I. PHYSICAL ELECTRONICS
Prof. W. B. Nottingham
S. Aisenberg
A. R. Hutson
W. J. Lange
R. N. Noyce
L. E. Sprague
A. ELECTRON EMISSION PROBLEMS
1. Magnetic Velocity Analyzer Investigation of Thermionic Emission from Tungsten
Construction of the final tube is nearly completed. A spot-welding jig has been constructed to make the final welds of the crystallized filament to its mounts.
A. R. Hutson
2. Photoelectric Study of Surface States on Insulators
A 935 phototube has been calibrated against a vacuum thermocouple so that it may now be used as a secondary standard in determining the energy of the light used to irradiate the specimen being studied. For the particular phototube being used, the spectral sensitivity was found to be markedly different from that published for the tube type.
A new ultraviolet source has been obtained: the General Electric type C-H3. This is a capillary mercury arc operating at medium pressure at 85 watts. There is only slight broadening of the spectral lines. For the major lines in the range from 2500A to
3000A, with the monochromator set for a 50-A bandwidth, the light output is between
1 and 0. 1 iw. This is several times the output using the Xenon arc, and probably has better spectral purity.
The first data are being taken, but as yet no results are satisfactory because of difficulties with the electrometer being used. A new tube with slight changes in design is under construction to replace the present tube in which the ion gauge is broken.
R. N. Noyce
B. EXPERIMENTAL TECHNIQUES
1. Ionization Gauge Studies
The distinguishing feature of the Bayard-Alpert ionization gauge is the use of an ion collecting electrode of very small surface area so placed with respect to the other elements of the tube that it nevertheless is an efficient ion collector. This reduction in surface area as compared with the conventional gauge used for so many years reduces, by a factor of about 200, the background spurious current caused by the reversed photoelectric effect. As a result of this improvement, vacuum measurements at pressures of
-11 the order of 5x10 mm of Hg become feasible. Without making any radical departures from the Bayard-Alpert principle but by introducing improved design principles, one can gain enough in sensitivity so that with all factors considered it is anticipated that
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(I. PHYSICAL ELECTRONICS)
ITH GLASS GUARD)
Fig. I-i
Elements of ionization gauge. (a) Filaments: 1. 25 inches long made of 0. 007-inch tungsten. Plane of filaments perpendicular to a radius. (b) Electron collecting grid: 1-inch diameter,
1. 5 inches long; 0. 005-inch tantalum wire wound 10 turns per inch on two 0. 020-inch tantalum supports with closed ends made of 0. 005-inch tantalum wire. (c) Ion collector: 0. 003-inch tungsten extends 0. 5 inch inside of electron collector. Lead-in shielded with glass but not in contact with it. (d) Screen:
1. 75-inch diameter, 1. 75 inches long; 0. 005-inch tantalum wound
10 turns per inch on 0. 030-inch tantalum supports.
a vacuum as low as 10
1 2
mm of Hg can be investigated with the improved gauge design.
The essential features of the best structure investigated to date are illustrated in
Fig. I-1. The important dimensions are incorporated in the caption. The main elements in the tube include:
(a) Two hairpin filaments, either of which can be used as an electron emitter or a surface on which condensible gases can be allowed to collect and further information concerning the properties of the residual gas in the gauge can be determined by the flash filament method.
(b) The electron collector is a cylindrical grid structure with a grid network closing both ends of the cylinder. This electron collecting structure serves as an excellent ionization space which so effectively traps the ions that the ion collection efficiency is very high in spite of the small area of the collector itself. It is obvious that an increase in all of the dimensions of this electron collector by a factor of 2 will increase the gauge sensitivity by a factor of about 8 with no sacrifice in the x-ray background level. The dimensions proposed in Fig. I-1 represent a practical compromise between sensitivity on the one hand and the over-all size of the gauge on the other. The material used in our gauges is tantalum. It is claimed that the use of molybdenum for the grid structure is advantageous and that it is thought to be easier to outgas. We have no quantitative experience to confirm or deny this view.
(c) The ion collector is a very small tungsten wire supported on a tungsten lead

(I. PHYSICAL ELECTRONICS)
Table I
Gauge Constants for Nitrogen
Gauge K in (mm)
-l through the glass. This support lead is guarded by a glass sleeve which is close to the lead-in wire but not in physical contact with it. The purpose of this glass sleeve is to minimize the emis-
1. Open end
2. Closed ends
3. Screen and closed ends
18
30
50 sion of electrons from the lead resulting from the absorption of light from the hot filaments or soft x-rays from the electron collecting grid.
(d) The screen grid which is shown surrounding this entire structure is an important improvement not previously described; it both increases the sensitivity of the gauge and permits the operator to maintain the potential of the glass wall negative with respect to the filaments under all conditions of normal operation.
Direct comparisons have been made over a wide range in pressure conditions between various gauges that differ from each other in certain of their structural features. All three gauges had electron grid structures of equal wire size, equal turns per inch, and equal enclosed volume. One tube had an open-ended structure similar to that originally proposed by Bayard and Alpert, and no external screening grid. The second gauge was similar to the first in all respects except that grid structures were supplied on both ends of the electron collector. The third gauge included both the gridcovered ends of the electron collector and the external screening shield. This gauge is the one illustrated in Fig. I-1. The equation used f0 or thei express
4
UI on o
V 1- e pressure i n h1 t e gaug as a function of the ion current- electron current ratio is i+ 1
(1
= p in mm of Hg.
I
S,
100
;
200
VOLTS
300
I
40
_
500
Fig. I-2
Gauge constant as a function of electron energy for nitrogen.
In this equation (i +/i_) is the observed ion currentelectron current ratio in which both currents are expressed in the same units. The gauge constant
K is given in reciprocal pressure units and the pressure p is expressed in millimeters of mercury
1
I I
L
Iz
K1 d q d i
Il bl±r I
.
V I UV 1;U1
The master curve shown in Fig. I-2 gives quantitative data for the gauge constant K as a function of the filament to electron grid potential. This applied potential is a measure of the maximum electron energy used to ionize the nitrogen.
The energies of the ionizing electrons range from this
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(I. PHYSICAL ELECTRONICS) value down to the ionization potential. The maximum in the gauge constant occurs at
100 volts and falls on either side of this maximum in a manner consistent with the expected ionization efficiency curve characteristic of nitrogen. The curve shown was accurately reproduced over a range of gas pressure from 10-8 mm to 10-3 mm. At pressures greater than 10-3 mm the effective yield of ions for high energy electrons is greater, indicating that when the mean free path is equal to or even shorter than the electron path in the ionization region, multiple collisions play a part in increasing the ionization efficiency for high energy electrons. At pressures below 10
- 8 mm the presence of contaminating gases interfered with the accuracy of the results.
A determination of the photoelectric current emitted from the ion collector in the presence of the light from the hot filament indicates that this factor in the background spurious current will have to be considered as a correction factor only at a pressure value of 2 X 1014 mm. The x-ray background of gauge No. 3 has not yet been determined, but it is estimated that this background will be a limitation only for pressures well under 1011 mm when the gauge is operated with an electron accelerating potential of 100 volts or less.
An accurate calibration curve was established for nitrogen over the range of pressure 10
- 6 to 10
- 2 mm as determined by carefully taken McLeod gauge readings. Excellent linearity is found to a maximum pressure of 3 x 10-
3 mm and a decrease in effective
-2 gauge sensitivity takes place between this pressure and 10 mm, bringing the sensitivity down by a factor of 2.
S. Aisenberg, W. J. Lange, L. E. Sprague, W. B. Nottingham
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