Measurement of7Be by accelerator mass spectrometry

Earth and Planetary Science Letters, 89 (1988) 103-108 Elsevier Science Publishers B.V., A m s t e r d a m - Printed in The Netherlands

103

[6]

Measurement of 7Be by accelerator mass spectrometry G . M . R a i s b e c k a n d F. Y i o u Laboratoire Rend Bernas, Centre de SpectromOtrie NuclJaire et de Spectrom~trie de Masse, 91406 Orsay (France)

Received September 16, 1987; revised version received March 14, 1988

We describe an accelerator mass spectrometry (AMS) technique for measuring the cosmogenic isotope 7Be (half-life 53.2 days). Our detection limit of - 104 atoms with this technique is - 100 times lower than that obtained by "/ counting. The potential advantages of this increased sensitivity, combined with A M S measurements of 1°Be (half-life 1.5 My) in the same samples, are discussed in the context of the application of 7Be/l° Be as a time-dependent tracer of various environmental transport processes.

1. Introduction

The technique of accelerator mass spectrometry (AMS) has revolutionized the measurement and potential applications of long-lived (half-life > 100 year) cosmogenic nuclides [1-5]. For shorter-lived isotopes, it is generally believed that classical radioactive decay counting is still the most sensitive technique. Nevertheless there are some cases, particularly where the decay process leads to radiation that is non-specific (beta decay) or difficult to measure (electron capture), where AMS may still offer significant advantages. The isotope 7Be (half-life 53.2 days) is one such case. Only 10.4% of 7Be decays lead to a readily measurable 478 keV 3,-ray, while the remaining electron capture branch gives a very low energy X-ray. We report here AMS measurements of 7Be that are 10-100 times more sensitive than previously reported decay counting procedures. We also discuss the potential benefits of such increased sensitivity, together with concurrent measurements of 10Be in the same samples, for the application of 7Be as an environmental tracer. 7 B e is one of the few radioactive nuclides ( 3 H , 14C, 1°Be) that can be formed by nuclear reactions on nitrogen and oxygen, the principal components of the atmosphere, and has a half-life longer than a few minutes. As such it has many 0012-821X/88/$03.50

© 1988 Elsevier Science Publishers B.V.

potential applications in studies of environmental transport processes (see for example, Lal and Peters [6] for a general discussion, and references to some early studies). Indeed 7Be has been measured in the atmosphere [7-15], in rainwater [16-19], seawater [20-23], and on particles associated with lacustrine [24], and estuarine [16,18,19] environments. The measurements have been used to investigate atmospheric residence times [7,8], stratospheric/tropospheric exchange [9-15], distribution of aerosol associated species to land [17-19] and ocean [20-22] surfaces, particle associated transport in fluvial, estuarine and coastal environments [16,18,19,22,23], sediment mixing [16], and possible solar effects on climate [15]. For such applications, the 7Be radioactivity, with or without prior chemical separation, has been counted in either NaI or Ge(Li) detectors. The former have the advantage of larger possible efficiencies, while the latter have much better resolution, thus greater specificity. In both cases the quantity of 7Be measured is usually > 107 atoms, with a minimum detection limit of - 1 0 6 atoms. Often the applications mentioned (and other potential ones) are hampered by the relatively large samples ( > 103 m 3 of tropospheric air, > 1 m 3 of ocean water) necessary to provide the > 107 atoms of 7Be required for counting. Using the AMS procedure described here, it should be fairly

104

straightforward to measure samples containing 106 atoms of 7Be, and an ultimate detection limit appears to be - ] 0 4 atoms.

30

2 . 9 , 10 6 a t o m s

(a)

o f 7Be

25

2. Experimental procedure 20

We have previously described a procedure for measuring a°Be (half-life 1.5 My) with a low-energy Tandetron AMS facility [25,26]. We have used the same facility to measure 7Be. Initial samples were prepared by mixing a known quantity of 7Be (Amersham Ltd), together with 0.5 mg of stable 9Be carrier, to - 1 g samples of marine sediment. The 7Be was originally destined to serve in some chemical tracer studies (see section 5). Beryllium extraction, purification and transformation to BeO were carried out as for 1°Be and a known fraction of the BeO (and thus known quantity of 7Be) was loaded into the ion source. 7BeOions were injected into the accelerator and stripped to 7Be2+ at the terminal, which was operated at 2.2 MV. 9BeO- ions were periodically injected into the accelerator and the 9Be2+ current used to normalize the measurements. The potentially most serious problem for measuring 7Be by AMS is interference from even minute quantities of 7Li inevitably present in the sample a n d / o r ion source. Several years ago we pointed out that, in situations where an interfering isobar has a lower atomic number than the nuclide being studied, complete stripping followed by magnetic analysis can be used to eliminate the unwanted isobar [27]. Using relatively high-energy accelerators, we and others have used this procedure to make AMS measurements of 26A1 [27], 41Ca [28], 36C1 [29] and 59Ni [30]. In the present case, the low atomic number of 7Be means that a relatively large efficiency for complete stripping can be obtained even with the modest energy available with our Tandetron facility. The 7Be2+ ions were thus passed through a 200 / l g / c m 2 carbon foil located between the two high-energy analyzing magnets (the same foil used for reducing I°B in our 1°Be studies [25]), and the resulting 7 B e n + ions focussed into the detector. Rate of energy loss ( A E ) and residual energy ( E ) signals from this detector were fed into a two-dimensional multichannel analyzer to give the characteristic 7Be identification.

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Fig. 1. (a) Rate of energy loss ( A E ) vs. residual energy ( E ) spectrum for a sample containing 2.9×106 atoms of 7Be, counted for 1000 seconds. The numbers ( n ) indicate the number of events in a given channel ( N ) by 2 " - 1 < N < 2". (b) Spectrum for sample processed in the same manner as (a) except no 7Be added, counted for 5000 seconds. A single event in the window would correspond to 4000 7Be atoms in sample.

In Fig. l a we show a spectrum for a sample containing 2.9 × 10 6 atoms of 7Be, which is comparable to the detection limit given for decay counting systems. Fig. l b shows a spectrum from a blank sample, where a single event would imply 4000 atoms of 7Be in the ion source. What is remarkable is the total absence of background events in the blank sample. Thus, unlike decay counting systems, the sensitivity obtainable with

105

this procedure appears to be limited only by the statistics of the number of 7Be events detected (and thus ultimately the system efficiency) and not by background. In order to estimate our overall efficiency, we have decay counted the ion source cathode containing a concentrated ( - 109 atoms) 7Be sample before and after an extended (3000 seconds) AMS measurement, during which we integrated the number of 7Be events reaching the detector. The result was an efficiency (TBe detected per 7Be removed from cathode) of - 1 × 10 -3, which is composed of an ion source efficiency for producing BeO of - 2 % , and an accelerator transmission of - 5%. It should be noted that this overall efficiency, while quite satisfactory in the context of the present application, is still almost two orders of magnitude lower than that calculated by Fireman et al. as being necessary for measurement of 7Be in a proposed solar neutrino detection experiment [31].

prove, and possibly open up new applications of 7Be as an environmental tracer. To demonstrate these ideas, we have made AMS measurements of 7Be and 1°Be on two samples: (1) 418 g of rainwater collected during a few hours with a plastic funnel on the roof of our laboratory at Orsay, (2) a 13.5 mg aliquot of the particulate matter collected with a sediment trap exposed at 1000 m in a 2200 m water column in the Mediterranean Sea, - 1 5 miles off the coast of Calvi, Corsica (42°43.9'N, 08°31.3'E) [33]. The beryl-

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15.

3. Normalization with roBe In applications where one attemps to obtain time information from 7Be (or any other single radioactive isotope), one is often faced with an ambiguity; a decreased 7Be concentration in an environmental sample can arise either from decay, or by removal or dilution processes. We have discussed earlier how the use of the ratio 7Be/l° Be can help remove this ambiguity in the case of atmospheric samples [32]. Similar advantages will occur for a number of the other applications mentioned above. Because 7Be and 1°Be are formed together and have the same chemical behavior, 1°Be (which is essentially stable over the lifetime of 7Be) can be used to "normalize" the 7Be concentration. Thus, in a given system, if the initial 7Be/l°Be can be measured, or estimated, any decrease in this ratio can be attributed to decay, since removal or dilution will affect both isotopes virtually identically. In addition, if 7Be/I° Be measurements are made on the same sample, several other potential sources of uncertainty (sample collection efficiency, chemical yield) will cancel out, resulting in a measurement precision limited essentially by the number of 7Be and roBe ions recorded. We thus believe that the use of AMS measurements of 7Be/X°Be can facilitate, im-

112

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Fig. 2. (a) Spectrum for 7Be extracted from 418 g of rainwater collected at Orsay, counted for 2000 seconds. Symbols as in Fig. 1. (b) Spectrum for 7Be extracted from 13.5 mg of particulate matter collected in sediment trap exposed at 1000 m in Mediterranean sea, counted for 12,300 seconds.

106 TABLE 1 Measured 1°Be and 7Be in rainwater and sediment trap matter Sample

Weight (g)

Measured aoBe in sample (atoms)

Measured 7Be in sample (atoms)

7Be/lO Be at collection time (atom/atom)

Rainwater Mediterranean sediment trap matter

418 0.0135

8.6 ___1.0 × 106 4.0 + 0.5 x 106

4.0 + 0.5 x 106 ~< 7000

0.53 + 0.09 ~<0.01

lium was extracted as described above, except only 0.25 mg of 9Be carrier was used. The 7Be and a°Be measurements were made within a few hours of each other, using the same BeO sample. The 7Be spectra are shown in Fig. 2. The rainwater spectrum (which represents only - 3 0 minutes of counting time on the accelerator) illustrates the relative ease of measurement - 4 × 106 atoms of 7Be, while the sediment trap result confirms that our detection limit for 7Be under realistic experimental conditions is indeed - 1 0 4 atoms. The 7Be and t°Be results are summarized in Table 1. The 7Be/l°Be ratio in the rainwater is in the range of values reported earlier (0.35-0.64), where the 7Be was determined on order of magnitude larger samples by "/counting [34]. If we make the reasonable assumption that the ratio of 7Be/l° Be input to the surface of the Mediterranean is approximately the same as the in the rainwater at Orsay, then the measured 7Be/l° Be from the sediment trap implies a lower limit on the apparent "age" or mean transport time (t) of the trapped cosmogenic 7-1°Be since introduction to the sea as: 53.2 0.53 t> ~ ln~>__300 days

lieve the present results illustrate the considerable advantages of AMS measurements of 7Be/l°Be for such studies.

4. Application to other isotopes Given the much lower detection limit obtained here for 7Be by AMS, compared to conventional decay counting, one naturally wonders whether similar advantages could be available for other relatively short lived nuclides. As mentioned in the introduction, the fact that only - 10% of 7Be decays lead to an easily measurable 7-ray favors the AMS technique compared to counting. However, even allowing for this fact, 7Be has approximately the same specific "y activity as an isotope having a 100% y-ray branching ratio, and a half-life of - 1 . 5 years. Thus, if AMS procedures with overall efficiencies comparable to that obtained here for 7Be can be developed, AMS could probably improve detection limits for a number of natural or anthropogenic radionuclides with half-fifes >__1 year. This could have potentially important applications in both monitoring these isotopes, and their use as environmental tracers.

5. l°Be in commercial 7Be While we do not wish to discuss in detail the implications of these measurements (which in fact represent the test stage of a more elaborate project) we can note that this transport time corresponds to a mean sinking speed of >__3 m / d a y , which is considerably smaller than that observed in the upper 200 m of the water column for several Chernobyl fallout radionuclides, using similar sediment traps at the same location [33]. Additional 7Be/l°Be measurements at other locations and depths will be necessary to elucidate the detailed behavior of cosmogenic Be isotopes compared to other oceanic tracers. However we be-

As pointed out in section 2, the 7Be used in the initial stages of this work was originally destined to serve as a tracer during trials of some modifications in our procedures for extracting 1°Be from marine sediments. During some of our early AMS studies we had discovered that 7Be sold by Amersham (England) contained a small but measurable quantity of 1°Be. ( 1 ° B e / 7 B e - 10- 5). Such a concentration (verified for several different "batches" at that time), while significant for some studies [35], was estimated to be negligible compared to the natural 1°Be in the sediments we were

107 t e s t i n g . M u c h t o o u r c h a g r i n , h o w e v e r , w e discovered while making AMS measurements in the s e d i m e n t s m e n t i o n e d a b o v e t h a t t h e p r e s e n t 7Be apparently had a l°Be/7Be ratio of -1 a t its p u r c h a s e d a t e o f D e c e m b e r , 1986. After much difficulty we were finally able to determine that between our early experiments and more recently, Amersham had changed the mode o f p r o d u c t i o n o f t h e i r 7Be. I n s t e a d o f u s i n g l o w e n e r g y n u c l e a r r e a c t i o n s o n Li, t h e y w e r e n o w u s i n g VBe e x t r a c t e d f r o m t h e c a r b o n b e a m s t o p a t a high-energy accelerator. This naturally led to the simultaneous production of approximately equal q u a n t i t i e s o f l°Be. W e h a v e b e e n i n f o r m e d (P. M a x i m , p r i v a t e c o m m u n i c a t i o n ) t h a t A m e r s h a m is n o w w i t h d r a w i n g t h i s 7Be f r o m t h e m a r k e t . H o w ever, we strongly advise anyone planning to use 7Be t o d e v e l o p c h e m i c a l p r o c e d u r e s f o r l ° B e m e a surements to first check the level of l°Be in the 7Be b e i n g u s e d .

Acknowledgements W e t h a n k J. L e s t r i n g u e z , D . D e b o f f l e a n d Zhou for help in preparation and measuring p l e s , a n d P. B u a t - M e n a r d f o r p r o v i d i n g t h e m e n t t r a p s a m p l e . T a n d e t r o n o p e r a t i o n is p o r t e d b y t h e C N R S , C E A a n d I N z P 3.

S.Z. samsedisup-

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