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INTERACTION OF FAST HYDROGEN IONIC
CLUSTERS WITH MATTER
M. Chevallier, N.V. de Castro Faria, B. Farizon-Mazuy, M. Gaillard, J.-C.
Poizat, J. Remillieux
To cite this version:
JOURNAL
DE
PHYSIQUEColloque C2, suppl6ment au
n02,
Tome 50, f6vrier 1989INTERACTION OF FAST HYDROGEN IONIC CLUSTERS WITH MATTER
M. CHEVALLIER, N.V. de CASTRO FARIA, B. FARIZON-MAZUY, M.J. GAILLARD, J.C. POIZAT and J. REMILLIEUX
Institut de Physique Nucleaire (and IN2P3), Universite Claude Bernard Lyon-l, 43, Bd du 11 Novembre 1918, F-69622 Villeurbame Cedex, France
R6urn6
- L e s a g r h g a t s rapides d ' h y d r o g i n eH
'
n'interagissent pas a v e c 1a m a t i & r e d e 1a m @ m e m a n i i r e que d e s protons d e m @ m e v i t e s e . Nous prksentons l e s r i s u l t a t s obtenusi
Lyon c o n c e r n a n t I'interaction d'agrhgats Ibgers dlhydrog&ne (52
n_<
23) a v e c une c i b l e g a z e u s e ou une feuille mince, des vitesses voisines d e l a vitesse d e Bohr (dynamique d e l a f r a g m e n t a t i o n , &tat d e c h a r g e des f r a g m e n t s Cmergents, section e f f i c a c e d e dissociation dans un gaz). Nous discutons aussi d'expkriences f u t u r e s et e n particulier plus h a u t e knergie.Abbtm.19 - F a s t ionic c l u s t e r s H+ i n t e r a c t with m a t t e r in a specific way which is observed t o n
d e v i a t e strongly f r o m t h e i n t e r a c t i o n of a t o m i c ions at t h e s a m e velocity. We present s o m e results obtained a t Lyon about foil and g a s i n t e r a c t i o n s of hydrogen c l u s t e r s (5
<_
n2
23) a t projectile velocities close t o t h e Bohr velocity, i.e. dynamics of t h e c l u s t e r fragmentation, c h a r g e state of a t o m i c f r a g m e n t s and absolute dissociation c r o s s s e c t i o n s in gas. We a l s o discuss f u t u r e experiments specially at higher velocities.R e c e n t advances in experimental techniques allow to produce a t o m c l u s t e r s of various s i z e s a n d compositions. This h a s c r e a t e d a new a r e a of r e s e a r c h s i n c e c l u s t e r s c a n be regarded a s l a r g e molecules o r a s f r a g m e n t s of condensed m a t t e r . The a c c e l e r a t i o n of c l u s t e r s s t a r t e d when Henkes /l/ s e n t CO2 c l u s t e r ions through a potential d i f f e r e n c e up t o 2 KV a n d analysed t h e i r m a s s spectrum. These e x p e r i m e n t s soon led t o various e f f o r t s t o a c c e l e r a t e hydrogen ions f o r use in nuclear fusion technology. T h e hydrogen c l u s t e r s a r e t h e simplest ionic molecular complexes. Their s t r u c t u r e h a s generally been described as H2 molecules surrounding a
H;
ionic core, i.e. H ; ( H ~ ) ~ 121.Hydrogen c l u s t e r b e a m s of energies ranging f r o m 40 t o 700 keV a r e c u r r e n t l y delivered by t h e Cockroft-Walton c l u s t e r a c c e l e r a t o r of t h e Institut d e Physique Nuclkaire d e Lyon 131. T h e a c c e l e - r a t e d ionic c l u s t e r beam is s e l e c t e d in energy a n d mass by e l e c t r o s t a t i c a n d m a g n e t i c analysers. In a f i r s t experiment
141,
we have measured t h e transmission of light hydrogen c l u s t e r ions through a argon g a s t a r g e t , f o r projectile velocities b e t w e e n 1.5 t o 5 10' c m l s , i.e. around t h e Bohr velocity. These m e a s u r e m e n t s allowed us t o study t h e dissociation cross-sectionsa
d as a function of t h e mass of t h e c l u s t e r and i t s velocity. Determining ad is useful not only f o r practical reasons, but a l s o because t h e velocity dependence ofad
f o r a given c l u s t e r c a n help understand t h e break up processes. Moreover t h e variation ofa
with n, t h e c l u s t e r proton number, at a given velocity, yields informationd on t h e c l u s t e r structure.
On figure 1, we show t h e mass dependence of t h e dissociation c r o s s section f o r 2 0 keV/p H: c l u s t e r s in a n argon gas t a r g e t . N o t e t h a t this figure includes ad f o r 12 keV/p c l u s t e r s with n = 21 e t 23 (square symbols) s i n c e w e know t h a t in this velocity r a n g e ad remains a l m o s t independent of t h e velocity. If o n e e x c e p t s t h e o values obtained f o r n d = 15 a n d n = 19, t h e variation of
ad
with n is very well described by t h e lawad
= KN~
with a = 0.66-
213 a n d K = 8.65i2
(see figure 1). Since t h e c l u s t e r orientations with r e s p e c t t o t h e beam direction a r e randomly distributed, w e mayJOURNAL
DE
PHYSIQUEFiguie
I-
Mabb dependence
06the dibbociation c z o ~
section
06H:
chbterb in azgon gab. The
dabhed line cozzebpondb to a
6it
with a n2I3 Paw (bee text].
BO- m
5
-
m 'v' E 0 '0 40- '0, b D 20- o t 0 1 l I 010-
l
/'
-
%--l='
0,,.iO-i-'j
f
-,p'
,v0
20 keV/p /' m 12 keV/p-
/ /' / / / . . . ~ . l . . B S l . t t ' ~ . * ~ . ~ I . ~ ' 5 X) 15 20 n-
Figuie
2 -Mabb dependence
06the FWHM
06t h e angu9a.r dibtzibutionb
06neutza! atom6
zebdting 6zom the dibbociation
0630 to 120
keUlp
H ;clubteza in a
2.1Fg.cm-2 thick
cazbon 602.
e
Figuie
3 -Mu44 dependence
06the n e u t d
dzaction in the atomic 6zagmentb zebdting 6zom
the
dibbociation
0630 to 120 keUIp
H ;chstezb in a
2.1pg.crn-2 thick cazbon boil.
consider t h a t t h e clusters a r e statistically "spherical" a s f a r a s t h e dissociation is concerned. The observation of a n2J3 dependence f o r t h e dissociation cross section reflects t h e invariability of t h e cluster compactness a s n varies, just like t h e dependence of t h e nuclear radius reflects t h e invariability of t h e density of t h e nuclear m a t t e r . The value found for K leads t o a density of 0.052
O 3
atoms/A if one makes t h e hypothesis t h a t t h e dissociation cross section is equal t o t h e geometrical cross section of "spherical" clusters. This value is in good a g r e e m e n t with t h e previous e s t i m a t e by
"3
Van Lumig and Reuss 151 (0.025 H2/A ). It is c l e a r t h a t more experimental work bearing particularly on t h e dissociation fragments would be needed t o identify t h e various dissociation processes. In a second group of experiments, we focused our a t t e n t i o n on some consequences of t h e penetration of e n e r g e t i c clusters into thin carbon foils. These experiments generally involve a detailed analysis of emerging fragments and usually require a detailed analysis of t h e velocity distributions and of t h e charge s t a t e distribution of t h e emerging fragments. These distributions c a n be compared with measurements performed with a t o m i c projectiles of t h e s a m e velocity.
When a n hydrogen ion cluster penetrates into a condensed medium most of i t s electrons a r e immediately stripped off and t h e binding between t h e a t o m s constituting t h e cluster is disrupted. The ionic fragments a r e repelled from e a c h others by a screened Coulomb interaction while they fly through t h e solid. When they e m e r g e from t h e e x i t surface of t h e foil some of these fragments bind electrons t h a t may originate from t h e incident projectile or t h a t have been picked up from t h e target. The first process is dominant in t h e c a s e of very small dwell t i m e s in t h e foil and is directly related t o t h e transmission probability of t h e projectile electrons 161. In t h e "pick-up" regime corresponding t o thicker foils, t h e close proximity between t h e fragments a t t h e exit surface tends t o decrease their a v e r a g e final c h a r g e s t a t e 17-91. In t h e particular c a s e of hydrogen molecules, t h e s e e f f e c t s a r e known t o increase t h e neutralization probability of a t o m i c fragments.
We have measured t h e angular distribution and t h e yield of t h e hydrogen fragments emerging from a self-supporting 2.1 pg.cm-2 carbon foil bombarded with H: projectiles a t velocities ranging from 1.2 t o 2.2 v. /10,11/. In this velocity range t h e emergent fragments a r e mainly H0 and
H+.
We note t h a t t h e t a r g e t is thick enough t o consider t h a t all t h e emergent H' result from a c a p t u r e of a t a r g e t electron.We show first (figure 2) t h e FWHM, Q:, of t h e angular distributions of neutral a t o m s for n values up t o 21 and for velocities between 30 and 120 keV/p, a f t e r traversal of t h e 2.1 pg.cm-2 carbon foil. For e a c h velocity Q ~seen t o increase f i r s t with n and then t o reach a saturation value for n S
>_
5. The1
enhancement above t h e Q. value is due t o t h e (screened) repulsion between t h e fragments inside t h e carbon foil, since it is quite reasonable t o assume t h a t t h e interaction in t h ? post foil region between a neutral with e i t h e r protons or neutrals is negligible. The saturation e f f e c t above n
-
5, observed also for thicker foils, means t h a t t h e average kinetic energy gained by e a c h fragment of a H: cluster is t h e s a m e when n>_
5. Moreover t h e n dependence ofon
is q u i t e smooth and does not confirm t h e assumption deduced from gas dissociation studies 141 about t h e possibility t h a t 1 5 and 19 could be. magic numbers corresponding t o denser clusters. However multiple s c a t t e r i n g could wash out such small effects.In figure 3, we show for various velocities t h e variation of t h e neutral fraction +>ith n
C2-192 JOURNAL DE PHYSIQUE
cluster a r e randomized in t h e target, due t o both repulsion and multiple scattering effects. Then if t h e protons a r e randomly distributed inside a sphere, t h e number of closest neighbours of a proton located a t t h e c e n t e r is 12. The number of closest neighbours is much less for a proton located in t h e outer part of t h e sphere, and t h e average number of closest neighbours is estimated t o be 6.5, quite in a g r e e m e n t with our observation.
So, with these foil experiments, we show t h a t t h e information. one c a n e x t r a c t a r e of t w o kinds. The angular distribution of t h e emergent f r a g m e n t s depends on what happens in t h e first layers of t h e foil and could shed some light on t h e s t r u c t u r e of t h e incident clusters when multiple scattering e f f e c t s a r e not t o o large. On t h e contrary, t h e c h a r g e state distribution a t emergence depends strongly on t h e respective distances between t h e fragments in t h e last layers of t h e foil and may allow t o help understanding t h e fundamental processes in t h e interaction of ionic projectiles with solid targets. The a t o m i c excitation of t h e fragments a f t e r t h e break-up of ion clusters in thin carbon foils a r e being measured by conventional beam foil spectroscopy technique and will give new information on collective e f f e c t s on a t o m i c collisions in solids.
However, some theoretical and experimental difficulties a r e due t o t h e too low velocity of our cluster ion beam. We know for example t h a t multiple s c a t t e r i n g e f f e c t s severely limit t h e accuracy of t h e informations one can e x t r a c t from t h e data, namely those concerning t h e cluster ion structure. The available ranges of cluster ion masses and energies a r e rather limited with t h e existing e l e c t r o s t a t i c accelerator. It has t h e r e f o r e been proposed t o a c c e l e r a t e cluster ions by means of a Radio Frequency Quadrupole (RFQ) a c c e l e r a t o r [H.O. Moser and A. Schempp, contributed paper t o this Workshop]. The beam of 500 keV H: cluster ions (n
<_
50) produced in t h e Lyon a c c e l e r a t o r will bei n j e c t e d i n t o a R F Q p o s t - a c c e l e r a t o r d e s i g n e d t o g i v e a f i n a l e n e r g y of up t o 5 MeV. Finally, we would review quickly what could be done with f a s t e r
H
:
ions. We s e l e c t only four topics :--
Chbtez b t ~ u c t u ~ e
-
When very thin foils and swifter ionic clusters a r e t o b e used, electron c a p t u r e by protons a t emergence wlll be negligible, t h e Coulomb repulsion between t h e protons will occur mainly in vacuum downstream t h e target, multiple scattering inside in t h e foil will be reduced and t h e joint angle-energy distributions of t h e break-up products will give a direct information on t h e s t r u c t u r e of t h e incident cluster ion. Ion cluster dissociation induced by a transverse electron beam and by a g a s jet will also be used f o r s t r u c t u r e studies.--
C4ubtez dopping powez
- One c a n give a n interpretation of t h e energy loss of a n ionic cluster in m a t t e r by dividing t h e electronic energy loss in t w o parts : t h e energy loss due t o close collisions with individual t a r g e t electrons - this part involves t o o small impact parameters t o be concerned by any proximity e f f e c t s-
and t h e energy loss due t o distant excitation of t h e t a r g e t electrons in collective modes by t h e swarm of protons. This last process involves distances larger than t h e mean distance between protons in t h e swarm so t h a t proximity e f f e c t c a n be manifested. This interpretation allows a quick estimation of t h e stopping power for a n ion cluster but t h e theoretical limits between close and distant collisions a r e not a c c u r a t e enough t o allow a full quantitative description of t h e stopping power t o be observed.in t e r m s of a "vicinage function" c a l c u l a t e d f r o m i n t e r f e r e n c e e f f e c t s b e t w e e n plasmon wakes. Due t o t h e c l o s e proximity of t h e f r a g m e n t s in c l u s t e r beam-foil e x p e r i m e n t s with f a s t projectiles t h e i n t e r f e r e n c e should b e observed. Indeed a n e n h a n c e m e n t h a s a l r e a d y been observed in various e x p e r i m e n t s
1131
with molecular ions like H; o r H;. It would b e i n t e r e s t i n g t o e x t e n d t h i s study t o l a r g e r s y s t e m s s u c h as ionic clusters.-- Electton Spcct206copy
- Although t h e production of e l e c t r o n s by f a s t ion i m p a c t i s of principal i n t e r e s t in many b r a n c h e s of physics, i t is only r e c e n t l y t h a t e l e c t r o n s p e c t r o s c o p y h a s received a g r e a t d e a l of a t t e n t i o n . In particular, t h e so-called cusp e l e c t r o n s which a r e e m i t t e d with nearly z e r o velocity r e l a t i v e t o t h e projectile, w e r e a c t i v e l y investigated b o t h e x p e r i m e n t a l l y a n d t h e o r e t i c a l l y 1141. I t would b e i n t e r e s t i n g to c o m p a r e t h e r e s u l t s obtained with heavy ions a n d with H: c l u s t e r s of t h e s a m e velocity.-- Channelng
06
dabt ionic clubtet4
will considerably modify t h e dynamics of t h e explosion inside t h e foil by breaking down i t s cylindrical s y m m e t r y . P l a n a r channeling potential will p r e v e n t t h e repulsion along t h e t r a n s v e r s e direction of protons channeled b e t w e e n t h e s a m e a t o m i c planes. Then channeling e x p e r i m e n t s should provide a unique way of sampling t h e incident p r o j e c t i l e i n t o s l i c e s of known t r a n s v e r s e width ( t h e planar spacing dp), which should give i n f o r m a t i o n s o n t h e s t r u c t u r e and t h e vibration motion of t h e incident clusters.New s t u d i e s a r e now in progress a n d a new g e n e r a t i o n of e x p e r i m e n t s will b e possible with t h e e x p e c t e d higher e n e r g y clusters, concerning t h e knowledge of t h e ionic c l u s t e r s t h e m s e l v e s a s well a s t h e investigation of c o l l e c t i v e e f f e c t s in t h e p a r t i c l e - m a t t e r interaction.
/ I / Henkes, W. Z. Naturforschq. 1 6 a (1961) 8 4 2
/2/ Yamaguchi, Y., Gaw, J.F., ~ e ~ x n ~ t o n , R.B. a n d S c h a e f e r , H.F. J. Chem. Phys.
86
(1987) 9 /3/ Chanut, Y., Martin, J., Salin, R., Moser, H.O. S u r f a c e Sci.106
(1981) 563/4/ Chevallier, M., Clouvas, A., Frishkorn, H.J., Gaillard, M.J., Poizat, J.C. a n d Remillieux, 3.
2. Phys.
D-2
(1986) 8 7/ 5 / Van Lumig, A. a n d Reuss, J. Int. J. Mass. Spectrom. Ion Phys.
25
(1977) 137 / 6 / Gaillard, M.J., P o i z a t , J.C., Ratkowski, A., Remillieux, J. a n d Auzas, M.Phys. Rev.
A-16
(1977) 2323/7/ Maor, D., Cooney, P.J., Faibis, A., K a n t e r , E.P., Koenig, W., Zabransky, B.J. Phys. Rev.
A-32
(1985) 1 0 5/S/ Gaillard, M.J., P o i z a t , J.C., Ratkowski A. a n d Remillieux, J. Nucl. Instr. Meth.
132
(1976) 6 9 /9/ d e C a s t r o F a r i a , N.V., F r e i r e , F.L., Montenegro, E.C. a n d d e Pinho, A.G.3. of Phys.
B-19
(1986) 1781/10/ M a z u y , B., B e l k a c e m , A., C h e v a l l i e r , M., G a i l l a r d , M.J., P o i z a t , J.C. a n d R e m i l l i e u x , J. Nucl. Instr. Meth. in Phys. Res.
8-28
(1987) 497/ I I / M a z u y , B., B e l k a c e m , A., C h e v a l l i e r , M., G a i l l a r d , M.J., P o i z a t , J . C . a n d R e m i l l i e u x , J. Nucl. Instr. Meth. in Phys. Res.
1
( t o be published)/12/ Brandt, W. a n d R i t c h i e , R.H. Nucl. Instr. Meth.
132
(1976) 4 31131 S e e f o r e x a m p l e : Brandt, W., Ratkowski, A. a n d Ritchie, R.H. Phys. Rev. L e t t .
