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Submitted on 1 Jan 1979
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LAMB SHIFT STUDIES IN Cl16+
D. Murnick, C. Patel, M. Leventhal, O. Wood, H. Kugel
To cite this version:
JOURNAL
DEPHYSIQUE
Colloque
C1,
supplement
au
n o 2, Tome
40,
ftcvrier 1979, page
C1-34
LAMB SHIFT STUDIES IN c116+
D.
73.Murnick, C. K. N. Pate1 and M. Leventhal
Bell Laboratories
Murray Hill, New Jersey
07974
0.R. Wood I1
Bell Laboratories
Holmdel, New Jersey
07733
H. W. Kugel
Rutgers, The State University
New Brunswick, New Jersey
08903
~isumd
-
Les dtudes exp&rimentales du d&placement Lamb dans les ions hydrog6noides ont encore de l'int6rSt
etl'importance pour le sondage des effets dlectrodynamiques quantiques (QED) dans le rggime de haut
champ. Des ex~griences
sur la mesure du deplacement Lamb dans ~116f
sont en cours au laboratoire de
l'accdl6rateur Tandem double
MP
B
Brookhaven. Les ions metastables 2.3112 sont produits par ca ture
6lectronique dans les feuilles de carbone minces (5pgm/cm2)
2 l'aide d'un faisceau d'ions C117'
form6 par
stripping d'un faisceau de clP4+
a
190 MeV sortant de l'acc&l&;ateur.
Le contrBle de la population
metastable est effectug par l'observation de la disintegration spontan6e 2E1 et M1
a'l'aide d'un spectro-
m'etre Si(Li)
5
rayons X. On trouve que la durie de vie de l'itat est 4,92
+
0,04 ns en accord avec les
calculs. Un systirme laser au C02 Q-commutg unique, se composant d'un &age d'oscillateur avec une grille
intracavitg et un amplificateur de haute puissance produit une puissance de rayonnement de 3 kW dans 1'1R
avec un cycle de dgbit de 10%. L'angle rayon laser-faisceau d'ions est de 20° de f a ~ o
2ce que l'ion
puisse accorder les frequences par effet Doppler autour de la valeur lr6vue 1064,95 e
m
-
'
.
La resonance
est dltectee par l'augmentation du taux de comptage sur Lyman
ade C1
6+, dans deux compteurs Ge intrins-
eques de 300mm2
x7 m .
Abstract
-
Experimental studies of the Lamb Shift ( 2 s 1 / 2 - 2 ~ ~ / ~
energy splitting) in hydrogenic ions
continue to be of interest and importance in probing quantum electrodynamic (QED) effects in the high
field regime. Experiments are in progress at the double
MPTandem accelerator laboratory, Brookhaven
National Laboratory, to measure the Lamb Shift in cll6+.Metastable 2 ~ 1 1 2
state ions are produced by
electron pickup in thin (5pgm/cm2) carbon foils by a beam of 17+ C1 ions formed via stripping the 14+ C1
beam at 190 MeV emergent from the accelerator. The metastable population is monitored by observing the
spontaneous 2E1 and MI decay using a Si(Li) x-ray spectrometer. The lifetime of the 2s1/2 state is found
to be 4.9211s (2.04) in agreement with calculations.
A
unique Q-switched C02 laser system consisting of an
oscillator stage with intracavity grating and a high power amplifier produces3kWT.R radiation at a 10% duty
cycle. The laser beam intersects the relativistic particle beam at an angle of 20° such that the Doppler
shifted fixed frequencies of the C02 laser can be discretely tuned about the expected Lamb Shift, 1064.95
cm-l. The resonance condition is determined by an enhanced count rate at 2.96keV, the c116+ Lyman a
transition, in two 300mm2
x7mm intrinsic Ge counters.
INTRODUCTION
The Lamb shift, or the 2 ~ ~ , , ~
-
plI2 energy
splitting in atomic hydrogen or hydrogenic ions is
a prime quantity to test the theory of quantum
electrodynamics (QED).
In recent years, advances in
beam-foil techniques and instrumentation have made
possible an extension of La*
shift studies to
hydrogenic ions of increasingly higher nuclear
charge (2) using heavy ion accelerators.[lI In addi-
tion, new calculational techniques have been applied
to high
Zsystems. [2,3] This has stimulated wide
intefest in this subject as the ( a ~ ) ~
+
(higher
order in aZ) dependence of the Lamb shift allows
sensitive stydies of QED interactions in a high
field regime and suggests the possibility of observ-
ing new and unexpected phenomena.[41
Methods for measuring high Z hydrogenic ion
Lamb shifts include Stark effect lifetime measure-
ments, anisotropy of quench radiation and laser
resonance spectroscopy. Gould [5] has reported at
this conference, on the Berkeley work on ~
r
~
~
+
using the Stark effect lifetime technique achieving
S = 31,3476112 A€ = 9 . 2 5 ~ I O ~ G H Z
TzS
= 4.91 nsec 'Czp = 0.02 psec E(Lya )
= 2.95 keV '5112A
Fig. 1
-
Hydrogenic ion with parameters for c116+.
CO; LASER BEAM Fig. 2
-
Schematic diagram of experimental system.an experimental s e n s i t i v i t y of about 1.5%. Even a t Yo i s t h e l a s e r frequency, y l ( B ) i s t h e frequency t h i s l e v e l one of two c a l c u l a t i o n s i n t h e l i t e r a - observed by t h e moving i o n , v i s t h e i o n v e l o c i t y t u r e [3] appeared t o be i n disagreement with exper- and 9 i s t h e a n g l e between t h e two beams. Concep- iment. The l a s e r resonance spectroscopy method t u a l l y , t h e n , t h e Lamb s h i f t resonance experiment which was reviewed a t t h e Gatlinburg Beam F o i l i s s t r a i g h t - f o r w a r d . Metastable hydrogenic i o n s
Spectroscopy Conference 161 o f f e r s t h e p o t e n t i a l i n t e r s e c t a t u n a b l e l a s e r beam and Lyman a r a d i a - f o r achieving experimental s e n s i t i v i t i e s of 0.1% o r t i o n i s induced a s t h e l a s e r i s tuned t o t h e Lamb b e t t e r . This paper summarizes o u r r e s u l t s t o s h i f t resonance frequency. Tuning can be achieved d a t e on l a s e r resonance spectroscopy of by changing yo, v , o r 0 , however changes i n y
a r e l e a s t s e n s i t i v e t o s y s t e m a t i c e f f e c t s . THE HYDROGENIC I O N
c116+
F i g u r e 1 shows t h e r e l e v a n t n = 1 and n = 2 EXPERIMENTAL SYSTEM
l e v e l s of a hydrogenic i o n n e g l e c t i n g h y p e r f i n e The i o n beam from t h e a c c e l r a t o r i s s t r u c t u r e and numerical v a l u e s of t h e important s t r i p p e d by a 50 ~ c ~ m / c m ~ C f o i l a f t e r energy
+
p a r y n e t e r s f o r t h e s p e c i f i c c a s e of From a n a l y s i s and t h e 17 (10%-20% of t h e t o t a l beam) a n experimental p o i n t of view, t h e important p o i n t s component i s m a g n e t i c a l l y s e l e c t e d . A schematic t o n o t e a r e : 1 ) t h e Lyman a energy, 2.96keV a l l o w s diagram of t h e a p p a r a t u s i s shown i n F i g u r e 2. A e f f i c i e n t d e t e c t i o n w i t h modest r e s o l u t i o n ( 5 t o t h i n adder f o i l i s used t o p r e f e r e n t i a l l y p o p u l a t e 10%) u s i n g s o l i d s t a t e d e t e c t o r s ; 2) t h e 2s1l2 hydrogenic i o n s i n t h e 2s1/2 m e t a s t a b l e s t a t e . s t a t e l i f e t i m e , 4.91 n s e c , i s s u f f i c i e n t l y long t o Lower s t a t e s of i o n i z a t i c n , p a r t i c u l a r l y Helium- a l l o w resonance spectroscopy; 3) t h e s i n g l e photon
M 1 decay i s e a s i l y observable b u t i s i n d i s t i n g u i s h - a b l e from 2p s t a t e decay; 4 ) t h e p r e d i c t e d Lamb s h i f t i s almost c o i n c i d e n t w i t h a C02 l a s e r t r a n s i - t i o n a t 1046.85 cm-I and 5) The r a p i d 2p s t a t e
r
decay i m p l i e s a broad resonance l i n e s h a p e (- S %.25, where
r
i s FWHM and S i s t h e Lamb s h i f t ).
K. R. Jones [7] has d e s c r i b e d t h e Brookhaven N a t i o n a l Laboratory tandem a c c e l e r a t o r l a b o r a t o r y , where our experiments a r e c a r r i e d o u t , and where
+
beams of c h l o r i n e i o n s (12' t o 1 5 ) a t e n e r g i e s up t o about 220 MeV can be provided. A t t h e s e
e n e r g i e s on t h e o r d e r of 40% of t h e i o n beam can be converted t o hydrogenics by f o i l s t r i p p i n g . I n a d d i t i o n , t h e beam q u a l i t y i s such t h a t t h e Doppler s h i f t can be used w i t h crossed p a r t i c l e and l a s e r beam t o provide some frequency t u n a b i l i t y a s given by:
Channel Number
JOURNAL DE PHYSIQUE
F i g . 4
-
Schematic diagram of C02 l a s e r system.l i k e , a r e a major s o u r c e of background r a d i a t i o n Under our c o n d i t i o n s , assuming a m e t a s t a b l e y i e l d n e a r t h e Lyman a energy. The spectrum observed i n e q u a l t o 1% of t h e hydrogenic y i e l d , Ymax should b e t h e 300 mm2 x 7 mm Ge d e t e c t o r s ( F i g u r e 3a) con- about 1 2 / s e c . The Lyman a l i k e background s i n g l e s s i s t s of two photon and M I r a d i a t i o n from t h e 2 s counting r a t e should b e 2-3 kHz.
s t a t e , Lyman a r a d i a t i o n from t h e 2p l e v e l s
and r a d i a t i o n from n = 2 c1I5+. A 152 micron RESULTS AND OUTLOOK
Beryllium window a b s o r b s 85% of t h e dominant two The system d e s c r i b e d above h a s been photon decay from t h e 2 s s t a t e . For d i a g n o s t i c t e s t e d u s i n g a l O n A beam of c l 1 7 + i o n s a t 190 MeV purposes, e i t h e r of t h e Ge detectors could b e r e - i n c i d e n t on a C02 l a s e r beam focussed t o about placed by a s m a l l t h i n window S i ( L i ) X-ray s p e c t r o - 0.5 mm diameter a t t h e i n t e r s e c t i o n p o i n t . The meter y i e l d i n g t h e spectrum shown i n F i g u r e 3b l a s e r frequency was s e t a t 1046.85 cm-l (P(20) l i n e where t h e two photon continuum i s more completely of t h e 00°1+0200 band of Cog). The i n c i d e n t a n g l e r e s o l v e d . By i n t e g r a t i n g t h i s p o r t i o n of t h e spec- was 20°. A s t h e two beams d e f i n e a v e r t i c a l p l a n e , trum and moving t h e a d d e r f o i l along t h e beam, t h e t h e i n t e r a c t i o n r e g i o n i s p a r t i c u l a r l y s e n s i t i v e t o l i f e t i m e of t h e 2 ~ 1 1 2 l e v e l was determined t o b e h o r i z o n t a l i n s t a b i l i t i e s i n t h e p a r t i c l e beam. A 4.92'1.04 n s i n agreement w i t h one e l e c t r o n c a l c u l a - d e f l e c t o r p l a t e and s l i t feedback system was t h u s t i o n s
.
implemented t o keep t h e beam s p o t f i x e d*
0.1 mm.The l a s e r system ( F i g u r e 4) c o n s i s t s of a n Under t h e s e c o n d i t i o n s , a s i g n a l r a t e of 5 t o 6 / s e c o s c i l l a t o r s t a g e w i t h i n t r a c a v i t y d i f f r a c t i o n h a s been achieved. S i n g l e s r a t e s a r e on t h e o r d e r g r a t i n g and p r o v i d e s 140 Watts of peak power i n of 1 0 kHz, however. These h i g h r a t e s a r e due p r i - 180 psec d u r a t i o n p u l s e s a t 480 Hz. The a m p l i f i e r m a r i l y t o t h e l a r g e f i e l d of view of t h e Ge detec- s t a g e i s a modified Coherent R a d i a t i o n Model 43 C02 t o r s and t h e i n a b i l i t y t o b e t t e r d i s c r i m i . ~ a t e l a s e r p r o v i d i n g 1 3 m of a m p l i f i c a t i o n p a t h . The a g a i n s t t h e two photon continuum.
a v e r a g e o u t p u t power i s 240 Watts. D i s c r e t e l i n e s With t h i s system a resonance c u r v e c a p a b l e from t h e l a s e r between 1090.03 cm-I and 914.42 cm-I of a few p a r t s p e r thousand r e s o l u t i o n f o r t h e Lamb c a n b e c h o ~ e n by t h e computer c o n t r o l l e d i n t r a c a v i t y s h i f t should e v e n t u a l l y b e p o s s i b l e We have g r a t i n g . I n t h i s way about one
r
(FWHM) of t h e r e c e n t l y begun such a measurement. Before f i n a l resonance can be covered a t a f i x e d a n g l e and beam r e s u l t s can b e quoted, p o s s i b l e s y s t e m a t i c e r r o r s energy. must b e e v a l u a t e d by v a r y i n g t h e beam energyThe expected s i g n a l r a t e , Ymax, a t t h e and t h e beam c r o s s i n g a n g l e . peak of t h e resonance can be w r i t t e n a s :
Ymax = n F l t 7 ~ where n i s t h e m e t a s t a b l e i o n f l u x , )J ACKNOWLEDGEMENT
t h e induced t r a n s i t i o n r a t e , t t h e i n t e r a c t i o n time, We a r e g r a t e f u l t o t h e Brookhaven s t a f f ,
and encouragement.
REFERENCES
[l]
H. W.
Kugeland D. E. Murnick, Rep. Prog. Phys.
40, 297 (1977).
-
[2] P. J. Mohr, Phys. Rev. Letters
2,
1050 (1975).
[3] G.
W.
Erickson,
J.
Phys. Chem. Ref. Data
6 ,
831 (1977).
[4]
J.Reinhardt and W. Greiner, Rep. Prog. Phys.
40, 219 (1977).
-
[ 5 ]
