LASER SPECTROSCOPY OF UNSTABLE ISOTOPES
R. Neugart
To cite this version:
R. Neugart. LASER SPECTROSCOPY OF UNSTABLE ISOTOPES. Journal de Physique
Colloques, 1979, 40 (C1), pp.C1-38-C1-45. <10.1051/jphyscol:1979111>. <jpa-00218390>
HAL Id: jpa-00218390
https://hal.archives-ouvertes.fr/jpa-00218390
Submitted on 1 Jan 1979
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Colloque C l , suppl6ment au no 2 , Tome 40, fgvrier 1979, page C1-38
JOURNAL DE PHYSIQUE
LASER SPECTROSCOPY OF UNSTABLE ISOTOPES
R. Neugart
I n s t i t u t fiir Physik, Johannes Gutenberg U n i v e r s i t a t , Mainz, Germany
A b s t r a c t . High-resolution methods of o p t i c a l spectroscopy i n u n s t a b l e i s o t o p e s a r e discussed,
w i t h emphasis on fast-beam l a s e r spectroscopy. The c h i e f goal i s t o o b t a i n information about
n u c l e a r ground-state p r o p e r t i e s from hyperfine 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 be 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
available.
R6sum6. Des mgthodes de s p e c t r o s c o p i e o p t i q u e de 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 rapide. 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 hyperfine e t l e dbplacement i s o t o p i q u e . Ces m6thodes doivent s ' a c c o r d e r aux quant 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 could 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 favourable 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 spectro-
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 experi-
c a r r i e d out by means of r f spectroscopy.
ments of H.J.Andrl e t a l . L1J aiming a t a good time
r e s o l u t i o n . Recent experiments t a k e advantage of t h e
collinear-beam geometry L2-53.
Applications of t h i s
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 spectroscopy t o
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
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.
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
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 .
The method has 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 n c l u d e high 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
2.1 Hyperfine s t r u c t u r e
The h f s energy of a s t a t e w i t h t o t a l angular momen-
F=
tum
I
W =-KA
F 2
s u i t a b i l i t y f o r use w i t h on-line 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 out s o f a r on neutron-rich
Rb and Cs isotopes
willbe
with
other ex-
periments using d i f f e r e n t methods.
with
is given by
+
$K(K+I) - I(I+I)
+
. J(J+I)
B
(1)
21(21-1)J(2J-I~
. .
K = F(F+l) - I ( I + l ) - J ( J + I )
.
It contains t h e nuclear s p i n I , t h e magnetic d i p o l e
interaction constant
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
The h i s t o r y of atomic spectroscopy a s a t o o l of
n u c l e a r physics c 9 1 s t a r t e d about 50 years ago when
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
A =
(2)
IJ
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
9
B = e Q < -a'
>v
s
2
az
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 rnagneti c coupling,between t h e nucleus and t h e e l e c t r o n
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
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 could be
has t o know t h e magnetic hyperfine 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
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
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
l e d t o t h e discovery of quadrupole i n t e r a c t i o n ,
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
connected w i t h a n u c l e a r e l e c t r i c quadrupole moment,
element. Magnetic moments a r e a v a i l a b l e i n p r a c t i -
and of t h e nuclear volume e f f e c t i n i s o t o p e s h i f t s
c a l l y a l l cases from n u c l e a r and atomic beam magnee
( 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
i c resonance, and an i n c r e a s i n g number of s p e c t r o -
of o p t i c a l l i n e s ( = I GUz).
s c o p i c quadrupole moments i s determined from h f s of
For 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,
mesic atoms C101. Most quadrupole moments a r e s t i l l .
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979111
based on c a l c u l a t i o n s of
r a t e w i t h i n 10 % t o 30 %
< a 2V/az 2>
1 1 11 .
which a r e accu-
+
Reactor wall
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 multip 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 nuclear mass [12]
Beam
For i s o l a t i n g t h e f i e l d s h i f t one has t o know t h e
mass s h i f t which i s n o t very important i n t h e heavi-
CPS
t
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 accounting 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" one-electron mass
shift M = ~11836.
The f i e l d s h i f t i s connected with t h e change of t h e
2 AA'
by
n u c l e a r rms charge r a d i u s 6 < r >
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
Fig.1: On-line mass s e p a r a t o r f o r f i s s i o n products.
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.
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 equation holds a l s o f o r t h e r e l a t i v i s t i c case, i f a c o r r e c t i o n f a c t o r depending on
Z and A i s introduced.
A
I Y(0) 1
has 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
cence spectroscopy using a frequency-doubled
pulsed
dye l a s e r C141 had t o ensure s e n s i t i v i t y .
Generally, the 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
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
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
n o t p r a c t i c a b l e [I 1 1
half-life.
.
Disregarding t h e s e complications one can expect 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
general 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
V e r s a t i l e sources of u n s t a b l e n u c l i d e s
a r e t h e on-line 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 iso-
t o p i c a l l y pure samples i n tfie form of i o n beams.
changes i n nuclear r a d i i . This statement i s a l r e a d y
As an example, Fig.1 shows t h e set-up used a t Mainz.
a p a r t o f a n answer t o t h e question:
It has been developed f o r making a v a i l a b l e neutron-
3. Unstable i s o t o p e s
-
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.
why and how?
The r e a c t o r , operated by t h e n u c l e a r chemistry
The fundamental nuclear p r o p e r t i e s which manifest
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 near 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 nuclear 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 information about nuclear ground s t a t e s i s
rather scarce.
i n s t i t u t e , i s used a s a source of thermal neutrons.
2 3 5 ~imbedded i n porous g r a p h i t e i s placed i n a
tungsten oven c l o s e t o the r e a c t o r c o r e and exposed
2
t o a f l u x of 2 x 10'' neutronslcm s e c . A t temperat u r e s of about 1 2 0 0 ~t h~e a l k a l i s d i f f u s e o u t of
t h e t a r g e t and evaporate from t h e tungsten s u r f a c e
a s p o s i t i v e ions. Accelerated t o 3-lOkeV
New unexpected deformation e f f e c t s were discovered
they a r e
guided o u t of t h e r e a c t o r i n t o a s e p a r a t i o n magnet,
i n t h e f i r s t s y s t e m a t i c study of IS f o r very neu-
and t h e i s o t o p i c a l l y pure beam i s a v a i l a b l e i n
tron-deficient
q u a n t i t i e s shown i n t h e lower p a r t of t h e f i g u r e .
i s o t o p e s of Hg C13,141
. I n these
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 could 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 -
s amuch more e f f i Although ISOLDE I1 a t C ~ ~ ~ l G e n ei v
c i e n t ( c f . s e c t i o n 5 ) , t h i s y i e l d can be taken a s
t y p i c a l f o r s i m i l a r f a c i l i t i e s . Therefore, o p t i c a l
methods designed f o r general use w i t h r a d i o a c t i v e
c 1-40
JOURNAL DE PHYSIQUE
nuclides should a t l e a s t be a b l e t o work with
10
about 10"
atoms per 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 over 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 satur a t e t h e t r a n s i t i o n s , and t h e linewidth achieved by
frequency s t a b i l i z a t i o n i s about
t u r e s allow
1MHz. These fea-
h i g h l y s e n s i t i v e spectroscopy w i t h
natural-linewidth
tense i n t e r n a l beam of t h e c v c l o t r o n . Samples of
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 ultra-high p u r i t y a r e used a s matrix mater i a l from which Ba i s heated 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
6s2 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
resolution.
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 systematic 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-
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 high d e t e c t i o n e f f i c i e n c y and low background
has t o be taken. The o v e r a l l photon d e t e c t i o n e f f i ciency i s about 2 % . By c a r e f u l e l i m i n a t i o n of
t e n s i o n s a r e obvious.
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
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.
, using
e x c i t a t i o n of
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 than 100counts / s e c . The r e s u l t
t h e D l i n e s i n a collimated atomic beam which was
i s demonstrated i n a resonance curve ( F i g . 2 )
analyzed by a s i x p o l e magnet and a mass spectro-
t a i n e d w i t h i n 6min from a sample of about 10
meter. Two y e a r s l a t e r i t was extended t o t h e neu-
atoms C2 17
tron-rich
ob11
.
i s o t o p e s up t o 3 1 ~ produced
a
by high-
energy proton r e a c t i o n s i n a uranium t a r g e t . Quite
r e c e n t l y , t h e &me scheme has been a p p l i e d t o neut 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 neutron-deficient
Ba i s o t o p e s
a t Karlsruhe, 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
i s o t o p e s [I91
.
Rb and C s
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 fluorescence 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
Fig.2: Spectrum of 1 2 6 ~ a S t a b l e 13'Ba
a reference.
i s used a s
Measurements were performed f o r t h e r a d i o a c t i v e iso-
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-
topes and isomers 1 3 5 m ~ a 71 3 3 ~ a , 133%a,
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
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
beam and c r o s s i n g i t with t h e l a s e r beam one a r r i v e s
s t a b l e i s o t o p e s . R e s u l t s a r e included i n Fig. 15.
13'Ba,
a t a s t a n d a r d arrangement o f sub-Doppler spectroscopy 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
A c o l l i m a t e d atomic beam, c r o s s e d with t h e l a s e r
by fluorescence.
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
5 . O p t i c a l pumping of atomic beams
.
Instead
of working 6n l i n e w i t h the a c c e l e r a t o r t h i s experiment 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 takes advantage of t h e in-
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
introduced 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 )
.
The atomic beam passes through a s i x p o l e magnet,
~ .
were evaporated by h e a t i n g a t about 9 0 0 ~ About
atoms
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 region. It was a m a t t e r of convenience
#
I7
d~aphragm
//
N
!/laser
beam
atoms/sec
i n t h e region under c o n s i d e r a t i o n .
Ions from
target
and 10''
t h a t t h i s y i e l d i s between 10"
I/
6 . Fast-beam l a s e r spectroscopy
t
SIX-pole
magnet
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
I
to the
spectrometer
from the problem of e f f i c i e n c y i n preparing 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 given 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 obtained, 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 long
Fig.3: Experimental set-up and schematic diagram
of o p t i c a l h f s pumping detected by s t a t e
selection.
enough. Therefore, t h e l a s e r beam has t o be superposed c o l l i n e a r l y t o t h e output beam of t h e separat o r . And, s i n c e resonance l i n e s of ions a r e u s u a l l y
which focusses 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 source of a small
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 has
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 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 h e apparatus a r e d e t e c t e d by i o n counting. I f t h e
t r a n s f e r i n an alkali-vapour c e l l .
t o b e converted i n t o a f a s t atomic beam. This neu-
l a s e r i s tuned t o a l i n e connected with 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 belonging t o m J = + 1 / 2 s t a t e s i s increased. 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
ance of any l o s s of m a t e r i a l between i s o t o p e separ 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 with 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.
l e v e l give r i s e t o a negative s i g n a l .
This non-optical
The s t r i k i n g advantage of t h i s concept i s t h e avoid-
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
Since t h e spread 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
5 0 % of t h e beam a r e counted by t h e d e t e c t o r and,
with 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 background i s about
1 count/sec. Thus, compared t o opti-
c a l d e t e c t i o n , t h e signal-to-background
ratio is
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 spread 6 v , o r likewise t h e product of t h e
i n c r e a s e d by roughly a f a c t o r of lo3, provided one
Doppler s h i f t AvD and t h e Doppler width 6 v D a r e
photon p e r atom i s emitted.
c o n s t a n t s of the motion. I n o t h e r words: The Doppler
I n t h e most r e c e n t experiments on neutron-deficient
Cs i s o t o p e s hf s and IS i n the D2 l i n e 6s
*?s1
-
"
6p L ~ 3 1 2 ( A = 852. I-nm ) could be measured f o r t h e
s
maglong chain of i s o t o p e s ranging from 3 3 7 ~with
width i s considerably reduced from i t s o r i g i n a l
value 6v '(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 sources)
i c neutron 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 r e d u c t i o n f a c t o r i s about 400 f o r a source tem-
The apparatus had been connected t o ISOLDE. The
p e r a t u r e of
mass-separated CS+ beam was stopped i n s i d e a t a n t a -
5 k V . It corresponds t o a Doppler width of
lum tube coated with y t t r i u m from which C s atoms
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
1500K and a n a c c e l e r a t i o n v o l t a g e of
4MHz
4
C 1-42
JOURNAL DE PHYSIQUE
n a t u r a l linewidth of
1.2MHz. Natural linewidths 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. It 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 reso l u t i o n i s comparable t o Doppler-free methods.
I t i s presumed, o f course, t h a t t h e charge-transfer
process doesn'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 crosss 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
resonant case, e.g.
the beam energy remains unchanged, whereas i n t h e
n o r r e s o n a n t case, 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 exactl 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 e
2
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 energ i e s i n t h e range of keV
.
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 along a path l e n g t h of
20cm by a c y l i n -
I n any case, 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
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
t h e a n g u l a r divergence i s not a f f e c t e d by charge
a l i g h t pipe.
t r a n s f e r . However, t h e Doppler-shifted sharp l i n e s
Varying t h e Doppler s h i f t a t f i x e d l a s e r frequency
may be s p l i t by t h e energy-loss spectrum correspond-
o f f e r s a convenient way of scanning the a b s o r p t i o n
i n g t o d i f f e r e n t r e a c t i o n channels.
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-accelera t i n g the 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 isotope 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 give 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
Beam
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
1;;Fi'-1 D L Q ~ ~
Plpe
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
Fig.4:
Experimental set-up f o r fast-beam l a s e r
spectroscopy
s
a s e p a r a t e i o n source i s run
s t a b l e 3 3 3 ~from
through the apparatus 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
6.1 The experiment
i s o t o p e beam. U n c e r t a i n t i e s bf beam-energy
calibra-
Fig.4 shows t h e experimental set-up used w i t h t h e
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
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
obtained from t h e mass s e p a r a t o r .
f i s s i o n l s e e Fig.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
heated charge-exchange c e l l c o n t a i n i n g a l k a l i metal
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 counting fluorescence photons,
6 . 2 Measurements
Measurements were performed i n the t r a n s i t i o n s
2
2
6s -Sll2
7p _P3,2 (A = 455.5 nm) of Cs and
-
5s L ~ 1 1 2 - 6p L ~ 3 f 2 (A = 420.2 m ) of Rb f o r t h e
i s o t o p e s 138-142cs and 89-94~b. T h e i r
neutron-rich
h a l f - l i v e s extend from 32 min f o r 1 3 8 ~tso 1.7 s e c
f o r 1 4 2 ~ 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
8 9 ~ band 9 4 ~ b .The i s o t o p e s w i t h magic neutron number,
1 3 7 ~and
s 8 7 ~ b ,a r e chosen f o r c a l i b r a t i n g t h e
isotope 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
(Fig.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 with t h e
1 3 3 ~rse 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
U I Volts
from which t h e h f s parameters and t h e I S can be
evaluated. Atomic masses a r e known with 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 [251.
The linewidth 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 exciteds t a t e h f s i s b e t t e r than
IMHz, y i e l d i n g t h e very
small quadrupole i n t e r a c t i o n c o n s t a n t s with an accuracy of about 2 0 %
. Errors
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 seve r a l MHz.
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 products
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 the
r e a c t i o n p a r t n e r s C5, 261
. In
f a c t , these high-resw
l u t i o n measurements can be used t o e v a l u a t e branchi n g r a t i o s of charge-transfer
r e a c t i o n channels.
7. R e s u l t s
The information obtained throughout t h e described
i s t y p i c a l f o r a l l odd
The h f s s p l i t t i n g of I3'cs
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
i s o t o p e s i n t h i s region Which have s p i n s I = 712
quadrupole moments a s w e l l a s d i f f e r e n c e s of nuclear
and t h e r e f o r e s i m i l a r magnetic moments.
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 previously been measured by
o
W
V)
221 .
C. Ekstriim e t a l . C271 using atomic beam resonance
2 5 xlOaatorns lsec
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.
IOOMHz
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 with J L l
, and
i s o t o p e s h i f t meas-
urements have t o involve d i f f e r e n t atomic s t a t e s .
It would be 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
U I Volts
Fig.6:
Spectrum of 13'cs
(I=7/2)
t h e i r a n a l y s i s i s f a r from being complete. Publicat 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
results.
A d i f f e r e n t c a s e i s 140cs f o r which the magnetic
and e l e c t r i c hyperfine 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 small magnet'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 general 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 be given. Fig. 8 contains 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
d i f f e r e n t symbols. The gross run of 6 < r > values
near neutron-shell c l o s u r e i s c h a r a c t e r i z e d by a
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 by
1.4 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 , giving r i s e t o a l o s s of k i n e t i c energy equal
sharp i n c r e a s e above t h e magic neutron number N=82.
This behaviour i s c e r t a i n l y due t o an i n f l u e n c e of
neutrons i n t h e opened s h e l l on t h e proton d i s t r i bution. It i s a l s o observed f o r t h e i s o t o n i c s t a b l e
JOURNAL DE PHYSIQUE
reached. Therefore 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 -
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
t h e o t h e r method w i l l depend on t h e element and atomic 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 gener a l remarks can be made:
1 ) The d e t e c t i o n of fluorescence 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 absorbing s t a t e . M u l t i p l e excit 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
IS
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 has 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 decisive condition f o r the sophisticated detection
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 ~ ,21 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 hardly
Fig.8: Change of rms charge 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)
be
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.
f o r cs ( o ~ ~ ~ ~ with
- I isomers
s ~ ~ ~ ~ )
0 ,f o r C s (Mainz)
3 ) Fast-beam spectroscopy i s based on t h e r e d u c t i o n
F u l l symbols denote s t a b l e i s o t o p e s .
of Doppler width occuring by a c c e l e r a t i o n . The o p t i nuclides a t N = 50 and N = 82. A f u r t h e r s t r o n g inmum 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
2
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
linewidth, 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
nuclear deformation near N=90.
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<rL> j u s t below N = 8 2
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
had been known from standard o p t i c a l spectrascopy
been chosen f o r convenience of l a s e r a c t i o n and
on t h e s t a b l e and long-lived
photon d e t e c t i o n , t h e observed Doppler width (de-
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 Otten C281 i n
termined by beam divergence) i s l a r g e r than t h e
terms of c o l l e c t i v e nuclear models. The new high-
n a t u r a l linewidth of 1.2 MHz by a f a c t o r of 10.
r.esolution measurements r e v e a l a p e r f e c t correspond-
Extension of t h i s method t o o t h e r elements may r e -
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
q u i r e gas-discharge i o n sources which have a broa-
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
d e r energy d i s t r i b u t i o n . This, of course, e f f e c t s
should be 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
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
s h i f t might considerably change t h e a b s o l u t e scaling
be achieved by use of v e l o c i t y - s e l e c t i n g
(up t o
* 20X)independently
f o r both c a s e s , b u t not
filters.
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
t h e s t r u c t u r e of t h e curves.
according 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 range 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 evident 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 nuclear defor-
troscopy i s a s e n s i t i v e method f o r r a t h e r general
mation [29]
.
use with i o n s o r atoms. The charge-transfer
neutra-
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. Discussion
Three d i f f e r e n t high-resolution
d i e d elements. I n cases where ground-state resotechniques of l a s e r
spectroscopy have proved s u c c e s s f u l i n systematic
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
q u i t e promissing f o r f u t u r e work.
The r e s o l u t i o n l i m i t of n a t u r a l linewidth i s almost
nance l i n e s a r e not a c c e s s i b l e t o cw l a s e r s t h e
population of metastable s t a t e s [30]may
meet with
help to
a b s o r p t i o n i n t h e v i s i b l e . Recent ex-
periments on ~ e L+4 3 and ~ b +E3ljtook advantage of
t h e c o n s i d e r a b l e p o p u l a t i o n of metastable s t a t e s i n
the discharge ion source.
R.J. Powers, Hyperfine Interactions 4, 123(1978)
Further effort may tackle the problem of sensitivi-
E.-W. Otten, Hyperfine Interactions
ty by improving the efficiency of light collection
K. Heilig and A. Steudel, Atomic Data and
Nuclear Data Tables 16,613 (1974)
J. Bonn, G. Huber, H.-J. Kluge, E.-W.Otten,
Z. Physik
187 and 203 (1976)
and suppressing the background. Useful techniques
may be the excitation and observation at different
wavelengths where it is possible, and, for longerlived 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 recovered 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-resolution laser techniques have reached a stage of
sensitivity sufficient for investigating long isotopic chains which still provide key information
about nuclear structure.
Lochmann, G. Moruzzi, R. Neugart, E.-W. Otten, L.
von Reisky, B. Schinzler, K.P.C. Spath, J. Steinacher and D. Weskott. I like to thank the Karlsruhe and Orsay teams for providing unpublished
material, and their permission to present it here
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2,
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G. Huber, private communication
A,
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E.-W. Otten, in Atomic Physics Y, Proc. 5th
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