EXOTIC ATOMS

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EXOTIC ATOMS

G. Backenstoss

To cite this version:

G. Backenstoss. EXOTIC ATOMS. Journal de Physique Colloques, 1971, 32 (C5), pp.C5b-255-C5b-

260. �10.1051/jphyscol:19715139�. �jpa-00214716�

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U n i v e r s i t y of Karlsruhe, Germany and

CERN, Geneva, Switzerland.

Nous présentons ou r a p p e l l o n s l e s p r o p r i é t é s des atomes

u-,

IT-, K-,

5

^{e t}^C-. Nous d i s c u t o n s l e s a p p l i c a t i o n s de l ' é t u d e des atomes e x o t i q u e s à l a ~ h ~ s i q u e des p a r t i c u l e s é l é m e n t a i r e s e t à l a physique n u c l é a i r e .

Pro'perties of p-, 7-, K

- ,

^f5 ^and atoms a r e p r e s e n t e d and r e f e r r e d t o . A p p l i c a t i o n s of t h e s t u d y of e x o t i c atoms t o elementary p a r t i c l e and n u c l e a r physics a r e d i s c u s s e d .

INTRODUCTION

The s t u d y of muonic atoms has been going on f o r many y e a r s w i t h a r a p i d l y i n c r e a s i n g p r e c i s i o n and a l s o i n v e s t i g a t i o n s of p i o n i c atoms have been c a r r i e d o u t t o an e x t e n t t h a t t h e i r b a s i c p r o p e r t i e s a r e known and understood. I n t h e p a s t y e a r s t h i s f i e l d has expanded s o much t h a t even a new name should b e i n t r o d u c e d which covers a l 1 types of atoms observed s o f a r . Whereas c o n s i d e r a b l e p r o g r e s s was made i n measuring t h e s t r o n g i n t e r a c t i o n e f f e c t s f o r K-mesonic atoms a l s o C-hyperonic and a n t i p r o t o n i c atoms have been observed.

I n o r d e r t o avoid r e p e t i t i o n and i n o r d e r t o keep t h i s r e p o r t reasonably s h o r t r e f e r e n c e i s made t o t h e published work.

The main reason why i n my opinion t h i s f i e l d i s a t t r a c t i v e i s t h e f a c t t h a t i t connects s o many p a r t s o f p h y s i c s and correspondingly has a p p l i c a t i o n s i n very d i f f e r e n t d i r e c t i o n s . Therefore, 1 w i l l t r y t o

cover a l a r g e r p a r t of t h e f i e l d i n i t s v a r i o u s as- p e c t s by s e l e c t i n g some p a r t i c u l a r p o i n t s .

The b a s i c concept i s very simple. The f a c t t h a t we t a l k about atoms i m p l i e s t h a t t h e r e i s some- t h i n g i n common w i t h normal e l e c t r o n i c atoms. I n f a c t i t i s t h e Coulomb f i e l d which determines t h e atomic p r o p e r t i e s . T h e r e f o r e , we could expect t h a t any n e g a t i v e l y charged p a r t i c l e which l i v e s s u f f i - c i e n t l y long w i l l form such an e x o t i c atom. The de- t a i l e d mechanism of formation i s n o t y e t known. How- e v e r , t h e atom i s produced i n a h i g h l y e x c i t e d s t a t e witli t h e p r i n c i p a l quantum number n

q,

where m = mass of t h e p a r t i c l e , m = e l e c t r o n mass. The time f o r t h e d e - e x c i t a t i o n of t h e atom i s given by t h e t r a n s i t i o n p r o b a b i l i t y f o r E l t r a n s i t i o n s

Wx 0.5 x 10l0 2' sec"

.

e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19715139

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Thus the heavier the particle the faster the transi- tion in a given nucleus Z. For muonic lead, for example, the entire cascade starting from a state with n n 14 down to n = 1 takes about 10-l3 sec. In Table 1 the lifetimes of negatively charged particles are shown and are al1 large coinpared with the time that elapsed during the cascade to the ground state.

The principal properties of al1 these atoms are already contained in the simple Bohr formula for the binding energy E

n,R

and the Bohr radius r n

where p is the reduced mass of the orbiting particle.

It is important to note that E a m and r l/m, which is the main reason for the interest exotic

atoms have received. Masses of the particles and the Bohr radius for the ground state (n = 1) are given in Table 1. Already for relatively light nuclei r l becomes comparable with nuclear radii.

The energies range from a few eV for highly excited levels to several MeV for the low-lying states.

Therefore in the initial steps of the cascade, energies characteristic of chemistry and solid-state physics are important, and effects can be expected and have been seen caused by the chemical and physi- cal property of the material in which the exotic atom is formed. Mesochemistry will certainly play its role in future. In the intermediate range of the cascade one finds the domain of atomic physics;

here the atomic cascade can be studied. But also properties of the orbiting particle, mass, magnetic moment, spin, or detailed questions of the long range

interactions such as the vacuum polarization can be studied. Because of the short-range nature of nuclear forces, the strong interaction will set in rapidly from a certain atomic state onwards. The low-lying states, therefore, will provide information about the nuclear properties. In Table 1 some of the properties of the particles, the interactions and the nuclei are indicated in an obvious way.

Studies on muonic atoms provide, together with electron scattering experiments, the best information on the charge distribution p of spherical nuclei.

Muonic atoms yield even charge distributions of de- formed nuclei and excited states (isomer shift) and of the distribution of nuclear magnetic moments p+

Fi (Bohr-Weisskopf effect). These topics could provide the subject matter for a separate lecture, but are described in detailed review articles [1,2J.

Pionic and kaonic atoms are subject to strong interaction. Being bosons they obey the Klein-Gordon equation, where the strong interaction can be treated by inserting an optical potential V

N

where V is the electromagnetic potential of the extended nucleus. For pions VN is rather compli- cated [ 3 , 4 ] , owing to the fact that p-wave interaction plays an important role and two-nucleon absorption is dominant, since one-nucleon absorption is forbidden by energy-momentum conservation. From pionic atoms one could expect information on the basic T-N interaction at low energies, and data on the matter distribution pN as well as short-range pair correlations (p ) . This field has been re-

ZN

viewed recently [5J. The best measurement of the pion mass made so far originates from studies on pionic atoms [ 6 ] yielding

m = (139.549 t 0.008) MeV.

TI

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For K mesons t h e o p t i c a l p o t e n t i a l [7] i s s i m p l e r , a s one-nucleon a b s o r p t i o n i s p o s s i b l e , f o r example, v i a t h e process

and p-wave i n t e r a c t i o n i s n o t important:

rnK and

$

denote t h e masses of K meson and nucleon, r e s p e c t i v e l y , p ( r ) and p ( r ) a r e t h e n u c l e a r neutron

n P

and proton d i s t r i b u t i o n , and A. and A l a r e t h e

fi

s-wave s c a t t e r i n g l e n g t h s f o r i s o s p i n zero and one, r e s p e c t i v e l y . T y p i c a l v a l u e s a r e

which can b e s i m p l i f i e d under t h e assumption pn : pp:

p A = N : Z : A t o

The s t r o n g i n t e r a c t i o n e f f e c t s d e s c r i b e d by such a p o t e n t i a l produce a s h i f t of t h e atomic energy l e v e l s and a n a t u r a l l i n e width of t h e l e v e l s due t o t h e a b s o r p t i o n of t h e p a r t i c l e by t h e nucleons. I t is t h e main emphasis of t h e work on h a d r o n i c atoms t o measure t h e s e e f f e c t s . According t o Eq. ( 5 )

,

t h e s e q u a l i t i e s a r e r e l a t e d t o t h e low-energy s c a t t e r i n g amplitudes and t o t h e n u c l e a r d e n s i t i e s . S i m i l a r p o t e n t i a l s should b e a p p l i c a b l e a l s o f o r b a r y o n i c (5,C) atoms. S i n c e C- can only be absorbed by pro- t o n s , whereas

?

can be absorbed by p r o t o n s and neu- t r o n s , important d i f f e r e n c e s e x i s t which could b e e x p l o i t e d s y s t e m a t i c a l l y i n o r d e r t o e x t r a c t d a t a f o r s c a t t e r i n g l e n g t h s and n u c l e a r d i s t r i b u t i o n s

from t h e s p e c t r a of e x o t i c atoms.

Another important f e a t u r e of t h e hadronic atoms i s t h e f a c t t h a t a b s o r p t i o n t a k e s p l a c e i n S t a t e s w i t h n i n c r e a s i n g and R i n c r e a s i n g correspondingly, when t h e mass of t h e o r b i t i n g p a r t i c l e becomes h e a v i e r . T h i s i s a r e s u l t of Eq. (2). S i n c e mr(E) ^ar R t h e o v e r l a p i n t e g r a l

i s l a r g e a t t h e n u c l e a r s u r f a c e . T h i s s e n s i t i v i t y t o t h e n u c l e a r s u r f a c e becomes more pronounced t h e h e a v i e r t h e p a r t i c l e i s . K-meson a b s o r p t i o n t a k e s p l a c e o u t s i d e t h e 10% d e n s i t y r e g i o n , whereas C a r e absorbed i n r e g i o n s where t h e n u c l e a r d e n s i t y amounts only t o 1% of i t s v a l u e i n t h e c e n t r e of t h e nucleus.

EXPERIMENTAL RESULTS ON X-RAYS OF EXOTIC ATOMS The experimental technique f o r measuring t h e X-rays o r i g i n a t i n g from e x o t i c atoms i s simple and s t r a i g h t f o m a r d , and i t i s s i m i l a r f o r t h e d i f f e r e n t types of atoms. It i s d e s c r i b e d i n v a r i o u s review a r t i c l e s [1,5]. For muons and pions i t is u s u a l l y s u f f i c i e n t t o i d e n t i f y t h e momentum-analysed p a r t i - c l e s by t h e i r range u s i n g a s c i n t i l l a t i o n c o u n t e r t e l e s c o p e . I n a beam p r o v i d i n g slow K- mesons o r a n t i p r o t o n s , t h e r e a r e 100 t o 1000 times more pions p r e s e n t . Therefore, t h e i d e n t i f i c a t i o n of t h e slow K- mesons and a n t i p r o t o n s must-be achieved by adding t o t h e t e l e s c o p e cerenkov c o u n t e r s which v e t o t h e passing pions [ 8 , 9 ] . A f u r t h e r r e j e c t i o n of t h e f a s t e r p a r t i c l e s can b e o b t a i n e d by a pulse-height d i s c r i m i n a t i o n i n t h e t h i n p l a s t i c t e l e s c o p e counters.

Vacuum p o l a r i z a t i o n

As an example of t h e more fundamental problems which can b e i n v e s t i g a t e d w i t h e x o t i c atoms, t h e d e t e r m i n a t i o n of t h e vacuum p o l a r i z a t i o n i n muonic atoms may be considered. The vacuum p o l a r i z a t i o n c o n t r i b u t e s with 2.6% t o t h e Lamb s h i f t i n t h e hy- drogen atom, b u t i t dominates s t r o n g l y i n muonic

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atoms. The reason f o r t h i s i s t h a t t h e muon s t a y s much c l o s e r t o t h e nucleus i n a much s t r o n g e r Coulomb

f i e l d , where t h e vacuum p o l a r i z a t i o n i s l a r g e r , whereas t h e self-energy i s p r o p o r t i o n a l t o l / m 2 , m being t h e mass of t h e p a r t i c l e . The c h a r a c t e r i s t i c range w i t h i n which t h e e f f e c t i s p r e s e n t i s t h e Compton wavelength of t h e e l e c t r o n s :

S i n c e t h e Bohr o r b i t of t h e muon i s

t h e vacuum p o l a r i z a t i o n e f f e c t i s n o t i c e a b l e i n heavy n u c l e i even f o r high l e v e l s , and d e c r e a s e s l e s s r a p i d l y w i t h n a s t h e f i n i t e s i z e e f f e c t . Therefore i t i s p o s s i b l e t o choose muonic t r a n s i t i o n where t h e s h i f t due t o t h e vacuum p o l a r i z a t i o n domi- n a t e s a l 1 o t h e r e f f e c t s . For t h e 5g7 ⁺4 f 5 t r a n s i -

h h

t i o n i n 82Pb of 437.806 keV, t h i s s h i f t has been c a l - c u l a t e d t o b e 2.219 keV. This t r a n s i t i o n can be measured with an accuracy of 140 eV. A l 1 o t h e r cor-

r e c t i o n s can b e c a l c u l a t e d with an u n c e r t a i n t y small- e r than t h i s e r r o r . A number of t r a n s i t i o n s have been measured and a v a l u e

has been deduced [IO]. The t h e o r e t i c a l c o n t r i b u t i o n s of h i g h e r o r d e r s i n a , ( a 2 , a 3 ) , amount t o about 2%.

They have been included i n t h e t h e o r e t i c a l value.

The agreement between experiment and theory i s b e t t e r i f t h e higher-order terms a r e included.

K-mesonic atoms

Experimentally t h e most s t r i k i n g p r o p e r t y p a r t i - c u l a r t o hadronic X-ray s p e c t r a i s t h e t e r m i n a t i o n of t h e X-ray t r a n s i t i o n s a t a c e r t a i n l e v e l n due t o n u c l e a r a b s o r p t i o n , which a l s o r e s u l t s i n a reduc- t i o n of t h e i n t e n s i t y of t h e l a s t observed t r a n s i t i o n .

Measurements of t h i s type have been performed by Wiegand [ll]. However, i t i s r a t h e r d i f f i c u l t t o o b t a i n , from t h e s e measurements, a c c u r a t e information on t h e K-meson a b s o r p t i o n from t h e upper l e v e l of t h e t r a n s i t i o n , s i n c e a knowledge of t h e p o p u l a t i o n of t h i s l e v e l i s n e c e s s a r y and t h i s i m p l i e s a d e t a i l e d s t u d y of t h e mesonic cascade. More a c c u r a t e measurements of K-mesonic X-ray t r a n s i t i o n could be performed a t a s p e c i a l l y designed, p a r t l y s e p a r a t e d , low- energy K-meson beam a t t h e CERN PS. It was p o s s i b l e t o measure 1 8 1 d i r e c t l y t h e n a t u r a l l i n e width of t h e 4f ^-+3d t r a n s i t i o n i n 16S. The i n s t r u m e n t a l l i n e width could b e determined from nearby y-rays of radio-

a c t i v e sources which have been i r r a d i a t e d simultaneous- l y . By a computer a n a l y s i s , t h e n a t u r a l L o r e n t z i a n l i n e width could be unfolded from t h e Gaussian-shape i n s t r u m e n t a l width. I n o r d e r t o deduce t h e energy s h i f t caused by t h e s t r o n g i n t e r a c t i o n , a p r e c i s e energy d e t e r m i n a t i o n i s n e c e s s a r y , r e q u i r i n g an e x a c t c a l i b r a t i o n . This can most e a s i l y b e provided by t h e h i g h e r X-ray t r a n s i t i o n s , p a r t i c u l a r l y t h o s e f e e d i n g t h e n = 4 l e v e l (5 ⁺4, 6 ^-+4, 7 + 4, e t c . ) , s i n c e t h e s t r o n g i n t e r a c t i o n e f f e c t s on t h e n = 4 l e v e l a r e a l r e a d y n e g l i g i b l e . Moreover, t h e e n e r g i e s and i n t e n s i t i e s of t h e 8 + 4 and 9 -+ 4 t r a n s i t i o n s , which b r a c k e t t h e i n t e r e s t i n g 4f + 3d t r a n s i t i o n s and c a u s e some d i s t u r b a n c e t o t h i s l i n e , can be c a l c u l a t e d and s u b t r a c t e d s u f f i c i e n t l y w e l l . Recent p r e l i m i n a r y and unpublished d a t a Cl21 a r e shown i n Table 2. For comparison a l s o t h e v a l u e s c a l c u l a t e d by s o l v i n g t h e Klein-Gordon e q u a t i o n (4) w i t h t h e f u l l complex poten- t i a l (5) a r e g i v e n f o r t h e Re

a

a s suggested by Eq. (6a) and f o r comparison f o r some o t h e r c h o i c e s of Re

A.

The v a l u e s f o r Re À = +0.42 fm a r e i n b e t t e r agreement w i t h t h e experiment. C l e a r l y t h e q u e s t i o n must be s t u d i e d f u r t h e r .

C-hyperonic atoms

The X-rays of C-hyperonic atoms appear t o g e t h e r

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w i t h t h e k a o n i c X-rays i n t h e same spectrum. The sigmas o r i g i n a t e m a i n l y from one of t h e f o l l o w i n g r e a c t i o n s

K - + p + c - + T + (8)

K- + n + C- + n o ( 9 )

I n e m u l s i o n s [13] t h e r e a c t i o n s (8) and ( 9 ) y i e l d a b o u t 80% of t h e produced C- w i t h an energy d i s t r i - b u t i o n c e n t r e d around 20 MeV. The energy of t h e sigmas from r e a c t i o n (10) i s c o n s i d e r a b l y l a r g e r . The decay "me of t h e p a r t i c l e s i s of t h e same o r d e r of magnitude a s t h e slowing-down t i m e i n m a t e r i a l . T h i s l e a d s t o t h e f a c t t h a t a p a r t of t h e produced sigmas may decay b e f o r e t h e y a r e c a p t u r e d and form a C-hyperonic atom.

I n a former experiment [II], an i n d i c a t i o n f o r a 6 + 5 C t r a n s i t i o n i n a k a o n i c p o t a s s i u m s p e c t r u m was found. The energy o f t h e l i n e c o i n c i d e d w i t h

t h e t h e o r e t i c a l l y p r e d i c t e d one; t h e 7 + 6 t r a n s i - t i o n , however, c o u l d n o t b e o b s e r v e d , s o t h a t a c l e a r ,proof f o r t h e e x i s t e n c e of C-hyperonic X-ray l i n e s was n o t p o s s i b l e . I n t h e CERN e x p e r i m e n t [14], s i x C-hyperonic l i n e s were found i n t h e k a o n i c S, C l , and Zn s p e c t r a , t h e e n e r g i e s and i n t e n s i t i e s of which f i t t h e c a l c u l a t e d ones v e r y w e l l . From t h e s e l i n e s t h e e x i s t e n c e of C atoms i s unambiguously proved.

A f u r t h e r aim i s t h e measurement of C t r a n s i t i o n s w i t h reduced y i e l d s i n o r d e r t o g e t i n f o r m a t i o n a b o u t t h e low-energy C-N i n t e r a c t i o n , which i s p r a c t i c a l l y unknown up t o now. A f i r s t p r e l i m i n a r y r e s u l t was b b t a i n e d [12] f o r C-Ca where t h e 6 + 5 t r a n s i t i o n was o b s e r v e d w i t h a r e d u c e d i n t e n s i t y 1 ( 6 + 5 ) / f ( 7 + 6) = 0.75 I 0.10.

l n t i p r o t o n i c atoms

The X-ray s p e c t r a of l i g h t and heavy a n t i p r o - o n i c atoms h a v e b e e n o b s e r v e d [9]. As was t h e c a s e o r t h e f i r s t K-mesonic X-ray measurements, t h e

t e r m i n a t i o n of t h e X-ray s e r i e s a t a c e r t a i n l e v e l c o u l d b e o b s e r v e d , and t h e a t t e n u a t i o n of t h e l a s t t r a n s i t i o n s c o u l d b e measured i n s u i t a b l e atoms.

P r e c i s e e n e r g y measurements y i e l d e d t h e b e s t v a l u e s o f a r f o r t h e mass o f t h e a n t i p r o t o n

lm- - m

1

⁵^0.5^{M e V}

.

P P

An i n d i c a t i o n of t h e f i n e s t r u c t u r e s p l i t t i n g due t o t h e m a g ~ i e t i c moment of t h e a n t i p r o t o n c o u l d b e s e e n i n t h e (10 + 9) t r a n s i t i o n i n T l a t 386 keV The l i n e s h a p e a g r e e s w i t h t h e s p l i t t i n g i f one assumes f o r t h e t h e same m a g n e t i c moment a s f o r t h e p r o t o n .

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2,

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(USA)

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-

[5] G. B a c k e n s t o s s , Annu. Rev. N u c l e a r S c i .

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^B, 233 (1970).

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[IO] G. Backenstoss, S. Charalambus, H. Daniel, Cl31 European K Collaboration, Nuovo Cimento

2,

Ch. von der Malsburg, G. Poelz, H.P. Povel, 315 (1959).

H. Schmitt and L. Tauscher, Phys. Letters 31 B, 233 (1970)

-

^{Cl43 G}

^.

Backenstoss

,

^T

.

Bunaciu, S. Charalambus,

J. Egger, H. Koch, A. Bamberger, U. Lynen, [ll] C.E. Wiegand, Phys. Rev. Letters 22, 1235 (1969). H.G. Ritter and H. Schmitt, Phys. Letters

33 B, 230 (1970) Cl21 G. Backenstoss, A. Bamberger, 1. ~ergstrom,

-

P. Bounin, T. Bunaciu, J. Egger, S. Hultberg, H. Koch, U. Lynen, H.G. Ritter, A. Schwitter and R. Stearns,Phys. Letters to be published.

Table 1

Table 2

Preliminary experimental data and theoretical values for the widths and shifts of kaonic energy levels. Energies in keV.

Calculations with "standard potential" of Eq. (6a) or with Re

A

^{as noted.}

C

-

1.5 x IO-'' 2343 22.612 Particle

Li£ etime (sec) Mass (me)

Bohr radius rl (fm)

K

-

1 . 2 x 1 0 - ~ 966 54.712 particle

interaction

-

P

Q>

1836 28.812 m +-

"K

^{m-, Ei-}P P m ~ * Matter distribution at nuclear

Pe, pii W. P ~ N

I

^surface

I

Vac. pol. particle-nucleus interaction, particle-nucleon

{

interaction at iow energies.

1 -

Fi

2 . 2 x 1 0 - ~ 207 25612

I

Elem.

'

! B

l i ~

15P 16' 17Cl 30Zn

'n

-

2.6x10-' 273 19412

Level

2p 2p 3d 3d 3d 4f

Level shift Level width

Exp.

-0.28 I 0.03 -0.27

+

^0.03

-0.36 f 0.05 -0.51 ? 0.15 -0.85 I 0.25

Exp.

0.78 I 0.07 0.68

+

^0.07

1.45 I 0.15 2.7 5 0.3 4.75

+

^1.0

1.7 C 0.6

Calculated Calculated

Stand.

-0.23 -0.26 -0.43 -0.76 -1.20 -0.63

Stand.

0.31 0.36 0.70 1.05 1.62 1.0 Re

A

= O

-0.50 -0.91 -0.43

Re À = O

1.54 2.32 1.4 Re À = +0.42£

-0.37 -0.80 -0.30

Re

A

^=+0.42

2.20 3.24 1.9