US3267326A

US3267326A - Vacuum gauge

Info

Publication number: US3267326A
Application number: US306756A
Authority: US
Inventors: Wesley H Hayward; Sherman L Rutherford
Original assignee: Varian Associates Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1963-09-05
Filing date: 1963-09-05
Publication date: 1966-08-16
Anticipated expiration: 1983-08-16

Description

A 1966 w. H. HAYWARD ETAL 3,

VACUUM GAUGE Filed Sept. 5, 1963 FIG. 3

INVENTORS 34 WESLEY H. HAYWARD 3e 35 SHERMAN L. RUTHERFORD BY MMA TORNEY United States Patent 3,267,326 VACUUM GAUGE Wesley H. Hayward, Mountain View, and Sherman L.

Rutherford, Palo Alto, Calif., assignors to Varian Associates, Palo Alto, Calif., a corporation of California Filed Sept. 5, 1963, Ser. No. 306,756 14 (Iiairns. (Ql. 315-111) This invention relates to vacuum gauges and more particularly to ionization gauges for use in the ultra-high vacuum range.

During the mid-forties, the only instrument generally available which was capable of measuring pressures below torr was the ion gauge. At that time, the standard gauges consisted of: a hot wire cathode for emitting electrons which in the course of their journey collide with the molecules of the gas whose pressure is being measured to form positive ions; a positively charged grid surrounding the cathode for accelerating and eventually collecting the electrons; and, a metallic shell surrounding the grid for collecting the positive ions, the number collected being an index to molecular density, i.e., pressure. The whole arrangement was enclosed within an envelope connected to the system whose pressure was to be determined.

Such gauges never indicated pressures below l0" torr and this was generally attributed to a failure at this pressure of the apparatus then available for pumping to ultrahigh vacuum. It was then suggested that the failure was one of measurement rather than one of pumping, and that there existed a residual current to the ion collector caused by photoelectrons ejected from the ion collector by soft X-rays produced by electrons bombarding the grid. 7

Subsequently, various schemes have been devised for minimizing the errors in measurement attributable to the X-ray effect. In one, the cathode is placed outside the grid, while the collector, now of small surface area, is placed inside the grid. In this way the amount of X-radiation intercepted by the ion collector can be made arbitrarily small by making the ion collector arbitrarily small. In another, a grounded metal tube is placed around the collector between the place where it passes into the envelope and the grid in order to shield the collector throughout this length from X-rays. In still another, a suppressor grid is placed next to and made sufficiently negative with respect to, the ion collector so that photoelectrons emitted by the ion collector when struck by X-rays are returned to it. The difl'lculty with this scheme, however, is that, at the same time the ion collector is being struck with X-rays to produce photoelectrons, the suppressor grid is being struck with X-rays thereby producing photoelectrons. Since the collector is positive with respect to the suppressor grid, the photoelectrons emitted by the suppressor grid are attracted to the ion collector, giving rise to a negative X-ray effect.

In accordance with the teachings of the present invention, an ionization gauge is constructed employing a fourth electrode maintained at a potential below that of the cathode. The soft X-rays generated at the grid cause photoelectrons to be emitted from this fourth electrode.

Some portion of these photoelectrons are energetically t capable of reaching the collector, giving rise to a component of residual current opposite in direction to the normal X-ray current. This produces a net reduction in the X-ray limit of the gauge. With appropriate combinations of gauge geometry and operating voltages, the photoelectric current reaching the collector may be adjusted so as to just cancel the normal X-ray current, and thereby completely eliminate any X-ray effect.

ACCOlCllngljQlt is the object of this invention to provide an ionization gauge which completely eliminates any inaccuracies in pressure measurement due to X-ray effects. One feature of the present invention is the provision of an ionization gauge employing a fourth electrode for producing photoelectrons when struck by X-rays.

Another feature of the present invention is the pro vision of an ionization gauge of the above type wherein the fourth electrode is of large surface area as, for example, a conductive sleeve member.

A further feature of the present invention is the provision of an ionization gauge of the above type which includes a sealed envelope adapted to be connected to the system whose pressure is to be measured, and wherein the fourth electrode is a conductive coating on the inside wall of the envelope.

Another feature of the present invention is the provision of an ionization gauge of any of the above types including means for varying the potential of at least one of the electrodes of the gauge.

These and other objects and features of the present invention and further understanding may be had by referring to the following description and claims, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a top view of a novel gauge of the present invention;

FIG. 2 is a fragmentary cross sectional view of a novel gauge of the present invention connected to a system whose pressure is being measured, and including the associated circuitry in schematic; and,

FIG. 3 is a fragmentary cross sectional view of another embodiment of the present invention.

Referring now to FIGS. 1 and 2, there is shown an ionization gauge 11 constructed in accordance with the teachings of the present invention. The ionization gauge 11 includes a short straight thin wire 12 of conducting material, as, for example tungsten. This wire is the ion collector electrode 12, the purpose of which is to collect positive ions. Surrounding the collector electrode 12 is a helically wound grid 13, as, for example, platinum clad tungsten supported on grid frame members 14 forming a cage. This grid 13 is the accelerator or grid electrode, the purpose of which is to accelerate and eventually collect electrons. Disposed outside the grid is a coiled filament 15, as, for example, tungsten. The filament 15 is fabricated on a connector 16 having a set screw 17 to allow easy replacement of the filament 15 without weld-ing. The filament 15 acts as a source of primary electrons which collide with gas molecules to form positive ions and more electrons.

Each of the above electrodes are connected, as, for example, by welding to lead-in

wires

18, 19, 20. The lead-in wires are insulated from each other by insulator members 21, as, for example, alumina ceramic, sandwiched between a pair of

cup members

22, 23. The upper cup member 22 is brazed to the insulator 21 and the respective lead-in wire while the lower cup member 23 is brazed to the insulator member 2'1 and a header assembly 24. The collector lead-in wire 18 is further shielded by a conductive sleeve member 25, as, for example, stainless steel brazed at its lower end to the header member 24.

The header assembly 24 comprises a cylindrical insulating member 26, as, for example alumina ceramic and a flange member 27, of the type shown and disclosed in US. patent application, Serial No. 144,458, filed October 11, 1961, and assigned to the same assignee as the present invention.

Surrounding the first three electrodes is a conductive sleeve member 28, as, for example, stainless steel secured at one end by welding to the system 29 whose pressure is being determined and secured at its other end by welding to a mating flange member 30.

In operation, the collector 12 is maintained at some negative potential, for example, --45 volts by means of power supply 31, the grid 13 at some positive potential, for example +130 volts by means of power supply 3 2 and the cathode at or near ground, for example, +6 volts by means of power supply 33. The sleeve member 28 or fourth electrode is maintained at some potential below filament or cathode voltage, typically to -200 volts by means of power supply 34.

The bulk of the electrons emitted by the cathode 15 are caused by the grid 13 to oscillate back and forth in the gauge 11 and in the course of their journey collide with the molecules of the gas whose pressure is being measured to form positive ions and more electrons. These ions are collected at the highly negative collector 12, the number collected being an index to molecular density, i.e., pressure.

As is now well known in the art, electrons bombarding the grid 13 produce low energy X-rays. Some of this radiation strikes the ion collector 12 and releases electrons from its surface. So far as the current meter in the external collector circuit is concerned, the departure of a negative electron from the collector 12 has the same elfect as the arrival of a positive ion, thus giving rise to inaccurate readings in pressure.

However, in the present invention, a certain amount of X-ray radiation also strikes the fourth electrode or sleeve member 28 and thus releases electrons from its surface. In the preferred embodiment shown in FIG. 1 the fourth electrode 28 is of large surface area and of such potential that a certain proportion of the photoelectrons emitted by the sleeve 28 are energetically capable of passing through the grid 13 and reaching the collector 12, thereby giving rise to a component opposite in direction to the normal Xray current. This produces a net reduction in the X-ray limit of the gauge 11. With appropriate combination of gauge geometry and operating voltages, the net X-ray limit can be reduced to zero.

The energy with which the primary electrons emitted by the cathode 15 strike the grid is determined by the potential difference between the cathode 15 and grid 13. But the energy of these electrons also determines X-ray energy distribution which in turn determines the energy distribution :of the photoelectrons emitted when the X-rays strike the collector 12 and the fourth electrode 28.

It has been observed that as the potential difference between cathode 15 and grid 13 is increased, the number of photoelectrons energetically capable of reaching the collector 12 from the fourth electrode 28 of FIG. 1 increases less rapidly than the number of photoelectrons .leaving the collector 12. Therefore, assuming the reverse X-ray current does not quite cancel all the normal X-ray current, than either decreasing the grid potential as,

- for example, by varying the resistance of variable resistor 36 or increasing the cathode potential as, for example, by varying the resistance of variable resistor 37 will tend to reduce the X-ray limit of the gauge to zero.

Also, it has been observed that by increasing the potential difference between the collector 12 and the fourth electrode 28, either by increasing the collector potential as, for example, by varying the resistance of variable resistor 38 or decreasing the fourth electrode potential as, for example, by varying the resistance of variable resistor 39, more photoelectrons emitted from the fourth electrode 28 are energetically capable of reaching the collector.

A11 ionization gauge of the type shown in FIG. 1 has been constructed and is capable of pressure measurements to 1.5 1O torr. Also, sensitivity is increased by 50% 4 since because of the negative potential of the fourth electrode 28, electrons spend a greater time within the grid cage producing more ions for a given pressure.

Referring now to FIG. 3, wherein like numerals refer to like parts, the ionization gauge 11 includes a sealed glass envelope 40 surrounding the collector 12, grid 13, and cathode 15, and is adapted to be connected to the system whose pressure is to be measured. The fourth electrode is a conductive coating 28', as, for example, stannic oxide on the inside wall of the glass envelope.

Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred for-ms has been only by way of example and that numerous changes in the detail of construction and the combination and arrangement or parts may be resorted to without parting from the spirit and the scope of the invention as hereinafter claimed.

What is claimed is:

1. An ionization gauge for measuring pressure within an enclosed system including: four spaced apart electrodes; the first electrode for collecting ions; the second electrode for accelerating and collecting primary elec trons, being maintained at a potential positive with respect to said first electrode; the third electrode for producing primary electrons, being maintained at a potential intermediate the potential of the first and second electrodes; and, the fourth electrode of large surface area for producing photoelectrons caused by X-rays hitting the fourth electrode, said fourth electrode being maintained below the potential of said third electrode, said second and third electrodes spatially interposed between said first and fourth electrodes.

2. The gauge according to claim 1, wherein said fourth electrode is a conductive sleeve member.

3. The gauge according to claim 1 including a sealed envelope surrounding said electrodes and adapted to be connected to the system whose pressure is being measured, and wherein said fourth electrode is a conductive coating on the inside wall of said sealed envelop 4. The gauge according to claim 1, including means for varying the potential of at least one of said electrodes.

5. The gauge according to claim 4, wherein said electrode whose potential is varied is the first electrode.

6. The gauge according to claim 4, wherein the elec trode whose potential is varied is the second electrode.

7. The gauge according to claim 4, wherein the electrode whose potential is varied is the third electrode.

8. The gauge according to claim 1, wherein the electrode whose potential is varied is the fourth electrode.

9. An ionization gauge for measuring pressure within an enclosed system including: four spaced apart electrodes; a first electrode for collecting ions; a second electrode for accelerating and collecting electrons, being maintained at a potential positive with respect to said first electrode; the third electrode for producing primary electrons, being maintained at a potential intermediate the potential of the first and second electrodes, the fourth electrode for producing photoelectrons caused by X-rays hitting the fourth electrode and being maintained below the potential of said third electrode; said second and third electrodes spatially interposed between said first and fourth electrodes; and means for varying the potential of at least one of said electrodes.

10. The gauge according to claim 9, wherein the electrode whose potential is varied is the first electrode.

11. The gauge according to claim 9, wherein the electrode whose potential is varied is the second electrode.

12. The gauge according to claim 9, wherein the electrode whose potential is varied is the third electrode.

13. The gauge according to claim 9, wherein the electrode whose potential is varied is the fourth electrode.

14. An ionization gauge for measuring pressure within an enclosed system including: a first electrode of 5 6 small surface area for collecting ions; a second electrode References Cited by the Examiner surrounding said first electrode for accelerating and col- UNITED STATES PATENTS 'lectmg electrons, being maintained at a potential pos1- tive with respect to said first electrode; a third electrode 2,721,972 10/ 1955 Rthte1n 317-7 external to said second electrode for producing primary 5 3,001,128 9/1961 Nottmgham electrons being maintained at a potential intermediate 3,153,744 10/1964 Tomey 313-4 X the potential of the first and second electrodes and, a fourth electrode of large surface area surrounding said DAVID GALVIN 'm Exammer' first three electrodes for producing photoelectrons caused GEORGE WESTBY, Examiner.

by X-rays hitting the fourth electrode, said fourth elec- 1O trode being maintained below the potential of said third SCHNEEBERGER j ig jf k electrode.