US3026436A

US3026436A - Light source

Info

Publication number: US3026436A
Application number: US798379A
Authority: US
Inventors: Hughes John Duncan Horsfall
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1958-03-12
Filing date: 1959-03-10
Publication date: 1962-03-20
Anticipated expiration: 1979-03-20

Description

March 20, 1962 J. D. H. HUGHES 3,026,436

LIGHT SOURCE Filed March 10, 1959 2 Sheets-Sheet 1 INVENTOR JOHN DUNCAN HORSFALL HUGHES BY Oz ws74;

March 20, 1962 J. D. H. HUGHES LIGHT SOURCE 2 Sheets-Sheet 2 Filed March 10, 1959 ZZ 2Z M AVW INVENTOR JOHN DIEICAl-F HCHSFI-LLL HUGI"ES 3,026,436 Patented Mar. 20, 1962 3,026,436 LIGHT SOURCE John Duncan Horsfall Hughes, Wantage, England, as-

signor to United Kingdom Atomic Energy Authority, London, England Filed Mar. 10, 1959, Ser. No. 798,379 Claims priority, application Great Britain Mar. 12, 1958 13 Claims. (Cl. 31354) The invention relates to light sources in which the source of energy producing the light is a radioactive gas or vapour emitting beta radiation.

One such known source comprises a transparent bulb coated internally with a thin layer of phosphorescent material and filled with radioactive gas. The layer of phosphorescent material is thus activated by radiation on one of its surfaces and light is emitted from the other of its surfaces. The bulb may be placed at the focus of a parabolic reflector.

According to the present invention, a light source comprises a gas-tight container, a beta-radioactive gas within said container, and a coating of phosphorescent material on a portion of the internal surface of said container, the container being light-transparent at least over a portion of the remainder of said surface. The invention enables the phosphorescent coating to be of optimum thickness irrespective of its opacity to light, that is, to be of a thickness to absorb substantially the whole of the beta-radiation from the radioactive gas which is incident thereon. Most of the light emitted from the phosphorescent material is thus emitted from the surface of the coating on which the beta-radiation is incident. Emission of substantially all the light from this surface may be ensured by inter-posing a layer of light-reflecting material between the phosphorescent material and the surface of the container.

The radioactive gas may comprise any radioactive isotope emitting beta radiation, but the isotope is preferably one which emits very little, if any, gamma radiation, is chemically non-toxic, and has a reasonably long halflife. The preferred isotope is krypton-85, which has only a low intensity gamma radiation associated with its beta radiation, is chemically non-toxic, being an inert gas, and has ahalf-life of about years.

The phosphorescent material for coating the internal surface of the container may be, for example, zinc sulphide, cadmium sulphide, zinc phosphate or zinc silicate, activated by small amounts of other metal compounds as is well known in the m. The required thickness of the layer of phosphorescent material to absorb substantially all the beta radiation of krypton-85 is about 2 mm. A suitable thickness to absorb a major part of the beta radiation is 1 mm. A suitable light-reflecting material for interposition between the layer of phosphorescent material and the internal surface of the container is a dense white pigment such as titanium dioxide.

In one particular advantageous form of the invention, the portion of the internal surface of the container which is coated with the phosphorescent material is made a major portion of the surface, the container being lighttransparent over substantially all the remainder of the surface. Thus the light emitted from the phosphorescent material is efiectively collimated through the transparent portion and the apparent brightness of the light source is greater than the intrinsic brightness of the surface of the phosphorescent material. In this form, the container for the radioactive gas consists of a body with a cavity in it having an aperture, the internal surface of the cavity being coated with a layer of the phosphorescent material, and the aperture being closed by a transparent cover. The cover preferably has a plane internal surface to confine the radioactive gas to the cavity. The area of the aperture is less than the area of the coating of phosphorescent material and it may with advantage be less than half, or even less than a quarter, of the area of the coating in order to obtain greater intensity of light transmitted through the aperture.

A proportion of a quarter, or less, can be achieved for example by making the cavity rectangular with a square aperture, part-spherical with a circular aperture, cylindrical with the axis of the cylinder perpendicular to the plane of a circular aperture, cylindrical with a hemispherical bottom and a circular aperture, or rectangular with a semi-cylindrical bottom and a square aperture, provided that the maximum depth of the cavity is made at least equal to the side of the square aperture or the diameter of the circular aperture.

A proportion of half, or less, is achieved by making the cavity rectangular, or rectangular with semi-cylindrical ends, provided that the maximum depth of the cavity is made at least equal to half the width of the aperture.

A proportion of or less, is achieved by making the cavity part-cylindrical with the axis of the cylinder parallel to the plane of the aperture, or part-cylindrical with part-spherical ends, or rectangular with a semi-cylindrical bottom, or rectangular with semi-cylindrical ends and a semi-cylindrical bottom with quarter-spherical ends, provided that the maximum depth of the cavity is made at least equal to half the width of the aperture.

The cavity may be formed in a body of any suitable material, such as a metal or a plastic, which is impermeable and chemically resistant to the radioactive gas or vapour. A suitable plastic is polymethyl methacrylate.

The material sunrounding the cavity may itself be made thick enough to reduce the transmission of any gamma radiation through it to a safe, low level, or further shielding, for example lead, may surround it.

The aperture may be closed by a cover of any suitable gas-impermeable material, transparent to visible radiation, such as glass or transparent plastic, e.g. polymethyl methacrylate, but if the glass or plastic is adversely affected by beta radiation from the radioactive gas, an intermediate layer of transparent radiationuesistant material, for example ceria-stabilised glass, may be interposed between it and the gas. The glass or plastic which forms the cover may also comprise a phosphorescent material, which will enhance the brightness of the light source. For example, a phosphor glass may be used.

The radioactive gas or vapour may be introduced into the cavity by any suitable means. Preferably it is introduced into the cavity, after closure of the aperture, by means of a capillary which is sealed subsequently.

The nature of the invention and the manner in which it is to be performed will be made more apparent by the following description of two particular embodiments of the invention, by way of example, which are illustrated in the accompanying drawings, in which:

FIG. 1 is a section taken on a diameter of a circular light source;

FIG. 2 is a section on the line IlII of FIG. 1

FIG. 3 is a longitudinal section through another form of the invention;

FIG. 4 is a section on the line IVIV of FIG. 3, which is itself a section on the line IIIIII of FIG. 4; and

FIG. 5 is a partially cut-away view of the embodiment shown in section in FIGS. 3 and 4.

Referring first to the embodiment shown in FIGS. 1 and 2, the light source comprises a polymethyl methacrylate cup 1, defining a cavity 2 in the form of a calibration standards.

hemisphere 2a with a short cylindrical part 212 surmounting the hemisphere. The cup 1 is coated on its inner surface with a layer of phosphor 3 about 1 mm. thick. The cup is about 2 thick, has an internal diameter of 2.2 cm., and is provided with a capillary 4 which communicates initially with the outside of the cup. The cup is closed by a plane polymethyl methacrylate cover 5 to which is cemented a protective disc of ceriastab ili'sed glass 6 fitting within the aperture of the cup The closed cup is surrounded on all sides, except that of the aperture, by a lead shield 7, provided with a hole 8 to accommodate the capillary 4.

The cavity 2 is connected byrneans of the capillary 4, first to an exhaust pump to reduce the pressure within the cavity to a low value, and then to a source of krypton gas containing krypton-85 whereby radioactive 'gas is admitted to the cavity. The capillary 4 is then heat sealed at 9 to retain the gas within the cavity.

In a light source constructed as in this particular embodiment, the area of the cavity which is coated with phosphor is about 9 sq. cm., and the area of the aperture is @314 sq. cm; thus the area of the aperture'is approximately a third of the area of the phosphor coating. This proportion may be further decreased, and the brightness of the light source further increased, by increasing the length of the cylindrical part 2b of the cavity. a

Referring now to the embodiment shown in FIGS. 3, 4 and 5, a self-luminous direction sign comprises a polymethyl methacrylate sheet 21 in which are cut cavities consisting of channels having a semi-cylindrical cross-section of radius'S mm, forming the letters 22a, b, c and d. The cavities forming the letters are each coated with a l'ayer of phosphor, 23a, b, c,'d about 1 mm. thick. The sheet 21 is cemented aroundits slightly raised periphcry to a second sheet of polymethyl methacrylate 25, leaving a gap between the central parts of the

sheets

21 and 25 to provide a free gas path between the cavities forming the individual letters. One of the cavities forming the letter 22a, is provided with a capillary 4 which communicates with a socket 26 in the opposite face of the sheet 21.

The cavities forming the letters 22a, 12, c, d are connected by means of the capillary 24, first to an exhaust pump, connected by suitable means to the socket 26, to reduce the pressure in the cavities to a low value, and then to a source of krypton gas containing krypton-85, whereby radioactive gas is admitted to the gap between the

sheets

21 and 25 and to the cavities. A plug is then fixed in the socket 26 to retain the gas within the cavities forming the letters 22a, b, c, d.

In the direction sign constructed according to this particular embodiment, the proportion of the area of the aperture for each of the letters to the area of the cavity proportion is This proportion could be decreased by forming the cavities as channels with a U-shaped cross-section, i.e., by increasing the depth of the cavities.

Light sources using a radioactive gas or vapour as their source of energy have numerous'applications where electric power is not'available or is undesirable, where electric power failure may be embarassing, or where long power cables are a disadvantage. For example, such light sources may be used as personnel, package or safety markers in mines, stores, dark rooms, etc.;' on door handles and switches for location in the dark; as signalling lamps; on large clock faces; or as photometric Thelight sources of the present invention have particu- 4 lar advantages in that the whole of the visible radiation emitted from the phosphor is transmitted as a beam from a small aperture with some directional properties. "the directional properties of the beam may be further improved by fitting a lens over the aperture. The intensity of light sources made in accordance with the invent on 13 greater than that of previously known sources using a radioactive gas as energy source. Intensities of up to 30 111K (3000 micro-lamberts) have been achieved in light sources constructed as the specific embodiment illustrated in FIGS. 1 and 2, and containing 0.8 curie of krypton-85. Sources of such intensity are visible at distances of 500 to 1000 yards in darkness or twilight, when fitted with a lens over the aperture.

Signs and lettering embodying the invention, constructed as the specific embodiment illustrated in FIGS. 3, 4 and 5 have particular value as direction indicators, warning signs, and the like.

Light sources constructed in manner similar to either of the embodiments may be further modified by the incorporation of opaque signs, lettering or numerals m the transparent cover for the aperture, the l1ght source thus forming a luminous background for the signs,.letters, etc, and throwing them into sharp relief.

I claim:

1. A light source comprising a gas-tight container, at beta-radioactive gas within said container, and a coat1ng of phosphorescent material on a portion of the internal surface of said container, the container being lighttransparent at least over a portion -ofthe remainder of said surface. e

2. A light source as claimed in claim 1, in wh1ch sa1d coating is of a thickness sufficient to absorb substantially all the beta-radiation from said gas incident on said coating.

3. A light source as claimed in claim 1, in which a layer of light-reflecting material is interposed between said coating and said surface. l p

4. A light source as claimed in claim 1, in which the portion of the internal surface of the container which 15 coated with the phosphorescent material is a majorportion of said surface and the container is light-transparent over substantially all the remainder of said surface.

5. A light source as claimed in claim 4, in which said container comprises a body defining a recess cavity in its surface and a light transparent cover closing the aperture of said cavity, the walls of said cavity being coated with said phosphorescent material.

6. A light source as claimed in claim 5, in which said cavity is a hemispherical cavity.

7. A light source as claimed in claim 5, in which said cavity is part-spherical with a circular aperture, the maximum depth of said cavity being at least equal to the diameter of said aperture.

8. A light source as claimed in claim 5 in which said light-transparent cover has a plane internal surface.

9. A light source as. claimed in claim 5, in which said cavity is in the form of a channel of semicircular crosssection with closed ends.

10. A light source as claimed in claim 9 in which said channel is shaped to form readable indicia of a sign.

11. A light source as claimed in claim 5, in which said cavity is in the form of a hemisphere surmounted by a right circular cylinder of diameter equal to that of a said hemisphere.

References Cited in the file of this patent UNITED STATES PATENTS Goldstein Mar. 1, 1945 Linder Feb. 16', 1954