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LEGAL NOTICE
This report was prepared as an account of
Government sponsored work. Neither the
United States, nor the Commission, nor any.
person acting on behalf of the Commission:
A. Makes any warranty or representa-
tion, expressed or implied, with respect to
the accuracy, completeness, or usefulness
of the information contained in this report,
or that the use of any information, appa-
ratus, method, or process disclosed in this
report may not infringe privately owned
| rights; or
B. Assumes any liabilities with respect
to the use of, or for damages resulting from
the use of any information, apparatus,
method, or process disclosed in this report.
As used in the above, “person acting on
behalf of the Commission” includes any em-
ployee or contractor of the Commission, or
employee of such contractor, to the extent
that such employee or contractor of the
Commission, or employee of such contractor
prepares, disseminates, or provides access
to, any information pursuant to his employ-
ment or contract with the Commission, or
his employment with such contractor.
:
*
.
.
.
.
.
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2
ORAL-P-1336
CONF-6506.37-/
MASTERS
metri intotiwa
new windo*
CHANNELING EFFECTS ON THE ENERGY LOSS OF HIGH ENERGY (20-80 Mev)
79Br AND 1271 IONS IN GOLD*
W
O XIDALANCA
ommummomarganyapo matuohon
JUN 24 1965
8. Datz, T. S. Noggle and C. D. Moak
Oak Ridge National Laboratory
Oak Ridge, Tennessee
*-.she -Win!;. isinin birincisini, Do's.
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The channeling effect of an ordered crystal lattice on the
trajectory of energetic ions was first predicted in the computer
calculations of Robinson and Oen on the ranges of multikilovolt
ions in copper. These effects were observed in the transmission
of 75 Kev protons through gold and by precise range measurements
with low energy heavy ions. 5.4 Evidence has been found for channel-
ing effects with (por) and (p, n) reactions and in studies of yields
of x-rays induced by proton bombardment.”- Other observations
with fast light particles in Si have been reported by Dearnaley,
Erginsoy, Wegner and Gibson and Schiffer and Holland. 10
In the present work we have used multicomponent beams of
Br and I ions from the Oak Ridge tandem accelerator with energies
up to 80 Mev to study the effect of crystalographic orientation
...-','1
5-7
',catiep
.
. .
Dearnalev. 8
.
on energy losses in thin gold single crystals.
The application of the tandem van de Graaf accelerator to the
production of heavy ion beams with energies from 10-120 Mev was
deve loped at ORNL by Moak and co-workers." The experimental
arrangement is shown in Fig. 1. Negative Br or I ions are injected
from the source, accelerated to 7 Mev at the stripper gas canal.
From the canal they may emerge with'ionic charge of 4*, 5* or
*Research sponsored by the U. 3. Atomic Energy Commission under se to
LBSTERIAI IR ARDPRODEANPROCEDURES
contract with the Union Carbide Corporation. THE PUBLICSTA
AREM EILE IN THE RECC:VING SECTION, ·
perhaps higher. At this point, a departure from usual procedure
is made. If the gas pressure in the accelerator tune is high enough,
some of the particles will continuously lose electrons by collision,
reaching higher and higher charge states. As the energy increases
the most probable charge state increases, and as the charge increases
the rate of acceleration or acceleration per unit voltage increases.
This leads to a kind of energy compounding so that a few of the
particles reach energies as high as 120 Mev with charges up to 20+.
The beam energy distribution thus attains continuous high energy
tail which extends above 100 Mev.. The useful aspect is that high
energies and high ionic charges go together so that the magnetic
rigidity of each energy region of the beam is about the same. Note:
.
ME
from the equation
- KH“, that if the magnet is set to pass
.
charge 57 at 25 Mev, then it will simultaneously pass charge 67 at
36 Mev, charge 7+ at 49 Mev, charge 8+ at 64 Mev, etc. A different
magnet setting simply changes the series of energies for which the
various charge states pass. Thus, once the magnet constant kis
precisely determined from proton thresholds and resonances, we have
a multi-component beam consisting of a whole series of accurately
known energies. Incidence of this beam onto a si surface barrier
detector produces a pulse height spectrum characteristic of the
beam energy distribution. In practice it is necessary to divert
the main accelerator beam which would burn up our detector if it
happened to come through the analyzer. For this reason we connect
the slit regulator system up backwards so that the terminal voltage
3
will avoid those values which cause a main beam to come round through
the magnet onto the slits.
A portion of the resulting spectrum for 1271 ions is shown in
the upper part of Fig. 2. Each of the lines is broadened because of
the rather poor energy resolution which solid state detectors exhibit
for very heavy ions. Were it not for this effect the lines would be
a lmost ten t.imes as sharp. A number of sharp, low energy peaks is
present in all the spectra, due to oxygen, nitrogen and other light
contaminant ions. The spectrum for Br ions is similar except that
double peaks are observed due to the mixture of 79Br and 81Br isotopes.
However, with suitable adjustment of the ion source tuning and slit
system, either one or the other isotope can be a lmost completely
eliminated.
For a measurement of the stopping power of any material, it is
only necessary to place the material between the accelerator and the
detector and measure what happens to each peak in the spectrum. Notice
that we are not concerned with the height of the peaks. No attempt is
made to regulate the accelerator voltage, so the exact composition of
the high energy tail distribution will vary from run to run. Our
only concern is with the shapes and positions of the various peaks.
The spectra shown in the lower half of Fig. 2 were obtained for
a polycrystalline gold foil approximately 200 ug/cm² thick and for
12'1 ions." Notice that the peaks are shifted and somewhat broadened.
Such spectra can be used to give accurate estimates of dE/dx for
various materials. Since the peaks are well separated, the use of
.
1-
2NS
R
.
"
ete
CU
hi
multicomponent beams with energy sensitive detectors permits us to
perform several experiments at different energies at one time.
For single crystal experiments, the absorber was mounted on a
goniometer so that it could be rotated in the beam for the experi-
ments being reported here, only one direction of rotation was
available for each crystal. The gold crystals used were epitaxially
grown on freshly cleaved, ratiation-hardened rock-sa ít faces. The
rock salt was dissolved away and the crystals were mounted free.
An x-ray map was used to determine the crystal axes and to analyze
the imperfections in each crystal. The crystals were formed with a
(100) plane parallel to their surfaces and were 1.1 mg/cm thick
(ca. 2000 atoms) as determined by a particle energy loss measurements
and by weighing. One foil was mounted with the goniometer axis lying
on a (100) crystal axis in the plane of the foil; a second foil was
mounted with the goniometer axis lying on a (110) axis in the plane
of the foil. The goniometer axis was parallel to the (100) planes
within 0.2° and was perpendicular to the beam axis within 0.2º.
The angular position error was I 0.1°. The foil dianeter was 3 mm,
and this defined the beam divergence of approximately 1.5 mm in
10 meters (ca. 0.01°). The collimator arrangement consisted of the
target, 3 mm dia., followed by a 1 cm dia. aperture at a distance
of 10 cm, followed by the 2 cm dia. detector at a distance of 5 cm
from the aperture.
An illustration of the crystal orientation available is shown
in Fig. 3. The two axes of rotation are shown in the upper figure.
With the goniometer set at oº, a square channel is seen for both
crystals. Since the crystal is 2000 atoms deep, an extremely
accurate angle sitting would be required for any sort of trans-
parency to occur; fortunately, this is quite unnecessary for the
appearance of strong channeling effects. As the crystals are
rotated away from oº, planes are viewed edge-on and for most angles
the picture is as shown in the figures marked planar channel. Por
the (100) axis rotation, a very large diamond shaped (110) channel
opens up at 45º. Finally, for the (100) rotation the rectangular
1112) channel is exposed at 35° 16',
A number of experiments on channel effects on energy losses
of light ions have been previously reported. 8-10 The typical result
with channe led light ions in Si single crystals in the reports
published until now was characterized by an emergent particle spectrum
consisting of a peak due to normal energy loss with a high energy tail
due to channe led particles which did not lose as much energy as they
would have lost in amorphous or polycrystalline material. The high
· energy particle spectrum usually exhibited a sma 11 component of
rather definite energy loss consisting of a peak in the distribution
at an energy loss which was always more than half the normal loss.
The experiments being reported here give lor the first time
some indication of what channeling effects are for fast, multiply
charged, very heavy ions. The particles ''Br and 1271 simulate
rather well the masses and energies of fission fragments, and thus
give an idea of the crystalline effects to be expected with fission
.
.
24.
fragments. .
IF
-
E
Typical spectra obtained with "Br ions incident on a single
crystal gold target are shown in F18. 4. The top curve is the .
spectrum without absorber. Barllor dx/dx measuromonts on poly-
crystalline samples were used to calculate the normal energy losses
expected for polycrystalline targets corresponding to the thickness
of specimens used in the present work. These expected losses are
indicated by the arrows.
As can be seen from the figure, channeling offects are quito
pronounced and often they completely predominate over all other
types of energy loss.
· The spectrum of Fig. 4c iliustrates the effects produced when
the 1001) crystal direction was aligned to a ''Br bean. The energy
losses for the various groups are characterized by single peaks.
The losses are substantially less than corresponding normal (poly-
crystalline) l08808 for a foil of this thickness. Although the
peaks are asymmetric, the fraction of particles with normal energy
loss was less than 0.1. Almost identical patterns have been observed
for bombardment in the (110) and the (112) directions. This type of
channeling, associated with all low index crystal directions observed,
has been named pipe channeling. For the case of the (111) direction,
the spectra were unusable because the excessive sample thickness,
due to the oblique angle, caused overlap of the energy groups.
The spectrum of Fig. 4b was obtained with the crystal rotated
9° about a (110) axis from the (001) direction; in this case the
(110) planes were "edge-on" to the bean. Each peak has shifted and
broken up into two distinct groups, and in this case some of the
ions are channeled between shexts of atons. The spectrum consists
of double peaks, one set of which corresponds to the normal energy
lossor. This spectrum 19 typical of all those taken at angles
..
.
..
.
.
Some corresponding spectra obtained for I ions are shown in
Pig. 5. Once again single peaks of reduced energy loss occur for
pipe channel directions, Fig. 5c; for planar channels, Fig. 5b,
double peaks appear corresponding to reduced energy loss and normal
enorgy loss
The fact that these double peaks are resolvable into two groups
is demonstrated in Fig. 6, which shows a computer analysis of the
data shown in Fig. 4b. The analysis consisted of a non-linear
least-squares fit of gaussian distributions, and indicated that the
observed distributions could be very closely described by a set of
gaussian peaks.
The effect of planar channel dimension is shown in Fig. 7 for
1211 in the (100) and (110) planes. In the wider (100} planar
channel, the fraction channeled is higher and the difference in the
energy loss for channeled and unchanns les particles, as can be seen
from the distance between the two peaks in a given group, is greater.
The effect of small displacements from a pipe channel direction
is shown dramatically in Figs. 8 and 9. In Fig. 8 we explored the
region of (110) direction with ''Br cons. It can be seen that just
a 0° 30' displacement is sufficient to change the pattern from the
typical pipe channel single distribution in the lower curve to a
typical planar channol distribution of the upper eurvo. In the
case of 1271 (Fig. 9), the situation is more complex because the
position of the normal energy loss group from a given initial energy
actually lies below the channe led loss portion of the preceding
energy group. This mixing of groups leads to a maximum in confusion
abouč 1° away from the (110) direction. However, when the channel
18 in line we again see the single group characteristic of pipe
channeling energing (patterns obtained at 469 and 47° are identical
to those obtained at 44º and 43°, respectively).
For the experiments reported here, two axes of rotation were
not available simultaneously. Without rotation around two axes,
sna11 risalignments could not be corrected. Although these mis-
#lignments are thought to be less than 0.1°, several spectra have
indicated that even these small errors might affect the relative
populations of channeled and unchanneled groups. It will be neces-
sary to perform rotations around two axes in order to obtain the
detailed structure of channel acceptance angles. In spite of the
possible wise ligament, it is noteworthy that in the cases studied
where planar channeling was involved a major portion of the ions
were channeled. As can be seen from the relative peak heights for
a given energy group in Figs. 7, 46 and 5b, the fraction of the long
which are channe led decreases with increasing ion energy. These
results seem to give at least qualitative verification to arguments
of Lindhard's, which indicate an expected (v) a dependence for
channel acceptance angle. For direct incidence into a pipe channel,
the fraction undergoing normal loss is too small to be measured
accurately at any of the incident energies; at ang les slight ly oft
Dishes
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axis (1.e., 0° 30'), a normal loss component became observable
which increased with increasing energy. These observations suggest
a positive channeling mechanism as opposed to a simple trans-
parency model and indicates that the probability of channeling
decreases with increasing transverse momentum of the incident ion.
Moreover, the fact that two distinct groups were observed, rather
than a' continuum of energy losses, implies that most of the channeled
particles were channeled a lmost immediately upon entering the crystal
and remained in channels during the remainder of their trajectory
through the solid. Channeling of ions involves correlated collisions
with rows of atoms which steer the ions on paths with reduced proba-
bility of low impact parameter events; however, the effective size
of the "strings"ds of atoms, in this energy range, is too sma 11 to
confine a particle to a single channel. Thus, in a pipe channel
direction, movement of the particle to adjacent channels is essentially
unhindered. Interplanar movements are more restricted because wander-
ing is limited to only one direction.
Although the trajectory of the particle is controlled by nuclear
......
..
.
-
- wo
collisions, the energy loss suffered by the Cons in this energy range
.
...
.
is primarily due to electronic stopping and the decreased energy loss
associated with channeling iherefore reflects the lower average
electron density encountered by the channeled particle. However, if
the ions are merely conc irained to stay away from strings, it is
curious that the channe led group of ions displays such a definite
energy loss, as opposed to a continuum on the high energy side of
the normal loss group. Of course, if the transverse oscillations
. "Wimmo-
n irin intermed
mari
the content
ee
it will s
10
were damped, all the ions would tend to settle into common pathways,
but we have been unable to find any mechanism which would cause
damping.
Detector resolution in these experiments was insufficient to
allow obafrvation of the detailed structure and shape of the channe led
and unchanneled groups of ions; however, in those cases where the two
groups appeared to be well separated, a set of numbers called the
channel energy loss fraction (f - AE channe led/AE normal) was derived
from the data. The energy loss fraction in various channels is
plotted against incident energy in Fig. 10.
The values all 1.9
between 0.5 and 0.85, show a slow rise with energy, and seem to have
lower overall values for broader channels. Planar channels seem to
.
-
.
---
.
--
give the same energy loss as pipe channels of the same dimension.
It is also noteworthy that the energy loss fraction in planar channels
did not change when the crystal thickness was changed by 40% in accord
with the assumption that the ions are channeled early and remain chan-
ne led. The loss fraction curve for 79Br ions in a given channel is
somewhat higher than that for 1271 ions; an effect which may be caused
by the higher velocity of the ''Br ion at a given energy. The fact
that the values are larger than those recently measured for protons
and a-particles 14 is perhaps caused by the fact that the heavy ions
are not bare nuclei but have electrons which spend part of their
-
..
.
-
-
-
-
-
..
-
time in dense regions alongside the channels, while the fact that the
values are not constant with energy may indicate that the ion charge
states are not following the same schedule which they would follow
in amorphous solid matter. This latter effect could be brought about
..
.
on the
by the avoidance of small impact parameter collisions in which the
ion would tend to lose electrons. Presumptive evidence for this
possibility can be adduced from studies of inelastic collisions in
gases in the region 25-100 kev from which 1t has been demonstrated
that the charge state increases rapidly as the distance of closest
approach decreases. 15, 16 Since the electronic stopping power increases
with ionic charge, a decrease in the equilibrium charge state could
contribute significantly to the lowered energy loss of the channeled
particles. Experiments designed to test this hypothesis are presently
.
.
-
-
underway.
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12
References
1. M. T. Robinson and 0. 8. Oen, Bull. Am. Phys. soc. ?, 171 (1962);
Appl. Phys. Letters 2, 30 (1963); Phys. Rev. 132, 2385 (1963).
2. R. 8. Nelson and M. W. Thompson, Phil. Mag. 8, 1677 (1963).
.3. G. R. Piercy, F. Brown, J. A. Davies and M. McCargó, Phys. Rev.
Letters 10, 399 ( 1963). ,'
4. H. Lutz and R. Sizmann, Phys. Letters 5, 113 (1963).
5. E. Bøhg, J. A" .e's and K. 0. Nielsen, Phys. Letters 12, 129
( 1964).
M. W. Thompson, Phys. Rev. Letters 13, 756 (1964).
Brandt, Khan, Potter, Worley and Smith, Phys. Rev. Letters 14,
42 ( 1965).
G. Dearna ley, IEEE Trans. Nucl. Sci. 11., 249 (1964). ..
9. C. Erginsoy, H. E. Wegner and W. M. Gibson, Phys. Rev. Letters
13, 530 ( 1964); Bull. Am. Phys. Soc. 10, 43' (1965).
10. J. P. Schiffer and R. E. Holland, Bull, Am. Phys. Soc. 10, 54
: (1965).
11. Moak, Neiler, Schmitt, Walter and Wells, Revs. Sci. Instr. 34,
853 (1963).
12. C. D. Moak and M. D. Brown, Phys. Rev. Letters 11, 284 (1963).
13. J. Lindhard, Phys. Letters 12 126 ( 1964).
Both Erginsoy, Wegner and Gibson, and Sattler and Dearna ley have
recently observed proton channel energy losses far below the
minimum value of 0.5 predicted by the equipartition rule.
Lindhard and A. Winther, Kgl. Danske Videnskab. Selskab, Mat.-
Fys. Medd. 34, No. 4 ( 1964). We are indebted to these authors
for communication of their data prior to publication.
13
.15. G. H. Morgan and E. Everhart, Phys. Rev. 128, 667 (1962).
16. V. V. Afrosimov, Iu. 8. Gordeev, M. N. Panov, and N. V. Fedorenko,
Zh. Tekhn. Fiz. 34 1613, 1624, 1637 (1964).
.
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Figure Captions
Fig. 1. Arrangement for the production of multicomponent high-energy
heavy ion beams in the tandem van de Graaf accelerator.
Fig. 2. 1??1 pulse height spectra obtained with (a) no absorber and
(b) an Au polycrystalline absorber (205 Mg/cm). The q
values given on the upper curve are the charge states on the
ions prior to entering the absorber. The peaks labeled O
are due to oxygen ion inpurities.
Fig. 3. Representative of a face centered cubic lattice demonstrating
the various channeling directions (for Au, a - 4.07 Å).
Fig. 4. 79Br pulse height spectra obtained with (a) no absorber,
(b) an Au single crystal absorber (1.1 mg/cm², rotated gº
about a (110) axis in the (100) surface toward (111) and
(c) with the (001) axis in line with the beam. The shifts
indicated by the arrows drawn from the peak positions on (a)
show the expected energy loss for a polycrystalline Au
sample of the same thickness.
Fig. 5. 1271 pulse height spectra obtained with (a) no absorber,
(b) an Au single crystal absorber rotated gº about a (100)
axis toward (110) and (c) with the (001) axis in line with
the ion beam direction.
Fig. 6. A computer analysis of the data in Fig. 4b. The single
curves aro gaussians for a given energy group. The upper
curve is the sum of the gaussians and the points are the
experimental data smoothed over a 5-point average. (Note
that in this curve we are using a linear abscissa as
against the logarithmic scales of the other figures.)
.
.
.
(
a
Fig. 7. A comparison of the (100) and the (110) planar channeling
of 1271 tons.
Fig. 8. The effect of small rotations on the pulse height spectra
of 79Br ions in the region of the (110) pipe channel. .
Fig. 9. The effect of small rotations on the pulse height spectra
of 1271 ions in the region of the (110) pipe channel.
Fig. 10. Fractional energy loss for 1271 and 79Br ions for several
channel directions in Au vs incident son energy.
within the ministeri memnunarsto sito.hentermine
tment and consisting
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A
ORNL-LR-DWG 78271
.
Br", 1° ions
Br8+, TO 15.+
19+ TO 19+
20°
ANALYZER
MAGNET
90° ANALYZER
MAGNET

NEGATIVE
SLITS
MAGNETIC
LENS
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SOURCE
Br4+, 5+
14+, 5+.
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HIGH VOLTAGE
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STRIPPER GAS CANAL
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MAGNETIC LENS
MOVEABLE ALPHA PARTICLE
CALIBRATING SOURCE
DETECTOR
CONDITION FOR PASSAGE
THROUGH ANALYZER
MAGNET
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MULTICHANNEL
ANALYZER
DOUBLE
LINE
AMPLIFIER
CHARGE
SENSITIVE
PREAMPLIFIER
02
PRECISION
PULSE GENERATOR
Arrangement for Measuring Heavy Ion Energies in Semiconductor Detectors
ORNL-DWG 65-2194
10
20
1271 ION ENERGY (Mev)
40
50
30
60
.
70
80
DETECTOR
RESPONSE
NO ABSORBER
(a)
-
q=10
9=12
19=8
------
4
-
COUNTS PER CHANNEL
POLYCRYSTALLNET
Au 205 kg/cm2
(b)
WWWVYVYN
50
400
450
200
250
CHANNEL NUMBER
300
350
400
450
500

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50
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100
150
200
250
CHANNÉL NUCABER
300
350
450
500
OROL - DWG GS-2033
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-
1277 ION ENERGY (Mor)
40
50
80
DETECTOR
RESPONSE
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m
COUNTS PER CHANNEL
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9* FROM 1000
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IC
1001)
CHANNEL
50
100.
150
200
250 300
CHANNEL NUMBER
350
400
450
500
550

WANNCL NUMOCN
ORNL-DWG. 65-5601
20-
79Br ION ENERGY ( Mev)
30
40
50
50
.
9° FROM [001)
TOWARD (111)
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50100
150
200
300
250
CHANNEL NUMBER
350
400
400

ORNL-DWG 65-2094
1271 ION ENERGY (Mevi
40
20
50
60
70
80
NORMAL ENERGY
9° FROM (001) TOWARD (110)
(100) PLANER CHANNEL
COUNTS PER CHANNEL
©
9° FROM (001) TO''ARD (111)
[110] PLANER CHANNEL
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50
100
150
200 250 300
CHANNEL NUMBER
350
400
450
500

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ORNL-DWG 55-2092
79Br ION ENERGY (Mev)
30
40
70
-UNSHIFTED PEAK POSITIONS —
Oc30' FROM [140]
TOWARD [001]
COUNTS PER CHANNEL
..
[110] DIRECTION
100
150
200 250
CHANNEL NUMBER
300
350
400
450
500

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COUNTS PER CHANNEL
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100
150
200
300
350
250
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400
450
500

ORNL-DWG. 65-5390A
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20
30
40
50
60
70
80
r
O
ION ENERGY (Mev)
END
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1.
DATE FILMED
9/ (10/65







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