A Surface Ionization Mass Spectrometer for Production Control M. W. ECHO and T. D. MORGAN Atomic Energy Division, Phillips Petroleum Co., Idaho Falls, Idaho
b A mass spectrometer using a surface ~ . . . ionization source has been constructed at the Notional Reactor Testing Statio in Idaho. The design i s based on th 12-inch radius, 60" type o f instrumer developed b y M. G. lnghram at th Argonne National Laboratory for us in the analysis o f nongaseous sample, Vacuum lock improvements resulted from experience with equipment at the Idaho chemical processing plant. Changes in source design were limited to improvements in methods of assembly. The instrument has been used successfully for production criticality control and accountability o f uranium. .
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mass spectrometer was constructed at the U. 8. Atomic Energy Commission National Reactor Testing Station in Idaho for use a t the chemical processing plant. The instrument is used for the measurement of concentration and isotopic composition of uranium to provide criticality and accountability information on feed and product lines of the plant. Considerable experience in this method of
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Vacuum lock assembly
control badibcen gained at the Argonne National Laboratory (3). Thousands of analyses were made with a mass spectrometer, but i t became evident that an additional machine was required to handle the sample load and ensure fulltime instrument availability. DESCRIPTION
General. The design of the surface ionization mass spectrometer is based on the first-order direction focusing, 12-inch radius magnetic deflection, 60" type of instrument developed by M. G. Inghram a t the Argonne National Laboratory (3). Figure 1 is a photograph of the entire instrument.
Figure 1.
Mass spectrometer and control console
The permanent magnet field is varied by a shunt which is positioned by 8 worm and gear drive. Scanning is accomplished hy auxiliary field coils on the magnet. A vacuum lock introduces nongaseous samples. The analyzer tube is made of 2-inch outside diameter, '/tcinch wall Inconel flattened to inch in the magnet region- Three Consolidated Electrodynamics Carp. Model GHG-15 mercury diffusion pumps, three liquid nitrogen traps, and four mechanical pumps provide evacuation. The signal from the ion collector is amplified by an Applied VOL. 29, NO. 1 1 , NOVEMBER 1957
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Figure 3. Vacuum lock showing source carriage supported in carriage guide, ready for insertion Physics Gorp. vibrating reed electrometer and fed to a Brown Instrument Co. Electronik recorder. The console consists of three 66-inch cabinet racks placed a t 45" with respect to each other and fitted with a Formica surfaced table shout 29 inches from the floor. The most frequently used controls are located within easy reach of an operator. Vacuum Lock. The vacuum lock described by Stevens (3)was the basis for the design of the system. Basically this system provides a means for transferring a sample-loaded filament into the source region with only a small increase in pressure. The sample carriage is in the form of a piston which fits closely in a cylinder. Three pumping stations increase the degree of vacuum surrounding the sample as it is moved toward the source region. A s"l;A piston fills the vacuum lock whent?ver the sample carriage is removed. A sketch of the vacuum lock systern is shown in Figure 2; Figure 3 is a phi,tograph of a portion thereof. The original design had two sample carriages wliich were inserted alternately and one Dnk carriage. Only one sample carriage has been provided in this instrument. The blank carriage is long enough to fill the entire vacuum lock when the source is removed. In this manner the removable portion of the source is exposed to atmosphere only long enough to exchange samples, resulting in lower pump-down time. The cylinder wall and the source cup wall were made thicker to improve dimensional stability. The annular grooves a t the pumping stations were shortened to 3/g inch and the pump leads staggered at 30" from each other. This decreases the over-all length of the vacuum lock to 15 inches, thus lessening the machining and aligning problems. Because the vacuum lock was to he used only with a single filament surface ionization source, there was no disadvantage in reducing the space in the source region. All parts of the vacuum lock are Inconel with the exception of the blank
carriage which is Type 310 stainless steel. The removable portion of the source fits freely into the source cup so that a shoulder on the carriage presses against the end of the cup during insertion. After insertion of a sample, the blank carriage is extracted by an amount greater than the depth of the source cup. This is done with the aid of a cable-connected counter balance. The cable is disconnected whenever the sample carriage is removed from the vacuum lock, so that the blank carriage can move freely and fill the vacuum lock in the absence of the sample carriage. The housing of the counter
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balance is oil filled to provide damping. The contacting surfaces of the vacuum lock were given a hard chrome plate of 0.0035 inch and finish ground, honed, and polished. The diametral clearance between the cylinders and pistons is about 0.0004 inch. There are no gaskets or 0 rings between these parts. Within 5 minutes after introducing a sample through the vacuum lock, the analyzer pressure is sufficiently low for operation-Le., about 3 X 10-7 mm. of mercury. The source pressure is about 3 X 10-f nun. of mercury during operation with the filament on. After pump down, the outer or highest pressure pumping stage can be vented completely without a noticeahle increase in source or analyzer pressure. A very small accumulation of dirt will impede the action of the vacuum lock pistons. The pistons and cylinders must be cleaned about every 6 months to maintain free action. It was found in routine operation that it was difficult to prevent bumping the sample holder against the sides of the vacuum lock cylinders during removal. The guide shown in Figures 2 and 3 was installed to support the sample carriage as it is extracted. A nylon insert in the carriage guide allows the carriage head to be inserted and removed easily. Source. The critical dimensions of the source are the same as those in the original instruments (9). Changes in source design were concerned primarily with methods of assembly. The removable portion of the source-i.e., all except the defining slits-is shown attached to the sample carriage in Figure 4 and disassembled in Figure 5. It is mounted on a borosilicate glass spacer for high voltage insulation and held in place by connection to the center lead of the Iiovar seal as shown in Figure 4. The washers on this lead are bent to provide some mechanical shock absorption and allow thermal expansion. Positioning of the focus plates is secured hy the use of glass balls. The metallic pieces were machined out of solid stainless steel. An attempt was made to use sapphire balls for high voltage insulation but this resulted in arcing difficulties. A borosilicate glass insulator prevented arcing. Protection System. .A schematic diagram of the cooling water control system is shown in Figure 6. This system, which can he seen in Figure 3, was designed to prevent damage from abnormal conditions such as water failure, a broken glass mercury diffusion pump, or a disconnected hose.
Figure 4. Source carriage and two views of source
This control system includes a pressure regulator, flowmeter, Lucite tube, solenoid valve, and pressure sensing device. The Lncite tube of the water control system has an orifice near the bottom. If inlet water pressure is lost due to some exterior malfunction, a pressure switch will turn off the pump heaters and turn them on again when the pressure returns. If a pump water
be reset manually. At about 25 gallons ner hour the Lucite tube overflows.
stable with time. RESULTS
Figure 5. Source por
of about 6 inches. When the flow falls below this value, the pressure-sensing device trips, causing the control to turn off the heaters and close the inlet line solenoid valve after a 3kecond delay. From this condition, normal flow must
jacket breaks or hose connection comes loose, the incoming pressure remains normal but the water level in the Lucite tube begins to drop. At about 10 gallons per hour the orifice at the bottom of the Lucite tube will maintain a head
More than 1500 analyses of uranium samples have been completed on this instrument, most of which vere for concentration of uranium in solution as well as for isotopic distribution. An isotope dilution technique (1) aas used to measure the concentration and isotopic distribution of samples containing fission products. A uranium control standard containing uranium-234, uranium-236, uranium-238, and about 90% uranium-235 was used to determine the precision for isotopic distribution measurements. The 95% confidence internal on a single determination of n r e nium-235 in this standard mas *.11 percentage points. Twenty-three samples of normal uranium oxide were analyzed. This material was obtained from the U. s. Atomic Energy Commission, New Brunswick Laboratory, where it was designated as Sample Number 15 of the available analyzed ore samples. The 95% confidence limit for the ratio of uranium-238 to uranium-235 peaks was 138.8 +.3. ACKNOWLEDGMENT
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The successful design and constrnction of this instrument was possible because of the efforts of many individuals and groups at the National Reactor Testing Station. These included R. W. Sliger, design engineer, R. J. Hall who constructed most of the electronic instrument, and W. D. Laney, supemisor of the machine shop. The authors wish to thank R. C. Shank for supporting this work, and C. M. Stevens for his assistance in making final adjustments on the instrument.
.n- n JRE CITED
(1) Goris, P., Duffy, W. E., Tingey, F. H.. ANAL.CHEM.29.1590 (1957). (2) Stevens: C. M., RW. &i. I&. 24, 148-51 (1953). (3) Stevens, C. M., Inghram, M. G., U. S. Atomic Energy Commission, Rept. ANL-5251 (1954).
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Figure 6. Cooling water control system
RECEIVED for review October 29. 1956. Accepted June 20, 1957. Fourth Annual Meeting of ASTM Committee E-14 on Mrtss Spectrometry, Cincinnati, Ohio, M m 28. 1956. Work performed under Co
