ELECTRON LOSS FROM FAST, LOW-CHARGE-STATE
IONS COLLIDING WITH GASES
R.D. DuBois, A.C.F. Santos
University of Missouri-Rolla, Rolla, MO 65409, USA
Th. Stöhlker, F. Bosch, A. Bräuning-Demian, A. Gumberidze, S. Hagmann,
C. Kozhuharov, R. Mann, A. Oršić Muthig, U. Spillmann, S. Tachenov
Atomic Physics Division, GSI, 64291 Darmstadt, Germany
W. Barth, L. Dahl, B. Franzke, J. Glatz, L. Gröning,
S. Richter, D. Wilms, , A. Krämer
Accelerator Division, GSI, 64291 Darmstadt, Germany
and
K. Ullmann and O. Jagutzki
Instutüt für Kernphysik der J.W. Geothe Universität Frankfurt, August-Euler-Str. 6, Frankfurt am Main, Germany
Abstract. Absolute cross sections for single and multiple projectile loss from MeV/u low-charge-state argon and xenon
ions are presented. The impact energies were 0.74 and 1.4 MeV/u and the incoming charge states were 1,2 and 3. Using
the present and data from the literature, it was shown that the multiple loss cross sections scale as (ΣBE)-n , where ΣBE is
the sum of the binding energies required to remove a certain number of electrons. The power n was found to be velocity
dependent, varying roughly from 2.5 to 1 for velocities ranging from approximately 0.2 to 20 (units of 108 cm/s).
velocity. In 1948, Bohr [2] used semi-theoretical
arguments to show that the electron loss cross sections
for light particles interacting with intermediate Z
targets should scale according to Ztarg2/3 Zproj-1 vproj-1.
Here the subscripts indicate target or projectile
properties.
INTRODUCTION
As has been known since the early days of atomic
physics, electron capture and loss processes lead to a
decrease or an increase in the mean charge state of a
beam of energetic ions as they pass through matter.
Experimentally, at high energies where electron loss
dominates, Rutherford [1] showed that the electron
loss cross sections for alpha particles passing through
air decreased as v-1 where v was the alpha particle
In 1978, Olson et al. [3] provided a scaling rule
based on electron loss by hydrogen atoms colliding
with a wide variety of projectile ions. They not only
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
64
of electron loss by fast, low charge state heavy
projectiles has been initiated between the University of
Missouri-Rolla and GSI Darmstadt. In this paper, we
report results of our initial work where absolute single
and multiple electron loss cross sections have been
measured for Ar+, Ar2+ and Xe3+ ions interacting with
Ne, Ar, and N2 targets. In this study, the collision
energies were 0.74 and 1.4 MeV/u which are
considerably higher than those previously available for
heavy low-charge-state ions.
provided absolute cross sections but also showed that
the energy scaling ranged from no velocity
dependence at “low” velocities to a vproj-2 behavior at
“high” velocities, as expected from the Born formula.
They also showed that there was a range of validity for
a particular v behavior and this range depended on the
collision system. More recently, for MeV/u Xeq+ - N2
collisions [4], a vproj-1 behavior was experimentally
demonstrated for q = 18.
This behavior was
reproduced by CTMC calculations which also
provided information as to how the electron loss cross
sections behave as a function of projectile charge. The
cross sections decreased as q-1 for large q but were
relatively constant for small q. It was also shown that
the cross sections scale inversely with the ionization
potential of the projectile.
EXPERIMENTAL METHOD
The measurements were performed at GSI by using
the gas stripper and analyzing magnet located at the
end of the UNILAC section of the accelerator. Slits
were inserted before the gas stripper to collimate and
reduce the beam intensity. In addition, slits were
inserted after the gas stripper to reduce background
scattering effects and to define the beam axis and
divergence. These slits also provided differential
pumping between the target and the beam transport
regions. A high rate position sensitive detector was
inserted on axis immediately after the charge state
analyzing magnet which is located roughly 2 meters
downstream from the gas stripper. Two-dimensional
position spectra were recorded using a fast
histogramming TDC and PC. These spectra provided
information about the charge state intensity
distribution after the beam interacts with the target gas.
From the pragmatic side, one of the driving forces
for studying electron loss is accelerator technology and
development. A positive aspect is that multiple
electron loss (stripping) can be exploited to accelerate
heavy ions to high energies. On the negative side, any
changes in charge state of the beam leads to loss of
beam intensity and can induce severe problems
associated with wall heating or increased background
gas pressure. Therefore, beginning in the 1960s many
measurements were performed where electron loss
was studied for a variety of projectiles, targets, and
impact velocities. Much of this work is compiled in
references 5 - 6. In addition, numerous other studies
have been performed, most notably by the groups at
Crhus and Oak Ridge; see ref. 7-9 and references
therein. One important finding was that multiple
electron loss processes not only occurred but,
particularly for heavier systems, often dominated the
overall loss cross section.
The basic procedure consisted of accelerating
specific argon and xenon ions to energies ranging from
0.74 to 1.4 MeV/u, passing them through the target
region while monitoring the pressure at the periphery
of the interaction chamber. Because of the differential
pumping, the target was a “gas cell” with an effective
target density proportional to this measured pressure.
The method used to calibrate the effective density is
described below. The outgoing charge state spectra
were measured as a function of target density, i.e., data
were collected using the growth curve method.
Counting rates were typically 100 kHz which were
well within the capabilities of our histogramming
TDC. The 2D spectra showed clearly separated
islands containing the various charge states of the
beam and small background intensities between the
islands. Counts contained in the various islands were
integrated, background subtracted, and converted to
charge state fractions. Independent of incident ion
species and charge state, ten well-resolved charge
states could be detected without repositioning the
detector or adjusting the analyzing magnet.
Recently, plans in the USA and Germany to use
intense beams of energetic heavy ions to heat and
compress DT plasmas or for nuclear physics studies
have generated a renewed interest in electron loss,
particularly for heavy, low-charge-state ions traveling
at high energies. For example, in the US Heavy Ion
Fusion Program it has been proposed to use an intense
beam of high energy heavy ions to heat and compress
a DT pellet in a reaction chamber and thereby induce
nuclear fusion. However, this requires accelerating,
transporting, and injecting tightly focused intense
beams of ions. Electron loss introduces loss of usable
beam intensity and destroys a tight beam focus due to
increased space charge effects; therefore, it is highly
detrimental.
To address this particular problem and to provide
information in a regime where currently no
experimental information exist, a collaborative study
65
The charge state fractions were then plotted versus
the target density and converted to absolute electron
loss cross sections using the linear term of a
polynomial fit to the growth curves. Typically the
target density was increased until loss of the primary
beam was approximately 30%. Thus, a second or third
order polynomial fit was required to account for
multiple collision effects. As a crosscheck whether
multiple collisions influenced our extracted cross
sections, particularly those for the loss of many
electrons in a single collision, cross sections were also
calculated by solving the coupled equations given by:
∑
1E-15
+
Ar - Ar
1
2
q in + 10
Absolute cross sections for single and multiple
projectile electron loss in Ar+ - Ar collisions are shown
in Figure 1. Data for collision velocities less than 3 x
108 cm/s are from data compilations [5-6] and those
above 109 cm/s are our present measurements.
Cross Section (cm )
Fq (π ) =
RESULTS AND DISCUSSION
π Fq ' (π ) σ q ' q
(1)
q ' = q in − 2
Here Fq(π) is the measured fraction for charge state
q at target density π, qin is the incident charge state of
the beam, and σq’ q is the cross section for the
projectile going from charge state q’ to charge state q.
The sum includes all single collision contributions
from double capture to loss of ten electrons. Using the
measured fractions at different target densities, a
matrix of equations of the form above was written to
include two, three, or four dominant channels, i.e., qin,
qin+1, qin+2, qin+3 listed in declining importance.
Internal consistency between these results and with our
polynomial fit results indicated that single collision
conditions generally dominated our extracted cross
sections except for a few cases involving loss of many
electrons. In those cases, the matrix equation results
were averaged and this average value used.
1E-16
2
3
4
5
6
7
1E-17
8
9
1E-18
1
8
10
Collision Velocity (10 cm/s)
FIGURE 1. Projectile electron loss cross sections for Ar+ Ar collisions. Data are present measurements (above 109
cm/s) and from Ref 5-6 (below 3 x 108 cm/s). The numbers
at the right indicate how many electrons are lost in a single
collision.
Figure 1 is representative of available information
prior to our present work, namely a total absence of
experimental electron loss cross sections information
for low-charge-state, heavy ions interacting at high
velocities. It is also important to note that using only
the previously available data, extrapolation to high
velocities would be impossible.
The effective target gas density for each target
investigated was calibrated by measuring growth
curves for 0.74 MeV/u He+ ions. By comparing the
measured fractions to those calculated using absolute
electron capture and loss cross sections for He+ impact
on Ne, Ar, and N2 taken from the literature, our
background pressure scale was converted to an
absolute effective target density scale.
It was
demonstrated that the major uncertainties in this
procedure were directly proportional to uncertainties
in the absolute single electron loss cross sections for
He+ impact. These were taken to be ± 30%. As a
result, the absolute cross sections presented here have
a ± 30% uncertainty due to target density plus an
additional uncertainty associated with our polynomial
fit. Comparison of our polynomial curve fitting results
with our matrix manipulation results indicated that
uncertainties due to cross section extraction were
typically less than 5% except for selected cases
involving low statistics and loss of many electrons.
+
0.74 MeV/u Ar
+
1.4 MeV/u Ar
2+
1.4 MeV/u Ar
3+
1.4 MeV/u Xe
2
Cross Section (cm )
1E-16
1E-17
1E-18
100
1000
Binding Energy (eV)
FIGURE 2. Cross sections for loss of one or more electrons
plotted versus the energy required for their removal. Data
are for various projectiles colliding with argon.
66
was also shown that a simple scaling procedure could
be applied to obtain a “universal” curve capable of
describing the loss of up to 9 electrons. Such
information has direct applications in the fields of
accelerator technology, nuclear physics, and controlled
nuclear fusion.
As seen, multiple electron loss is significant at all
velocities. Figure 2 demonstrates that the multiple loss
cross sections scale fairly well as a function of their
binding energy (BE). For our present high energy
data, the scaling is approximately (BE)-1. It is also
seen that this scaling is rather independent of projectile
charge or type, at least for the low charge state
projectiles used in the present experiment.
1E-15
q+
1.4 MeV/u P on Argon
The lower velocity data in Figure 1 were similarly
tested for a (BE)n scaling behavior. It was found that
the multiple loss cross sections could be scaled at all
velocities but that the power was velocity dependent.
For the limited dataset used, n = 2.24 – 0.79 log(vproj)
for velocities given in units of 108 cm/s. After
applying this scaling to normalize all the multiple loss
data shown in Figure 1 to the single loss values, Figure
3 shows that all cross sections now fall within a factor
of two of a “universal” curve, rather than differ by up
to two orders of magnitude as in Figure 1.
Total Loss
2
Cross Section (cm )
1E-16
Double Loss
1E-17
5 electrons Lost
8 electrons Lost
1E-18
+
Ar
0
1
2+
3+
Ar
Xe
2
3
4
Projectile Charge State
+
FIGURE 4. Electron loss cross sections for 1.4 MeV/u
low charge state ions colliding with argon.
2
Scaled Cross Section (cm )
Ar - Ar
1E-15
ACKNOWLEDGMENTS
This work supported by the U.S. Department of
Energy, Grant No. ER54578. A.C.F.S. is grateful for
support obtained from CNPq (Brazil).
1E-16
1
8
Collision Velocity (10 cm/s)
10
REFERENCES
FIGURE 3. Projectile electron loss cross sections scaled as
described in the text. Symbols are as in Figure 1.
1. Rutherford, E., Phil. Mag. 47, 277 (1924).
In Figure 4 we plot our single and multiple electron
loss cross sections as a function of the projectile
charge. As seen, they are nearly independent of q,
when q is small. This, and Figure 2 imply that the
scaling we used for Ar+ impact should be applicable to
any low charge state heavy ion.
2. Bohr, N., Mat. Fys. Medd. Dan. Vid. Selsk. 18, 1 (1948).
SUMMARY
5. Lo, H.H. and W.L. Fite, Atomic Data 1, 305-28 (1970).
3. Olson, R. E., K. H. Berkner, W. G. Graham, R. V. Pyle,
A. S. Schlachter, and J. W. Stearns, Phys. Rev. Lett. 41,
163-166 (1978).
4. Olson, R.E., R.L. Watson, V. Horvat and K.E. Zaharakis,
J. Phys. B 35, 1893-1907 (2002).
6. Dehmel, R.C., H.K. Chau, and H.H. Fleischmann,
Atomic Data 5, 231-89 (1973).
In conclusion, we have presented new
measurements of electron loss. For fast, low-chargestate heavy ions, these data provide the first
experimental information for energies above
approximately 0.3 MeV/u. It was demonstrated that
the cross sections depended strongly on the number of
electrons lost but weakly on the projectile charge. It
7. Datz, S. et al., Phys. Rev. A 2, 430-8 (1970).
8. Alton, G.D. et al., Phys. Rev. A 23, 1073-8 (1981).
9. Knudsen, H. et al., Phys. Rev. A 19, 1029-37 (1979).
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