RADIATIVE ELECTRON CAPTURE BY FAST HIGHLY STRIPPED HEAVY IONS CHANNELED IN A THIN CRYSTAL

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RADIATIVE ELECTRON CAPTURE BY FAST HIGHLY STRIPPED HEAVY IONS CHANNELED IN

A THIN CRYSTAL

S. Andriamonje, M. Chevallier, C. Cohen, J. Dural, M. Gaillard, R. Genre, M.

Hage-Ali, R. Kirsch, A. l’Hoir, B. Mazuy, et al.

To cite this version:

S. Andriamonje, M. Chevallier, C. Cohen, J. Dural, M. Gaillard, et al.. RADIATIVE ELECTRON CAPTURE BY FAST HIGHLY STRIPPED HEAVY IONS CHANNELED IN A THIN CRYSTAL.

Journal de Physique Colloques, 1989, 50 (C1), pp.C1-285-C1-293. �10.1051/jphyscol:1989131�. �jpa-

00229328�

JOURNAL DE PHYSIQUE

Colloque C1, suppl6ment au n o l , Tome 50, janvier 1989

RADIATIVE ELECTRON CAPTURE BY FAST HIGHLY STRIPPED HEAVY IONS CHANNELED IN A THIN CRYSTAL

s . ANDRIAMONJE, M. CHEVALLIER * , C. COHEN* * , J. DURAL * * * ,

M.J. GAILLARD*, R. GENRE*, M. HAGE-ALI""', R. KIRSCH*, A. L'HOIR*", B. M A Z W * , J. MORY***"" , J. MOULIN**, J.C. POIZAT*, J. REMILLIEUX*, D. SCHMAUS * * and M. TOULEMONDE***

Centre D'Etudes Nucleaires de Bordeaux and IN2P3, F-33170 Gradignan, France

' ~ n s t i t u t de Physique Nucleaire and IN2P3, UniversitB Claude Bernard Lyon-1, F-69622 Villeurbanne Cedex, France

* * ~ r o u p e de Physique des Solides de 1'Ecole N o m a l e SupBrieure, F-75251 Paris Cedex 05, France

Centre Interdisciplinaire de Recherches avec les Ions Lourds.

F-14040 Caen Cedex, France

* * * * Groupe Physique Appliquge aux Semiconducteurs, Centre de Recherches

NuclBaires, F-67037 Strasbourg Cedex, France

* * * * * Laboratoire des Solides IrradiBs, Ecole Polytechnique. F-91128

Palaiseau Cedex, France

RBsurnB

- On s a i t que I ' i n t e r a c t i o n d e s ions a v e c une cible monocristalline est t r 6 s sensible B I'orientation du f a i s c e a u incident par r a p p o r t aux directions particuligres du cristal. Nous avons m o n t r e q u e la canalisation d'ions lourds d e t r b s h a u t e Bnergie modifie l e r a l e n t i s s e m e n t e t 1'Bchange d e c h a r g e d e s ions. Non s e u l e m e n t l e s ions canalisds n e p a r c o u r e n t que d e s regions B f a i b l e densit6 Blectronique m a i s l e s d l e c t r o n s r e n c o n t r e s n e p e u v e n t & r e c a p t u r e s a u moyen du p r o c e s s u s ordinaire non radiatif. Dans une expgrience oh d e s ions XBnon d e c h a r g e 52+, 5 3 e t 54' s o n t envoyds s u r un c r i s t a l m i n c e d e Silicium, nous observons que pour u n e incidence quelconque l e s ions t r a n s m i s a t t e i g n e n t l'dquilibre d e c h a r g e m a i s q u e l e s ions canalises s o n t pour l a plupart t r a n s m i s s a n s avoir c h a n g e d ' d t a t d e charge. D'une p a r t ils n e p e u v e n t p a s p e r d r e d'dlectron e t d ' a u t r e p a r t l e seul processus d e c a p t u r e qui l e u r est a c c e s s i b l e est l a c a p t u r e r a d i a t i v e ( R E C ) , u n p r o c e s s u s d ' o r d i n a i r e m a s q u e p a r l a p r e d o m i n a n c e d e l a c a p t u r e rnecanique (MEC). Nous avons o b s e r v e l e s photons Bmis par R E C e t qui c o r r e s p o n d e n t B d e s c a p t u r e s d a n s l e s c o u c h e s K, L ou M d e s ions XBnon. L a s t r u c t u r e d e s r a i e s o b s e r v e e s r e f l e t e l a distribution d e s vitesses d e s Blectrons captures. L a canalisation p e r m e t donc d'btudier l a c a p t u r e r a d i a t i v e p a r d e s ions d e c h a r g e bien definie d a n s une c i b l e d e n s e d'blectrons quasi-libres. C e t t e opportunite p o u r r a i t 6 t r e Btendue & d ' a u t r e s processus c o n c e r n a n t d e s ions lourds t r k s rapides.

Abstract - I n t e r a c t i o n of moving ions with single c r y s t a l s is known to b e very sensitive t o t h e o r i e n t a t i o n of t h e incident b e a m with r e s p e c t t o t h e c r y s t a l l i n e directions of t h e t a r g e t . We h a v e shown t h a t channeling conditions strongly modify t h e slowing down and t h e c h a r g e exchange processes of high e n e r g y heavy ions. The r e a s o n is t h a t channeled p a r t i c l e s a r e p r e v e n t e d f r o m approaching t h e t a r g e t a t o m s , a n d t h e n c a n i n t e r a c t only w i t h loosely bound t a r g e t electrons. This r e s u l t s n o t only in drastically reducing t h e e l e c t r o n density experienced by t h e m , which lowers e n e r g y loss a n d e l e c t r o n loss, b u t also in inhibiting t h e radiationless c a p t u r e of bound electrons, s i n c e most of t h e available e l e c t r o n s a r e quasi-free valence electrons. In a n e x p e r i m e n t w h e r e hig5 e n e r g y xeq+ ions, with q

52, 53, 54, a r e d i r e c t e d o n t o a thin Si single c r y s t a l w e observe t h a t , w h e r e a s unchanneled ions r e a c h c h a r g e equilibrium very rapidly, m o s t channeled ions k e e p t h e i r incident c h a r g e state unchanged all along t h e i r passage through t h e crystal. On o n e hand t h e loss of t h e i r electron(s) (q = 52, 53) is impossible. On t h e o t h e r hand t h e only way t h a t is l e f t t o t h e m t o c a p t u r e quasi-free e l e c t r o n s is t h e r a d i a t i v e e l e c t r o n c a p t u r e (REC). This process d o e s o c c u r s also f o r unchanneled ions but is q u i t e difficult t o observe, particularly b e c a u s e of t h e overwhelming non radiative Mechanical E l e c t r o n Capture(MEC). R E C photons h a v e been observed in channeling conditions and correspond t o e l e c t r o n c a p t u r e into t h e K, L a n d M shells of t h e projectiles. The shape of t h e R E C photon lines r e f l e c t s t h e m o m e n t u m distribution of t h e e l e c t r o n e n c o u n t e r e d by channeled ions. Channeling o f f e r s a unique opportunity t o study t h e r a d i a t i v e e l e c t r o n c a p t u r e s i n c e i t allows ions of well defined c h a r g e s t a t e t o t r a v e l through a d e n s e 'quasi-free e l e c t r o n t a r g e t . This opportunity could be e x t e n d e d t o o t h e r processes involving high energy heavy ions.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989131

JOURNAL DE PHYSIQUE

We describe a n e x p e r i m e n t a l work, p e r f o r m e d a t GANIL, in which t h e c h a r g e e x c h a n g e process of highly s t r i p p e d 25 MeV/u Xenon ions in a thin Silicon c r y s t a l is observed by m e a n s of t w o c o m p l e m e n t a r y methods, t h e m e a s u r e m e n t of t h e c h a r g e s t a t e distribution of t r a n s m i t t e d ions a n d t h e s p e c t r o m e t r y of t h e projectile X-rays e m i t t e d during t h e t r a v e r s a l of t h e target. In t h e s e e x p e r i m e n t a l conditions t h e profound modification of t h e ion-solid i n t e r a c t i o n t h a t is brought by channeling e f f e c t s r e s u l t s f i r s t in t h e a l m o s t c o m p l e t e "freezing" of t h e incident c h a r g e s t a t e . This is d u e t o t h e d r a s t i c r e d u c t i o n of e l e c t r o n loss a n d c a p t u r e probai,ilities. Channeling conditions also r e d u c e strongly t h e e x c i t a t i o n probability of t h e projectiles, b u t t h e main consequence of channeling in our study lies in t h e e m e r g e n c e of t h e R a d i a t i v e E l e c t r o h C a p t u r e (REC), a process usually difficult t o observe b e c a u s e i t is overwhelmed, in amorphous t a r g e t s (or unaligned crystals), by t h e Mechanical E l e c t r o n C a p t u r e process (MEC). We r e c a l l f i r s t t h e g e n e r a l p r o p e r t i e s of channeling and t h e e x p e c t e d consequences on t h e i n t e r a c t i o n of s u c h f a s t heavy ions with a solid t a r g e t . Then, a f t e r a s h o r t description of t h e e x p e r i m e n t a l set-up w e describe o u r m e a s u r e m e n t s of c h a r g e s t a t e distributions. A f t e r w a r d s w e describe t h e X-rays s p e c t r a in which (np - Is) t r a n s i t i o n s and R E C lines a r e observed, and discuss f u r t h e r developments of s u c h studies.

2 -- CHANNELING IN BRIEF

Channeling of positive ions in a c r y s t a l o c c u r s when t h e direction of t h e incident b e a m is c l o s e t o a plane o r a n a x i s d i r e c t i o n of t h e crystal. Channeling p r o p e r t i e s a r e s k e t c h e d in figure 1 w h e r e a positive ion e n t e r s a c r y s t a l in a parallel d i r e c t i o n with a n axis (a), w i t h a n i m p a c t p a r a m e t e r p l w i t h r e s p e c t t o t h e a t o m i c string. If p l is l a r g e r t h a n t h e t h e r m a l vibration a m p l i t u d e of t h e t a r g e t a t o m s , t h e i m p a c t p a r a m e t e r s of t h e successive collisions with t h e row a t o m s a r e n o t randomly distributed b u t a r e s e e n t o i n c r e a s e slowly along t h e row, and t h e incident ion, repelled by t h e a t o m i c s t r i n g a s a whole, is said t o b e channeled. T h e ion t r a j e c t o r y resulting f r o m t h e s e c o r r e l a t e d multiple s c a t t e r i n g e v e n t s c a n b e c a l c u l a t e d q u i t e a c c u r a t e l y by using t h e continuum p o t e n t i a l approximation. The c h a n n e l e d p r o j e c t i l e s a r e in a potential V(p) t h a t is t h e zero-order t e r m of t h e F o u r i e r s e r i e s in which t h e periodic ion-atom p o t e n t i a l a t a d i s t a n c e p along t h e s t r i n g c a n b e decomposed. The p o t e n t i a l V(p), r e p r e s e n t e d in figure Ib, would b e infinite f o r p = 0 if t h e t a r g e t a t o m s w e r e s t a t i c . The t h e r m a l motion of t a r g e t a t o m s c a n be shown t o result in a f i n i t e value of V(0).

Fig. 1 - Channeling t r a j e c t o r y of a positive ion incident in a parallel direction with a c r y s t a l axis, with t h e i m p a c t p a r a m e t e r p l . The a r r o w s on o n e of t h e a t o m s of t h e s t r i n g s i m u l a t e t h e t h e r m a l m o t i o n cf t a r g e t a t o m s (a). Variation of t h e continuum p o t e n t i a l with

, t h e d i s t a n c e t o t h e axis, f o r a s t a t i c r o w ( d o t t e d l i n e ) a n d w i t h t h e t h e r m a l m o t i o n i n c l u d e d ( s o l i d c u r v e ) (b).

0 ^{- -} -

^- - ^-

v ( p l h tatic row

V rnax

0

p

L r

p 2 p3

The entrance conditions, incidence angle t o the axis direction and impact parameter, define the transverse energy o f each projectile, that can be considered, i n a f i r s t approximation, as being constant during the passage of the ion through a thin crystal. The value o f the transverse energy determines the closest distant of approach t o the atomic rows. The best channeled particles have the smallest transverse energy and then are maintained f a r f r o m the atomic rows. The channeling c r i t i c a l angle Y , defined by E Y '

Vm x, the potential maximum, is the maximum incidence angle t o the axial &rection f o r an 'ion t o %e channeled. This equation gives the energy dependence o f the angular acceptance f o r channeling. (In the experimental conditions of this work, the c r i t i c a l angle was around 1 mradian).

I n f a c t a more complete description o f axial channeling must take more than one atomic string into account. On figure 2 we show the map o f the target electron density averaged along the <'l10>

direction of a silicon crystal (a) and the map o f the ion potential f o r a single charge ion, also averaged along the same direction (b). The electron density has been calculated by means of the electron wave functions i n solid silicon and the potential has been deduced f r o m the electron density. Along this axial direction, w i t h the particular arrangement o f the atomic rows i n a diamond type crystal, the best channeled particles, with, f o r example, a transverse energy smaller than 1.5 eV (contour 2 o f figure 2b) are confined w i t h i n this contour and then between a given set o f six atomic strings t h a t is called a channel. Figure 2a shows t h a t these particles encounter

very low electron density (less than- 0.24 A-') compared w l t h the density encountered by a p a r t i c l e travelling randomly i n silicon (0.7 A

Then we can summarize the main properties o f channeled ions : as they cannot approach the atomic rows, they do n o t interact closely w i t h the target nuclei and w i t h the innershell target electrons ( i n particular the suppression o f Rutherford scattering is, f r o m a practical point of view, the most useful feature o f low energy channeling). Also the lowering of the encountered electron density reduces the energy loss r a t e by a factor up t o two or three. The e f f e c t s o f channeling on the electron capture and loss of moving ions w i l l be discussed later.

@

@

Fig. 2 - Map of the electron density i n a silicon crystal, averaged along the <110> direction.

Contour lines 1 t o 0 : 0.032, 0.1, 0.24, 0.5, 1, 3, 10 and 30 ( A - ~ ) (a).

Map of the continuum potential f o r a u n i t charge along the <110> direction o f Si. Contour lines 1 t o 8 : 0.5, 1.5, 2.5, 7, 20, 50, 100, 200 (eV) (b).

3 - EXPERIMENTAL SET-UP

The experimental arrangement, already described i n a previous report 121, is schematized i n

figure 3. We used t h ? LISE beam line a t GANIL, a line specially desi nerl t o deliver highly stripped

i o n beams of excellent space and momentum definition. 25 MeVlu Xe4+ ions, w i t h q

52, 5 1 or 54,

were produced by the post-stripping i n a Be f o i l of the incident 27 MeV ~ e beam. The charge ~ ~ +

selection was operated by means o f the magnetic spectrometer M 1 a f t e r t i g h t collimation of the

beam (angular divergence less than 0.2 m a d , diameter 4 mm). Unfortunately, as the mean charge

state of the beam transmitted+through the Be f o i l was around 50, the beam intensity went d3wn

quite rapidly f r o m 52' t o 54 beams. A f t e r passage through a r o t a t i n g beam-chopper used t o

Cl-288 JOURNAL DE PHYSIQUE

monitor the beam intensity, either by the beam current measurement or by the counting i n a Si-Li detector o f the X-rays produced i n the silver coating o f the r o t a t i n g blades, the beam was sent onto the crystal, a 1 7 p m thick (111) Si crystal hold by a three-axis goniometer. The crystal was vie;ued b y a solid state Ge detector positioned a t 90° t o the beam. The transmitted beam was energy and charge analyzed by means o f the msgnetic spectrometer M Z and a w i r e chamber. By stepping t h e intensity i n M p the energy loss spectrum associated t o each charge state, and the charge state distribution could be obtained.

BEAM-CHOPPER

CRYSTAL

A M2

L

STRIPPER FOIL (Be) CHAMBER

27MeVf u

~e~~

Fig. 3 - Experimental set-up.

4 -

I n figure 4 we show the cherge state distributions obtained w i t h 52+, 53', 54' Xe ions, respectively, incident along the <110> direction o f the Si crystal and also along a "random" direction, i.e. a direction f r e e o f channeling effects. I n b o t h cases the projectile pathlength through the crystal was 21 p n . The "random" charge distribution is measured t o be the s a n e f o r the three incident charges, which means t h a t the charge state equilibrium i s reached. The average charge state is 49.5. I n this w e l l known case a large number of capture and loss events have taken place. The dominant capture process is the Mechanical Electron Capture (MEC) o f innershell target electrons, i n which the incident ion, the captured electron and the ionized target atom share the available energy /3/. The impact parameters are small, of the order the magnitude of the orbital of the electron t o be captured. The Radiative Electron Capture (REC), i n which the photon emission allows f o r energy and momentum conservation /4/ can also occur, b u t i t s probability is much smaller a t this i o n velocity although a much larger range o f impact parameters contributes t o R E C (J.E. Miraglia, R.

Gayet and J. Salin, p r i v a t e communication). As f o r the projectile electron loss f r o m i t s K- and L- shells it is due, as MEC, t o low impact parameter collisions w i t h target atoms.

On the contrary the three distributions obtained f o r axial alignment are quite d i f f e r e n t f r o m each other, although o f analogous shapes, and are then f a r f r o m equilibrium. I n each case the distribution is peaked on the incident charge and presents a rapid decrease on the low charge side. On the other hand a l l the 53+ and 52' incident ions keep their(s) K electron(s). Moreover the detailed observation of the energy spectrum o f transmitted ions of each particular charge state shows t h a t ions "frozen"

i n their i n i t i a l charge state were channeled i n the crystal, as revealed by their reduced energy loss,

which is 0.45 times t h e "normal" energy loss ( i n agreement w i t h values measured w i t h l i g h t e r ions

a t lower velocities). The ions that had captured one electron were channeled f o r a p a r t o f t h e m

only, whereas the rest of the transmitted ions were not channeled. Then the charge state

distributions are easily understood

the high charge states correspond t o channeled ions t h a t have

capture zero or one electron and the l o w charge states correspond t o unchanneled ions t h a t have

experienced "random-type" charge exchange processes. It must be noted t h a t the f i r s t observation o f

channeled "frozen" ions has been performed by Datz e t al. /5/ w i t h oxygen ions of much lower

velocities transmitted through a t h i n gold crystal. I n this case also the suppression o f close

collisions f o r channeled particles inhibits the MEC process, and then reduces strongly the electron

capture probability since the only process available is REC. I n our experiment t h e loss o f K-shell

electrons by channeled Xe ions is n o t only reduced b u t forbidden since collisions w i t h a valence or an outershell target electron cannot transfer enough energy (even excitation is impossible, a feature t h a t w i l l be discussed later). Finally the probability f o r channeled ions of capturing an electron by R E C is seen t o decrease f r o m the 54' case t o the 52' case which is (qualitatively) understood because the REC i n t o the Xe K-shell is a large p a r t o f t o t a l REC, as also discussed below.

Emerging ion charge

Fig. 4 - Charge state distributions obtained w i t h 25 MeV/u 52+, 53' o r 54' X e incident ions, i n random conditions and f o r <110> alignment a f t e r a pathlength o f 21 p m through the Si crystal.

5 - ^STUDY

X-RAY SPECTRA

The f i r s t observation of R E C by heavy ions i n solids is due t o Schnopper e t al. / 6 / . The f i r s t experiment designed f o r the study of R E C using channeling conditions was performed by the Oak Ridge group /7/ i n the wake of their previous work about channeling e f f e c t s on charge exchange.

However the experimental conditions (projectile atomic number and velocity) did n o t allow them t o observe REC spectra i n a very detailed way. The present experiment was then undertaken t o yield a b e t t e r observation o f REC properties.

We r e c a l l the energetics o f the radiative capture of a f r e e electron assumed i n i t i a l l y a t rest by a moving ion. The photon e m i t t e d i n the radiative capture i n t o a state o f binding energy E has the enefgy E R E ? Ep + El, where E' is the kinetic energy o f an electron t h a t would %ave the p r o j e c t ~ l e ve o c ~ t y . This value is calculated i n the i o n rest frame b u t the corresponding energy o f the photon detected a t 90° is quite close t o this value.

The X-ray spectra o f the solid state Ge detector viewing the crystal are shown i n figures 5 t o 7 f o r incident 52+,

<110> incidence, respectively, and f o r the same number of incident ions.

The information contained i n these spectra is quite abundant, even f o r random incidence, t h a t we

consider first.

C1-290 JOURNAL DE PHYSIQUE

(a) "Random" spectra

We start w i t h the simplest case, the random spectrum obtained w i t h 52' ions (figure 5), that w i l l be used t o describe the general features o f this set o f data. The spectrum is dominated by a series o f lines t h a t we f i n d convenient t o c a l l the Lyman+ series (a, 8, y), although the ions are n o t H-like ones. On the contrary, as we know t h a t these 52 ~ n c i d e n t ions reach charge equilibrium i n the target around an average value o f 50, these lines result mainly f r o m collisional excitation o f K-shell electrons. N o t e t h a t the linewidths are larger than the energy resolution o f the detector and are due t o the slight charge dependence of the transition energies. Weaker lines are also observed, t h a t correspond t o the radiative electron capture i n t o K, L, and M shells o f the projectile. The L- and M- REC peaks represent the contribution o f R E C t o the electron capture necessary t o reach the average equilibrium charge state o f 50, even i f electron captul'e is dominated by MEC. As f o r the very weak K-REC peak,

corresponds t o the REC contribution t o the capture following collisional loss o f a K-shell electron. N o t e that, as i n a l l the presented spectra, the small peak a t 18 keV is due t o an escape e f f e c t f r o m the Ge detector i n the detection of the L y a photons.

PHOTON ENERGY (keV

Fig. 5 - X-ray spectra f r o m the Ge detector viewing the Si crystal bombarded by 25 MeV/u x e S 2 + i o n s i n r a n d o m and < 1 1 0 > a l i g n m e n t c o n d i t i o n s , r e s p e c t i v e l y ( n o t e t h e l o g a r i t h m i c scale).

;Lye I I

25 W / u xea*

...

Y 1 1 0 >

-

K-REC

1

lo2

- . .

I I I . I I

10 20 -. 30 40 50 60 70

PHOTON ENERGY (keV 1

Fig. 6 - As f o r figure 5 w i t h 53+ ions.

Lastly the strong continuum spectrum underlying the lines is i n f a c t composed o f t w o parts t h a t cannot be distinguished. The f i r s t one is due t o primary bremsstrahlung, the radiation e m i t t e d by target electrons accelerated by the Coulom'r, f i e l d of the projectile. This process can be considered as a radiative electron "capture" i n t o the continuum of the projectile and the photon energy distribution decreases rapidly f r o m zero t o E', the l o w energy l i m i t of the REC line series.

The second one is due t o secondary bremsstrahlung, i.e. the bremsstrahlung emission o f the target electrons knocked on by the incident ions. The resulting photon energy distribution decreases also rapidly, but f r o m zero t o - 4 El. There i s unfortunately a t h i r d contribution t h a t i s due t o nuclear background. This is negligible i n the 52' case but not i n the 53' and specially i n the 54' case f o r which the beam intensities are lower.

L y e

I ^25MeVIu

fl

...

10

20

PHOTON ENERGY (keV )

Fig. 7 - As f o r figure 5 w i t h 54' ions.

If we consider now the random spectrum obtained w i t h 53' ions (figure 61, why should i t be d i f f e r e n t f r o m the previous case ? The difference between the t w o situations is t h a t the f i r s t charge exchange event w i l l be, w i t h a high probability, an electron capture, MEC or less probably REC. Afterwards, the t w o cases are identical because the crystal thicknzss is large compared w i t h t h e mean pathlength between the entrance i n the crystal and this f i r s t event. But we know f r o m measurements f r o m Meyerhof e t al.

communication) t h a t i n our experimental conditions the MEC process involves mainly the excited states o f Xe ions. Then (except f o r the 2s state) these excited states decay very rapidly (in comparison w i t h the projectile dwell t i m e i n the target) and contribute t o the Lyman emission. I n f a c t we observe experimentally t h a t the comparison of the Lyman yield f r o m 52+ and 53+ ions incoming along a random direction is colnpatible w i t h the idea t h a t an incident 53' ions produces one more L y m a n photon i n the target, on the average, than a 52' ion.

may be interesting t o remark t h a t the precise subtraction of t h e 52' spectrum f r o m the 53' one would give the Lyman spectrum due t o the electron capture by a 53' io,, which could be, more generally a method f o r studying capture without the necessity o f using gas or very t h i n solid targets.

The last random spectrum, obtained w i t h 54' ions ( f i g w e 71, is also, i n an snalog way, the sum of a 53' spectrum and o f the Lyman spectrum associated with the electron capture by a bare 54' ion.

The p a r t i c u l a r i t y o f this s p e c t r u n is the f i n e structure appearing i n the Lyman lines which is due t o

the higher energies o f the H - l i k e Lyman transitions. I n the case o f n = 3 + 1 transitions f o r

example this high energy l i n e is due t o t h s 3p - I s transition a f t e r the f i r s t electron capture by an

incident 54' ion. I f one supposes t h a t the electron has been captured i n t o the 3p state (then

ignoring cascade effects), the radiative H-like decay can be observed only i f it occurs before a

second electron capture takes place. I f not, the photon w i l l contribute t o the lower energy peak o f

Lyf3. This interesting competition between radiation decay and electron capture could be studied

quantitatively but is com7licated by cascade e f f e c t s because the f i r s t electron can be captured i n t o

a higher level and decay through the 3 p state. We w i l l come back t o this point i n the study of the

C1-292 JOURNAL DE PHYSIQUE

spectra obtained i n alignment conditions. 4 f i n a l remark about random spectra is t h a t a l l o f the incident ions have empty L- and M- shells. As a consequence the L- and M - R E C probabilities and then the number of L- and M - REC photons per incident ion shz~uld be the same i n the three random sp&ctra. We do observe t h a t and deduce t h a t the 54' Lyman spectrum is mainly due t o the electron capture process.

(b)

"Aligned"

Although we have seen t h a t i n alignment conditions the incident ions present a broad distribution o f transverse energies, we may simplify the discussion by distinguishing only between channeled and non channeled particles.

F i r s t o f a l l one observes a strong reduction o f the bremsstrahlung yield w i t h respect t o the random case. This is o f course due t o the reduction o f the electron density encountered by channeled particles whereas the non channeled p a r t o f the beam experiences a normal electron density.

Another feature common t o the three spectra is the strong reduction of the Lyman event probability. This is n o t surprising since both K electron excitation and mechanical electron capture require small impact parameter collisions w i t h target atoms. I n the 52' case where the Lyman photons are due t o excitation only, the channeled particles do n o t contribute a t a l l and the channeling e f f e c t on the L y m a n yield is stronger than the e f f e c t on bremsstrahlung. I n the 53' and 54+ cases, where Lyman transitions induced by the non channeled particles are also due t o MEC, the channeled particles contribute by means o f M - and L- REC (cf. below), and the channeling e f f e c t is less pronounced.

We note t h a t the structures i n the L y m a n lines o f t h e 54' random spectrum can be observed again, b u t it is quite intriguing t h a t i n both f o r L y a and LyB, the r a t i o between the t w o components is changed i n favour o f the H- l i k e one. We believe t h a t the competition between t h e deexcitation process o f the f i r s t captured electron and the capture o f second electron i s d i f f e r e n t f o r particles incident along a random direction and f o r non channeled particles of an aligned beam : although random and non channeled particles may have the same average probability of close collision w i t h target atoms i n the crystal, the statistics o f collisions is quite particular f o r non channeled particles, which spend short times i n areas of high nuclear density (the atomic strings) and long times i n areas of zero density (between atomic strings). This property of the unchanneled p a r t o f an aligned beam had never been described or observed before.

A t last we come t o the study o f the R E C photons, which f o r m prominent lines. I n the 52' spectrum, only M - and L-REC lines show up whereas strong K - REC peaks appear i n the cases o f incident H-like 53' and bare 54' ions, as

could be expected.

The position o f the peaks are w e l l understood : the M - and L - REC photons have higher energies than f o r random incidence and the s h i f t increases w i t h the incident charge. This is due t o the charge state dependence o f the binding energy o f the L- shell electrons. I n the same way the s h i f t between the 5 3 and 54' K - REC peaks reflects the difference between the binding energies o f the I s electrods) o f H- l i k e and He- l i k e Xe ions.

The shape o f the R E C lines is also o f great interest. I n particular the K- R E C peaks, located i n an energy range where the continuum is low, appear rather wide and asymmetrical w i t h a t a i l on the l o w energy side. The width is due t o the momentum distribution (Compton profile) o f the captured target electrons. The asymmetry results f r o m the dependence of the REC cross section upon the r e l a t i v e velocity of the Xe i o n and the electron t o be captured. The shape indicates that the main contribution t o REC comes f r o m valence target electrons. Moreover, if one assumes t h a t the angular distribution o f K- R E C photons follows a sin2 Olab law, the K - REC intensity is found equal t o 0.2 times the theoretical value deduced f r o m Bethe and Salpeter estimates. This can be explained by the f a c t t h a t the mean electron density experienced by channeled ions is calculated t o be equal t o 0.2 times the mean electron density i n silicon.

On the other hand

is noticeable t h a t the L- R E C intensity is very l i t t l e a f f e c t e d by channeling effects. Since the L- shell is empty i n channeling conditions and nearly empty i n random conditions, t h e steadiness of the L- R E C intensity seems t o indicate t h a t the radiative electron capture i n t o t h e L- shell i s n o t sensitive t o the l o c a l electron density, a surprising result t h a t further experiments should help t o elucidate. More generally the study o f t h e radiative eiectron capture i n t o (n >

and the photon angular distribution.

Further work on associated charge state measurements and X-ray observation under channeling

conditions should include coincidence measurements, a study of photon angular distribution, a

precise study o f the crystal-beam orientation dependence of capture and excitation processes

(previous results / 2 / have n o t been discussed here). Other charge states could be used f o r the

incident ions.

6 - CONCLUSION

We have described the charge exchange processes involved i n the interaction of helium-like, hydrogen-like and bare Xenon ions of high energy w i t h a t h i n silicon single crystal. I n particular we have shown t h a t heavy ions channeled along a major axial direction interact mainly w i t h quasi-free target electrons. Then i n this case the ion-solid interaction becomes rather an ion-electron interaction and this type o f target, equivalent t o a dense electron target ( i n comparison w i t h electron beams i n crossed beam experiments), could be used f o r other studies.

One o f them i s the study o f Resonance Electron Transfer (RTE), the inverse Auger process, i n which t h e energy available i n the capture of a quasi-free electron by a heavy i o n is transfered i n t o e x c i t a t i o n of another electron o f the ion. This resonant process could be studied i n channeling conditions and yield then sharper resonances than i n gas targets.

Another interesting possibility would be t o use channeling conditions f o r the study of the electron loss process w i t h incident ions carrying a large number of electrons.

A l a s t and i m p o r t a n t r e m a r k i s t h a t t h e e x p e r i m e n t a l s i t u a t i o n d e s c r i b e d i n t h i s w o r k i s kinematically equivalent t o the case of experimental works i n which highly charged heavy ions, produced i n an i o n source and basically a t rest, are bombarded by low-energy electrons, f o r instance f o r electron loss /9/ o r excitation / l o / studies. The complementary o f the two methods shoud be quite useful i n the study o f t h e interaction o f highly charged ions w i t h electrons.

We thank the G A N I L s t a f f f o r delivering t o us a high quality beam. This work was specifically supported by CNRS and IN2P3, and p a r t l y supported by GDR 202.

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