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POLARIZATION OF STABLE AND RADIOACTIVE NOBLE GAS NUCLEI BY SPIN EXCHANGE WITH
LASER PUMPED ALKALI ATOMS
W. Happer
To cite this version:
W. Happer. POLARIZATION OF STABLE AND RADIOACTIVE NOBLE GAS NUCLEI BY SPIN EXCHANGE WITH LASER PUMPED ALKALI ATOMS. Journal de Physique Colloques, 1985, 46 (C1), pp.C1-261-C1-267. �10.1051/jphyscol:1985125�. �jpa-00224498�
JOURNAL DE PHYSIQUE
Colloque C1, supplbment a u n O l , Tome 46, janvier 1985 page C 1-26 1
P O L A R I Z A T I O N OF STABLE AND R A D I O A C T I V E NOBLE GAS N U C L E I BY S P I N EXCHANGE WITH LASER PUMPED A L K A L I ATOMS
W. Happer
Department o f Physics, Princeton University, Princeton, NJ 08544, U.S.A.
Resume - Les noyaux d e s gaz r a r e s peuvent E t r e fortement p o l a r i s e s p a r dchan- ge de s p i n avec des vapeurs a l c a l i n e s suffisamment denses e t p o l a r i s k e s p a r pompage o p t i q u e . Seule une f a i b l e f r a c t i o n du moment a n g u l a i r e de s p i n des atomes a l c a l i n s e s t t r a n s f e r e e v e r s l e s p i n n u c l e a i r e des atomes de gaz r a r e . La p l u s grande p a r t i e de c e moment d i s p a r a i t dans l e mouvement r e l a t i f de l ' a t o m e a l c a l i n p a r r a p p o r t 2 l ' a t o m e d e gaz r a r e . Pour l e s gaz r a r e s lourds, l a p l u s grande p a r t i e du t r a n s f e r t de moment a n g u l a i r e a l i e u dans l a mole- c u l e de Van d e r Waals formee p a r l e couple a l c a l i n - g a z r a r e . L ' e f f i c a c i t b de ce t r a n s f e r t e s t determinee p a r l e taux d e formation e t de d e s t r u c t i o n d e s mol6cules de Van d e r Waals dans l e gaz ambiant. Nous avons dbveloppe des mgthodes e x p e r i m e n t a l e s de mesure de l V e f f i c a c i t 6 du t r a n s f e r t du s p i n . Des noyaux de gaz r a r e s r a d i o a c t i f s o n t 6 t b p o l a r i s e s p a r c e s mgthodes e t l a p o l a r i s a t i o n a 6tB d e t e c t e e g r l c e 2 l l a n i s o t r o p i e d e s ~ r o d u i t s de l a d6sin- t e g r a t i o n r a d i o a c t i v e . Nous en avons d k d u i t d e s v a l e u r s t r e s p r e c i s e s des moments magnetiques d e s noyaux r a d i o a c t i f s .
A b s t r a c t - The n u c l e i of noble g a s e s can be s t r o n g l y p o l a r i z e d by s p i n ex- change w i t h s u f f i c i e n t l y dense o p t i c a l l y pumped a l k a l i vapors. Only a small f r a c t i o n of t h e s p i n a n g u l a r momentum of t h e a l k a l i atoms i s t r a n s f e r r e d t o t h e n u l c e a r s p i n of t h e noble gas. Most of t h e s p i n a n g u l a r momentum i s l o s t t o t r a n s l a t i o n a l a n g u l a r momentum of t h e a l k a l i and noble g a s atoms about each other. For heavy noble gases most of t h e angular momentum t r a n s f e r occurs i n alkali-noble-gas van d e r Waals molecules. The t r a n s f e r e f f i c i e n c y depends on t h e formation and breakup r a t e s of t h e van d e r Waals molecules i n t h e ambient gas. Experimental methods t o measure t h e s p i n t r a n s f e r e f f i c i e n - c i e s have been developed. Nuclei of r a d i o a c t i v e noble g a s e s have been polar- i z e d by t h e s e methods,-and t h e p o l a r i z a t i o n has been d e t e c t e d by o b s e r v i n g t h e a n i s o t r o p y of t h e r a d i o a c t i v e decay products. Very p r e c i s e measurements of t h e magnetic moments of t h e r a d i o a c t i v e n u c l e i have been made.
I n 1960 Bouchiat Carver and Varnuml showed t h a t it w a s p o s s i b l e t o t r a n s f e r s p i n a n g u l a r momentum from o p t i c a l l y pumped Rb vapor t o t h e n u c l e i of 3 ~ e b u f f e r gas by c o l l i s i o n s . Unfortunately, t h e t r a n s f e r r a t e s were very slow and many hours were r e q u i r e d t o b u i l d up d e t e c t a b l e n u c l e a r s p i n p o l a r i z a t i o n i n 3 ~ e . Some 18 y e a r s l a t e r rover^ r e p o r t e d t h a t t h e s p i n t r a n s f e r r a t e s from o p t i c a l l y pumped Rb vapor t o t h e n u c l e i of t h e heavy noble g a s e s 1 2 9 ~ e and 8 3 ~ r were many o r d e r s of magni- t u d e f a s t e r , and s u b s t a n t i a l n u c l e a r s p i n p o l a r i z a t i o n could be b u i l t up i n a few minutes of o p t i c a l pumping i n f a v o r a b l e cases.
Once t h e noble g a s nucleus i s s p i n p o l a r i z e d it can r e t a i n i t s p o l a r i z a t i o n f o r a v e r y long t i m e , hours o r even days i n c e l l s w i t h p r o p e r l y t r e a t e d walls. Such slowly r e l a x i n g s p i n s can be used t o make very p r e c i s e measurements of s m a l l terms i n t h e s p i n Hamiltonian of t h e noble gas. For example, i f t h e r e l a x a t i o n time i s one hour and t h e s i g n a l t o n o i s e r a t i o i s 100, one can measure t h e resonance f r e - quency t o a s t a t i s t i c a l u n c e r t a i n t y of r 1 x ( 1 h r ) - I x ( l o o ) - ' = 1 v Hz. Thus, one can e a s i l y sense t h e r o t a t i o n r a t e , 11.6 Hz, of t h e e a r t h , and s e r i o u s e f f o r t s have been made t o develop commercial gyroscopes based on s p i n p o l a r i z e d noble gas
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985125
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n u c l e i . Small upper bounds on t h e e l e c t r i c d i p o l e moment of t h e 12'xe nucleus have been s e t by looking f o r some i n f l u e n c e of an e x t e r n a l e l e c t r i c f i e l d on t h e n u c l e a r magnetic resonce frequency.4 There a r e o t h e r i n t e r e s t i n g uses of s p i n p o l a r i z e d noble gases i n n u c l e a r p h y s i c s a s t a r g e t s f o r s c a t t e r i n g experiments and a s h i g h l y p o l a r i z e d sources of r a d i o a c t i v e atoms. I n t e r e s t i n g s t u d i e s of s u r f a c e i n t e r a c t i o n s a r e p o s s i b l e . It would probably be p o s s i b l e t o l i q u i f y o r f r e e z e some of t h e heavy noble gases without much l o s s of p o l a r i z a t i o n , and very i n t e r - e s t i n g s p i n i n t e r a c t i o n s could be expected i n t h e s e h i g h l y p o l a r i z e d , condensed n u c l e a r s p i n systems.
An important reason f o r t h e l a r g e enhancement of t h e s p i n t r a n s f e r r a t e s between o p t i c a l l y pumped a l k a l i atoms and heavy noble g a s e s i s t h e c o n t r i b u t i o n of l o o s e l y bound alkali-noble-gas van d e r Waals molecules t o t h e s p i n r e l a x a t i o n , a phenome- non f i r s t discovered by Aymar, Bouchiat and ~ r o s s e l . ~ The n a t u r e of t h e i n t e r a c - t i o n i s i n d i c a t e d i n Fig. 1
Fig. 1 - Alkali-noble g a s van d e r Waals molecules a r e formed i n three-body c o l l i s i o n s a t a r a t e of T ~ p e r a l k a l i - ~ atom and TK-I p e r noble gas atom. They a r e broken up a t a r a t e T-I by c o l l i s i o n s with o t h e r atoms o r molecules.
Angular momentum can b e t r a n s f e r r e d from t h e a l k a l i e l e c t r o n s p i n S t o t h e n u c l e a r s p i n K of t h e noble g a s atom during t h e r e l a t i v e l y long molecular l i f e t i m e T.
: X e ;
'-->
1 I
>N~(
I '.-.' I
To i n v e s t i g a t e t h e p h y s i c s of t h e s p i n t r a n s f e r i n van d e r Waals molecules we have made use of t h e a p p a r a t u s 7 sketched i n Fig. 2.
CHART RECORDER LOCK-IN
7 I
U) REFERENCE ( 4 2 KHz) I
Fig. 2 - The a p p a r a t u s used t o s t u d y s p i n t r a n s f e r between a l k a l i atoms and noble gas molecules. CP i s a c i r c u l a r p o l a r i z e r ; P i s a l i n e a r p o l a r i z e r used i n con- j u n c t i o n with t h e p h o t o e l a s t i c modulator t o d e t e c t c i r c u l a r l y p o l a r i z e d l i g h t ; 4
i s a compensator p l a t e t o n u l l o u t s t r a y c i r c u l a r p o l a r i z a t i o n during t h e probe phase.
For s y s t e m a t i c s t u d i e s of s p i n t r a n s f e r t o 1 2 9 ~ e we p r e f e r not t o use a l a s e r , b u t i n s t e a d t o u s e a t r a d i t i o n a l , q u i e t and inexpensive e l e c t r o d e l e s s d i s c h a r g e lamp.8 Laser pumping i s e s s e n t i a l f o r s t u d i e s of o t h e r noble gas i s o t o p e s and e s p e c i a l l y f o r s t u d i e s of r a d i o a c t i v e i s o t o p e s . Each c i r c u l a r l y p o l a r i z e d D l photon absorbed by t h e vapor d e p o s i t s approximately 9 u n i t s of a n g u l a r momentum i n t h e a l k l a i atoms. A c e r t a i n f r a c t i o n of t h i s a n g u l a r momentum i s t r a n s f e r r e d t o t h e n u c l e i of t h e noble g a s e s , mainly during t h e l i f e t i m e of van d e r Waals molecules sketched i n Fig. 1. The l a r g e s t p a r t of t h e a n g u l a r momentum i s l o s t t o 3 , t h e r o t a t i o n a l a n g u l a r momentum of an a l k a l i - n o b l e gas p a i r about each o t h e r . The s p i n t r a n s f e r r a t e s a r e s o slow t h a t we o r d i n a r i l y wait f o r 10 o r 15 minutes f o r e q u i l i b r i u m n u c l e a r p o l a r i z a t i o n t o be e s t a b l i s h e d i n t h e noble gases. Once t h e s p i n s a r e p o l a r i z e d we remove t h e c i r c u l a r p o l a r i z e r from t h e i n p u t o p t i c s of Fig. 2 s o t h a t u n p o l a r i z e d D l l i g h t e n t e r s t h e c e l l . During t h i s probe phase, s p i n a n g u l a r momentum flows back from t h e noble g a s n u c l e i t o t h e e l e c t r o n s p i n s of t h e Rb atoms and a small amount of s p i n p o l a r i z a t i o n i s main- t a i n e d i n t h e a l k a l i vapor. This a l k a l i p o l a r i z a t i o n i n t u r n induces a weak c i r c u l a r p o l a r i z a t i o n of t h e probe l i g h t which i s d e t e c t e d with t h e p h o t o e l a s t i c modulator. To e l i m i n a t e t h e e f f e c t s of slow d r i f t s i n t h e e l e c t r o n i c a m p l i f i e r s and i n t h e thermally-induced p o l a r i z i n g p r o p e r t i e s of t h e o p t i c a l elements we p e r i o d i c a l l y i n v e r t t h e noble g a s p o l a r i z a t i o n w i t h a chirpped audiofrequency pulse. R e p r e s e n t a t i v e s i g n a l s a r e shown i n Fig. 3.
Fig. 3 - Decaying 1 2 9 ~ e n u c l e a r s p i n p o l a r i z a t i o n observed i n K , Rb and C s ;;
vapor. The n u c l e a r p o l a r i z a t i o n i s 5
i n v e r t e d p e r i o d i c a l l y t o e l i m i n a t e a
t h e e f f e c t of slow d r i f t s i n t h e - e u
r e c o r d i n g system. By u s i n g d i f f e r e n t i n t e r v a l s between i n v e r s i o n s t h e 0 k s l i g h t s p i n d e s t r u c t i o n p e r i n v e r s i o n &' (7 1 % ) can be accounted f o r . 0 -a J
e
W
u'
Z
X
"d
Decay t r a n s i e n t s such a s t h o s e of Fig. 3 can be s t u d i e d a s a f u n c t i o n of t h e temperature of t h e sample c e l l . We f i n d t h a t t h e n u c l e a r s p i n r e l a x a t i o n r a t e i s a l i n e a r f u n c t i o n of t h e a l k a l i number d e n s i t y a s i n f e r r e d from s a t u r a t e d vapor
p r e s s u r e curves. R e p r e s e n t a t i v e d a t a i s shown i n ,
I_il
Fig. 4.Fig. 4 - Spin r e l a x a t i o n of 1 2 9 ~ e a s a func- 13 - - - C E L L # 3 3 luncootedl
t i o n of a l k a l i number d e n s i t y . The wall-
coated c e l l contained 14.9 Torr of N2 and t h e ,,
uncoated c e l l contained 21 Torr of N2. Both ,,
c e l l s contained 0.5 Torr of Xe, i s o t o p i c a l l y 1
e n r i c h e d t o 63% l2'xe. 9 /
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The e x t r a p o l a t e d r a t e a t z e r o a l k a l i d e n s i t y i s due t o s p i n i n t e r a c t i o n s on t h e c e l l w a l l s . This w a l l r e l a x a t i o n can be s i g n i f i c a n t l y suppressed w i t h s i l i c o n e c o a t i n g s . l o
The measured r e l a x a t i o n r a t e s depend on t h e third-body p r e s s u r e a s one would expect from t h e p h y s i c a l p i c t u r e sketched i n Fig. 1. A t low third-body p r e s s u r e s t h e r a t e s a r e slow because t h e molecules form t o o slowly. A t high third-body p r e s s u r e s t h e r a t e s a r e a g a i n slow because t h e molecules a r e broken up t o o quick- l y . An example of t h e dependence of t h e r a t e s on t h i r d body p r e s s u r e i s shown i n Fig. 5.
5 - Measured s p i n r e l a x a t i o n r a t e s of e i n v a r i o u s a l k a l i vapors a s a f u n c t i o n of n i t r o g e n p r e s s u r e . A l l r a t e s a r e given f o r an a l k a l i number d e n s i t y of 1012 atoms/cm2 a s i n f e r r e d from s a t u r a t e d vapor
p r e s s u r e curves. 9
,
-::
:oLoo
NITROGEN PRESSURE ( t o r r )The r e l a x a t i o n r a t e a l s o depends s t r o n g l y on t h e magnitude of t h e e x t e r n a l magne- t i c f i e l d i n which t h e sample c e l l i s l o c a t e d . An example i s shown i n Fig. 6 .
Fig. 6 - Slowing down of t h e r e l a x a t i o n r a t e of 12'xe i n 8 7 ~ b vapor i n an e x t e r n a l magne- t i c f i e l d H. The r a t e s a r e normalized t o t h e i r low-field values.
$ Magnetic Field H (Gauss) Z
The slowing down of t h e n u c l e a r s p i n r e l a x a t i o n occurs f o r t h e same reason a s t h e slowing down of t h e e l e c t r o n i c s p i n r e l a x a t i o n of Rb i n v a r i o u s b u f f e r gases f i r s t observed and explained by Bouchiat e t a l . l 1 The magnetic f i e l d decozples t h e e l e c t r o n i c s p i n 5 of t h e Rb atom from t h e r o t a t i o n a l a n g u l a r momentum N i n t h e long-lived van d e r Waals molecules.
The s i m p l e s t s p i n Hamiltonian which i s c o n s i s t e n t w i t h p r e s e n t l y known d a t a on s p i n r e l a x a t i o n and s p i n t r a n s f e r i n mixtures of a l k a l i vapors and noble g a s e s i s
The p h y s i c a l s i g n i f i c a n c e of t h e v a r i o u s a n g u l a r momentum v e c t o r s of ( 1 ) is i l l u s t r a t e d i n Fig. 7 .
A
Fig. 7 - A v e c t o r model of t h e s p i n coupling B of Eq. 1. The s p i n r o t a t i o n frequency i s wl
= yN/h and t h e e l e c t r o n Larmer frequency i n t h e e x t e r n a l f i e l d i s w,,. The Breit-Rabi parameter of Eq. 1 i s x = $. The s t a t i s - t i c a l weight of t h e a l k a l i nucleus is [ I ] = 21+1.
+
The e l e c t r o n i c s p i n S of t h e a l k a l i atom i n t h e molecule i s coupled t o t h e
9 + n u c l e a r s p i n I of t h e a l k a l i atom by t h e magnetic d i p o l e i n t e r a c i t o n AI-S. The+
e l e c t r o n s p i n of t h e a l k a l i i s a l s o coupled tz :he r o t a t i o n a l a n g u l a r mymenturn N of t h e molecule by s p i n - r o t a t o n i n t e r a c t i o n yN*S. The n u c l e a r s p i n of K of t h e noble g a s atom i s cou$l$d t o t h e e l e c t r o n s p i n of t h e +alkali by t h e magnetic d i p o l e i n t e ~ a c q i o n aAK*S. An e x t e r n a l magnetic f i e l d B couples t o t h e magnetic
. .
moments o f S , I and K a s shown i n ( 1 ) . The g f a c t o r s g1 and g~ w i l l be some t h r e e o r d e r s of magnitude s m a l l e r t h a n gs. A l l of t h e coupling c o e f f i c i e n t s , A , y, a..., w i l l depend somewhat on t h e v i b r a t i o n a l and r o t a i t o n a l s t a t e of t h e van d e r 4 w a a l s molecule o r on t h e i n t e r n u c l e a r s e p a r a t i o n and v e l o c i t y of a n unbound c o l l i d i n g p a i r .
By s y s t e m a t i c a l l y i n v e s t i g a t i n g t h e s p i n r e l a x a t i o n i n v a r i o u s a l k a l i noble g a s p a i r s w i t h t h e methods o u t l i n e d above we have been a b l e t o determine t h e key parameters which govern t h e s p i n t r a n s f e r r a t e s i n a number of a l k a l i - n o b l e g a s systems. For example, f o r 8 7 ~ b l Z 9 x e we f i n d
Breit-Rabi parameter: x = = 3.1(3)
P r e s s u r e - l i f e t i m e product: T ~ ( N ~ ) = 1.5 ( 2 ) x 1 0 - ~ s e c Torr 3-Body formation r a t e : = 4 . 8 ( 5 ) ~ 1 0 - ~ ~ c m ~ s e c - ~
TKIRbl [NZI S p i n - r o t a t i o n coupling: = 110(10) MHz
We have r e c e n t l y extended t h e s e p o l a r i z a t i o n methods t o r a d i o a c t i v e Xe i s o t o p e s . Our a p p a r a t u s i s shown i n Fig. 8.
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Fig. 8 - The a p p a r a t u s used t o p o l a r i z e r a d i o a c t i v e Xe
i s o t o p e s . P.H.A. i s a p u l s e H e l m h o l z Coils h e i g h t a m p l i f i e r .
5 0 9 D y e
Pyrex sample c e l l s , 12 mm i n diameter were prepared by d i s t i l l i n g a s m a l l amount of 8 7 ~ b metal i n t o t h e c e l l s and adding about 10 II C i of r a d i o a c t i v e 1 3 3 ~ e a l o n g with' t r a c e s of 131mxe and 1 3 3 m ~ e . The i s o t o p e s were purchased from t h e
General E l e c t r i c Company i n g l a s s ampules. About 50 Torr of N p g a s was a l s o added t o t h e c e l l t o s e r v e as a t h i r d body and t o quench t h e f l u o r e s c e n c e of l a s e r e x c i t e d atoms. The c e l l was heated t o a temperature of 150°C t o p r o v i d e adequate s p i n exchange r a t e s . A t t h e s e temperatures t h e c e l l is some 100 o p t i c a l d e p t h s t h i c k s o l a s e r o p t i c a l pumping i s e s s e n t i a l t o burn through t h e vapor and maintain high a l k a l i s p i n p o l a r i z a t i o n . A l i t h i u m - d r i f t e d germanium d e t e c t o r was used t o d e t e c t gamma r a y s , and when t h e Xe n u c l e i were s p i n p o l a r i z e d t h e s p a t i a l d i s t r i b u t i o n of t h e gamma r a y s changed. Thus we could use t h e changes i n t h e a n i s o t r o p i c gamma r a d i a t i o n t o d e t e c t n u c l e a r magnetic resonance i n t h e radioac- t i v e Xe atoms. An example of such a reesonance i s shown i n Fig. 9.
Fig. 9 - Nuclear resonance o f 131mxe observed w i t h o
t h e a p p a r a t u s of Fig. 8. "
OJ
-
E I I t I I
0
c3 2 94 295 2 96
F r e q u e n c y ( H E )
The e x c e l l e n t s i g n a l t o n o i s e r a t i o of Fig. 9 shows t h a t s p i n exchange l a s e r pumping of r a d i o a c t i v e noble gas n u c l e i is a promising experimental t o o l which w i l l b e u s e f u l i n many o t h e r experiments.
This work was supported by t h e U.S. A i r Force O f f i c e of S c i e n t i f i c Research under g r a n t M O S R 81-0104-C
References
1. M.A. Bouchiat, T.R. Carver and C.M. Varnum, Phys. Rev. L e t t e r s $ 373 (1960).
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