Miniaturized Electric Source

Abstract

A miniaturized electric source having a radioactive source of .beta. -radiation and a collector for said radiation, said source and said collector forming the terminals of the electric source, said source comprising a metal disc having coated thereon a tritium compound in solid state and a layer of a solid - state dielectric being disposed between said source and collector.

Description

This invention relates to a miniaturized electric source of the type having a radioactive source emitting .beta.-radiation, a collector for receiving this radiation and output electrodes formed by said collector and a terminal connected to said source.

Various sources of this type have been proposed in the past, but no satisfactory results have been found for a miniaturized source adapted for use in a watch as an example, and meeting with all conditions such as safety, reliability suitable power output and high life time in the order of 10 to 20 years.

Tritium (H.sub.1.sup.3, termed T in the following specification) would be suitable as a radioactive source from a number of points of view, particularly its half-life time and the possibility of screening stray radiation, but no suitable technical solution has been known allowing use of this element in a miniaturized electric source. Due to its high ability of diffusion, vacuum used as a dielectric between the source and the collector would be destroyed within short time. Accommodation of a sufficient amount of radioactive material in a miniaturized source used to be a serious problem.

This invention aims to overcome the above difficulties and to provide a miniaturized source of high power output and life time, extremely low volume and simple construction. This is achieved by providing a source comprising a tritium compound in a solid state, a layer of said composition being deposited on at least one surface of a carrier, and the space between said source and said collector comprising a layer of a solid - state dielectric having a thickness and density proper for allowing passage of said .beta.-radiation at an energy in the order of up to 18 keV and for electrically insulating said source from said collector. When using T in a chemically bound solid state the problem of diffusion does no longer exist or may in any case be neglected. Even though T and He produced during operation will diffuse into the space between the radio-active source and the collector during operation, such gases will be contained in such a small amount between the radioactive source and the collector that passage of the .beta.-radiation and ionisation are no problem. As far as ionisation of minute cushions of gases occurs, this is without effect because the electrical insulation between the radioactive source and the collector is obtained by said layer of solid - state dielectric. For these reasons and since the space between radioactive source and collector need not be evacuated, the casing of the source must not be tightly sealed. Gases developed in the source such as He and T do not build up pressure in the casing but will easily diffuse or flow through small leaks into the ambient atmosphere. Therefore, substantially atmospheric pressure exists in the source this being particularly advantageous because chemically all available compounds of T are relatively unstable under low pressure.

This invention will now be explained in detail for one embodiment of which a cross section is illustrated on a substantially enlarged scale in the accompanying drawing.

The source which is of circular shape has a metal disc 1, for instance of copper, serving as a carrier for the radioacive material. This material is applied to both surfaces of disc 1 in very thin layers 2. The thickness of such layers 2 is suitably selected in relation to the absorption of .beta.-radiation of the available energy therein such that the radiation emitted from the innermost portion of such layers still leaves the layers in an significant amount.

The radioactive material used is a compound of T with an element of suitable affinity. LiT.sub.3 is used in the illustrated embodiment, but any other composition may be used. Similar compounds exist with Zirconium, Yttrium and other elements. The use of the one or other of the available compounds is a question of chemical stability and price. In the illustrated embodiment LiT.sub.3 is applied to carrier 1 in a density of 200 mC/cm.sup.2. The coated surface of disc 1 is 2 cm.sup.2 so that the total amount of LiT.sub.3 is 400 mC in the new state of the source, and it will still be in the order of 200 mC after about 10 to 15 years.

The radioactive source formed by carrier 1 and layers 2 is enveloped by foils 3 of plastic material flanged down and over-lapping each other along the edge of disc 1. Foils 3 are extremely thin in the order of 0.2.mu. when their density is 1g/cm.sup.3, this thickness being proper on one hand for allowing passage of the .beta.-radiation from layers 2 through foils 3 and on the other hand for electrically insulating the radioactive source from the collector. The collector is formed by a metallic casing 4 for instance of steel comprising two shells frictionally engaging each other along the rim. Each shell of the casing is coated at its inner surface with a layer 5 of graphite serving as an absorbtion medium avoiding reflection of electrons from the collector back towards the radioactive source.

The casing 4, layer 5, upper plastic foil 3 and upper layer 2 of LiT.sub.3 have a hole for passage of a terminal wire 6 soldered to plate 1. A metallic screen 7 is fixed on terminal wire 6 for collecting possible radiation passing through the hole of the collector. Screen 7 and the inner end of terminal 6 are imbedded in a bead 8 of plastic material, this material filling also the space between terminal 6 and the collector 4, 5, whereby all parts are properly insulated from each other.

In operation the .beta.-radiation emitted by layers 2 passes through foils 3 and is collected by collector 4, 5. In this way the collector is charged to a high negative potential relatively to disc 1. Due to the extremely thin insulating foils 3 between the radioactive source and the collector the electric source has a relatively high capacity in the order of 1,000 pF. This is advantageous particularly when the source is loaded by an oscillator as is usual in electric timepieces,where the current consumption is highly variable.

Manufacture of the source is of similare simplicity as its design. After application of LiT.sub.3 - layers 2 and of terminal wire 6 to disc 1, foils are loosely applied and then clamped between layers 2 and layers 5 of the collector and simultaneously flanged round the edges of disc 1 by applying and closing casing shells 4. No evacuation or sealing procedure is required except for application of the insulating bead 8.

The source is extremely compact and flat and is thus highly suitable for use in watches where it may be accommodated at one side of the timepiece, or in any other miniaturized electric or electronic instrument.

Claims

What is claimed:

1. A miniaturized electric source utilizing the energy of .beta.-radiation emitted by radioactive tritium, comprising:

a. a source of .beta.-radiation comprising a metal disc having coated on at least one surface thereof a thin layer of a solid tritium compound;

b. a collector element comprising a metallic casing enclosing said disc;

c. a dielectric layer of plastic foil enveloping said disc, interposed between said disc and said collector casing and also enclosed by said collector casing, and having a thickness and density adapted to allow passage of said .beta.-radiation at an energy level in the order of up to 18 keV.

2. A source according to claim 1, wherein said di-electric is a plastic foil of a thickness in the order of 0.2.mu..

3. A source according to claim 1, wherein said source comprises a compound of tritium with one element of the group consisting of lithium, zirconium and yttrium.

4. A source according to claim 1, wherein said compound is applied to said metal disc in an amount of 200 mC/cm.sup.2.

5. A source according to claim 1, wherein said tritium compound is applied to opposite surfaces of said metal disc at a density of about 400 mC/cm.sup.2.

6. A source according to claim 1, wherein said source is enclosed in two plastic foils flanged down each over the rim of said metal disc.

7. A source according to claim 1 wherein said collector is a metal capsule enclosing said source and plastic foil and contacting said plastic foil.

8. A source according to claim 7 wherein said collector has a hole for passage of said terminal, the space between said terminal and collector being sealed and insulated by means of a bead of plastic material.

9. A source according to claim 1, wherein the collector surface facing said source has a coating of an absorption medium preventing reflection of electrons back toward the radioactive source.