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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS - 1963
1055.2020

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MASTER
PREPARATION OF ISOTOPE TARGETS BY ELECTRON-BOMBARDMENT TECHNIQUES*
O MI - AEC - OFFICI41
E. H. Kobisk
ORNL - AIC - OFFICIAL

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Isotopes Development Center RELEASED. POR ANNOUNCAT
Oak Ridge National Laboratory
Oak Ridge, Tennessee 1. IN NUCLEAR SCIENCE ABSMILACTS
Problems encountered in the preparation of thin films of normal elements
by vacuum svaforation-condensation techniques are significantly magnified
when using isotopically enriched materials. These latter materials re-
quire maximum efficiency in evaporation and condensation and to achieve
this end, the Oak Ridge National Laboratory Targeć Center developed an
electron-bombardment vaporization source using high electron currents and
low acceleration voltages. A minimum of 20% and a maximum of 90% effi-
ciency of vapor condensation have been obtained with this system. The
vaporization source uses special crucib).es to attain mechanical collima-
tion, permitting the use of milligram quantities of evaporant in cases
where previously several hundred milligrams had been required with other
evaporation techniques. Materials having very high vaporization tempera-
tures (~3000°C) have been evaporated successfully by this method, e.g.,
neptunium oxide, uranium oxide, plutonium oxide, and thorium oxide. Since
the electron-bombardment heat source can be installed in all standard
vacuum chambers at low cost and is suitable for use at pressures of 10
torr, it has proven a highly versatile tool.
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The following characteristics of an evaporation system are desirable for
pre pazing thin-film targets from isotopic materials.
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1. Variable evaporation temperatures between 300 and 3000°C.
2. Collimation of vapors for maximum recovery of starting material.
3. Compatibility of evaporant and crucible material to minimize loss
of isotopes by chemical reaction or alloy formation.
4. Elimination or minimization of contaminants in the condensed films. :
5. Maximum uniformity of film density.
6. Compatibility of the size of the evaporation system with the
vacuuma chamber.
Most of these criteria have been achieved with the vaporization system
illustrated in Fig. 1..
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From the Knudsen laws of gaseous flow through tubes, mechanical collima-
tion of material can result if a tubular crucible is employed whose length
is much greater than its internal diameter (L/D ~10). The evaporation
crucible shown in Figs. 1 and 2 approximates these requirements.
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*Research sponsored by the U. 8. Atomic Energy Commussion under
ontract with the Union Carbide Corporation.
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However, choice of crucible materials is critical and was limited to
those that would not alloy during fusion and vaporization of the spacific .
isotope. The uniporization gun uses ring-type. or hairpin-type, electron- :::.........
emission filaments, fabricated from 0.020- to 0.040-in.-dia thoriated
tungsten wire and resistively heated with 25-100 amp ac to achieve the
appropriate emission temperatures. Electrons drawn from the filament are
rd the crucible by imposing a positive potential of 100 to
1000 volts dc between the crucible and the filamenti Increased electron
efficiency is achieved by surrounding the entire assembly with a 0.010-in.-
alum box, electrically at earth potential, which acts as an
electrostatic shield and increases the electron current to the crucible
by -30%. To raise the crucible temperature above 2000°C, an electron
current of w1.5 amp at a potential difference of 800 volts between the
filaments and the crucible was required.
To attain appropriate heat distribution and to minimize thermal losses,
the crucible configuration illustrated in Fig. 2 was adopted. Crucibles
were made from a tungsten rod 0.75 to 1.0 in. long and 0.25 to 0.125 in.
in diameter. Heat conduction from the crucible was limited by a 0.040-in.-
dia tungsten wire which also supported the crucible and resulted in a more
uniform temperature distribution. Because of the high temperatures re-
quired, crucible materials were usually limited to tungsten, tantalum, and
molybdenum. If crucible and evaporant were incompatible, ceramic liners
(Fig. 3) could be used. One of the most versatile materials used for
crucibles was found to be spectroscopic-grade graphite. . Further colli-.
mation of vapors could be achieved using an effusion plug (shown at the
right of Fig. 2), which proved very effective at short distances between
the crucible and the substrate.
A variety of evaporation techniques was found possible using this versatile
• heat source. For example, as shown in Fig. 4 it was desired to coat the
inside and outside surfaces of right circular cylinders with plutonium
oxide. With resistance heating, this task would have been infeasible,
· particularly since good uniformity of the films on the surfaces was desired.
However, with our electron-bombardment heating technique, successful deposi-
tion of the oxide over the inside and outside surfaces was achieved. At
the bottom of Fig. 4 is shown the boat-type tungsten crucible used to can-
tain the plutonium oxide while it was heated to ~2400°C by electron bombard-
ment using a hairpin-type electron-emission filament. Each cylinder in the
nest of cylinders was axially rotated at 100 rpm, with the evaporation
crucible coincident with the axis of rotation of the cylinder. A second
evaporation source of similar design was used to evaporate material onto
the outer surface of each cylinder independently. The tungsten crucible'
was 2 in. long and grooved down the center to allow even distribution of.
plutonium oxide powder before evaporation. Evaporation onto the outside
surface of a cylinder is illustrated in Fig. 5.
Figure 6 illustrates the glove box housing and Pyrex glass nyo" evaporator
system designed for general use with radioisotopes. The electron-bombardment
· Vaporization source is located centrally in the photograph. This particular
system was used to deposit 24cm on the end of a 0.063-in.dia copper wire. -
ORNL - AEC - OFFICIAL
OWL - AEC - OFFICIAL
The versatility of this technique is further illustrated by the fact that
0.2 ug of 252cf was evaporated with subsequent condensation of 50% of the
starting material on the substrate. Of course, vaporization-condensation
efficiencies of this high magnitude are not ordinarily achieved, particu-
larly when good uniformity of the deposited material is desired over a
large substrate.
One distinct advantage of this system is the low voltage employed to
&crelerate electrons toward, the crucible. In most commercial systems,
voltages of the order of 5000 volts dc are used. In practice, commer-
cial electron guis cause electrical breakdown or glow discharge in the
vacuum system following a burst of gas from the evaporant because of these
high acceleration voltages. With the electron-bombardment system developed
at the Target Center, it has been observed that discharge was avoided
until pressures of the order of 1 x 10-5 torr were attained. Further-
more, because of the filament and crucible geometry, a rather high degree
of ionization occurs in the evaporating material causing it to glow
visibly, thus allowing control by sight of the evaporation. In the
future it is hoped that, by redesign of the heat source, electrostatic
Vor magnetic collimation of the vapors can be realized. It is con-
ceivable that this technique can attain enhanced uniformity of deposition
by sweeping the ionized vapor beam over the surface of the substrate
rather than having it focused at one point and mechanically manipulating
the substrate to gain good uniformity.
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LEGAL NOTICE
This report mo prepared a aa account of Government sponsored work. Noither the United
Statos, bor the Commission, nor may person acting on behall of the Commission:
A, Makes nay warranty or representation, axprowad or implied, with respect to the accu-
racy, completeness, or wetulnost of the Information contained in this report, or that the we
of any information, apparatus, method, or process disclound in this report may not infringe
privately owned rights; or
B. Assumer any liabilities with respot to the use of, or for damages resulting from the
un of any information, apparatus, method, or procen disclosed in this report,
As used in the above, "person acting on behalf of the Commission" includes any on-
ployee or contractor of the Commission, or employ of such contractor, to the extent that
much employee or contractor of the Commisslon, or employee of much contractor prepares,
disseminatos, or provides acconto, may information pursuant to Me employmeat or contract
with the Commission, or dis omployment with such contractor.
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12/28/65
ORNI - AEC - OFFICIAL
FIGURE LIST
Fig. 2.. Electron-Bombardment Heat source Used for Evaporation
from a Tubular Crucible.
Fig. 2. Tungsten Evaporation Crucible: Components and Assembly.
Fig. 3. Crucible Liners That Have Been Used in Tantalum or
: Tungsten Electron Bombardment (left to right: Beo, ZrO2, and E).
Fig. 4. Slotted Tungsten Crucible and Electron-Emission Filament
(bottom) Used to Coat Fission Chamber Cylinders with 298 Puoa. (Fission
chamber assembly is shown in the upper portion of the photograph)
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Fig. 5. Evaporation of 238 Puoa onto Fission Chamber Cylinder in
a Pyrex Glass Cross' Evaporator. Cylinder was rotated at 100 rpm above
the vapor source.
Fig. 6. Glove Box Enclosed Evaporation System with Electron-
Bombardment Heat Source in Place. This particular unit was used to
· evaporate 244cm as oxide onto a 0.600-in.-dia copper disk (as shown
over the crucible).
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END
DATE FILMED
2 / 28 /66