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CONVOY ELECTRONS IN COINCIDENCE WITH OUTGOING PROJECTILE CHARGE STATES OF Ni
(15.6 MeV/u)
J. Kemmler, O. Heil, C. Biedermann, P. Koschar, H. Rothard, K.
Kroneberger, K. Groeneveld, I. Sellin
To cite this version:
J. Kemmler, O. Heil, C. Biedermann, P. Koschar, H. Rothard, et al.. CONVOY ELECTRONS IN COINCIDENCE WITH OUTGOING PROJECTILE CHARGE STATES OF Ni (15.6 MeV/u).
Journal de Physique Colloques, 1987, 48 (C9), pp.C9-223-C9-226. �10.1051/jphyscol:1987935�. �jpa-
00227353�
Colloque C9, suppl6ment au n012, Tome 48, dgcembre 1987
CONVOY ELECTRONS IN COINCIDENCE WITH OUTGOING PROJECTILE CHARGE STATES OF Ni (15.6 MeV/u)
J. KEMMLER, 0. HEIL, C. BIEDERMANN, P. KOSCHAR, H. ROTHARD, K. KRONEBERGER, K.O. GROENEVELD and I.A. SELLIN(')
Institut fiir Kernphysik der Joh. Wolfg. Goethe-Universitat, August-Euler-Strasse 6 , 0-6000 Frankfurt-am-Main 90, F.R.G.
RESUME: Nous avons CtudiC le nombre des dlectrons convoyCs Ye(qf) en fonction de lWCpaisseur (px) des cibles de carbon pour des projectiles incidents Niqi' 115.6 MeV/u) avec qi=27 et 28 en coincidence avec les projectiles Cmergeant de la charge qf=28 et 27. SimultanCment nous avons mesurd la distribution d'etats de charge F(qf,px) en fonction de la charge incident qi. Le domaine d'equilibre de charge est obtenu pour une epaisseur de cible plus grande que 650 ug/cm2. Dans le cadre du model des klectrons convoyCs la px dependance de Yelqi,qf) peut Stre expliquer si on tiend compte les deux processus ECC et ELC. 11 faut introduire une longueur de transport Ac, qui est douce fois plus grande que la longueur de la attCnuation Ae des electrones libres.
ABSTRACT: We have studied the target thickness (px) dependence of the convoy electron yield Ye(qf) for the incident projectiles Niqi+ (15.6 MeV/u) with qi=28 and 27 on carbon foils in coincidence with the outgoing projectiles with charge qf=28 and 27. Simultaneously the charge state evolution F(qf,px) dependent on the incident charge qi has been measured. For this collision system the charge state distri- bution saturates for target thicknesses larger than 650 ug/cm2. In the framework of the model for convoy electron production and transport the px dependence of the yield Ye(qi,qf) can be explained by assuming ECC and ELC processes. A transport length Ac must be introduced, which is twelf times larger than the attenuation length A, obtained with isotachic free electrons.
In the last years great experimental evidence had been achieved that convoy electron production takes place in the bulk of the solid.
Therefore the last-layer model for convoy electron production was discarded Ill.
If the convoy electrons are formed by the charge exchange processes of the projectile ion inside the solid the question arises how these electrons keep still correlated to the projectile ion in spite of the elastic and inelastic scattering processes inside the solid. The attenuation length for free electrons A, extends from several A for
("oak Ridge National Laboratory and University of Tennessee. Oak Ridge. TE 37831, U.S.A.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987935
C9-224 JOURNAL DE PHYSIQUE low energetic electrons
to 100 A for electrons of 10 keV [21. This is in general less than the mean free path for charge exchange ACc of heavy projectile ions.
In a recent measurement of the convoy electron yield Yc for Ni2*+ and
~ i ~ions ~ +(Ep=15.2 MeV/u) on carbon and alu- minum foils the slow in- crease of the yield Yc has been interpreted as an enhanced transport length for convoy elec- trons. A value for A, was obtained which is 20 times larger than A, for isotachic free electrons [ 3 1 .
For the interpretation of this result the charge exchange processes inside the solid had not been considered. Therefore it was not clear, if the px dependent increase of Yc
only describes the evolu- 10
loo
tion of a charge state
-TARGET THICKNESS
~ x [ p g l c m * I - + which produces mainlyconvoy electrons or is
related to a real Fig. 1: Charge state dis- enhanced transport tribution F(qf) for the
length. incident projectile ions
To clear this situation Ni2*+ (top) and ~ i ~ ~ + we have measured the px (bottom) Ep=15.6 MeV/u on
dependence of the convoy carbon foils. The numbers electron yield Ye(qf) for denote the outgoing charge
~ i * i + ( E =15.6 MeV/u)
P states qf. The lines are
with the incident charge drawn to guide the eye.
states qi=27 and 28 in coincidence with projec-
tiles of the outgoing charge state qf=27 and 28. Also the evolution of the charge state distribution F(qf) (fig. 1) was recorded. The experi- mental arrangement is the same than described in ref. [ 4 1 .
The yield Ye(qf) has been determined by integrating the electron velocity distribution over an interval of 2 4 vg centred at the peak
point subtraction of a
~ i ~ ~ ' ( 1 5 . 6 MeVluI- C
linear background arising
from ionisation elec- 10
-
trons. The number of con- voy electrons were norma-
lized to the total number c of projectiles N = c N ( ~ £ )
=I 6 -
i"
J i
associated with all final L O
charge states.
- -
The evolution of the Y -
u yields Ye(qi=28,qf=28)
-
and Ye(qi=27,qf=27) (fig.
2
2) proceeds much faster
and into a different di- -I
rection than the evolu-
w
tion of the related char-
z
10- ge state fractions qf=28 O LT and 27. In contrast thet
yields Ye(qi=27,qf=28) W -J
and Ye(qi=28,qf=27) in- W
crease in the same way as
& 6 -
the outgoing charge qf.
>
Z
Assuming the model of
8
charge exchange from I-
Allison [ 5 1 the mean free Z W
path ACc for the ~ O S S of
2 -
one electron ( ~ i ~
- >
~ +z
~ i ~ and for the cap- ~ + )
8
j8
27'n!: -
-
28T 1
,27/ 27-27
< /
: : : : I : : : : I :
! :!,;NiZ7+(
1 5 . 6M&IuI+ c i7\i-27 - '
-
/ 27-
-
T
2727/ 28
/
2827/1
28-28/ 28/-
28-28'
I 1 1 1 1 1 I l l 1
, ,
( 1ture of one electron
10 100
( ~ i ~
-
~ ~+ i ~ ~was + )- TARGET THICKNESS 9
x [ ~ g / c m 2 1 - * found to be nearly equal,with ACC=650 pg/cm2. Fig. 2: Target thickness Introducing the charge (px) dependence of the con- exchange processes in a voy electron yield Ye(qf model for convoy electron for the incident projectile
production [ 4 1 we ions ~ i (top) and ~~ ~ + i ~ ~ + can determine the rela- (bottom) E =15.6 MeV/u in
tive contribution of coincidence with P pro jec- electron loss t o tiles of the outgoing char- continuum (ELC) and elec- ge states qf=28 and 27. The tron capture to continuum lines are drawn to guide
(ECC) and the transport the eye.
length 1,.
If the convoy electrons
in the case of Ye(qi=28,qf=28) and Ye(qi=27,qf=27) are produced by pure ECC and A,=A,, after a very rapid increase the yield Ye should follow the dependence of the incident charge state qi. In the case of pure ELC the p x dependence of the yield should follow the charge
C9-226 JOURNAL DE PHYSIQUE
fraction F(qfVl). Therefore in the scope of the used model the in- crease of Ye(qi=28,qf=28) cannot be explained only by assuming ELC events, because the responsible F(qf-l) fraction increases to slow to explain the data. If a contribution of ECC and ELC is permitted the fitting procedure leads to a portion of 50% ECC and ELC. The increase of the yield point to an enhanced transport length with Ac=24+5 pg/cm2. The comparable value for isotachic free electrons is Ae=lOO A
[21. Interpreting the increase of Ye(qi=28,qf=28) as an indication for an enhanced transport length Ac of the convoy electrons we find that
Acc>Ac>Ae
These results are obtained from a simple Ansatz, which does not in- clude any type of excitation process of the projectile electrons. But the found enhanced transport length for convoy electrons supports the concept of refocusing of electrons which accompany higly charged ions in solids [61.
Many stimulating discussions with Wolfgang Meckbach Bariloche/Argenti- na and Peter Sigmund Odense/Danmark are gratefully acknowledged. We appreciate the support of P.Mokler and his group at GSI/Darmstadt during our experiment.
This work has been funded by the German Federal Minister for Research and Technology (BMFT) under the contract nr. 06 OF 173/2 Ti 476.
Support by NSF/Washington, CNEA Buenos Aires, DFG/Bonn, NATO/Brussells and GSI/Darmstadt is gratefully acknowledged.
References
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W-Meckbach, I.A.Sellin, ed), Berlin: Springer (1984) p. 109 J.Kemmler, O.Heil, C.Biedermann, P.Koschar, H.Rothard, K.Kroneberger, K.O.Groeneveld, A.Kover, G.Szabo, L.Gulyas, D.Berenyi, P.Focke, W.Meckbach,
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