 mg. Three million atoms of 26A1 must be loaded into the ion source to make a m e a s u r e m e n t where the counting uncertainty is equal to the transmission variability. This n u m b e r is based on an overall detection efficiency of 0.03% (0.25% ionization efficiency and 13% transmission) a n d a requirement of 100 particles reaching the detector. Thus, the 26A1 concentration is required to be greater than 1.5 × 10 7 a t o m s / g for samples with 1% A120 3, and greater than 5 × l 0 s a t o m s / g for samples with 30% A I 2 0 3. To get a count rate of 100 p a r t i c l e s / h for a sample with an 26AI/27A1 concentration of 10 -a4 requires a negative-ion current (again assuming 13% transmission) of 3.4 tzA. This is a b o u t 3 times the current we are currently able to achieve and is more than ten times the current obtained from most of the terrestrial samples we have measured. (They were measured before we discovered the effectiveness of high-temperature elimination of water.) Consequently for terrestrial samples, m e a s u r e m e n t precision is limited by the small n u m b e r of 26A1 particles detected, and m i n i m u m 26A1 concentrat i o n s / g are limited by the A1203 contents. For extraterrestrial samples, lower limits for sample size are determined solely by the requirement that a sufficient n u m b e r of 26A1 atoms be loaded into the ion source (3 × 106 atoms), and m e a s u r e m e n t precision is determined by transmission variability (_< 10%). The saturated chondritic value for 26A1 is - 3 × 10 l° a t o m s / g a n d typical A120 3 concentrations are - 1 % . For samples where there are substantial solar cosmic ray contributions (SCR) to the 26A1 production, 26A1 concentrations may be greater than 1011 a t o m s / g . Even for the typical chondritic value, however, the stable a l u m i n u m c o n c e n t r a t i o n is of little concern, a n d samples as small as 100 ~tg could be measured while still m a i n t a i n i n g one h u n d r e d counts in the detector. For samples with significant SCR contributions, size requirements may be reduced in some cases by as much as a factor of 10.
cathode was replaced with a blank containing reagent A120 3. This gave a count rate, during the first ten minutes, of 80 c o u n t s / m i n corresponding to a cross talk of 4 × 10 4. After ninety minutes of source operation, the count rate had fallen to twenty c o u n t s / m i n , corres p o n d i n g to a cross talk of 10 4
4. Some
An excellent o p p o r t u n i t y to measure the extent of ion-source cross talk presented itself while making measurements with a s t a n d a r d containing an extremely high c o n c e n t r a t i o n of 26A1. This particular sample gave an 26A1 count rate of 2 × l 0 s c o u n t s / m i n and was in the ion source for four hours. At the end of the run, the
typical
results
We have made measurements on several samples of b o t h terrestrial and extraterrestrial origin, but as the main purpose of this p a p e r is a description of the experimental technique, only two will be presented here as illustrative examples. 4.1. Stratospheric' air filter
In a recent publication [2] we reported measurements of the 26Al/l°Be ratios in two stratospheric air filters in which we determined the relative 26A1/1°Be p r o d u c t i o n ratio to be 3.8 × 10 -3. Fig. 6 shows a typical A E / E T spectrum obtained for filter 12412. During this particular run of forty minutes duration, 113 26A1 particles were counted and as is evident from the figure, were well resolved from the intense 27A1 group which arises from 2VAI of the same magnetic rigidity as the 26A1 and is produced by charge exchange in the high-energy tube of the accelerator. The measured 26AI/27A1 ratio was 6.88 × 10 13. Before this m e a s u r e m e n t was made a s t a n d a r d containing 26AI/27AI ratio of 4.36 x 10 11 was
-
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S T R A T O S P H E R I C AIR FILTER 12412 15 km ALTITUDE 58 ° - 65 ° N VOLUME = 5 4 7 m 5 (+lmg AL CARRIER) Z6A[/2~,L:e88xlOq3 ":
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435
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MEASUREMENT TIME = 4.0 26AL = 113 COUNTS
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I
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I 48
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Fig. 6. A typical particle identification spectrum where energy loss (AE) is plotted against total energy (ET) obtained with a stratospheric air filter. The numbers in the spectra are the logarithm base 2 of the number of events.
436
R. Middleton el al. / Accelerator mass spectromet(v with A I
run followed by a cathode containing reagent A1203 the latter to verify that cross contamination was negligible. (Zero counts were observed in twenty minutes corresponding to a contamination level of < 10 ~4.) The run on the filter was immediately followed by a second measurement of the standard. Three similar, but slightly shorter duration, runs were made on the filter yielding a total of 211 26A1 events and an average 26AI/27Al ratio of 6.8 x 10 13. From the known amount of 27A1 spike added to the filter during processing (see ref. 2) the total number of 26A] atoms in the filter was calculated to be (1.52 + 0.23) × 107. This corresponds to a decay rate of approximately twenty per year and clearly the 26AI content could only have been determined by direct atom counting. 4.2. Lunar grain
The selection of the lunar-grain spectrum, shown in fig. 7, was made to illustrate the ease with which extraterrestrial samples can be measured and to show that measurements can be made on extremely small samples. This particular grain had a total mass of 221 fig and, during processing, 1 mg of A1 carrier was added. In spite of this, the final sample was extremely small and after mixing with 10 mg of Ag powder produced an 27A1 current of about 190 hA. During a twenty-four minute run, 193 26A1 events were accumulated, corresponding to an 26A1/27AI ratio of (1.36 _+ 0.16) × 10- ]2. The number of 26A1 a t o m s / g in the grain was 1.42 × 10 II + 12%. It is
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LUNAR GRAIN 69921.1 221ffg (*1 mg Al CARRIER) 26A[/27Al
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believed that it might be possible to make similar measurements on grains as small as 25 fig.
5. Measurement of the half-life of 26AI 5.1. Introduction
Normally measurements on accelerator mass spectrometers, and indeed even on conventional mass spectrometers, are made relative to standards. It is possible however, to make " a b s o l u t e " measurements of isotopic ratios, though it is less desirable to do so as a routine practice because of the necessity of removing, or determining the extent of, systematic differences in the efficiency with which different isotopes are measured. In the case of radioactive isotopes, an absolute measuremerit of the number of radioactive atoms present in a sample of known activity allows a calculation of the radioactive element's half-life. Since there was reason to suspect the accepted value for the half-life of e6AI (primarily as a result of discrepancies in exposure ages determined for several meteorites based on measurements of their 26A1 contents and on their noble gas contents) [10], and since accelerator mass spectrometers have already been used to determine the half-life of at least one radioisotope [11] we decided to make a half-life determination for ~'6AI. 5.2. Sample preparation
Samples of certified activity were obtained from the National Bureau of Standards in Washington and Laboratoire de M&rologie des R a y o n n e m e n t s lonisants in Saclay, France. The NBS solution was diluted to produce a sample with an 26AI/27AI ratio of 4.82 X 10 s, weak enough to be counted without substantial dead time in our detector system, but capable of providing an intense enough Z6AI beam that it was possible to tune the accelerator using a rate meter. A sample with an 2~'Al/ZVA1 ratio of 4.36 x 10 -ix was produced from the L M R I standard. Both samples were made by careful dilution of the activity-standard with a certified-standard ZTAI solution and converted to the hydroxide with N H 4 O H . For the NBS sample, the dilution was - 1000 times, for the L M R I about t 0 0 0 0 times. The AI(OH)~ precipitate was dried and baked at - 500°C for several hours after centrifugation.
MEASUREMENT TIME : 24m,n 26AL : 193 COUNTS I
44
!
46
_
ET
I
I
48
I
510
~
(MeV)
Fig. 7. A particle identification spectrum similar to that shown in fig. 6 obtained from a 221 ~g lunar grain. It is thought that similar measurements might be possible with grains as small as 25 fig.
5.3. Possible sources o f 6Tstematic error
A series of preliminary measurements were performed to eliminate systematic errors that might affect an absolute measurement. In particular, attention was paid to the proper calibration of meters, assessment of dead time, and the possibility of multiple scattering
R. Middleton et al. / Accelerator mass ~pectrometrr with AI
losses in the detector. N o n e of these factors proved to be of serious concern. In order to minimize the velocity d e p e n d e n c e of the stripping efficiency at the terminal, a voltage of 7.5 MV was chosen since it is near the maximum in the probability distribution for the 7 + charge state in AI. In order to assess the extent of the velocity dependence of the stripping probability, we measured 27A1 transmission for the 6 + , 7 + and 8 + charge states with 7.5 MV and 7.79 MV on terminal (at the latter voltage the velocity of 27A1 is the same as >AI at 7.5 MV). Less than a 2% difference in transmission was observed at these two velocities, with a slight improvement at the higher velocity. As expected, the 6 + charge state decreased slightly ( - 3 . 6 % ) at the higher velocity, and the 8 + charge state improved ( - 8.7%). Ion-source fractionation and general mass dependence of the beam-transport system were assessed by measuring isotopic ratios for 29Si and 3°Si through the accelerator. This was undertaken both with and without tuning when switching from one beam to another. Retuning between isotopes does appear to improve transmission slightly ( - 2 % effect) and this was observed also for 26A1 and 2VAI using the NBS standard ( - 5%). Ion-source fractionation appears to be small under these conditions ( < 3%). 5.4. Measurement proeedure
The procedure adopted for the half-life measurement differed somewhat from the one we use for measuring isotopic ratios relative to a standard. After very carefully tuning the beam-transport system, a series of runs were initiated, alternating integration of the 27A1 current at the high-energy cup with the collection of 26A1 at the detector. For the NBS standard, the integration time of the 27A1 current was equal to the counting time for 2°Al. one minute. For the L M R I standard, the 26A1 was counted for two minutes. During the time that the >A1 was collected, the 27A1- current was integrated in the auxiliary cup. This procedure was repeated at least four times for each sample. Each standard was loaded into two separate cathodes, and a total of nineteen measurements were made on the two standards. 5.5. Results
The 26A1/27A1 ratio was calculated from the integral of the auxiliary cup and the average of the transmissions measured before and after each counting period. The NBS standard was measured with the beam-transport system " t u n e d " on 27A1 for one run, and on 26A1 for the second. When " t u n e d " for 2VA1, the measured ratio of 26A1 to 2VAI (taking into account the 2% transmission loss between the high-energy Faraday cup and the Faraday cup located behind the detector, and the 2.6% loss
437
of 26A1 due to the grid in the A E-chamber) was 4.54 × 1 0 - s . When " t u n e d " for 26A1, the ratio was 4.87 × 10 s. As one tuning method probably overestimates the ratio, while the other underestimates it, the average of 4.71 × 10 - s was adopted. This is 97.7% of the expected value assuming a half-life of 7.15 X 10 5 y, or a value for the half-life of 6.99 x 10 5 y. * The LMRI standard was also measured twice, but as its 26AI/27AI ratio was three orders of magnitude less than that of the NBS standard, it was impossible to tune on the 26A1 beam. Nonetheless, the two measurements give an average of 98.6% of the value expected if the half-life of 26A1 were 7.15 × 10 5 y or a half-life of 7.05 x l0 s y. * The average for both standards is 7.02 × 10 5 y. * Within the accuracy of these measurements, which considering all the known sources of systematic and random errors is believed to be - 8 % , the value we measure is consistent with the accepted value. A measurement of the half-life of >A1 is now in progress at Los Alamos, using a strong sample of 26A1 produced in a Si beam stop at L A M P F which has experienced a fluence of 10 23 protons. Their preliminary value for the half-life is 7.0 × 10 5 y ± 5% [12]. The authors would like to express their gratitude to Robert Zurmi]hle for pointing out the virtues of an axial field ( A E ) ionization chamber and for much help with its design and construction.
References
[1] D. Lal and B. Peters, Handbuch der Physik XLVI/2, ed., K. Sitte, (Springer-Verlag, Berlin, 1967) 551. [2] G.M. Raisbeck, F. Yiou, J. Klein and R. Middleton, Nature 301 (1983) 690. [3] R. McCorkell. E.L. Fireman and C.C. Langway, Jr, Science 158 (1967) 1690: S. Tanaka, K. Sakamoto, J. Takagi and M. Tsuchimoto, Science 160 (1968) 1348; J.-L. Reyss and Y. Yokoyama, Nature 262 (1976) 203; J.-L. Reyss, Y. Yokoyama and S. Tanaka, Science 193 (1976) 1119; F. Guichard, J.-L. Reyss and Y. Yokoyama, Nature 272 (1978) 155. [4] D. Lal, J. Ocean. Soc. Japan 18 (1962) 600. [5] W.D. Ehmann and T.P. Kohman, Geochim. Cosmochim. Acta 14 (1958) 340; M.A. Van Dilla, J.R. Arnold and E.C. Anderson, Geochim. Cosmochim. Acta 20 (1960) 115; D. Heymann and E. Anders, Geochim. Cosmochim. Acta 31 (1967) 1793: E.L. Fireman, Geochim. Cosmochim. Acta 31 (1967) 1691; R.W. Perkins, Nucl. Instr. and Meth. 33 (1965) 71: J.C. Evans, LA. Rancitelli and J.H. Reeves, Proc. 10th Lunar Planet. Sci. Conf. (1979) p. 1061. [6] J.H. Thomas, P. Parker, G. Herzog and D. Pal, Nucl. Instr. and Meth., 211 (1983) 511. * These values differ slightly from those in ref. 2 and are a result of a more detailed evaluation of the data.
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R. Middleton et al. / Accelerator mass spectrometry with A l
[7] G.M. Raisbeck, F. Yiou and C.J. Stephan, J. Phys. Eett. 40 (1979) L241; L.R. Kilius, R.P. Beukens, K.H. Chang, H.W. Lee, A.E. Litherland, D. Elmore, R. Ferraro and H.E. Gove, Nature 282 (1979) 488; M. Paul, W. Henning, W. Kutschera, E.J. Stephenson and J.E. Yntema, Phys. Lett. 94B (1980) 303. [8] J. Klein, R. Middleton and H.-Q. Tang, Nucl. Instr. and Meth. 193 (1982) 601. [9] R. Middleton, Nucl. Instr. and Meth., 214 (1983) 139. [10] K. Nishiizumi, S. Regnier and K. Marti, Earth Planet. Sci.
Lett. 50 (1980) 156; O. Mfiller, W. Hampel, T~ Kirsten and G.F. Herzog, Geochim. Cosmochim. Acta 45 (1981) 447. [11] D. Elmore, N. A n a n t a n a m a n , H.W. Fulbright, H.E. Gove~ H.S. Hans, K. Nishiizumi, M.T. Murrell and H. Honda, Phys. Rev. Lett. 45 (1980) 589; W. Kutschera, W. Henning, M. Paul, R.K. Smither, E.J. Stephenson, J.L. Yntema, D.E. Alburger, J.B. C u m m i n g and G. Harbottle, Phys. Rev. Lett. 45 (1980) 592. [12] T.E. Norris, A.J. Gancarz, D.J. Rokop and K.W. Thomas, Abst. 14th Lunar Planet, Sci. Conf. (1983) p. 568.