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NATIONAL BUREAU OF STAMOARDS -1963
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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.
1.
ا م ه
ا
ا ا
ا
ON
JUL 22 1563"
642-452322 -
1
AN UNCONVENTIONAL ULTRA-HIGH VACUUM FACILITY*
on in peace on
C. F. Barnett and J. A. Ray
Oak Ridge National Laboratory
Oak Ridge, Tennessee
U.S.A.
MASTER
In pursuing the problems of measuring atomic collision cross sections,
the need arose for a vacuum vessel that would maintain a pressure of 10-20
torr in the interaction region. The system should also satisfy two addi.
tional requirements: 1) The walls should be sufficiently removed from
the interaction region such that neutral particles released from particle
impact on wall surfaces would produce a minimum effect on the pressure
in the interaction region; 2) The vacuum system should be a subsidiary
component designed to let the experiment proceed without spending full
time in obtaining the low pressure required.
The first step in the design of the system was the selection of the
type of pump to.be used.
To evaluate the pumps, a test was initiated in
which two 6" diffusion pumps manufactured by two companies were compared.
under a variety of conditions. The pumps were blanked off and the ulti-
mate pressure and visible oil back streaming were observed after 48 hours
of operation. In Talle I 18 a comparison of the performance of the two
types of pumps with various baffle configurations.
*Research sponsored by the U.S. Atomic Energy Commission
under contract with the Union Carbide Corporation.
kamin natin na
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-LEGAL NOTICE

PATENT CLEARANCE OBTAINED. RELEASE YO
THE PUBLIC IS APPROVED. PROCEDURES
ARE ON EILE IN THE RECEIVING SECTION,
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tet, men , were show at Codes
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Why we or contracter of the Controlou, plyn al wel watroctor, I the one that
mai complexes or street at the Omnicom, e mpleyna a la nostra ter prepara
Mooto, ** portem , mert Wagwowotnet
C ater, « Memployment will contentur.
a aari kata na
2.

LABLE I
LUME #1
PUMP #2
Diffusion Ultimate Basel Diffusion Ultimate Base
Pump Fluid
Press. (Torr) Pump_Fluid Press. (Torr)
Test Conditions
Zeolite Traps Incl.
in System
Convoil-20
5 x 10*10
Dow 705
x 109
Traps Removed - H2O
Cooling on Baffle
Dow 705
5.8 x 10-8
Dow 705
1.3 x 10 9
traps Removed -
Baffle Refrigerated
K(-35° to -40°C)
Dow 705
4.5 x 1009
Dow 705
1.2 x 1009
Liquid Na cooled traps wei'e eliminated due to the attention required in
maintaining Na levels or flow through the baffles. Table I indicates only
small changes in pressure were obtained when elaborate baffles were used.
Armed with the information from the tests, the decision was made to use
the NRC 6" HK series pump with only water cooled optically thick baffles.
The pum, was also equipped with a water cooled cap above the upper jet.
The vacuum vessel shown schematically in Fig. I consisted of a stain-
less steel chamber two feet in diameter and eight feet long. The inside
of the vessel was polished to approximately a sixteen micro-inch finish to
assist in the cleanliness of the system. A liner inside the chamber was
constructed of copper with copper tubing attached by a heli-arc process
using a high-temperature solder, An inner copper container was suspended
inside the liner and both regions could be outgassed by passing steam
through the tubing cr cooled by flowing water or liquid Na. The diffusion
-3.
pumps and baffles were attached to the vessel directly with no intervening
val.ves. All flanges were sealed by compression type aluminum wire gaskets
made with 2S Al wire. Batch evaporation of Ti was accomplished by using
the usual ORNL Ta filanent wound with Ti wire and again all seals were
either Al wire gaskets or flange welded ceramic bushings. Shown also on
Fig. I are a set of Helmholtz coils to provide an axial magnetic field for
steering of charged beams. Notice also that the vessel is compartments-
lized. This was done so that with the use of beams of charged particles,
differential pumping could be used to partially isolate the beam dumping
region from the interaction region. Provisions have been made for insert-
ing movable probes and beam scanners all through stainless steel bellows
type arrangements. The fore vacuum was provided by Welch Duo-Seal #1398
pumps usually operating well below 1013 torr under static conditions. A
photograph of the system is shown in Fig. 2.
The procedure used in obtaining the required vacuum consisted in baking
the unit by passing steam at 100°C through the liners for 24 hours.
The
steam was turned off and water was allowed to flow through the lines. ,
Shortly thereafter, the pressure decreased to a constant value of 5 x 1009
torr. Ti was evaporated onto the water cooled surfaces for a period of .
four hours after which the pressure stabilized at 5 x 10-20 torr as read
by a Veeco RG-75 ionization gauge. Further Ti evaporation on liquid Na
cooled surface produced no change in the observed pressure nor was any
improvement noted when the liquid nitrogen cooled surface was thermally
cycled.
A Veeco Residral Gas Analyzer was placed on the system to study the
individual ma88 peak behavior when in the pressure range of 5 x 10°10 torr.
In a typical scan before Ti evaporation, the predominate mass peaks were
28 and 18 and as shown in Fig. 3 on the top trace. After Ti evaporation
on the water cooled liner the predominate mass peak was 28 which was reduced
in amplitude and the usual hydrocarbon peaks in the mas8 40 region disap-
peared. In the lower trace is a comparison when Ti was evaporated on sur-
faces at liquid Na temperatures which produced no further significant
decrease in the remaining mass peaks. Under these conditions all of the
hydrocarbon peaks vanished and also note in the lower traces the absence of
masses in the 70-80 range, which was the expected mass fragment of the Si
oil decomposition. . To make observations more quantitative, the sum of the
total partial pressures was determined and found to decrease by a factor
of six when Ti was evaporated onto liquid Na cooled surfaces. From this
decrease in pressure we have concluded that the pressure in the vessel was.
of the order of 10-11 torr.
This vacuum system has been operated for several months with no visible :
traces of oil backstreaming into the vessel. Crude tests, such as increas-
ing the heater power have revealed no significant amounts of oil in the
system. The vacuum obtainable is reliable, easy to obtain and is adequate
to meet the requirements of the experimental program.
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