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LASER SPECTROSCOPY OF UNSTABLE ISOTOPES
R. Neugart
To cite this version:
JOURNAL DE PHYSIQUE
Colloque
C l ,
suppl6ment au no
2 ,
Tome
40,
fgvrier
1979,
page
C1-38
LASER SPECTROSCOPY OF UNSTABLE ISOTOPES
R. Neugart
I n s t i t u t f i i r Physik, Johannes Gutenberg U n i v e r s i t a t , Mainz, Germany
A b s t r a c t . H i g h - r e s o l u t i o n methods of o p t i c a l s p e c t r o s c o p y i n u n s t a b l e i s o t o p e s a r e d i s c u s s e d , w i t h emphasis on fast-beam l a s e r spectroscopy. The c h i e f g o a l i s t o o b t a i n i n f o r m a t i o n about n u c l e a r ground-state p r o p e r t i e s from h y p e r f i n e s t r u c t u r e and i s o t o p e s h i f t s . The methods have t o b e s u i t a b l e f o r use w i t h minute samples, due t o t h e small amount of r a d i o a c t i v e atoms a v a i l a b l e .
R6sum6. Des mgthodes de s p e c t r o s c o p i e o p t i q u e d e h a u t e r S s o l u t i o n s o n t d i s c u t b e s pour l ' a p p l i - c a t i o n aux i s o t o p e s i n s t a b l e s , en p a r t i c u l i e r c e l l e s de s p e c t r o s c o p i e l a s e r avec f a i s c e a u r a - p i d e . L ' i n t e r t t p r i n c i p a l v i s e l e s p r o p r i b t b s de l ' b t a t fondamental du noyau, a c c e s s i b l e s p a r l a s t r u c t u r e h y p e r f i n e e t l e dbplacement i s o t o p i q u e . Ces m6thodes d o i v e n t s ' a c c o r d e r aux quan- t i t b s minimes d'atomes r a d i o a c t i f s d i s p o n i b l e s dans l e s S c h a n t i l l o n s .
1. I n t r o d u c t i o n and t h e quadrupole i n t e r a c t i o n c o u l d be proved i n a Laser e x c i t a t i o n of f a s t beams of i o n s o r atoms has few f a v o u r a b l e c a s e s of s t r o n g l y deformed n u c l e i . become a n important technique of resonance s p e c t r o - Exhaustive i n v e s t i g a t i o n of h f s remained t o be scopy. It was i n i t i a t e d by t h e crossed-beam e x p e r i - c a r r i e d o u t by means of r f spectroscopy. ments of H.J.Andrl e t a l . L1J aiming a t a good time
The g e n e r a l a p p l i c a t i o n of o p t i c a l s p e c t r o s c o p y t o r e s o l u t i o n . Recent experiments t a k e advantage of t h e
s t u d i e s of n u c l e i r e q u i r e s a Doppler-free method collinear-beam geometry L2-53. A p p l i c a t i o n s of t h i s
which allows n a t u r a l l i n e w i d t h r e s o l u t i o n (~10MHz). method t o molecular i o n s a r e demonstrated i n t h r e e
c o n t r i b u t i o n s t o t h i s conference [6,7,81. 2.1 Hyperfine s t r u c t u r e
I n t h i s t a l k I s h a l l d i s c u s s t h e a p p l i c a t i o n of The h f s energy of a s t a t e w i t h t o t a l a n g u l a r momen- fast-beam l a s e r spectroscopy t o u n s t a b l e i s o t o p e s . tum F = + is given by
The method h a s s e v e r a l o u t s t a n d i n g f e a t u r e s which
I $ K ( K + I )
-
I ( I + I ).
J ( J + I )i n c l u d e h i g h r e s o l u t i o n and s e n s i t i v i t y , and i d e a l W = - K A F 2
+
2 1 ( 2 1 - 1 ) J ( 2 J - I ~ B (1).
.
s u i t a b i l i t y f o r use w i t h o n - l i n e i s o t o p e s e p a r a t o r s .
The experiments c a r r i e d o u t s o f a r on neutron-rich w i t h K = F(F+l) - I ( I + l )
-
J ( J + I ).
Rb and Cs isotopes will be with other ex- It c o n t a i n s t h e n u c l e a r s p i n I , t h e magnetic d i p o l e periments u s i n g d i f f e r e n t methods. i n t e r a c t i o n c o n s t a n t
p1<He(O)>
2. Nuclear p r o p e r t i e s from o p t i c a l s p e c t r a A = IJ ( 2 )
The h i s t o r y of atomic spectroscopy a s a t o o l of
and t h e e l e c t r i c quadrupole i n t e r a c t i o n c o h s t a n t n u c l e a r p h y s i c s c 9 1 s t a r t e d about 50 y e a r s ago when 9
t h e well-known s p l i t t i n g of s p e c t r a l l i n e s , c a l l e d h y p e r f i n e s t r u c t u r e ( h f s ) was a s c r i b e d t o a rnagnet-
i c coupling,between t h e nucleus and t h e e l e c t r o n s h e l l . Spins and magnetic moments had t o be i n t r o - duced a s b a s i c p r o p e r t i e s o f n u c l e i and c o u l d b e e v a l u a t e d from h f s s p l i t t i n g s . Improved r e s o l u t i o n l e d t o t h e d i s c o v e r y of quadrupole i n t e r a c t i o n , connected w i t h a n u c l e a r e l e c t r i c quadrupole moment, and of t h e n u c l e a r volume e f f e c t i n i s o t o p e s h i f t s ( I S ) . A l i m i t of r e s o l u t i o n was t h e Doppler width of o p t i c a l l i n e s ( = I GUz). F o r t h a t reason n u c l e a r volume s h i f t s were only observed i n heavy elements,
a'v
B = e Q < - >
s a z 2
For e v a l u a t i n g t h e n u c l e a r q u a n t i t i e s p1 and Qs,one h a s t o know t h e magnetic h y p e r f i n e f i e l d cH (O)>
2
z
e
and t h e e l e c t r i c f i e l d g r a d i e n t
<a
V / a z > produced by t h e e l e c t r o n s . A c a l i b r a t i o n i s p o s s i b l e , i f pI and Qs a r e known f o r a t l e a s t one i s o t o p e of an element. Magnetic moments a r e a v a i l a b l e i n p r a c t i - c a l l y a l l c a s e s from n u c l e a r and atomic beam magnee i c resonance, and an i n c r e a s i n g number of s p e c t r o - s c o p i c quadrupole moments i s determined from h f s of mesic atoms C101. Most quadrupole moments a r e s t i l l .2 2
based on c a l c u l a t i o n s of
< a
V/az > which a r e accu- r a t e w i t h i n 10 % t o 30 % 1 111
.
2.2 I s o t o p e s h i f t
The IS between t h e c e n t e r s of g r a v i t y of h f s m u l t i - p l e t s a r e n o t only a n e f f e c t of change i n n u c l e a r volume ( f i e l d s h i f t ) , b u t a l s o i n n u c l e a r mass [12]
For i s o l a t i n g t h e f i e l d s h i f t one h a s t o know t h e mass s h i f t which i s n o t v e r y important i n t h e heavi- e s t elements, b u t predominates i n t h e l i g h t ones. A c a l c u l a t i o n a c c o u n t i n g f o r t h e c o r r e l a t i o n of e l e c t r o n momenta i s d i f f i c u l t . Only i n simple c a s e s such a s s
-
p t r a n s i t i o n s t h e " s p e c i f i c " mass s h i f t i s of t h e o r d e r of t h e "normal" o n e - e l e c t r o n mass s h i f tM =
~ 1 1 8 3 6 .The f i e l d s h i f t i s connected w i t h t h e change of t h e 2 AA'
n u c l e a r rms charge r a d i u s 6 < r > by
where A
1
Y(0)1
i s t h e change of e l e c t r o n d e n s i t y a t t h e nucleus i n t h e e l e c t r o n i c t r a n s i t i o n . This n o n - r e l a t i v i s t i c e q u a t i o n h o l d s a l s o f o r t h e r e l a - t i v i s t i c c a s e , i f a c o r r e c t i o n f a c t o r depending onZ and A i s i n t r o d u c e d . A
I Y(0) 1
h a s t o be calcu- l a t e d , i f a c a l i b r a t i o n by X-ray i s o t o p e s h i f t s o r d a t a from muonic atoms o r e l e c t r o n s c a t t e r i n g i s n o t p r a c t i c a b l e [I 1 1.
D i s r e g a r d i n g t h e s e complications one can e x p e c t t h a t o p t i c a l I S i n a long s e r i e s of i s o t o p e s r e v e a l t h e g e n e r a l t r e n d s a s w e l l a s l o c a l i r r e g u l a r i t i e s of changes i n n u c l e a r r a d i i . This s t a t e m e n t i s a l r e a d y a p a r t o f a n answer t o t h e q u e s t i o n :
3. Unstable i s o t o p e s
-
why and how?The fundamental n u c l e a r p r o p e r t i e s which m a n i f e s t themselves i n t h e o p t i c a l s p e c t r a a r e thoroughly known n e a r t h e v a l l e y of s t a b i l i t y . This i s t h e r e g i o n where n u c l e a r models have been developed and a r e u s u a l l y t e s t e d . But i n t h e r e g i o n f a r from s t a - b i l i t y i n f o r m a t i o n about n u c l e a r ground s t a t e s i s r a t h e r s c a r c e .
New unexpected deformation e f f e c t s were d i s c o v e r e d i n t h e f i r s t s y s t e m a t i c s t u d y of I S f o r very neu- t r o n - d e f i c i e n t i s o t o p e s of Hg C13,141
.
I n t h e s e experiments, due t o t h e l a r g e h f s and I S of t h e h e a v i e s t elements, t h e Doppler width c o u l d be t o l e ? a t e d . But s o p h i s t i c a t e d methods of 8 - r a d i a t i o n d e t e c t e d o p t i c a l pumping (RADOP) C13] o r f l u o r e s -Reactor
+
wallCPS
t
Beam
Fig.1: On-line mass s e p a r a t o r f o r f i s s i o n p r o d u c t s . I n t h e y i e l d curve below, s t a r s i n d i c a t e t h e i s o - topes i n v e s t i g a t e d by l a s e r spectroscopy.
cence spectroscopy u s i n g a frequency-doubled pulsed dye l a s e r C141 had t o e n s u r e s e n s i t i v i t y . G e n e r a l l y , t h e a p p l i c a t i o n of s t a n d a r d methods t o u n s t a b l e i s o t o p e s i s l i m i t e d by t h e small amount of r a d i o a c t i v e atoms a v a i l a b l e , and by t h e i r s h o r t h a l f - l i f e . V e r s a t i l e s o u r c e s of u n s t a b l e n u c l i d e s a r e t h e o n - l i n e mass s e p a r a t o r s connected w i t h s p a l l a t i o n o r f i s s i o n t a r g e t s [15]. They y i e l d i s o - t o p i c a l l y pure samples i n tfie form of i o n beams. As an example, Fig.1 shows t h e set-up used a t Mainz. I t h a s been developed f o r making a v a i l a b l e neutron- r i c h Rb and Cs i s o t o p e s from f i s s i o n of 2 3 5 ~ [163. The r e a c t o r , o p e r a t e d by t h e n u c l e a r chemistry i n s t i t u t e , i s used a s a s o u r c e of thermal n e u t r o n s . 2 3 5 ~ imbedded i n porous g r a p h i t e i s p l a c e d i n a t u n g s t e n oven c l o s e t o t h e r e a c t o r c o r e and exposed
c
1-40 JOURNAL DE PHYSIQUEn u c l i d e s should a t l e a s t be a b l e t o work w i t h 10 t e n s e i n t e r n a l beam of t h e c v c l o t r o n . Samples of atoms p e r second.
The g r e a t advance f o r s e n s i t i v i t y and r e s o l u t i o n of o p t i c a l spectroscopy was t h e development of a cw dye l a s e r . This l a s e r i s now a v a i l a b l e i n s i n g l e - mode o p e r a t i o n o v e r t h e e n t i r e v i s i b l e range of t h e spectrum. The o u t p u t power i s s u f f i c i e n t t o s a t u - r a t e t h e t r a n s i t i o n s , and t h e l i n e w i d t h achieved by frequency s t a b i l i z a t i o n i s about 1MHz. These fea- t u r e s a l l o w h i g h l y s e n s i t i v e spectroscopy w i t h n a t u r a l - l i n e w i d t h r e s o l u t i o n .
Up t o now t h r e e d i f f e r e n t methods have been i n t r o - duced f o r s y s t e m a t i c s t u d i e s o f u n s t a b l e i s o t o p e s . The experiments a r e s t i l l going on and f u r t h e r ex- t e n s i o n s a r e obvious.
I n 1975, G. Huber e t a l . a t Orsay r e p o r t e d t h e f i r s t experiment on 2 1 - 2 5 ~ a 1171.
,
u s i n g e x c i t a t i o n of t h e D l i n e s i n a c o l l i m a t e d atomic beam which was analyzed by a s i x p o l e magnet and a mass s p e c t r o - meter. Two y e a r s l a t e r i t was extended t o t h e neu- t r o n - r i c h i s o t o p e s up t o 3 1 ~ a produced by high- energy p r o t o n r e a c t i o n s i n a uranium t a r g e t . Q u i t e r e c e n t l y , t h e &me scheme h a s been a p p l i e d t o neu- t r o n - d e f i c i e n t Cs i s o t o p e s from ISOLDE.A c o l l i m a t e d atomic beam i s a l s o used i n f l u o r e s -
cence experiments on n e u t r o n - d e f i c i e n t Ba i s o t o p e s a t K a r l s r u h e , f i r s t r e p o r t e d by G.Nowicki e t a l . i n 1977 [I81
.
F i n a l l y we a p p l i e d t h e c o l l i n e a r l a s e r e x c i t a t i o n of a f a s t atomic beam t o neutron-rich Rb and C s i s o t o p e s [I91
.
Since t h e C s and Ba experiments a r e c l o s e l y r e l a t e d t o each o t h e r i n t h e i r p h y s i c a l meaning, I s h a l l t a k e them a s examples f o r f u r t h e r d i s c u s s i o n . 4. Atomic-beam f l u o r e s c e n c e spectroscopy
The p r e p a r a t i o n o f a sample has t o s t a r t from t h e mass-separated i o n beam. A s t r a i g h t f o r w a r d method i s t o c o l l e c t t h e i o n s on a t a r g e t and t o reevapo- r a t e them a s n e u t r a l atoms. By forming a c o l l i m a t e d beam and c r o s s i n g i t w i t h t h e l a s e r beam one a r r i v e s a t a s t a n d a r d arrangement o f sub-Doppler s p e c t r o - scopy 1 2 0 1
.
Resonance a b s o r p t i o n may be d e t e c t e d by f l u o r e s c e n c e .This i s e s s e n t i a l l y t h e scheme used by t h e K a r l s - ruhe group i n t h e experiments on Ba C183
.
I n s t e a d of working 6n l i n e w i t h t h e a c c e l e r a t o r t h i s experi- ment i s performed s t e p by s t e p . Thus, f o r i s o t o p e s w i t h h a l f - l i v e s 2 1 h it t a k e s advantage of t h e in-about 10" t o 1013 atoms of t h e r e s p e c t i v e i s o t o p e a r e produced by (d,xn) o r (a,xn) r e a c t i o n s i n Ba o r Xe, followed by o f f - l i n e mass s e p a r a t i o n . S i l i c o n d i s k s of u l t r a - h i g h p u r i t y a r e used a s m a t r i x mate- r i a l from which Ba i s h e a t e d o f f i n t h e atomic-beam oven.
Hfs and I S a r e i n v e s t i g a t e d i n t h e t r a n s i t i o n 1
6 s 2
Iso
-
6s6p P I(A
= 533.6 nm ).
I t s n a t u r a l l i n e - width i s 19MHz, and t h e beam c o l l i m a t i o n allows f o r Doppler broadening of about 8 M H z . Since a t most some lo4 atoms/sec a r e p r e s e n t i n t h e beam, s p e c i a l c a r e of h i g h d e t e c t i o n e f f i c i e n c y and low background h a s t o be t a k e n . The o v e r a l l photon d e t e c t i o n e f f i - c i e n c y i s about 2 % . By c a r e f u l e l i m i n a t i o n of s c a t t e r e d l a s e r l i g h t , incandescent l i g h t from t h e oven and y - r a d i a t i o n from t h e sample, t h e t o t a l background i s l e s s t h a n 100counts / s e c . The r e s u l t i s demonstrated i n a resonance curve ( F i g . 2 ) ob- t a i n e d w i t h i n 6min from a sample of about 10 1 1 atoms C217
.
Fig.2: Spectrum of 1 2 6 ~ a
.
S t a b l e 13'Ba i s used a s a r e f e r e n c e .Measurements were performed f o r t h e r a d i o a c t i v e iso- topes and isomers 1 3 5 m ~ a 7 1 3 3 ~ a , 133%a, 13'Ba, l Z 8 ~ a , 1 2 6 ~ a , i n a d d i t i o n t o remeasurements of a l l s t a b l e i s o t o p e s . R e s u l t s a r e i n c l u d e d i n F i g . 15. 5 . O p t i c a l pumping of atomic beams
A c o l l i m a t e d atomic beam, c r o s s e d w i t h t h e l a s e r beam, i s a l s o used by t h e Orsay group i n experiments on a l k a l i i s o t o p e s [17,22].They i n t r o d u c e d a new e f f i c i e n t d e t e c t i o n scheme based on o p t i c a l pumping
( Fig. 3 )
.
atoms
#
I7 I/ d~aphragm Ions from target//
tN
SIX-pole !/laser I magnet to the spectrometer beamFig.3: Experimental set-up and schematic diagram of o p t i c a l h f s pumping d e t e c t e d by s t a t e s e l e c t i o n .
which f o c u s s e s atoms i n t h e s t a t e m = + 1 / 2 i n t o a J
s u r f a c e i o n i z e r a c t i n g a s t h e i o n s o u r c e of a small mass s e p a r a t o r , and t h e atoms t r a n s m i t t e d through
t h e a p p a r a t u s a r e d e t e c t e d by i o n counting. I f t h e l a s e r i s tuned t o a l i n e connected w i t h t h e F = I - 1 1 2 h f s l e v e l of t h e ground s t a t e , p a r t of t h e a t o m s a r e pumped t o t h e F = I + 112 l e v e l , and t h e number of atoms b e l o n g i n g t o m J = + 1 / 2 s t a t e s i s i n c r e a s e d . A p o s i t i v e s i g n a l i s observed a t t h e i o n d e t e c t o r . S i m i l a r l y t h e t r a n s i t i o n s s t a r t i n g from t h e F = I + 1 / 2 l e v e l g i v e r i s e t o a n e g a t i v e s i g n a l .
This non-optical d e t e c t i o n i s c h a r a c t e r i z e d by an u l t i m a t e e f f i c i e n c y and very low background. Up t o 5 0 % of t h e beam a r e counted by t h e d e t e c t o r and, w i t h c a r e f u l s h i e l d i n g of r a d i o a c t i v i t y , t h e back- ground i s about 1 c o u n t / s e c . Thus, compared t o opti- c a l d e t e c t i o n , t h e signal-to-background r a t i o i s i n c r e a s e d by roughly a f a c t o r of lo3, provided one photon p e r atom i s e m i t t e d .
I n t h e most r e c e n t experiments on n e u t r o n - d e f i c i e n t Cs i s o t o p e s hf s and I S i n t h e D2 l i n e 6 s *?s1
-
"
6p L ~ 3 1 2 ( A = 852. I-nm ) could be measured f o r t h e long c h a i n of i s o t o p e s r a n g i n g from 3 3 7 ~ s with mag- i c n e u t r o n number t o 121cs, and f o r s e v e r a l isomeric s t a t e s L233. For r e s u l t s s e e F i g . 8 .
The a p p a r a t u s had been connected t o ISOLDE. The mass-separated CS+ beam was stopped i n s i d e a t a n t a - lum tube c o a t e d w i t h y t t r i u m from which C s atoms
were evaporated by h e a t i n g a t about 9 0 0 ~ ~ . About of t h e ISOLDE y i e l d was t r a n s m i t t e d t o t h e i n t e r a c t i o n r e g i o n . It was a m a t t e r of convenience t h a t t h i s y i e l d i s between 10" and 1 0 ' ' atoms/sec i n t h e r e g i o n under c o n s i d e r a t i o n .
6 . Fast-beam l a s e r s p e c t r o s c o p y
Quite a d i f f e r e n t approach t o s e n s i t i v i t y may s t a r t from t h e problem of e f f i c i e n c y i n p r e p a r i n g t h e sample. I n view of t h e f a c t t h a t u n s t a b l e i s o t o p e s a r e a v a i l a b l e a t mass s e p a r a t o r s one might t h i n k of a method d i r e c t l y u s i n g t h e f a s t i o n beam.
The s o l u t i o n of t h i s problem was g i v e n i n a paper by S. L. Kaufman i n 1976 [24], and soon a f t e r t h a t we performed t h e p i l o t experiment on f a s t beams of t h e s t a b l e Na and C s i s o t o p e s 151 .
The b a s i c i d e a i s v e r y simple: S u f f i c i e n t l y l a r g e e x c i t a t i o n r a t e s a r e o b t a i n e d , i f t h e time of i n t e r - a c t i o n between t h e beam and t h e l a s e r l i g h t i s l o n g enough. T h e r e f o r e , t h e l a s e r beam has t o b e super- posed c o l l i n e a r l y t o t h e o u t p u t beam of t h e separa- t o r . And, s i n c e resonance l i n e s of i o n s a r e u s u a l l y not a c c e s s i b l e t o cw dye l a s e r s , t h e i o n beam h a s t o b e converted i n t o a f a s t atomic beam. This neu- t r a l i z a t i o n i s e f f i c i e n t l y performed by charge t r a n s f e r i n an a l k a l i - v a p o u r c e l l .
The s t r i k i n g advantage of t h i s concept i s t h e avoid- ance of any l o s s of m a t e r i a l between i s o t o p e sepa- r a t i o n and l a s e r e x c i t a t i o n . But t h i s advantage i s only p r o f i t a b l e , i f a l l ( o r a s u f f i c i e n t l y l a r g e number o f ) atoms i n t e r a c t w i t h t h e l i g h t . This i s a q u e s t i o n of t h e a b s o r p t i o n Doppler width. Since t h e s p r e a d of k i n e t i c energy i n t h e beam remains unchanged under e l e c t r o s t a t i c a c c e l e r a t i o n
the product of t h e average v e l o c i t y v and t h e v e l o c i t y s p r e a d 6 v , o r l i k e w i s e t h e product of t h e Doppler s h i f t AvD and t h e Doppler width 6 v D a r e c o n s t a n t s of t h e motion. I n o t h e r words: The Doppler width i s c o n s i d e r a b l y reduced from i t s o r i g i n a l v a l u e 6 v '(0) i n t h e i o n source. For t h e i d e a l c a s e D of a thermal d i s t r i b u t i o n (which i s r e a l i z e d i n s u r f a c e - i o n i z a t i o n s o u r c e s )
The r e d u c t i o n f a c t o r i s about 400 f o r a s o u r c e tem- p e r a t u r e of 1500K and a n a c c e l e r a t i o n v o l t a g e of 5 k V . It corresponds t o a Doppler width of 4MHz f o r t h e b l u e Cs l i n e ( X = 4 5 5 . 5 n m ) , compared t o a
C 1-42
JOURNAL DE PHYSIQUEn a t u r a l l i n e w i d t h of 1.2MHz. N a t u r a l l i n e w i d t h s of t h e s t r o n g e s t resonance l i n e s a r e even l a r g e r by a f a c t o r of 10. I t may be concluded t h a t e s s e n t i a l l y a l l atoms i n t h e beam a r e e x c i t e d and t h a t t h e r e s - o l u t i o n i s comparable t o Doppler-free methods. I t i s presumed, o f c o u r s e , t h a t t h e c h a r g e - t r a n s f e r p r o c e s s d o e s n ' t change t h e v e l o c i t y d i s t r i b u t i o n . This i s a c t u a l l y t h e consequence of t h e l a r g e c r o s s - s e c t i o n of about 10 -I 4 cm2 which exceeds t h e k i n e t - i c c r o s s s e c t i o n by two o r d e r s of magnitude. I n t h e r e s o n a n t c a s e , e.g.
t h e beam energy remains unchanged, whereas i n t h e n o r r e s o n a n t c a s e , e . g .
CS+
+
Cs(6s) --C Cs(6p)+
CS+-
1.4 eV o r i n c o l l i s i o n s between d i f f e r e n t p a r t n e r s , t h e fast-beam k i n e t i c energy i s changed by almost exact- l y t h e amount of t h e energy d e f e c t A E . Conservation laws r e q u i r e t h a t i n forward s c a t t e r i n g t h e k i n e t i c energy t r a n s f e r r e d t o t h e t a r g e t atom i s of t h e2
o r d e r (AE) f e u , which i s n e g l i g i b l e f o r beam ener- g i e s i n t h e range of keV
.
I n any c a s e , t h e width of v e l o c i t y d i s t r i b u t i o n and t h e a n g u l a r divergence i s n o t a f f e c t e d by c h a r g e t r a n s f e r . However, t h e Doppler-shifted s h a r p l i n e s may b e s p l i t by t h e energy-loss spectrum correspond- i n g t o d i f f e r e n t r e a c t i o n channels.
Beam
1;;Fi'
-1
D L Q ~ ~
PlpeFig.4: Experimental set-up f o r fast-beam l a s e r spectroscopy
6.1 The experiment
Fig.4 shows t h e experimental set-up used w i t h t h e mass-separated beam of a l k a l i i s o t o p e s from n u c l e a r
f i s s i o n l s e e F i g . 1 ) . The i o n beam i s d e f l e c t e d t o merge w i t h t h e l a s e r beam and n e u t r a l i z e d i n t h e h e a t e d charge-exchange c e l l c o n t a i n i n g a l k a l i m e t a l a t a vapour p r e s s u r e of 1 0 - ~ T o r r . Laser l i g h t i s absorbed by t h e atoms whenever t h e Doppler-shifted frequency i s tuned t o one of t h e resonance l i n e s . This i s d e t e c t e d by c o u n t i n g f l u o r e s c e n c e photons,
Fig.5: Hfs and Doppler-scanned resonance curve f o r t h e example of a C s i s o t o p e w i t h I = 7 / 2 c o l l e c t e d a l o n g a path l e n g t h of 20cm by a c y l i n - d r i c a l l e n s and guided i n t o t h e p h o t o m u l t i p l i e r by a l i g h t p i p e .
Varying t h e Doppler s h i f t a t f i x e d l a s e r frequency o f f e r s
a
convenient way of scanning t h e a b s o r p t i o n s p e c t r a . For t h i s purpose a programmable v o l t a g e i s a p p l i e d t o t h e charge-exchange c e l l , pos t - a c c e l e r - a t i n g t h e i o n s p r i o r t o n e u t r a l i z a t i o n .Fig.5 i l l u s t r a t e s t h e h f s measurement f o r a C s i s o - tope with s p i n I=7/2. The ground and e x c i t e d s t a t e s p l i t t i a g s g i v e r i s e t o 6 h f s t r a n s i t i o n s i n d i c a t e d by t h e v e r t i c a l arrows. By scanning t h e a c c e l e r a t i o n v o l t a g e t h e s e t r a n s i t i o n s a r e observed a t a s c a l e
shown i n t h e lower p a r t of t h e f i g u r e .
Measurement of I S a d d i t i o n a l l y r e q u i r e s comparison t o a r e f e r e n c e i s o t o p e . For t h i s purpose a beam o f s t a b l e 3 3 3 ~ s from a s e p a r a t e i o n s o u r c e i s run through t h e a p p a r a t u s a l t e r n a t i v e l y t o t h e u n s t a b l e i s o t o p e beam. U n c e r t a i n t i e s bf beam-energy c a l i b r a - t i o n a r e e l i m i n a t e d by r e l a t i n g a l l I S t o 1 3 7 ~ s o b t a i n e d from t h e mass s e p a r a t o r . 6 . 2 Measurements
Measurements were performed i n t h e t r a n s i t i o n s
2
2
f o r 1 4 2 ~ s , and 8 9 ~ b and 9 4 ~ b . b e r , 1 3 7 ~ s and
s i m i l a r l y f o r t h e Rb i s o t o p e s between The i s o t o p e s w i t h magic n e u t r o n num- 8 7 ~ b , a r e chosen f o r c a l i b r a t i n g t h e i s o t o p e s h i f t s .
A spectrum of 13'cs may s e r v e a s a t y p i c a l example ( F i g . 6 ) . Both groups of h f s components a r e scanned a t a f i x e d l a s e r frequency, simultaneously w i t h t h e
1 3 3 ~ s r e f e r e n c e . The v o l t a g e s c a l e i s used f o r c a l - c u l a t i n g r e l a t i v e Doppler s h i f t s by
from which t h e h f s parameters and t h e I S can be e v a l u a t e d . Atomic masses a r e known w i t h s u f f i c i e n t accuracy from s t a n d a r d t a b l e s [ 2 5 1 .
The l i n e w i d t h corresponds t o lOMHz i n a frequency s c a l e . By a computer f i t t h e r e s o l u t i o n of e x c i t e d - s t a t e h f s i s b e t t e r t h a n IMHz, y i e l d i n g t h e v e r y s m a l l quadrupole i n t e r a c t i o n c o n s t a n t s w i t h an accu- racy of about 2 0 %
.
E r r o r s i n t h e l a r g e ground-state s p l i t t i n g s and i n i s o t o p e s h i f t s a r e u s u a l l y d e t e r - minedby t h e v o l t a g e c a l i b r a t i o n and amount t o sev- e r a l MHz.The h f s s p l i t t i n g of I3'cs i s t y p i c a l f o r a l l odd i s o t o p e s i n t h i s r e g i o n Which have s p i n s I = 712 and t h e r e f o r e s i m i l a r magnetic moments.
o 2 5 xlOaatorns lsec
221
IOOMHz W V).
U I Volts Fig.6: Spectrum of 13'cs ( I = 7 / 2 ) A d i f f e r e n t c a s e i s 140cs f o r which t h e magnetic and e l e c t r i c h y p e r f i n e i n t e r a c t i o n i n t h e e x c i t e d s t a t e a r e o f t h e same o r d e r , due t o t h e s m a l l mag- net'ic moment connected w i t h I = 1 . As shown i n Fig.7 p a r t of t h e s p l i t t i n g s n e a r l y compensate. A c h a r a c t e r i s t i c of a l l s p e c t r a a r e t h e s a t e l l i t e s of each l i n e , s e p a r a t e d from t h e main peak by1.4 v o l t . They a r e due t o t h e non-resonant branch of charge t r a n s f e r t o t h e f i r s t e x c i t e d b p s t a t e of C s , g i v i n g r i s e t o a l o s s of k i n e t i c energy equal U I Volts Fig.7: Spectrum of 140cs ( I = 1) t o t h e 6 s - 6 p energy d i f f e r e n c e . I n t h i s way t h e d i s t r i b u t i o n of c h a r g e - t r a n s f e r r e a c t i o n p r o d u c t s over f i n a l s t a t e s i s d i r e c t l y observed. We have shown t h a t t h i s d i s t r i b u t i o n s t r o n g l y depends on t h e r e a c t i o n p a r t n e r s C5, 261 . I n f a c t , t h e s e h i g h - r e s w l u t i o n measurements c a n be used t o e v a l u a t e branch- i n g r a t i o s of c h a r g e - t r a n s f e r r e a c t i o n c h a n n e l s . 7. R e s u l t s
The i n f o r m a t i o n o b t a i n e d throughout t h e d e s c r i b e d work i n c l u d e s s p i n s , magnetic d i p o l e and e l e c t r i c quadrupole moments a s w e l l a s d i f f e r e n c e s of n u c l e a r charge r a d i i . Most s p i n s and magnetic moments of Rb and C s i s o t o p e s had p r e v i o u s l y been measured by C. Ekstriim e t a l . C271 u s i n g atomic beam resonance on l i n e w i t h ISOLDE, and a few s p i n s were measured by o p t i c a l pumping (RADOP) C163. These q u a n t i t i e s a r e a c c e s s i b l e i n t h e atomic ground s t a t e .
The importance of l a s e r spectroscopy i s obvious: Quadrupole i n t e r a c t i o n can only be observed i n e x c i t e d s t a t e s w i t h J L l
,
and i s o t o p e s h i f t meas- urements have t o i n v o l v e d i f f e r e n t atomic s t a t e s . It would b e beyond t h e scope of t h i s t a l k t o discuss a l l r e s u l t s i n d e t a i l , a p a r t from t h e f a c t t h a t t h e i r a n a l y s i s i s f a r from b e i n g complete. Publica- t i o n s of a l l t h r e e groups comprise r e p o r t s of f i r s t r e s u l t s .A g e n e r a l review of n u c l e a r r a d i i i n t h e Cs-Ba r e g i o n can a l r e a d y b e given. Fig. 8 c o n t a i n s t h e r e s u l t s of t h e d i f f e r e n t experiments, marked by
2
JOURNAL DE PHYSIQUE
reached. T h e r e f o r e t h e methods have t o compete with each o t h e r mainly i n s e n s i t i v i t y and s u i t a b i - l i t y f o r t h e p a r t i c u l a r c a s e . A d e c i s i o n f o r one o r
i
t h e o t h e r method w i l l depend on t h e element and a- tomic t r a n s i t i o n under c o n s i d e r a t i o n , b u t some gene- r a l remarks can be made:1 ) The d e t e c t i o n of f l u o r e s c e n c e i s u s u a l l y l i m i t e d t o a few photonsfatom, s i n c e o p t i c a l pumping removes t h e atoms from t h e a b s o r b i n g s t a t e . M u l t i p l e exci- t a t i o n i s p o s s i b l e i n some s p e c i a l c a s e s such a s
I S
ground s t a t e s of doubly-even (I = 0 ) i s o t o p e s . This g a i n of d e t e c t i o n e f f i c i e n c y h a s been e x p l o i t e d i n t h e measurements on even Ba i s o t o p e s 128 and 126 ( c f . Fig. 2 ) .2) On t h e o t h e r hand, o p t i c a l pumping i s t h e d e c i - s i v e c o n d i t i o n f o r t h e s o p h i s t i c a t e d d e t e c t i o n v v
scheme used by t h e Orsay group, which r e l i e s on s t a t e s e l e c t i o n i n an inhomogeneous magnetic f i e l d . Limited t o paramagnetic atoms i t i s i d e a l l y s u i t e d t o 2 ~ 1 ,2 a l k a l i ground s t a t e s . Furthermore, e f f i c i e n t
1
s u r f a c e i o n i z a t i o n of focussed atoms may h a r d l yFig.8: Change of rms c h a r g e r a d i i r e l a t i v e t o N = 8 2 . work in o t h e r c a s e s . A p o s s i b l e alternative might Symbols:
0
f o r Ba (Karlsruhe)f o r
cs
( o ~ ~ ~ ~with - I isomers s ~ ~ ~ ~b e t h e d e t e c t i o n of atoms by r a d i o a c t i v i t y C271. ) 0 , f o r C s (Mainz)F u l l symbols denote s t a b l e i s o t o p e s . 3 ) Fast-beam spectroscopy i s based on t h e r e d u c t i o n of Doppler width occuring by a c c e l e r a t i o n . The o p t i - n u c l i d e s a t N = 5 0 and N = 82. A f u r t h e r s t r o n g in- 2 mum c o n d i t i o n i s a n i d e a l matching t o t h e n a t u r a l c r e a s e of 6 < r > i s expected a t t h e o n s e t of s t a b l e l i n e w i d t h , which i s e a s i l y f u l f i l l e d i n t h e s t r o n g n u c l e a r deformation n e a r N = 9 0 . t r a n s i t i o n s of a l k a l i atoms t o t h e f i r s t e x c i t e d
-
The extemely small v a l u e s of 6 < r L > j u s t below N = 8 2 had been known from s t a n d a r d o p t i c a l s p e c t r a s c o p y on t h e s t a b l e and long-lived i s o t o p e s of Ba, Cs and Xe. They were analyzed by U l l r i c h and O t t e n C281 i n terms of c o l l e c t i v e n u c l e a r models. The new high- r.esolution measurements r e v e a l a p e r f e c t correspond- ence between f i n e r s t r u c t u r a l e f f e c t s i n Ba and C s i s o t o n e s (odd-even s t a g g e r i n g ) . I n t h i s c o n t e x t i t should b e noted t h a t e r r o r s i n t h e s p e c i f i c mass s h i f t might c o n s i d e r a b l y change t h e a b s o l u t e scaling (up t o
*
2 0 X ) i n d e p e n d e n t l y f o r both c a s e s , b u t not t h e s t r u c t u r e of t h e c u r v e s .p s t a t e s . I n t h e b l u e Rb and C s l i n e s , which have been chosen f o r convenience of l a s e r a c t i o n and photon d e t e c t i o n , t h e observed Doppler width (de- termined by beam divergence) i s l a r g e r t h a n t h e n a t u r a l l i n e w i d t h of 1.2 MHz by a f a c t o r of 10. Extension of t h i s method t o o t h e r elements may r e - q u i r e gas-discharge i o n s o u r c e s which have a broa- d e r energy d i s t r i b u t i o n . T h i s , of c o u r s e , e f f e c t s t h e s e n s i t i v i t y , whereas t h e r e s o l u t i o n needed can be achieved by u s e of v e l o c i t y - s e l e c t i n g f i l t e r s . Another way i s t o i n c r e a s e t h e a c c e l e r a t i o n v o l t a g e a c c o r d i n g t o (6) a s f a r a s s u f f i c i e n t l y s t a b l e Measurements on Cs r a n g e t o t h e v e r y neutron-defi- high-voltage sources are
c i e n t i s o t o p e s , and p r e s e n t d a t a seem t o f i t t h e It seems t o be e v i d e n t t h a t fast-beam l a s e r spec- assumption of g r a d u a l l y i n c r e a s i n g n u c l e a r defor- troscopy i s a s e n s i t i v e method f o r r a t h e r g e n e r a l mation [29]
.
u s e with i o n s o r atoms. The c h a r g e - t r a n s f e r n e u t r a -l i z a t i o n i s by no means l i m i t e d t o p r e s e n t l y s t u - 8. D i s c u s s i o n
d i e d elements. I n c a s e s where ground-state r e s o - Three d i f f e r e n t h i g h - r e s o l u t i o n techniques of l a s e r
nance l i n e s a r e n o t a c c e s s i b l e t o cw l a s e r s t h e spectroscopy have proved s u c c e s s f u l i n s y s t e m a t i c
p o p u l a t i o n of m e t a s t a b l e s t a t e s [30]may h e l p t o s t u d i e s of ufistable n u c l i d e s , and f i r s t r e s u l t s a r e
meet w i t h a b s o r p t i o n i n t h e v i s i b l e . Recent ex- q u i t e promissing f o r f u t u r e work.
the discharge ion source.
Further effort may tackle the problem of sensitivi-
ty by improving the efficiency of light collection
and suppressing the background. Useful techniques
may be the excitation and observation at different
wavelengths where it is possible, and, for longer-
lived states, the pulsed excitation and observation
of the subsequent decay. A non-optical detection
scheme might combine fast-beam laser spectroscopy
with the RADOP technique [32j by implantation of
optically pumped atoms into a suitable crystal,
and observation of the asymmetry in the 8-decay
of polarized nuclei.
9. Conclusion
I tried to show that optical spectroscopy has re-
covered its importance for nuclear research. The
information obtained on nuclear ground and isomeric
states far from stability is hardly accessible to
other methods. Present experiments using high-re-
solution laser techniques have reached a stage of
sensitivity sufficient for investigating long iso-
topic chains which still provide key information
about nuclear structure.
The fast-beam experiments have been performed in
team work by J. Bonn, S.L.Kaufman, W. Klernpt, H.
Lochmann, G. Moruzzi, R. Neugart, E.-W. Otten, L.
von Reisky, B. Schinzler, K.P.C. Spath, J. Stei-
nacher and D. Weskott. I like to thank the Karls-
ruhe and Orsay teams for providing unpublished
material, and their permission to present it here
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