US 3329853 A
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July 4 1967 R. G. NEUHAUSER IMAGE ORTHICON WITH CESIUM GETTER ADJACENT ELECTRON MULTIPLIER Filed June 16, 1965 Smm? INVENTOR.
United States Patent O 3,329,853 IMAGE ORTHICON WITH CESIUM GETTER ADJACENT ELECTRON MULTIPLIER Robert G. Neuhauser, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Filed June 16, 1965, Ser. No. 464,291 3 Claims. (Cl. 313-179) ABSTRACT OF THE DISCLOSURE In an image orthicon tube having a photocathode, an electron multiplier, an electronically conducting glass target and an excess of cesium required in processing the photocathode; an antimony getter is positioned adjacent to a high voltage region of the multiplier and has an aiiinity for cesium that is greater than that of the target but less than that of the photocathode. The adjacency of the getter to the high Voltage region of the multiplier precludes harmful effects of the getter on the operation of the tube, and the selective aflinity of the getter for cesium precludes cesium collection on the target and assures adequate cesium `for desired operation of the photocathode.
BACKGROUND OF THE INVENTION (l) Field of the invention My invention relates to photoelectric tubes and particularly to photoelectric tubes having lphotoemissive cathodes including cesium, and to a method of making the same.
(2) Description of the prior art One kind of photoelectric tube having a photoemissive cathode including cesium is a pickup tube of the image orthicon type. This type of tube includes a target upon which electrons emitted -by the photoemissive cathode impinge and produce an electrostatic charge pattern conforming to an image to which the photoemissive cathode is exposed.
In order to preserve a desired delity of the electrostatic charge pattern thereon, it is important that the target be characterized by a relatively low order of lateral electrical conductivity, i.e., conductivity across the face or surface of the target. One cause for excessive lateral electrical conductivity of the target may be an unavoidable deposit thereon of a film of cesium. Such lm of cesium is derived from an excess amount of cesium used in processing the photoemissive cathode. Such excess amount of cesium is desirable to assure that an adequate amount thereof is available for completing a processing of the photoemissive cathode.
Such deposit of cesium on the target is tolerable in connection with some target materials, but is highly objectionable when the target is made of another and particularly desirable material. Target materials that can tolerate the cesium film thereon without objectionable lateral conductivity, are semiconductors such as aluminum oxide and magnesium oxide, and an ionically conductive glass that includes sodium or other alkali metals. However, these materials have limitations such as suitability for only special applications in the case of the semiconductors, and a prohibitively short life in the case of ionically conductive glass. A particularly desirable target material having a relatively long life and relatively wide application, is an electronically conductive glass free of any substantial amount of alkali metal ions. However, this glass responds to a lm of cesium in such a way as to result in prohibitive lateral conductivity.
Cesium is a material that has a relatively high vapor pressure so that when it is used in an electron tube it migrates throughout the tube, not only during the processing of a photoemissive cathode within the tube, but also during the operating life of the tube. Such migration of cesium causes wanted accumulation of cesium on the photoemissive cathode and unwanted accumulation on other parts. Such accumulations of cesium are relatively rapid during the processing of the photoemissive cathode and -relatively slow during the operating life of the tube. Where the tube includes a target made of electronically conductive glass, it is important that both types of accumulation of cesium on the target be reduced.
It is thus seen that it is desirable to reduce cesium deposits on a target made of electronically conductive glass. However, it is necessary that such deposits of cesium upon the photoemis-sive cathode be unimpeded.
SUMMARY Accordingly it is an object of the invention to provide a photoelectric tube having a cesium-containing photoemissive cathode in which unwanted deposits of cesium within the tube are reduced.
Another object is to provide a cesium getter for a photoelectric tube having a cesium-containing photoemissive cathode and an electronically conductive glass target, wherein the getter is made of a material that has a higher attraction for cesium than the glass target.
A further object is to provide and advantageous method of making an electron tube having a cesium-containing photoemissive cathode and an electronically conductive glass target.
According to a feature of the invention a photoelectric tube having a photoemissive cathode including cesium, and an electronically conductive glass target, is provided with a cesium getter that functions to reduce cesium deposits on the target while permitting `such deposits on the photoemissive cathode.
The material of which the getter is made may include antimony, -rubidiurn, bismuth or a silver-bismuth alloy including by weight nine parts of silver and one :part of bismuth. Of these materials, antimony is preferred.
Where the photoelectric tube is an image orthicon, the getter material may be supported at a location within the tube wherein any -tendency for photoemission of the compound formed by reaction of the getter material, will not introduce spurious photoelectrons in unwanted areas. Such location may be adjacent to the inner wall of the tube envelope at the bulbous image region thereof or at the neck portion of the envelope in which a multiplier system is positioned.
According to a method aspect of the invention, a getter.
for cesium is flashed after the release of cesium within the tube.
According to another method aspect of the invention, a getter for cesium is flashed prior to the release of cesium within an electron tube and subsequent to such release ffor providing a continuing source of getter material during the operating life of the tube.
BRIEF DESCRIPTION OF THE DRAWING Further objects and features of the invention will become evident from the following detailed consideration of :an exemplary embodiment taken in connection with the appended drawing in which:
FIG. 1 is an elevation, partly in section, of an image orthicon tube in which the invention is used; and
FIG. 2 is a transverse view taken along the line 2--2 of FIG. 1 and shows features ofthe image section of the tube.
(a DESCRIPTION OF THE PREFERRED EMBODIMENT The image orthicon tube shown in FIG. l comprises an envelope having in one end portion thereof, an electron gun 12 and an electron multiplier system 14. In the other end portion of the envelope is an image section comprising a photoemissive cathode 16 supported on an end wall 18 of the tube, and a target 20 positioned in the path of electron emission from the photocathode 16. An electrode 22 serves to accelerate the electrons emitted by the photocathode in their travel towards the target 20. The photoelectrons released by the photocathode 16 strike the target and release seconda-ry electrons therefrom which are collected by a Wire mesh electrode 24. `Such release of secondary electrons produces an electrostatic charge pattern on the target 20 conforming to the image to which the photocathode 16 is exposed. This electrostatic charge pattern includes regions that are more negative than other regions.
The electron gun 12 is adapted to produce a beam of electrons that is scanned across the target 20 by suitable means, not shown. A portion of the beam is reected back toward the gun 12 by the more negative regions of the target 20. Such reflected electrons enter the multiplier system 14 where they are multiplied and carried out of the tube through a -suitable lead 26 as an output signal.
The photocathode 16 -for example, may be one having an S-10 or S-20 response. A photocathode having S-10 response includes silver, bismuth, oxygen and cesium. A photocathode having S-20 response includes potassium, sodium, antimony and cesium. A photocathode of S-10 response is described in U.S. Patent 2,682,479 to Johnson, and a photocathode of S-20 response is described in U.S. Patent 2,770,561 to Sommer. In making these types of photocathodes, electrical current is passed through -a Wire 28 (FIG. 2) supporting a plurality of pellets 30. The pellets 30 consist of the bismuth-silver alloy referred to for a photocathode of S-10 response 0r of antimony for a photocathode of S20 response. In both types of photocathodes, cesium is introduced by heating 'boats 32 containing cesium chromate and a reducing material such as silicon, to a temperature of about 160 C. This results in the evolution of cesium within the envelope 10 `and a deposit of cesium on the photoemissive layers previously Iformed.
At the temperature indicated, cesium becomes fugitive 'and besides depositing on the photocathode, deposits on tube elements Where it is not wanted. Such unwanted deposits occur even though it is attempted to limit the amount of cesium introduced to that required for sensitization of the photocathode. However, such -a limitation in the amount of cesium introduced into the tube is preferably accompanied by -a slight excess to make certain that a suicient amount of cesium for complete sensitization of the photocathode is available.
One of the tube elements on which a cesium -deposit is unwanted is the target 20, when this target is made of electronically conductive glass.
Targets made of electronically conductive glass acquire a prohibitive excessive lateral conductivity when cesium is deposited thereon. This is a serious problem because targets made of electronically conductive glass have utility in a large number of applications.
One type of electronically conductive glass found of particular advantage as a target material consists by weight of 33.1% barium oxide (BaO), 11.1% calcium oxide (CaO), 1.8% aluminum oxide (A1203), 39.8% silicon dioxide (SiOz), and 14.2% titanium oxide (TiO2). The surface of a-target made of this glass is relatively smooth so that cesium deposited thereon forms a conductive path thereacross, and no resistive compound is formed, as in the case of ionically conductive glass. The resultant high order of lateral conductivity adversely affects the electrostatic charge pattern formed on the target so that the output of the tube is characterized by a resolution that is inferior to an unacceptable degree to that of the pattern initially formed on the target by the emission `from the photocathode.
As herein disclosed, a photoelectric tube having a photocathode including cesium and wherein the envelope of the tube includes an amount of cesium in excess of that required lby the photocathode, and a tube element such as a target harmfully affected by such excess cesium, is provided with a getter for cesium which possesses an important property in relation to the photocathode and the tube element referred to. This property resides in the fact that the getter has ya preference or aflinity for cesium 4intermediate that of the photocathode `and the tube element. Thus, the getter material has a greater attraction for cesium than the tube element mentioned, but a smaller attraction for cesium than the photocathode. In this way, the free cesium is prevented from collecting on the tube element, and the photocathode is assured of adequate cesium for its proper function.
I have found that getter materials having this preferential attraction for cesium include antimony, rubidium, bismuth or a silver-bismuth alloy referred to before herein. Of these materials, antimony is preferred for reasons that will become apparent as the present description continues.
In one example of practicing the invention involving an image orthicon tube of the three-inch size, an antimony pellet 34 weighting about 12 milligrams is supported on a platinum clad molybdenum wire 36 vof 10` mil size, either in the bulbous image portion of envelope 10 or adjacent to the multiplier section 14 in the neck portion of the envelope, as shown in FIG. 1. An'electric current of about three amperes is fed across the support wire 36 to cause the antimony pellet 34 to be heated at least to 500 C., at which temperature it evaporates or ashes. The evaporant forms an antimony coating on the inner wall of the envelope 10 4at a region adjacent to the antimony pellet 34. I have found that this coating has a greater reactive attraction for cesium than the electronically conductive glass target 20, -but a smaller reactive attraction for cesium than the photoemissive cathode 16.
In practice, it is found that only one antimony pellet of the size indicated is required for satisfactory gettering of excess cesium Within the envelope 10. When the antimony getter lis disposed in the bulbous image region of the envelope 10, it is positioned between the inner wall of this region and a target cup 38. In this way, the target cup isolates the antimony coating formed on the inner Wall of the envelope 10 as well as that `forme-d on the outer surface of the target cup 38, from the region in which emission takes place from the photocathode 16 to the target 20. Therefore, if there should be'any tendency of the antimony getter coating to emit photoelectrons, such emission will not affect the normal emission from the photocathode 16 directed to the target 20.
'When the antimony getter 34 is positioned in the neck portion of the envelope 10, it is adjacent to a relatively high voltage region of the electron multiplier 14. The position of the getter material should be arranged so that any photoemission from the getter in this region Will have no appreciable effect on the output signal. This can be accomplished by placing the getter near the high voltage portion of the multiplier or by electrically connecting the getter surface to a high voltage electrode available in the multiplier section.
The thickness of each antimony coating formed by evaporation of the antimony getter pellet 34 should not be too thick since this will cause the coating to peel. However, it should be sufficiently thick to react with all excess cesium generated within the tube envelope 10. A satisfactory coating thickness is found by applicant to be roughly one micron.
The antimony getter 34 may be evaporated or ashed both before release of cesium from the boats 32 (FIG. 2)
as well as after such release and operation of the photocathode 16. If two evaporations are employed, a part only of the getter is flashed during each evaporation by an appropriate temperature-time relationship observed in each dashing step. It is desirable to getter the cesium before it settles on the target 20, and therefore ashing the getter 34 before any cesium is released in the tube 10 is of advantage. However, tlashing the getter before the release of cesium in the tube is not as important as hashing it after cesium has been released. 'I have found by tests, that best results are obtained when two dashing steps referred to above, are carried out. Less desirable, but satisfactory, results are obtained when the second flashing step alone is practiced.
The ashed coating of antimony forms a stable cesiumantimony compound at room temperature. It is believed that there is formed on the antimony coating, produced by flashing the getter 34, the equivalent of many monatomic layers of cesium-antimony compound. It is desirable that the antimony coating consumed in forming the cesiumantimony compound be the rst of two layers so that the second layer of antimony may be formed over the consumed antimony layer. In this way, the antimony getter will provide a continuous cesium gettering action during the life of the tube and a highly desirable electronically conductive target Z may be employed advantageously in image orthicon tubes.
While the present cesium getter is seen to be of particular value in a tube having an electronically conductive glass target, it is also of some advantage in connection with other types of targets. While such other targets may tolerate cesium to a greater degree than electronically conductive glass, the present getter will serve to prevent cesium deposits thereon beyond a tolerance range.
Furthermore, while the invention has been shown to be of advantage in connection with photoelectric tubes having S40 and S20 responds, it is also useful in photoelectric tubes having photocathodes characterized by other types of response.
1. A pickup tube comprising an elongated single chamber envelope, v
(a) a photoemissive cathode including cesium positioned in one portion of said envelope,
(b) an electron multiplier having a relatively high voltage region positioned in the other end portion of said envelope,
(c) an electrostatic charge-holding target positioned rintermediate said cathode and multiplier, said target being adversely affected by cesium contamination, and
(-d) a getter for cesium having a photoemissive property positioned'adjacent to said high voltage region and having a greater aflinity than said target for cesium,
(e)v whereby said target is preserved from cesium contamination and photoemission from said getter has negligible effect on the output -from said multiplier.
2. A pickup ltube according to claim 1 and wherein said getter comprises antimony.
3. An image orthicon tube having a `single chamber envelope, said envelope containing,
(a) a photoemissive cathode,
(-b) a target made of electronically conductive glass disposed in the path of electron emission from said cathode,
(c) an electron multiplier spaced from a side of said target remote from said photoemissive cathode and having a relatively high voltage region,
(d) cesium in excess of that required for sensitization of said cathode, and
(e) a getter for cesium made of a material that has stronger `atlinity for cesium than said target and a weaker ainity for cesium than said photoemissive cathode, whereby Aadequate cesium for sensitization of said cathode is assured,
(f) said getter being positioned adjacent to said high voltage region of said electron multiplier for assuring negligible etects of said getter on the operation of said tube.
References Cited UNITED STATES PATENTS 2,713,129 7/1955 Bruining et al. 313-65 2,887,596 5/1959 Rijssel et al. 313-65 3,061,752 1'0/ 1962 'Banks 313-65 FOREIGN PATENTS 860,551 2/ 1961 Great Britain.
JAMES W. LAWRENCE, Primary Examiner.
ROBERT SEGAL, Examiner.