US3478209A

US3478209A - Self-luminous tritium light sources

Info

Publication number: US3478209A
Application number: US475322A
Authority: US
Inventors: Irving Feuer
Original assignee: Canrad Precision Industries Inc
Current assignee: Canrad Precision Industries Inc
Priority date: 1965-07-22
Filing date: 1965-07-22
Publication date: 1969-11-11
Anticipated expiration: 1986-11-11

Description

United States Paten1: Oi

3,478,209 Patented Nov. 11, 1969 hee U.S. Cl. Z50- 77 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to self-luminous tritium light sources with improved light output and improved longevity wherein the loss of tritium in the tritiated light source is minimized by preventing exchange of tritium with hydrogen in water vapor which may become present in the tritium-activated lamp.

Lamps of the self-luminous type in which the light source comprises a phosphor which is excited by a radioactive material are known as beta lights, and beta lights which use either gaseous or solid tritiated luminous cornpounds can be considered as tritium beta lights.

It is a primary object of the present invention to increase the initial light output of such tritium beta lights, and also to improve the light output longevity of the same.

It is another object of the present invention to provide for a construction which minimizes the loss of tritium in a tritiated light source so as to improve the long term luminosity of the light source.

It is yet a further object of the present invention to provide sealed beta ray light sources Iwith a minimum of residual water vapor in the sealed source so as to result in increased initial luminosity and increased long term luminosity.

It is still anotnher object of the present invention to provide for the above improvements in beta ray light sources, in particular in the beta ray light source structure disclosed in my co-pending U.S. patent application Ser. No. 271,770, iiled Apr. 9, 1963.

Other objects and advantages of the present invention will be apparent from a further reading of the specification and of the appended claims.

With the above and other objects in view, the present invention mainly comprises as a self-luminous light source, a sealed casing which is at least partially transparent, and which is preferably of a material resistant to darkening under beta ray bombardment, a phosphor which is excited by beta rays to emit light, a source of tritium which gives off beta rays which in turn excite the phosphor to emit light, and a dehydrating agent within the casing to absorb Water vapor. The dehydrating agent may be any dehydrating agent which is non-reactive with any of the other materials in the casing, e.g. silica gel, calcium hydroxide, etc.

It is well-known that tritium (II-3) is an isotope of hydrogen, and it has been found that there is an exchange of tritium with the hydrogen in water or water vapor. Although this exchange is qiute low when one considers the rabsolute concentrations of tritium in the air, the exchange assumes importance, if one considers tritiated radioactive light sources.

It has been found according to the present invention that by providing a iield tritiated radioactive light source and further providing a dehydrating agent such as silica gel therein, the exchange of tritium from a tritiated light source with water vapor in the atmosphere and/ or in the eld tritiated radioactive light source is minimized, and in addition, the residual water vapor pressure in the iield source is decreased, so that the initial luminosity of the light source is increased and the long term luminosity is also increased. The increase in initial luminosity results from the decrease in the residual water vapor pressure in the iield source which decreases the absorptive barrier between the phosphor particles, for example the particles of luminous zinc sulphide.

In accordance with the preferred embodiment of the present invention, the invention mainly comprises an improved radioactive light source which comprises a casing having disposed therein a radioactive region containing a tritium-containing material giving oif beta rays, a phosphor region positioned in front of the tritium-containing region in the direction of light discharged from the source, the phosphor region being of suicient thickness to absorb a substantial portion of beta rays without substantial absorption of light rays, a light reflective and beta ray reilective heavy metal reilecting region positioned behind the radioactive region, the heavy metal having an atomic number of at least 45 and having a thickness suliicient to reflect beta rays and the reflecting region having a frontfacing light reflecting surface so that the reflecting region serves to reilect both light and beta rays forwardly, the forwardly directed beta rays exciting the phosphor region and being converted into light, and a dehydrating agent located within the light source for the absorption of water Vapor within the light source and for the prevention of exchange of tritium with water vapor.

As employed throughout the specication and claims, the terms forward and front are used to denote the areas or regions Ibetween the radioactive source and the external environment to be illuminated. Similarly, the term back region is employed to denote the area behind the radioactive material and away from the area or surface from which light is directed to the external environment.

In accordance with the present invention it is possible to provide capillary tubes of glass, vinyl plastics, styrene plastics, or the like, which are sealed and which contain the phosphor and either a gaseous or solid source of tritium, and which contain the dehydrating agent, such as silica gel either at the ends of the capillary tube or disposed throughout the phosphor. The source of tritium may be a gaseous or solid tritiated luminous material such as tritiated zinc sulphide. The capillary tube may be entirely transparent or it may be partially opaque to pro- Vide a split system, e.g. a dual or split source for spectrophotometric measurements.

The capillary tubes may be straight tubes or may be odd shaped structures, for example in the shape of letters of the alphabet.

The tritium-luminous material capillaries provide a maximum luminosity per quantity of radioactive material and can be used in a powder, non-gaseous tritiated medium.

It is also possible according to the present invention to employ filling gases which are dry and of low molecular weight (for example helium) in order to improve the light output initially and transiently.

By the use of the dehydrating agent, for example, silica gel, at the end of the tube where it is heat sealed, it is possible to accomplish the heat sealing under atmospheric conditions, which minimizes the deleterious effects of water vapor.

The back heavy metal reecting region which is used in the construction of the preferred embodiment of the present invention should desirably have a high atomic number of the metal, i.e. at least 45 and preferably greater than 76, in order to serve to back scatter the beta particles,

as well as to reflect light. The reflected beta particles then further excite the forward phosphor regions and ultimately this energy is discharged from the system in the form of light energy.

It is essential in this construction that the back scattering region be characterized as a metal or metal composite having an atomic number of at least 45 in order to reflect at least 60% of the back directed beta rays. Materials of lower atomic number such as aluminum cannot serve to effectively back scatter the beta particles. By use of a heavy metal region beta particles which would normally be absorbed outside of the phosphor light producing material are more efciently utilized within the phosphor regions. Thus, it is possible by the use of this system to use merely the heavy metal back scattering region and a front phosphor region in yet higher efficiencies in terms of light conversion than would be effected by the use of a two layer phosphor system with or without the use of an aluminum reecting surface, as for example, shown in U.S. Patent No. 2,953,634 to MacHutchin et al.

This construction is particularly suited to the use of relatively weak beta ray emitters such as tritium H-3, and the provision of the dehydrating or hygroscopic agent such as silica gel permits the maximum utilization of the tritium because of the minimizing of the exchange of tritium with hydrogen of water vapor.

In one aspect of the invention the tritium is deposited on an intermediate phosphor layer. This is desirable because the radiation from the tritium is maximally utilized and the upper non-activated phosphor layer serves as a protective barrier against radioactive contamination and mishandling.

Numerous types of phosphors or phosphor combinations such as zinc suldes, cadmium suldes, zinc silicates, zinc beryllium silicates, zinc oxides, calcium tungstates, etc., are employed in the present structure. The depth of the front phosphor region will vary somewhat depending on the energy level of the radioactive source but will be of suflicient depth so as to absorb beta rays but not light rays. This is made possible by the fact that the attenuation thickness of optical transmission is substantially greater than the beta ray thickness for complete absorption of weak beta rays from the radioactive source, e.g., tritium. Thus the depth of the front phosphor region may be controlled to fall within a region giving at least 90% absorption of the weak beta rays, without absorbing substantial quantities of the light ray. Thus, for example, when employing zinc sullide or cadmium sulde phosphors in combination with a tritium beta ray source, a thickness of l mil allows 90% of the light rays to pass through unabsorbed while at least 90% of the weak beta rays are absorbed, since the absorption thickness for weak beta rays is of the order of 1 0-20 microns (depending upon the nature of the absorber).

The average particle size of the phosphor preferably lies in the micron range, eg., 2 to 30, especially 10 to 30 microns. This is desirable because there is little self attenuation of the light in thin layers. However, if particle size is too small there are large light scatter losses.

As noted previously, it is essential that the back heavy metal region be of a metal or metal laminate having an atomic number greater than or equal to 45 in order to effectively return the beta rays to the forward part of the system and to eifectively convert their energy into light. Simultaneously the heavy metal reflects light forwardly, thus giving a highly elfective overall conversion of radio-active energy to light energy. It is particularly preferred to employ platinum, osmium, iridium, and their alloys, as the heavy metal back reflecting region. Alternatively, bismuth or lead or high atomic weight oxides such as lead oxide can be employed. Additionally, a bound laminate of aluminum deposited on a heavy metal such as platinum, bismuth, or lead can be utilized, the aluminum deposit serving to improve light reflection. It should be noted that the heavy metal region (compound of heavy metal with or without bound aluminum) is positioned closest to the back phosphor region (if one be employed) and is separated from the front phosphor region by the radioactive source. This is necessary since the back reflector region is employed to reflect both light and beta rays forwardly to the area where it is discharged from the structure in the form of light rays.

It is noted that since the front phosphor region is of suflicient depth so that at least some portions thereof are not radioactive, it serves as a protective cover absorbing the beta rays, as well as being a source of light and thus no additional protective covers are necessary. Normally, however, it will be desirable to use a front glass or plastic transparent cover such as one made of methyl methacrylate or mica. However, no distinct radio-active absorbing protecting structure is required. It is desirable, however, to coat the internal surface of the transparent cover with an anti-reflecting coating such as magnesium uoride so as to minimize the internal reilection of the emitted light rays and thus maximize the effective light sent outwardly to the external environment.

The various aspects and modifications of the present invention will be made more clearly apparent by reference to the following description and accompanying drawings, in which:

FIGURE l illustrates a capillary tube structure in accordance with the basic concept of the present invention.

FIGURE 2 illustrates a system characterized by the use of a single front phosphor region in combination with a heavy metal backed reflecting region in which the dehydyrating agent is distributed in the phosphor.

FIGURE 3 illustrates the use of multiple phosphor regions in combination with a solid radioactive source.

FIGURE 4 depicts a system amenable to the utilization of a gaseous radioactive material in which the preferred embodiment of the present invention is combined with the use of a dehydrating agent.

lIn FIGURE l a capillary tube 11, which may be made of glass, or any suitable transparent plastic such as a vinyl plastic or polystyrene, having a wall thickness of, for example, about 0.1-0.2 mm., the entire capillary tube having a diameter of about 1 mm., is heat sealed at its end 14. The tube is provided at the end 14 where it is heat sealed with a dehydrating agent 13, e.g. silica gel. As shown in FIGURE l, the tube can be heat sealed at both ends 14 and 14', in `which case both ends are provided with the dehydrating agent 13. This silica gel located at the end of the tube where it is heat sealed permits the sealing of the tube under atmospheric conditions and minimizes water vapor effects. The phosphor 12, which may for example be tritiated zinc sulphide is disposed throughout the tube. It is possible to have the dehydrating agent dispersed in the phosphor, as will be shown in the discussion of FIGURE 2.

With reference to FIG. 2, shown therein is a system characterized by the use of a front phosphor region and solid radioactive materials imbedded in a phosphor layer, there being no distinct back phosphor region employed in the illustrated system. The entire system is enclosed in casing 1 which may be made of any of a wide variety of materials such as glass, plastics, methacrylates, epoxy resins and metals, such as aluminum or iron. Casing 1 in combination with transparent glass or plastic cover 5 provides an enclosure for containing the system of the present invention whereby beta rays are converted into light. The source of radioactivity in region 7 are radioactive particles imbedded in or on the phosphor grains, which also have particles of silica gel dehydrating agent imbedded therein. The actual impregnation of the phosphor particle with the radioactive solid can be done by a wide variety of conventional techniques, as for example, (a) sedimentation and evaporation, (b) vacuum evaporation, (c) slush milling and evaporation, (d) spray coating, etc. The radioactive solid is a stearic type (or other organic or inorganic derivative) solid and a ZnS phosphor is employed. The radioactive material gives off beta rays having an energy range between 3 kev. to 17.9 kev. The phosphor particles preferably range between yl and 24 microns in size and the radioactive material comprises about -6 to 104% (by weight of the phosphor). Region 7 is approximately 5-18 microns in depth.

lPositioned forwardly from said radioactive source is front phosphor region 8. Region 8 may contain one or more layers of phosphor particles which are excited by the beta rays given off from region 7 and thus convert the radioactive energy into light energy which passes outwardly through transparent cover 5. At least a substantial portion of region 8 is free of radioactive materials so as to serve as a shield layer, preventing the weak beta rays from passing out through transparent cover 5. By the same measure the width of phosphor layer 8 is such that the light produced therein is not absorbed to a substantial degree and thus passes out to the external source. Phosphor particles 4 may be the same type of phosphor-containing material employed in the radioactive region or alternatively can be a different phosphor-containing material, as for example, in the present illustration, calcium tungstate. In general, there is no purpose for another phosphor in the coverage light source as another phosphor would yield another color. However, two different phosphors may be desirable where it is desired to obtain two color peaks, e.g. in the case of a double light standard source. In any event, either or both of the types of phosphors may have the dehydrating agent imbedded therein.

Number 6 in the drawing represents the radioactive substance deposited on or impregnated in phosphor particles 3. Numeral 9 in the drawing represents the dehydrating agent particles deposited on or imbedded in the phosphor particles 3 and the phosphor particle 4.

Since the beta rays are being given off in a variety of directions, normally only those passing forwardly would be seen by light producing phosphor region 4. However, in accordance with the present invention, region 2 containing a heavy metal, i.e., a platinum layer, is positioned behind the radioactive region 7 and serves to reflect both beta rays and light which may be directed inwardly from

phosphor regions

7 and 8. The reflected light and back scattered beta rays are reflected forwardly into phosphor region 8 and are effectively made use of, the latter being converted to light energy upon impinging the phosphor particles, and the former passing substantially unabsorbed out through transparent cover 5. In general, heavy metal reflecting region 2 will have a thickness of approximately 0.1 mil to l0 mils, preferably 0.1 to 2 mils, so as to effectively serve to reflect beta ray particles. Thus, in the present example, region 2 will have a depth of about 0.5 mil; region 7, a depth of about microns and region 8, a depth of about 15-30 microns. Substantially no beta rays thus pass out of the system through cover 5 while converting the beta rays of the radioactive solid source material to light rays.

In general, it is desired that the various regions, e.g., phosphor region, heavy metal reflecting regions be disposed in parallel relation in order to obtain uniformity of light discharged from the structure. While parallel curved surfaces can be employed, in general it is desirable to employ relatively flat regions.

Turning to FIG. 3, shown therein is a particularly preferred embodiment of the present invention employing a plurality of phosphor regions in combination with a heavy metal reflecting region. The source of the beta rays are zinc sulfide particles having a tritiated center (about 10-7 to 10-3 weight percent tritium based in zinc sulfide). The central radioactive solid source region is shown as a single layer of tritiated zinc sulfide particles although a plurality of layers could, of course, be employed. Throughout the structure various binders, plasticizers, etc., can be employed to bind the various particles to each other or to surfaces of the composite structure. Inorganic adhesives, such as sodium silicate and potassium silicate are particularly desirable because of their stability. Additionally, various resins such as epoxy resins or ethyl-cellulose can be employed. The binders, plasticizers, etc., are indicated by the numeral 103 in the drawing.

A front phosphor particle region 109 is positioned between radioactive materials 106 and the light discharging portion of the overall structure. The present example phosphor region 109 contains one or more layers of zinc sulfide phosphor particles 102. Particles 102 are 18 microns average, in size. The depth of region 109 is about 18 microns.

Positioned behind the radioactive source is a second phosphor region 108 similarly containing zinc sulfide particles. Beta rays given off by the tritium pass randomly and thus the presence of back phosphor layer 108 serves to convert beta rays passed backwardly into light energy. Light from

regions

108 and 109, together with beta rays which are not emitted in a forward direction, strike heavy metal reflecting region 101 which in the present example is a platinum reflector having a thickness of 0.5 mil. The heavy metal serves to reflect `both the light and the beta particles forwardly. The reflected beta particles then come into contact with the phosphor in

region

108 or 109 and are converted into light energy which passes out directly, or through reflection, through the front surface of the light producing system. Instead of platinum, lead oxide, platinum-iridium alloy rhodium, etc. could be employed for region 101. The phosphor particles are embedded with the dehydrating agent 110, such as silica gel.

A glass or a plastic, e.g. methyl methacrylate, cover 107 is normally employed at the front surface of the structure. Preferably the glass has an internal antirellecting region 10S which may take the form of magnesium fluoride which has been previously deposited on the internal portions of the glass. The magnesium fluoride insures that emitted light is not internally reflected into the central portions of the structure, but rather passes out through the glass covering plate. Enclosure surrounding the light source may be made of Lucite or any of a wide variety of conventional materials.

The relative dimensions of the system are as follows:

Approximate depth of front phosphor region The tritium radioactive material has a radioactivity ranging from 2.5 millicurie/cm.2 to a few hundred millicurie/cm2. By operating in accordance with the present invention a light brightness level (having a higher efficiency as previously stated) ranging from 5 microlamberts to a few hundred microlamberts is obtained. The efficiency of converting the beta rays into light energy can be better than 2 microlamberts per millicurie of solid tritiated compound in the low level light range. This is based on photometric measurements using an Aminco photomultiplier photometer and tritiated luminous standards.

FIG. 4 illustrates a structure particularly suitable for use in systems wherein a gaseous radioactive material, such as krypton-SS or tritium (H-3) are employed. The system of FIG. 3 is quite similar to FIG. 2 in that it contains two phosphor particle regions, 202 and 203 positioned on each side of radioactive region 206. Normally region 206 is evacuated through port 207 and thereafter radioactive gas is injected through inlet 207 to reach the pressure desired. Normally atmospheric or somewhat less than atmospheric pressure is utilized. Light source 200 similarly contains a heavy metal back reflecting layer 201 which serves to refiect both light and beta rays forwardly, light ultimately passing through transparent cover 205. The phosphor particles may be of any of a wide variety, eg., zinc sulfide, cadmium tungstate, etc. The thickness of the front phosphor region in particular is chosen so as to absorb substantially all the beta rays emitted from region 206 in a forward direction while allowing the light generated by the excitement of the phosphor particles to pass outwardly. Structure 200 may be enclosed by walls 204 which may be made of aluminum. A body of dehydrating agent 208 such as silica gel, is provided at the walls 204 to minimize Water vapor effects and prevent exchange of tritium with water vapor.

Cell 200 is gas-tight so that the effect of the dehydrating agent is at its maximum. In the present example, the space between

phosphor regions

203 and 202, i.e. the depth of the radioactive region 206 is of the order of 1 centimeter, and the phosphor regions have an approximate depth of about 18 microns. It is also to be noted that the overall depth of the cell, i.e. 1.5-3 centimeters is only a fraction of the other dimensions of the cell, eg., length, 25 cm.; width, 7.5 cm.; and thus maximum efficiency may be approached from the geometrical and reective properties of the configuration.

It should be clearly understood that the present light source can be employed in a variety of manners. They can be employed for railway and signaling purposes. They find application as a lantern or as a marker or sign; when employing it for the latter purpose a portion of the covering plate may be made opaque and so the transparent portion is illuminated and produces a self-luminous form such as a traffic speed indicator or directional signal, portable map reader or negative X-ray copier or reader.

Various modifications may be made to the present invention. For the more energetic medium energy beta emitter such as Kr-85 (gaseous type) and `thallium204 (solid type) one may employ the basic combination of a heavy metal back-scatterer and light reflector coupled with a single phosphor layer on the front fact to produce a more effective light source. In this case the light attenuation produced by both a front and back phosphor can be appreciable; hence one would Want maximum reflection of the beta rays.

With reference to the gas systems, one may utilize solely to take the form of a radioactive light source employing a weak beta ray source in Which substantially planar regions of heavy metal reflector, phosphor particles, and radioactive particles are utilized. The heavy metal region serves both as an electron and light reiiector. A minimal number of layers of a phosphorized material containing a radioactive source, i.e. tritiated phosphors can be employed with a front non-radioactive phosphor region serving as a source of light through *excitement by beta rays as well as substantially absorbing all forwardly directed beta rays and insuring safety of the overall device. A

Having described the present invention, that which is sought to be protected is set forth in the following claims.

What is claimed is:

1. An improved radioactive light source which comprises a casing defining an interior chamber having disposed therein a radioactive region containing tritium which gives off beta rays, a phosphor region positioned in front of said radioactive region in the direction of light discharged from. said source, said phosphor region being of sufficient thickness to absorb a substantial portion of beta rays without substantial absorption of light rays, a light reflective and beta ray reflective heavy metal reiiecting region positioned behind and enclosing the back portion of said radioactive region, said heavy metal having an atomic number of at least 45 and having a thickness sufficient to reflect beta rays, said metal being positioned adjacent to said radioactive region in direct contact with the beta rays given off by said radioactive region and having a front-facing light reflecting surface so that said reflecting region serves to reflect both light and beta rays forwardly, said forwardly directed beta rays exciting said phosphor region and being converted into light, and a dehydrating agent also located in said chamber defined by said casing so that it is at all times in direct contact with the gases therein, said dehydrating agent thus absorbing water vapor and minimizing exchange of tritium with the hydrogen of water vapor.

2. A radioactive light source structure comprising a casing defining an interior chamber having disposed therein a radioactive region containing tritium beta emitters, a phosphor region `positioned between said radioactive region and the area wherein light is discharged from said structure, said phosphor region being of sufiicient depth to absorb at least of the weak beta rays emitted from said radioactive region without substantially absorbing light rays, a light reflective and beta ray reliective heavy metal reliecting region positioned behind and enclosing the portion of said radioactive region away from the area of light discharged from said structure, said heavy metal region comprising a metal having an atomic number of at least 45 and being of a suicient'thickness to back scatter a major portion of the beta rays contacting its structure, said kmetal portion being adjacent said radioactive region and in direct contact with the beta rays given off by said radioactive region and having a forwardfacing light refiective surface so as to reect both light and beta rays forwardly, said reflected beta rays and beta rays emanating from said radioactive region serving to excite the phosphor region and be converted into light energy, and a dehydrating agent also located in said chamber defined by said casing so that it is at all times in direct contact with the gases therein, said dehydrating agent thus absorbing and minimizing exchange of tritium with the hydrogen of water vapor.

3. The structure of claim 1, wherein said dehydrating agent is dispersed in said phosphor region.

4. The light source of claim 1 wherein said dehydrating agent is silica gel.

References Cited UNITED STATES PATENTS 3,176,132 3/1965 Muller Z50-106 X 3,260,846 7/1966 Feuer 250--106 X ARCHIE R. BORCHELT, Primary Examiner U.S. C1. X.R.