US 3662206 A
An illustrative embodiment of the invention shown a method and apparatus for joining a crystalline magnesium fluoride faceplate to the envelope of a photomultiplier tube. The bialkali photocathode deposited on the faceplate is protected from the chemically incompatible silver chloride sealing compound by a preglazed glass frit that acts as a barrier between the photocathode and the seal.
Description (OCR text may contain errors)
O United States Patent 1151 3,662,206 Fleck 1 51 May 9, 1972 5 CATHODE RAY TUBE HAVING INERT 2,966,592 12/1960 Vogl et a1. ..250/2l3 3,275,878 9/1966 Wilbanks ..313/317 BARRIER BETWEEN SILVER 3,310,700 3/ 1967 Dresner et al. .....313/65 CHLORIDE SEAL AND PHOTO 3,519,866 7/1970 Leaman ..313/65 CATHODE 3,006,786 10/1961 S'oberg.... ....313/94 x 3155 859 11/1964 K 313/65 R oert....  lnvenm Fleck 3,213,308 10/1965 Feibelman ..313/65 R  Assignee: Weston Instruments, Inc., Newark, NJ.
Primary Examiner-Robert Segal  Fled: 24,1970 Attorney-William R. Sherman, Stewart F. Moore, John P. [21 1 APPL No; 16,997 Sinnott and Jerry M. Presson Related US. Application Data  ABSTRACT  Division of Sen No. 706,967, Feb. 20, 1968, Pat. No. An illustrative embodiment of the invention shown a method 3,510,925. and apparatus for joining a crystalline magnesium fluoride faceplate to the envelope of a photomultiplier tube. The bial-  [1,5, Cl ,313/94, 313/101 kali photocathode deposited on the faceplate is protected [51 1 Int. Cl. i l'l01j 39/02, H01j 39/00 from the chemically incompatible silver chloride sealing com-  Field of Search ..313 65 R, 101, 94, 65 R, 65 AB Pound y a Preglazed glass frit that acts as a barrier between the photocathode and the seal.  Reierences Cited 4 Claims, 4 Drawing Figures UNITED STATES PATENTS 2,951,962 9/1960 Miller et a1. ..313/89 PATENTEDHAY 9 ma FIG.2
INVENTOR Horsf G. Fleck CATHODE RAY TUBE HAVING INERT BARRIER BETWEEN SILVER CHLORIDE SEAL AND PHOTO CATHODE This application is a division of Ser. No. 706,967, filed Feb. 20, 1968, and now U.S. Pat. No. 3,510,925, issued on May 12, 1970.
BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to sealing methods and apparatus and, more particularly, to techniques for joining a far ultraviolet transmitting faceplate to the envelope of a photomul tiplier tube to fabricate broad spectral band sensitive detectors, and the like.
2. Description of the Prior Art Photomultiplier tubes ordinarily convert radiation, as for example, electromagnetic radiation or photons, into an electrical signal. Usually, these photons are admitted to the evacuated tube interior through a transparent glass window in the photomultiplier envelope. The protons (or quanta) strike a photocathode surface within the tube which then omits electrons in proportion to the intensity of the incident light.
An electrical potential accelerates these electrons toward a first plate in a series of dynodes." The bombarding photoelectrons knock new, or secondary electrons out of the surface of the dynode. The secondary electrons then are accelerated to the next dynode, and so on through successive stages until, at the final stage, an amplified charge pulse is produced that is proportional to the intensity of the initial photon input.
Tubes of this sort often are used to measure radiation intensities in the ultraviolet portion of the electromagnetic spectrum. Tube sensitivity through the entire range of wavelengths, however, is not uniform and continuous, but is a function of the faceplate transmission or bandpass characteristics and the response of the photocathode material to light quanta having wavelengths within the range under consideration. For example, available tubes that have quartz faceplates and alkali metal photocathodes do not produce a significant response to ultraviolet radiations with wavelengths of less than about 1,900 A. (where A. is the angstrom unit, or cm).
Consequently, it is an object of the invention to provide an improved photomultiplier tube that will respond to photons with wavelengths of less than 1,900 A.
1t is another object of the invention to provide a method and apparatus for producing an improved stable faceplate and photocathode combination for a photomultiplier tube that will respond to ultraviolet photons.
SUMMARY In accordance with the invention, a bialkali photocathode is evaporated onto a magnesium fluoride crystal faceplate. This specific combination provided a satisfactory response to photons with wavelengths at least as low as 1,216 A. The magnesium fluoride crystal, moreover, remains transparent to these photons even when subjected to high background radiations for long periods of time.
More particularly, magnesium fluoride crystals are transparent to ultraviolet radiation as low as 1,130 A. in wavelength without exhibiting the hygroscopic and radiation induced opacity features that have characterized the proposed lithium fluoride faceplates of the prior art. Successive layers of sodium and potassium are evaporated on one surface of the magnesium fluoride faceplate, moreover, to produce a bialkali photocathode that emits photoelectrons in response to incident ultraviolet radiation as low as 1,216 A. in wavelength.
Ordinarily, the crystalline faceplate is joined to the glass tube structure through an expansion mount fashioned from fine silver in the shape of an annular ring. The periphery of the silver ring is brazed to a Kovar flange,which, in turn, is fused to the glass envelope. The Kovar flange and the glass envelope have similar coefficients of expansion, while the silver expansion mount has a low yield strength and readily deforms. Consequently, thermal stresses established by differences in the expansion coefficients characterizing the glass and the crystal faceplate are compensated by the relatively soft expansion mount.
Fused silver chloride is used to bond, or join, the crystal faceplate to the expansion mount. The silver chloride, however, is chemically incompatible with the bialkali photocathode and many otherwise desired cathode materials. Typically, this unwanted chemical activity produces an observable deterioration in the photoelectric response of the cathode material and hence is a cause of shortened tube life. This problem is overcome through another aspect of the invention that provides for the interposition of an inert barrier of fused glass between the silver chloride and the bialkali photocathode.
Form a somewhat different standpoint, the invention comprises a method for assembling a vacuum seal in the presence of incompatible materials. For example, the silver expansion mount then is prepared to receive the faceplate on an inner shoulder or flat supporting ring by first depositing an annular barrier of solder glass, or frit, on the supporting surface of the ring. The frit then is preglazed to the adjacent silver shoulder by heating the frit and mount to 390 C. in an oven.
The periphery of the photocathode side of the faceplate is placed on the preglazed frit, and a preformed ring of silver chloride is interposed between the edge of the faceplate and the body of the expansion mount. The entire assembly then is heated in an inert atmosphere until the solder glass softens to establish a seal between the faceplate and the shoulder, which usually requires a temperature of about 450 C. A further increase in temperature, to about 460 C., causes the preformed ring of the silver chloride bonding agent to melt and flow between the edge of the faceplate and the body of the expansion mount.
On cooling, the frit and bonding agent solidify and finnly join the faceplate to the expansion mount. This double sealthe fused silver chloride and the glazed frit-provides a bond that has all of the desirable structural qualities that characterize a bonded silver chloride joint. After the tube has been fully assembled and evacuated, successive layers of sodium and potassium are evaporated and deposited on the interior surface of the faceplate. The glass frit now establishes an inert barrier between the chemically incompatible photocathode and the bonding agent that has, in prior art, severely limited the choice of possible cathode materials.
For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.
BRIEF DESCRIPTION OF THE DRAWING FIG. I shows a portion of a partly assembled photomultiplier tube faceplate structure in accordance with one embodiment of the invention;
FIG. 2 shows the photomultiplier tube structure in FIG. 1 in a more advanced state of assembly;
FIG. 3 shows the photomultiplier faceplate structure of FIG. 1 fully assembled; and
FIG. 4 is an enlarged detail view of the double seal shown in FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Turning first to FIG. 3, a double seal assembled in accordance with the invention completes the enclosure for a glass envelope I0. Preferably, for photomultiplier use, the envelope 10 is fashioned from a borosilicate glass as, for example, Corning Glass 7052. The open transverse end of the envelope 10 is fused to an annular metal welding flange 11. In these circumstances, the flange II often is formed of Kovar because the expansion coefficients of the envelope glass and the Kovar are about the same. These matched expansion coefficients prevent glass-shattering stresses from developing in the envelope as a result of temperature changes during operation.
A thin, ring-like expansion mount 12 having one or more corrugations to compensate for thermal expansion is formed preferably of fine silver, or the like. The periphery of the mount 12 is received on a shoulder 13 formed within the central aperture of the Kovar flange 11. The mount 12 is brazed to the shoulder 13 within the flange 11 in order to form a sturdy permanent point.
A circular central aperture 14 formed within the expansion mount 12 is circumscribed by a transverse mounting ring portion 15 (FIG. 4). A transversely disposed faceplate 16, preferably cut from a magnesium fluoride crystal, is supported on the transverse ring portion 15. The faceplate 16 has an inwardly oriented surface 17 on which a photocathode 20 is deposited after tube assembly and evacuation, for example, by evaporating successive layers of sodium and potassium on the surface 17. The photocathode 20, as hereinbefore mentioned emits electrons in response to the extremely short wavelength ultraviolet radiation passed through the faceplate 16. This photocathode evaporation technique, moreover, produces a layer that covers not only the surface 17 but also a portion of the inner surface of the silver expansion mount. An electrically conductive path thus is established that enables the photocathode to be maintained at an appropriate potential relative to the balance of the tube structure.
The faceplate 16 is joined or bonded to the expansion mount 12 by a seal 21 of fused silver chloride. It will be recalled that the silver chloride, although providing excellent structural integrity for the faceplate and expansion mount assembly, attacks or is chemically incompatible with many other wise excellent photocathode materials, of which the bialkali photocathode 20 and cesium-antimony are typical. To overcome this difficulty, one aspect of the invention provides an inert barrier 22 (FIG. 4) interposed between the photocathode 20 and the silver chloride seal 21, thereby establishing a double seal for the envelope 10. Preferably, the barrier 22 is provided by a solder glass or frit, as for example, 0.010 inches thick Vitta Tape, Type G-10l3.
In accordance with the invention, the double seal for the faceplate 16 and the envelope I0 is assembled as shown in FIGS. 1 through 3. Thus, a small layer 23 of the barrier frit is deposited on the outwardly disposed surface of the transverse ring portion 15 of the expansion mount 12. The layer 23 then is preglazed on to the transverse portion 15 by heating the deposited frit and the mount in a furnace to a temperature of about 390 C. and subsequently allowing the assembly to cool to a suitable handling temperature.
The crystal faceplate 16 next is placed on the preglazed layer 23 (FIG. 2) with the periphery of the surface 17 resting on the frit material. A suitable assembly fixture (not shown) aligns the center of the faceplate 16 with the center of the expansion mount aperture 14 and applies an appropriate force to engage the faceplate with the preglazed layer 23.
An annular corrugation 24 is formed in the expansion mount 12 between the transverse mounting ring portion 15 and the joint connecting the mount 12 to the shoulder 13 on the flange 1 1. A preformed ring of silver chloride 25 is placed in a recess formed by the corrugation 24 and the edge of the faceplate 16. The entire faceplate assembly then is placed in an inert atmosphere furnace and heated to a temperature of 450 C. in order to soften the preglazed glass frit 23. The heatsoftened frit fuses to the faceplate 16 (FIGS. 3 and 4). The thickness of the barrier 22 thus formed is a function of the force applied to the faceplate 16 by the assembly fixture (not shown).
After the glass frit has softened to form the barrier 22, the furnace temperature is increased to about 460 C. At this higher temperature the silver chloride ring 25 (FIG. 2) melts and fills the annular recess formed by the corrugation 24 and the rim of the faceplate 16, as shown in FIGS. 3 and 4. On
cooling, the face late 16 is *giued securely to the thermal expanslon mount 2 by the sed silver c lorlde seal 21. As
hereinbefore mentioned, the photocathode 20 is deposited on the faceplate surface 17, the inner surface of the barrier 22 and on at least a portion of the inner surface of the expansion mount 12 after the tube has been assembled and evacuated. The silver chloride, however, cannot attack the photocathode because the inert glass barrier 22 segregates the bonding compound from the electrically active portion of the photocathode 20.
Illustrative of the structural integrity of the photomultiplier tubes built in accordance with the principles of the invention is the ability of these tubes to withstand temperatures ranging from 50 to C. without failure. Vibration tests applying forces of thirty times the acceleration of gravity (G) through frequencies from 0 to 2,000 cycles per second have failed to damage those tubes. These tubes, moreover, have withstood 20G shock forces applied for 13 milliseconds duration.
Accordingly, the invention provides a sturdy, long-lasting photomultiplier faceplate structure that responds to ultraviolet radiation with wavelengths at least as low as 1,216 A. While there have been described what are at present considered to be preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, intended to cover all such changes and modifications as fall within the true spirit and scope of the invention. What is claimed is: 1. A tube structure comprising a faceplate, a photocathode formed on said faceplate, an expansion mount for supporting said faceplate and said photocathode adjacent thereto, a silver chloride seal chemically reactive with said photocathode for bonding said faceplate to said expansion mount, and an inert barrier means interposed between said seal and said photocathode to inhibit said chemical reaction,
wherein said expansion mount includes a mounting ring portion and said inert barrier means is formed from solder glass and is located on the mounting ring portion between the mounting ring portion and the faceplate, and
whereinthe seal is located on one side of the mounting ring portion, the photocathode is located on the other side of the mounting ring portion and the inert barrier means is interposed therebetween.
2. A structure according to claim 1 wherein said photocathode comprises at least two alkali metals.
3. A structure according to claim 2 wherein said two alkali metals comprise alternate layers of sodium and potassium evaporated on said faceplate.
4. A structure according to claim 1 wherein said faceplate comprises at least a portion of a magnesium fluoride crystal.