10 Counters, Accelerators, and Chemistry
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L. A . CURRIE and G. A . KLOUDA National Bureau of Standards, Center for Analytical Chemistry, Washington, D.C. 20234
Important advances in nuclear dating techniques and microchemical characterization are making major contributions to our ability to extract reliable infor mation from archaeological and environmental samples. The capability of measuring very much smaller and older samples promises major progress in our understanding of both anthropogenic and natural processes because of the tremendous increase in information content which comes about through multidimensional chemical/physical sample characterization and the isotopic analysis of individ ual chemical fractions. Following a brief discussion of the comparative performance of small sample liquid scintillation counting, gas proportional counting and direct atom (accelerator) counting (with respect to precision, sample size and destruction, and non-Poisson error components), we shall illustrate the critical role that serial and parallel chemical information has played in the modeling and interpretation of environ mental radiocarbon data. In our investigation of sources of carbonaceous gases and particles, serial data have included selective sampling (< 10 mg-C samples) followed by the determination of C in specific classes of compounds and particle size frac tions; parallel (multidimensional) data have included isotopic ( C ) , elementary and organic composition. 14
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During the past several years exciting advances have taken place in radioactive dating techniques—advances which have made i t possible to determine unusually small isotope ratios and to determine the radioisotopic composition of individual chemical fractions and particle size fractions of extremely small samples.
This chapter not subject to U.S. copyright. Published 1982 American Chemical Society. Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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N U C L E A R AND
C H E M I C A L DATING T E C H N I Q U E S
The i n c r e a s e d power t o e x t r a c t i n f o r m a t i o n from such samples, through combined microchemical and i s o t o p i c a n a l y s i s , i s enormous; and the a b i l i t y t o work w i t h t i n y samples a l l o w s us t o address numerous problems which were p r e v i o u s l y beyond our reach. These problems extend beyond simple d a t i n g . They i n c l u d e , as noted i n the Keynote Address [ l ] , understanding and c h a r a c t e r i z i n g the environmental system. The importance of t h i s has been manifest i n recent s o c i e t a l concerns r e l a t i n g t o the c o u p l i n g o f energy choices to environmental and c l i m a t i c consequences. One phase of t h i s question has been the focus of the recent work i n our l a b o r a t o r y . That i s , we have been attempting t o l e a r n more about the impact of human a c t i v i t i e s on carbonaceous gases and p a r t i c l e s i n the atmosphere through the use o f r e c e p t o r modeling together w i t h chemical, p h y s i c a l , and i s o t o p i c ( C , C ) c h a r a c t e r i z a t i o n of s e l e c t e d samples. F o l l o w i n g a comparative review o f the charac t e r i s t i c s of three advanced techniques f o r the measurement of radiocarbon i n small samples, we s h a l l summarize some of our recent o b s e r v a t i o n s r e l a t e d t o sources of carbonaceous pollu tants. 2
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THE REVOLUTION IN RADIOCARBON MEASUREMENT: COMPARISON OF METHODS The conventional approach t o radiocarbon d a t i n g u t i l i z e s l a r g e samples (5 g t o 50 g-carbon) which are measured i n gas p r o p o r t i o n a l or l i q u i d s c i n t i l l a t i o n c o u n t e r s , capable o f y i e l d i n g e x c e l l e n t p r e c i s i o n (0.2% to 0.5% RSD) [ 2 ] . Recent develop ments i n both of these l o w - l e v e l counting techniques (gas, l i q u i d ) have made i t f e a s i b l e t o perform r e l i a b l e measurements o f n a t u r a l radiocarbon i n small (10 mg t o 100 mg-carbon) samples [ 3 ] . S t i l l s m a l l e r samples (< 100 pg-carbon) may now be assayed by d i r e c t atom counting w i t h a c y c l o t r o n [4] or tandem a c c e l e r a t o r [ 5 ] . At t h i s p o i n t i n time i t appears t h a t the three small sample t e c h niques are somewhat complementary. D i r e c t atom counting i s i n a much e a r l i e r stage of d e v e l o p m e n t — i n terms of s t a b i l i t y , sample p r e p a r a t i o n , sources of contamination--than the other two, but i t is evolving rapidly. Besides s i g n i f i c a n t d i f f e r e n c e s i n minimum sample s i z e , the a l t e r n a t i v e techniques d i f f e r g r e a t l y i n instrument a v a i l a b i l i t y and c a p i t a l expenditure. (Any attempt a t economic comparison i n t h i s paper would be presumptuous. Although the c a p i t a l c o s t of a s u i t a b l e tandem f a c i l i t y exceeds t h a t of the other two by more than a f a c t o r of t e n , l a b o r c o s t s , shared uses and "market" f o r c e s may tend t o e q u a l i z e charges per sample [ 6 ] . ) P r i n c i p a l c h a r a c t e r i s t i c s of small sample l i q u i d s c i n t i l l a t i o n counting ( I s c ) , gas p r o p o r t i o n a l l o w - l e v e l counting (11c) and atom counting by a c c e l e r a t o r mass spectrometry (AMS) are summa r i z e d i n Table 1, and systems we have used are shown i n f i g u r e 1. The most important d i f f e r e n c e s ( a p a r t from c o s t and a v a i l a b i l i t y ) 2
F i g u r e s i n brackets i n d i c a t e the l i t e r a t u r e references a t the end of t h i s paper.
Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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Figure I. Small sample counting apparatus, (a) Liauid scintillation vial containing 1 mL of benzene; (b) quartz 5-mL gas proportional counter and Su^^rCu^UeU: (0 main acceleration tank of 3-MV Tandem van de Graaff used m atom-counting expe7mentof 40 μ C shown loaded in 0.5-mm cup in 12-sample wheel.(See text for references.)
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N U C L E A R AND
Table 1.
C H E M I C A L DATING T E C H N I Q U E S
A l t e r n a t i v e Methods f o r Small Samples
, 1 non. d e s t r . ( i d e a l form
(C-compound) CeHe
Mass-C
> 100
Counting
mg
serial
Time
1.8
hours
Modern/Background M).5%
