7 Krypton-81-Krypton Dating by Mass Spectrometry KURT MARTI
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University of California—San Diego, Chemistry Department, B-017, La Jolla, C A 92093
81
The Kr-Kr dating method is reviewed, the progress and experimental approaches are outlined, and some current applications and prospects are discussed.
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The discovery of cosmic ray produced K r in meteorites [I] introduced a new method of high sensitivity measurements of K r concentrations and cosmic ray exposure dating. The method consists of a direct measurement of both radioactive K r atoms (T = 2.13x10 y, [2] and of stable spallation Kr atoms by a mass spectrometer and the calculation of cosmic ray exposure ages from measured Kr isotope ratios. The method is analagous to the K- K method developed by Voshage [3] but provides much higher sensitivity, since static noble gas mass spectrometry is employed. Early measurements were carried out on meteorite samples corre sponding to 10 -10 atoms of K r [1,4]. The technique proved to be extremely useful in computing exposure ages of lunar samples returned by the Apollo program and i t allowed the precise dating of lunar craters. Over the years, the sensitivity of the method has been improved and background interferences were reduced, and at present i t is possible to measure samples corresponding to 10 atoms K r . On the other hand, attempts to detect K r by accel erator based high-energy mass spectrometry so far have not been successful. Progress has also been made in the understanding and calibration of cosmic ray induced reactions with the major target elements Sr, Y, and Zr in natural solid samples, and reasonably good agreement was obtained between predicted and observed pro duction rates for the Kr isotopes in lunar samples [5]. Kr measurements and Kr-Ar ages provided useful tools in studies of 8 1
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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 r e f e r e n c e s a t the end of t h i s paper.
0097-6156/82/0176-0129$05.00/0 © 1982 American Chemical Society Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
130
N U C L E A R A N D C H E M I C A L DATING T E C H N I Q U E S
the cosmic ray exposure h i s t o r i e s o f c h r o n d r i t i c m e t e o r i t e s and e l u c i d a t i n g the long-term average f l u x e s o f s o l a r and g a l a c t i c cosmic rays [6,7]. S p a l l a t i o n Systematics In n a t u r a l s i l i c a t e samples, cosmic ray s p a l l a t i o n Kr i s produced predominantly by high-energy r e a c t i o n s on S r , Y, and Zr. Low-energy r e a c t i o n s on Rb o f t e n make c o n t r i b u t i o n s mainly t o the heavier Kr i s o t o p e s , and slow neutrons w i t h energies below 0.5 MeV produce K r and K r by neutron capture r e a c t i o n s on Br. Because of the low cosmic ray f l u x a t the s o l i d s u r f a c e o f the e a r t h , s t u d i e s so f a r have been r e s t r i c t e d t o e x t r a t e r r e s t r i a l samples. The e x c i t a t i o n f u n c t i o n s f o r the p r o d u c t i o n o f s p a l l a t i o n Kr on the above l i s t e d t a r g e t elements were s t u d i e d by Régnier e t a l . , [ 5 ] . There i s a s y s t e m a t i c v a r i a t i o n o f the absolute and a l s o o f the r e l a t i v e p r o d u c t i o n r a t e s o f the Kr isotopes w i t h i n c r e a s i n g degree o f s h i e l d i n g due t o the c o n t r i b u t i o n by r e a c t i o n s o f secondary cosmic ray p a r t i c l e s . In the K r - K r d a t i n g method, which i s d i s c u s s e d i n the next paragraph, the p r o d u c t i o n r a t i o o f r a d i o a c t i v e and s t a b l e Kr isotopes can be e v a l u a t e d d i r e c t l y from the Kr s p a l l a t i o n spectrum i n a m e t e o r i t e a c c o r d i n g t o the r e l a t i o n [ 1 ] :
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8 0
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P
8 1
/P
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+ 2|93
[ (
80
K r
+
82
K r ) /
83
K r ]
( 1 )
here the f a c t o r 0.95 ± 0.05 i s the estimated i s o b a r i c f r a c t i o n yield. Régnier [ 8 ] and N i s h i i z u m i e t a l . , [ 6 ] s t u d i e d i n d e t a i l measured P versus P ( c a l c ) , as used i n equation ( 1 ) , and found t h a t the f a c t o r 0.95 i s a p p r o p r i a t e f o r a v a r i e t y of i n c i d e n t proton energies and chemical compositions, s p e c i f i c a l l y a l s o f o r c h o n d r i t i c m e t e o r i t e s . I t appears t h a t the p r o d u c t i o n ratio P i / P i s w e l l approximated by the i n t e r p o l a t i o n used i n equation ( 1 ) . However, i f s p e c i f i c neutron e f f e c t s , due t o neutron capture i n B r , are present on the s t a b l e i s o t o p e s K r and K r , equation (1) can no longer be used and a l t e r n a t i v e treatments are r e q u i r e d , such as i n t e r p o l a t i o n s between K r and K r [9,10]. Such a l t e r n a t i v e s , however, depend on the chemical composition o f a sample and need t o be a d j u s t e d a c c o r d i n g l y . 8 1
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The
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K r - K r Method
The method assumes t h a t the average cosmic r a y f l u x has been constant over the m e a n - l i f e o f K r (τ ι + 3.07 χ 1 0 y ) and over the o v e r a l l exposure time o f the sample cosmic rays. Furthermore, i t i s assumed t h a t the exposure geometry has remained f i x e d over the p e r i o d o f i r r a d i a t i o n , but no other assumptions are made regarding s h i e l d i n g . 8 1
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Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
7.
MARTI
81
Kr-Kr
Dating by Mass Spectrometry
Assuming constant
production
rates P
131 f o r t h e Kr i s o t o p e s ,
M
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the c o n c e n t r a t i o n of [ K r ] as a f u n c t i o n of time ( t ) i s given by [
8 1
Kr]
t
= (P
8 1
A
λ
8 1
) (1-e " 8 1 * ) ,
and the s p a l l a t i o n y i e l d o f a s t a b l e i s o t o p e , e.g.,
(2) 8 3
K r , by
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Therefore, a t the time of f a l l , we expect a r a t i o
e l-exp(-X T ) 8 1
e
and o b t a i n
FÎT
)
-
—
!
*
—
- - L
" l-expi-Xj,!,)
For
cosmic r a y exposure i n t e r v a l s Τ
»
(ϋϊΐ.
ÎS1
Ag, P
8 3
\81
K r
/
(4)
τ
5
τ ^ - = 3.07 χ 1 0 y r , t h e 81
exponential F(T )*T , e
term becomes n e g l i g i b l e and equation
( 4 ) reduces t o
e
(5)
c a n
i n
As discussed e a r l i e r , t h e production r a t i o Pgi/Psa > g e n e r a l , r e l i a b l y be obtained from equation ( 1 ) and, t h e r e f o r e , the K r - K r method avoids much o f t h e u n c e r t a i n t y a r i s i n g from unknown production r a t e s o r production r a t i o s . The method d e r i v e s exposure ages from Kr i s o t o p i c r a t i o s as obtained from mass spectrometry and does not r e q u i r e a knowledge o f the c o n c e n t r a t i o n of spallation K r , which makes t h e method i n h e r e n t l y more precise. 8 1
Mass Spectrometry Kr i s e x t r a c t e d e i t h e r by stepwise heating o r m e l t i n g o f a sample f o l l o w e d by u s u a l l y several gas clean-up steps and by s e p a r a t i o n o f Kr from other noble gases by s e l e c t i v e a d s o r p t i o n on charcoal a t cryogenic temperatures [11]. Kr i s analysed i n u l t r a - c l e a n s t a t i c mass spectrometers which a l l o w s r e c y c l i n g o f
Currie; Nuclear and Chemical Dating Techniques ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
132
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
non-implanted Kr atoms through the i o n source. A clean s e p a r a t i o n from abundant A r i s g e n e r a l l y r e q u i r e d , s i n c e i n t e r f e r e n c e s from 2+ + charge exchange (Ar + Ar ) between ion source and magnetic analyser may a f f e c t the Kr isotope a t mass 80 i n some mass spectrometers. I n t e r f e r e n c e s due t o i s o b a r i c background can be s i g n i f i c a n t l y reduced by rigorous bake-out techniques. Neverthe l e s s , the remaining i n t e r f e r e n c e s may be s i g n i f i c a n t , i f Kr amounts e q u i v a l e n t t o
