A film of either manganese or antimony is evaporated onto a substrate within an evacuated enclosure. Oxygen is introduced into the enclosure to oxidize the film. A layer of antimony is then deposited onto the oxidized film to a predetermined thickness measured by the transmission of light through the substrate. Rubidium and cesium are then evaporated onto the antimony layer after which the substrate is heated to promote an activating reaction between the rubidium, cesium and antimony. Photocathodes formed in this manner, without superficial oxidation, typically have sensitivities within the range of 80-130 microamperes per lumen.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a photocathode and more particularly to a 
photocathode comprising rubidium, cesium and antimony, and to methods of 
forming such a photocathode. 
2. Description of the Prior Art 
The rubidium-cesium-antimony (Rb-Cs-Sb) photocathode, useful in phototubes 
and other high vacuum photomultiplier tubes, has been known for some time. 
The particular advantages of this photocathode over the commonly used S-11 
(Cs-Sb), (K.sub.2 CsSb) and (Na.sub.2 KSb) photocathodes have been found 
to be in its better properties of long wavelength threshold, luminous 
sensitivity and relative ease of processing. (See Carl W. Morrison, 
"Technique For Producing High Sensitivity Rubidium-Cesium-Antimony 
Photocathodes", Journal of Applied Physics, Volume 37, Number 2, February 
1966, pages 713-715.) 
It has also been determined that the sensitivity and threshold wavelength 
of Rb-Cs-Sb photocathodes are increased by the well-known process of 
superficial oxidation. However, the sensitivities of these photocathodes 
produced by the common prior art techniques, including superficial 
oxidation, disadvantageously vary within a wide range from about 30 to 120 
microamperes per lumen. Such a broad range of photocathode sensitivities 
generally results in unacceptable non-uniformities in tube performance and 
relatively high scrap rates due to failure to meet required sensitivity 
levels. It is therefore desirable to consistently make photocathodes of 
relatively high sensitivity without superficial oxidation for ease of 
processing and improvement in performance. 
SUMMARY OF THE INVENTION 
A photocathode comprises a substrate and a film of antimony oxide or 
manganese oxide on the substrate. A layer of antimony is included on the 
oxide film and a deposit of rubidium and cesium is on the antimony layer. 
A method of forming such a photocathode is also provided.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 of the drawing, there is shown a photomultiplier tube 10 
comprising a photoemissive cathode 12 made in accordance with the present 
novel method. The photomultiplier tube 10 is utilized herein by way of 
example, it being understood that any photosensitive device incorporating 
a photocathode may be used. 
The tube 10 has a tubular glass envelope 14. One end of the envelope 14 is 
closed by a substantially flat transparent glass faceplate 16, and the 
other end of the envelope 14 is closed by a glass stem 18 having a number 
of electrical lead-in pins 20 and an exhaust tubulation 19, shown as being 
"tipped off". Along the interior of the tube 10 are spaced a series of 
dynodes 22. 
Mounted near the dynodes 22 are nickel wires 24 and 26 and a container 28. 
The nickel wires 24 and 26 are connected to, preferably, a tungsten wire 
filament 25 and a platinum-clad molybdenum wire filament 27, respectively. 
Attached to the tungsten wire filament 25, as by being fused thereon, and 
situated near the faceplate 16 is a pellet 30 of pure manganese metal. The 
pellet 30 may be commercial electrolytic manganese consisting of pure 
manganese containing a maximum of 0.1% of sulfur and iron. 
Attached to the platinum-clad molybdenum filament 27 is a pellet 32 of pure 
antimony. The pellet 32 may be high grade antimony having a composition of 
substantially 99.88% antimony and traces of iron, sulfur, arsenic and 
lead. The container 28 contains a substance for evaporating rubidium and 
cesium. The substance in container 28 is preferably a mixture of rubidium 
chromate, cesium chromate and silicon. The nickel wires 24 and 26 are 
suitably connected to electrical current sources (not shown) to pass a 
current through the pins 20 so that they can be activated separately by 
electrical resistance heating. The container 28 can be activated by 
electrical resistance heating or by RF induction heating. 
The photocathode 12 in one example of the present method is made in 
accordance with the following procedure, which is summarized in the flow 
chart shown in FIG. 2. 
The exhaust tubulation 19 is connected, prior to tip-off, to an exhaust 
system (not shown) and the tube envelope 14 is evacuated until the 
pressure in the envelope 14 is in the order of 10.sup.-6 Torr or less. The 
tube 10 is then baked between 275.degree.-400.degree. C. for two to three 
hours to remove occluded gases from the interior tube components. The tube 
10 is then cooled to room temperature. 
While the tube 10 is still being evacuated, a current is passed through the 
filament 25 as by the current sources (not shown) in order to heat the 
filament 25 to a temperature sufficiently high to evaporate the manganese 
from the manganese pellet 30 in the vacuum. The evaporated manganese 
condenses on the inner portion of the faceplate 16 and forms a thin film 
thereon. In order to measure the amount of material deposited on the 
faceplate 16, light transmission can be measured in a manner disclosed in 
U.S. Pat. No. 2,676,282 to Polkosky. A light indicator can be set to show 
a scale reading of 100 at full transmission of light through the faceplate 
16. The evaporation of manganese is continued until the transmission 
through the manganese film is preferably between 90-95% of its original 
value of 100%. 
Oxygen is next introduced into the envelope 14 through the exhaust 
tubulation 19 to a pressure of about 270-370 microns of mercury. The 
manganese film is then oxidized by the use of a high frequency electrode 
(not shown) placed over the faceplate 16. The high frequency of the 
electrode produces within the envelope 14 a gaseous discharge which causes 
the manganese to react with the oxygen in the envelope 14. The electrode 
is held over the faceplate for about five to twenty seconds. As a result, 
the manganese film is fully oxidized. This method of oxidizing metal films 
within an envelope is well known and completely described in U.S. Pat. No. 
2,020,305 to Essig. The oxygen within the envelope is then removed and the 
light transmission indicator is reset to 100. 
A layer of antimony is next deposited over the film of manganese oxide by 
evaporation of the antimony pellet 32. A current is passed through the 
filament 27 to heat the filament 27 to a sufficient high temperature to 
evaporate the pellet 32 of antimony. A film of antimony condenses on the 
oxidized film of manganese on the faceplate 16. The evaporation of 
antimony is continued until the light transmission through the faceplate 
16 is about 65 to 85% of its value without the evaporated antimony layer. 
Rubidium and cesium are then released into the evacuated envelope 14 for 
activation of the antimony film by heating the container 28 to a 
sufficiently high temperature to evaporate the substance contained 
therein. Preferably, the rubidium and cesium are released simultaneously, 
but the container 28 may be arranged for release of rubidium and cesium 
alternatively. The container 28 may be heated by passing a current 
therethrough or by an external RF electrode. Enough rubidium and cesium 
are released to completely react with the total amount of antimony on the 
oxidized manganese on the faceplate 16. In order to ensure that all the 
antimony has been reacted with the rubidium and cesium, an excessive 
amount of rubidium and cesium is preferably introduced into the envelope 
16. The tube 10 is then baked in an oven at a temperature of approximately 
140.degree. to 160.degree. C. for about 10 to 20 minutes. The temperature 
is then increased to about 180.degree. to 195.degree. C. and the tube 10 
is heated for about 25 to 50 minutes to promote an activating reaction 
between the rubidium, cesium and antimony. 
The tube 10 is next slowly cooled at a rate of about 5.degree.-10.degree. 
C. per minute to approximately 70.degree.-80.degree. C. at which 
temperature the tube is allowed to cool freely to room temperature. After 
the tube 10 is removed from the exhaust system and the exhaust tubulation 
19 sealed, the tube 10 is operative. 
In a preferred method of making a Rb-Cs-Sb photocathode, an antimony oxide 
is utilized as a base film instead of the manganese oxide. It has been 
previously determined, however, that an antimony film evaporated from a 
source of pure antimony is not readily oxidized upon the performance of 
the above-described oxidizing steps. Hence, the use of the manganese film. 
However, an antimony film may be readily oxidized when the film is 
evaporated from an antimony-platinum source. Thus, in accordance with the 
preferred method of making Rb-Cs-Sb photocathode, the pellet 32 is an 
antimony-platinum alloy comprising about 50% antimony and about 50% 
platinum, by weight. The pellet 32 is arranged to be situated near the 
faceplate and the manganese pellet 30 is eliminated. The steps for forming 
the photocathode are the same as described above with the exception that 
an antimony film is initially evaporated on the faceplate 16 instead of 
manganese. Also, the deposition of antimony on the oxidized film is from 
the same antimony-alloy pellet 32. 
Photocathodes made in accordance with the described method and in 
particular with the antimony oxide film have been measured to have 
sensitivities consistently within the range of 80 to 130 microamperes per 
lumen without superficial oxidation. It is also believed that the 
photocathodes formed by the present method are in substantially 
stoichiometric proportions according to the formula Rb.sub.x Cs.sub.3-x Sb 
and that the well-defined composition accounts for the consistency in its 
properties.