Z1 oscillations of the mean charge for isotachic ions in carbon foils Page: 3,355
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VOLUME 51, NUMBER 4
Z1 oscillations of the mean charge for isotachic ions in carbon foils
A. M. Arrale, Jianyue Jin, Z. Y. Zhao, D. L. Weathers, F. D. McDaniel, and S. Matteson
Ion Beam Modification and Analysis Laboratory, Department of Physics and Center for Material Characterization,
University of North Texas, Denton, Texas 76203
(Received 15 August 1994)
Oscillations in the mean-charge state of swift ions as a function of the atomic number Z1 are reported
for a wide range of ions of identical velocity (isotachic ions). A previously suggested mechanism for the
enhancement of the mean charge for certain ion-charge combinations that involves closed shells is
shown to be an inadequate explanation. Post-foil-Auger processes, however, are demonstrated to be a
more plausible explanation for the observed behavior of the mean charge of the ions.
PACS number(s): 34.70.+ e, 34.50.Dy, 79.20. - m
An ion impinging on all but the thinnest of target foils
experiences several inelastic charge-changing encounters
with target atoms before it emerges from the foil. Inside
the target, the charge state fluctuates as a result of the
competition between electron loss and capture. An equi-
librium charge-state distribution is established for thick
enough targets for which the mean-charge state (q) of
the ion does not change with increasing thickness (until a
thickness is reached for which appreciable energy loss
occurs). Due to the lack of a comprehensive theory to
treat the complex problem of the charge-state distribu-
tion, one is usually forced to rely on semiempirical ex-
pressions [1,2] for qf that often obscure Z1 oscillations
that have been reported previously [3,4] for constant ve-
locity (isotachic) ions emerging from thin carbon foils.
The present work was performed to shed light on the ori-
gins of these oscillations.
For low-energy isotachic ions (v =v0, v0 being the
Bohr velocity), Lennard and Phillips  have shown that
the mean charge state (qf) exhibits a nonmonotonic
dependence on the atomic number of the ion (Z1) and
has a broad peak at Z1 15. Lennard and Phillips attri-
buted the enhancement of qf to the filling of inner-shell
vacancies in the projectile ion in a post-foil-Auger decay
that involves simultaneous ejection of an Auger electron.
Their conclusion is partly supported by the absolute
Auger yield measurement of Schneider et al.  that
showed the L-vacancy fraction of v = v0 phosphorus ions
to be 0.63+0. 19, a markedly large fraction. Thus a phos-
phorus ion, for example, is expected to increase its mean
charge state by one after exiting the foil, due to this
Auger effect. Alternatively, Shima et al.  have report-
ed oscillations of the mean-charge state (qf) as a function
of Z1 at ion energies 0.55, 1, and 2 MeV/u, which they
argued are strongly correlated with shell structures of the
ion. Furthermore, they contend that the maxima occur
for ions whose mean number of residual electrons
ne = Z - qf is 2, 10, and 28, corresponding to full K, L,
and M shells, respectively.
In this Brief Report, we present results obtained using
a modified procedure for measuring the mean-charge
states of ions exiting foils. The aim of the experiment is
the resolution of the aforementioned dispute. The
present work grew out of other work focused on the
physical aspects of ion-induced electron emission (IIEE)
processes and their applications . The procedure
developed to investigate IIEE phenomena also presented
an efficient method of obtaining the mean-charge state of
ions exiting thin foils. The present experiments were per-
formed using the 3-MV tandem accelerator at the Uni-
versity of North Texas Ion Beam Modification and
Analysis Laboratory. Vacuum conditions were main-
tained to provide a chamber pressure below 10-9 Torr.
Positive ions of known velocity, charge, and mass were
incident on a self-supporting carbon foil. By carefully in-
tegrating all the space currents-the current of the emit-
ted electrons, the net current on the target, and the
current into the Faraday cup after the foil-one obtained
the mean-charge state of the exiting ions. This was com-
puted from the ratio of the charge collected in the Fara-
day cup to the algebraic sum of all charge collected.
More details of the experimental procedure are given in
Ref. . For each ion-energy combination used, five to
ten measurements were made and the resulting mean-
charge state averaged. The standard error of this mea-
surement average was estimated to be 0. 13 charge state.
For the ion energies employed in the present work, the
carbon foil was sufficiently thick ( _ 50 /g/cm2) to ensure
that the charge state reached its equilibrium value. No
dependence of qf on the projectile incident charge state
was observed in the case of any of the following test ions:
F+2 and F+3; C1+2 and C1+3; and Ag+7 and Ag+8 at
v = 2vo. Similarly, the pressure in the chamber, over a
range of one to two decades, was not found to influence
the exiting mean-charge state of the ions that were used
to check for the effect.
The measured equilibrium (mean-) charge state at con-
stant velocity is plotted as a function of Z1 in Fig. 1. For
v = 2vo, the mean-charge state increases nonmonotonical-
ly with Z1 with a small local maximum at Z1 = 5. Then,
with increasing Z1, another maximum appears in the
range 24 < Z1 <29. It is worth mentioning that the re-
@ 1995 The American Physical Society
PHYSICAL REVIEW A
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Arrale, A. M.; Jin, Jianyue; Zhao, Z. Y.; Weathers, Duncan L.; McDaniel, Floyd Del. (Floyd Delbert), 1942- & Matteson, Samuel E. Z1 oscillations of the mean charge for isotachic ions in carbon foils, article, April 1995; [College Park, Maryland]. (digital.library.unt.edu/ark:/67531/metadc139492/m1/1/: accessed April 28, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT College of Arts and Sciences.