Nuclear and Chemical Dating Techniques - American Chemical Society

4 Techniques for the Direct Measurement of Natural Beryllium-10 and Carbon-14 with a Tandem Accelerator Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 24, 2018 | https://pubs.acs.org Publication Date: January 29, 1982 | doi: 10.1021/bk-1982-0176.ch004

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J. R. SOUTHON , D. E. NELSON, and R. KORTELING Simon Fraser University, Burnaby, B.C., Canada V5A 1S6 I. NOWIKOW, E. H A M M A R E N , J. McKAY, and D. BURKE McMaster University, Hamilton, Ontario, Canada L8S 4K1

This paper is a review and progress report on our project to adapt a Tandem Van de Graaff accelerator for radiocarbon and radioberyllium dating. This project began in 1977 immediately following Muller's proposal [l] to use a cyclotron as a massspectrometer of such high sensitivity that many naturally occur ring radioisotopes could be detected directly. Measurement by direct detection, if practical, would have many advantages over the conventional specific activity measurement techniques. Since the type of measurements required for radioisotope dating are identical to those for stable isotope studies, with the additional requirements of much higher sensitivity, we realized that a Tandem Van de Graaff accelerator offered many practical advantages over the cyclotron for such purposes. In principle, it could be operated as a direct analogue of conventional isotoperatio mass spectrometers. A preliminary experiment [2] showed that a Tandem could provide the necessary sensitivity, and we then decided to modify this accelerator for radiocarbon and radioberyl lium dating. This modification was to proceed in stages because of funding and manpower restraints, and because we would then have an opportunity to test each idea in turn and not become locked into an unworkable scheme. At the same time, another group of researchers [3] (from the Universities of Rochester and Toronto, and the General Ionex Cor poration) working quite independently and unknown to us, also showed that a Tandem accelerator offered many advantages for direct detection radioisotope dating. Since those i n i t i a l experi ments, many groups have undertaken similar work, and the f i r s t commercial systems should appear within the next year. The measurement approach we have proposed [2,4] differs in detail from that proposed by the Rockester-Toronto-General Ionex group. Subsequently, most other researchers have adopted this latter technique. In the following sections, we discuss the 1

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Figures in brackets indicate the literature references at the end of this paper. Current address: McMaster University, Hamilton, Ontario, Canada L8I 4K1. 0097-6156/82/0176-0075$05.00/0 © 1982 American Chemical Society Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

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advantages and the disadvantages of each of these techniques, the r e s u l t s we have obtained on our own system, and our plans f o r f u r t h e r development.

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 24, 2018 | https://pubs.acs.org Publication Date: January 29, 1982 | doi: 10.1021/bk-1982-0176.ch004

The Measurement Technique In g e n e r a l , the Tandem systems a t t a i n high measurement s e n s i t i v i t y by p e r m i t t i n g the i d e n t i f i c a t i o n and counting of r a r e isotopes i n a sea of other p a r t i c l e s . This i s done by i o n i z i n g a sample, a c c e l e r a t i n g a beam of the r a r e isotope of i n t e r e s t t o an energy i n the region 10-40 MeV, s o r t i n g the ions w i t h an a n a l y z i n g magnet and then using s p e c i a l i z e d d e t e c t o r s t o d i s c r i m i n a t e between any remaining contaminant ions and the i o n of i n t e r e s t . The primary requirement i s t h a t the numbers of any contaminating ions do not exceed the count-rate c a p a b i l i t i e s of the d e t e c t o r . In a Tandem system, molecular ions are a u t o m a t i c a l l y d i s s o c i a t e d i n the a c c e l e r a t i n g process, and i s o b a r i c contaminants can be e l i m i n a t e d or reduced i n i n t e n s i t y by t h e i r i n s t a b i l i t y as nega t i v e ions (e.g., N ) , by the use of absorbers i n f r o n t of the d e t e c t o r s , or by other techniques. The r a r e isotopes can then be detected w i t h very high s e n s i t i v i t y . Our system d i f f e r s from others i n the method by which we propose to measure the r e l a t i v e c o n c e n t r a t i o n s of the i s o t o p e s . In our system, we i o n i z e , a c c e l e r a t e and d e t e c t beams of the s t a b l e and the r a r e isotopes simultaneously. In other systems, the isotopes are s e q u e n t i a l l y a c c e l e r a t e d and detected. The r a t i o n a l e u n d e r l y i n g our choice i s as f o l l o w s : In radiocarbon d a t i n g , the q u a n t i t y to be measured i s the r a t i o of the abundances of the r a r e i s o t o p e ( C ) t o t h a t of the s t a b l e isotopes ( C , C). These abundance r a t i o s are not measured on an absolute b a s i s , but are compared to t h a t of an i n t e r n a t i o n a l l y - a c c e p t e d standard. ( I t i s l i k e l y that a similar standard w i l l be adopted f o r B e dating.) These measurement requirements have s e v e r a l consequences: 1) i t i s not necessary t h a t the absolute e f f i c i e n c y of the measurement system be known, s i n c e a l l measurements of unknowns are referenced t o a measurement f o r the standard taken under i d e n t i c a l measurement c o n d i t i o n s 2) the absolute e f f i c i e n c y need not remain s t a b l e i n time 3) the e f f i c i e n c y f o r the r a r e i s o t o p e need not be i d e n t i c a l to t h a t f o r the s t a b l e i s o t o p e 4) the one requirement i s t h a t the measurement e f f i c i e n c y f o r the r a r e isotope r e l a t i v e t o t h a t f o r the s t a b l e isotope remain s t a b l e over times long compared t o the time r e q u i r e d f o r measure ments of both sample and standard. Systems w i t h these c a p a b i l i t i e s w i l l s a t i s f y the requirements for radioisotope dating. I f absolute i s o t o p i c abundances are r e q u i r e d f o r other work i t w i l l be necessary t o e i t h e r use a standard of a c c u r a t e l y known c o n c e n t r a t i o n s or t o determine the e f f i c i e n c i e s of the system. 1 4

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The requirements o u t l i n e d above a r e i d e n t i c a l t o those r e q u i r e d f o r measurements o f s t a b l e i s o t o p e s , such as C / C o r 0/ 0. In t h e conventional mass spectrometers developed f o r these measurements, very high measurement accuracy (< .1 /oo) i s achieved by i o n i z i n g , a c c e l e r a t i n g , a n a l y z i n g , and d e t e c t i n g the two isotopes o f i n t e r e s t simultaneously. These devices a r e designed such t h a t any changes i n e f f i c i e n c y (such as beami n t e n s i t y from t h e ion-source) w i l l a f f e c t both i s o t o p e s e q u a l l y and thus leave t h e i s o t o p e r a t i o unchanged. The sample and t h e standard are a l t e r n a t e l y measured a t s h o r t i n t e r v a l s . Our proposed Tandem system w i l l a l s o produce, a c c e l e r a t e , and analyze the beams o f the d i f f e r e n t isotopes simultaneously. This has the advantage t h a t the i n s t a b i l i t i e s t h a t are common t o Tandem a c c e l e r a t o r s w i l l a f f e c t t h e beams e q u a l l y and leave t h e r a t i o s constant. Any changes t h a t do occur w i l l be made r e a d i l y apparent by monitoring t h e beam c u r r e n t o f t h e s t a b l e i s o t o p e . (These changes would be more d i f f i c u l t t o d e t e c t by monitoring the countr a t e f o r the r a r e i s o t o p e , as they would be p e r c e i v e d as a small change i n an a l r e a d y - s m a l l r a t e . ) Since some changes i n these a c c e l e r a t o r s take p l a c e i n times o f the orders o f seconds o r l e s s , t h i s w i l l be an important advantage. F u r t h e r , f o r radiocarbon d a t i n g , a measure o f the constancy o f the r e l a t i v e i s o t o p i c e f f i c i e n c i e s f o r C and C can be made by c o n t i n u o u s l y monitoring the C / C r a t i o as t h e C / C measurement progresses. I f s u f f i c i e n t measurement accuracy can be obtained, t h i s measurement w i l l a l s o a l l o w the date t o be c o r r e c t e d f o r i s o t o p i c f r a c t i o n a t i o n processes. A t e c h n i c a l advantage i s t h a t t h e continuous s t a b l e i s o t o p e beam can be used t o c o n t r o l t h e e x i s t i n g a c c e l e r a t o r v o l t a g e s t a b i l i z a t i o n system. T h i s operates by sensing the p o s i t i o n o f t h e beam on a p a i r o f s l i t s downstream o f t h e a n a l y z i n g magnet, and feeding back an e r r o r s i g n a l i f the v o l t a g e changes and the p o s i t i o n o f t h i s beam s h i f t s . The major d i f f i c u l t y t o be expected f o r t h i s mode o f opera t i o n i s t h a t ions from t h e s t a b l e beams c o u l d , by a v a r i e t y o f circumstances, be s c a t t e r e d i n t o the beam o f the r a r e i s o t o p e . I f these cannot be e l i m i n a t e d , t h e s e n s i t i v i t y o f the technique may be l o s t . The a l t e r n a t i v e measurement technique [ 5 ] i s t o s e l e c t t h e ions o f i n t e r e s t from t h e i o n source and s e q u e n t i a l l y i n j e c t these i n t o t h e a c c e l e r a t o r . As only one beam i s i n the system a t any given time, there i s no p o s s i b i l i t y o f inter-beam i n t e r f e r ences. With such a system, i t w i l l be necessary t o ensure t h a t the c y c l e time between beams i s s h o r t compared t o t h e time-span over which any e f f i c i e n c y changes c o u l d occur. As w e l l , t h e normal method f o r s t a b i l i z i n g the a c c e l e r a t o r w i l l not be a v a i l a b l e , and other methods must be developed. Whatever system i s used, i t may prove v a l u a b l e i n some circumstances t o be able t o reduce t h e beam i n t e n s i t i e s o f t h e most abundant i s o t o p e t o reduce a c c e l e r a t o r l o a d i n g . T h i s can i n p r i n c i p l e be e a s i l y accomplished w i t h s e q u e n t i a l i n j e c t i o n by 1 3


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d e f i n i n g the length and r e p e t i t i o n r a t e of the abundant isotope to give the average beam c u r r e n t d e s i r e d . However, the e f f e c t of i n t r o d u c i n g s h o r t , sharp pulses of beam i n t o the a c c e l e r a t o r may i n p r a c t i c e cause s t a b i l i z a t i o n d i f f i c u l t y . For multi-beam detec t i o n , proposed methods f o r reducing the s t a b l e isotope beam i n t e n s i t i e s are discussed i n a f o l l o w i n g s e c t i o n . Our p r o j e c t to t e s t a Tandem a c c e l e r a t o r f o r multi-beam d e t e c t i o n was then to modify the a c c e l e r a t o r a n a l y z i n g magnet such t h a t the v a r i o u s beams c o u l d be dispersed and detected s i m u l t a neously, t o design an i n j e c t i o n system t o a p p r o p r i a t e l y process the d i f f e r e n t i s o t o p i c beams from the i o n source f o r i n j e c t i o n i n t o the a c c e l e r a t o r , and t o c o n s t r u c t an ion-source t o a l l o w us to r o u t i n e l y handle small samples. Our i n t e r e s t s a t present l i e i n radiocarbon and r a d i o b e r y l l i u m s t u d i e s , and so we have designed t h i s system to accommodate these isotopes. Present System An o v e r a l l view of our present system i s shown i n f i g u r e 1. Negative ions are sputtered from a small sample i n a Cs s p u t t e r ion source, e x t r a c t e d by a p o t e n t i a l of 25-30 kV, i n j e c t e d i n t o the a c c e l e r a t o r v i a a 20 degree i n f l e c t i o n magnet, and a c c e l e r a t e d t o the high v o l t a g e t e r m i n a l . The ions are s t r i p p e d of some of t h e i r e l e c t r o n s (and molecules are broken up) i n a gas and/or f o i l s t r i p p e r a t the t e r m i n a l ; the r e s u l t i n g p o s i t i v e ions are a c c e l e r ated f u r t h e r . A 90 degree a n a l y z i n g magnet s e l e c t s ions of the appropriate momentum-to-charge r a t i o and passes the stable isotopes t o Faraday cups and the r a r e isotope t o a s e t of e l e c t r o s t a t i c d e f l e c t o r s . These d e f l e c t o r s c a r r y out f u r t h e r f i l t e r i n g of the 'rare' beam according t o the energy-to-charge r a t i o of the ions. The ions passing t h i s f i l t e r are counted and t h e i r masses and atomic numbers are i d e n t i f i e d by a ΔΕ-Ε semi-conductor d e t e c t o r telescope. These d e t e c t o r s can handle counting r a t e s of ~30,000 p a r t i c l e s / s e c o n d . The Faraday cups can be moved t o i n t e r c e p t any beam of i n t e r e s t e x i t i n g the magnet. Our present technique f o r i n j e c t i n g several isotopes s i m u l t a neously c o n s i s t s of simply passing them a l l through the 20 degree i n f l e c t i o n magnet a t the low-energy end of the a c c e l e r a t o r . Although the d i f f e r e n t isotopes are bent d i f f e r e n t l y , a l l the emerging beams of i n t e r e s t overlap the acceptance phase space of the a c c e l e r a t o r t o some extent. This method i s f a r from i d e a l , as the beams are t r e a t e d very d i f f e r e n t l y . However, i t was a simple and easy technique t o use f o r i n i t i a l t e s t s . A b e t t e r i n j e c t i o n system, discussed below, i s planned but not y e t constructed. Results t o Date 14

Although we o r i g i n a l l y began our t e s t s t u d i e s on C , our work i s now p r i m a r i l y centered around r a d i o b e r y l l i u m measurements, f o r several reasons. F i r s t , the concentrations of B e i n n a t u r a l 10


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samples are s e v e r a l orders of magnitude higher than f o r C , which makes the experiments e a s i e r to perform. Second, l i t t l e i s known about B e i n the environment, and even low-accuracy (

Nuclear and Chemical Dating Techniques - American Chemical Society

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