Patent Number: 055901620
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 2A and 2B, the source assembly comprises a metallic collector 26 in the form of a hollow flat disk of metal containing the .beta.-emitting isotope material 28, which is electrically isolated from the metallic collector 26 by ceramic stand-off 30 and ceramic feed-through 32. Preferably, the .beta.-emitting isotope material 28 is centrally arranged inside the collector 26. A first electrical lead 34 is connected to the .beta.-emitting material 28 and penetrates the ceramic stand-off 30. A second electrical lead 36 has an end connected to the metallic collector 26. In accordance with one preferred embodiment, the .beta.-emitting isotope material 28 is formed as a solid thin disk. Alternatively, the .beta.-emitting isotope material 28 is deposited on a substrate formed as a solid thin disk, which substrate may be made of material which is not a .beta.-emitter. Also, the ceramic material may be replaced by any other suitable electrically insulating material able to withstand the thermal and radiological conditions of the reactor environment. In accordance with the preferred embodiment of the invention, the .beta.-emitting radioisotope is thallium, which decays directly to the ground state of Pb.sup.204 by 763-keV .beta.-decay with no .gamma.-emission. The resulting .beta.-particles are collected by the collector 26 to form a current. The collector material could be nickel or a nickel-base alloy. The ceramics could be alumina to thermally match the metal. These are typical materials, but other possible combinations exist, which allow the device to operate reliably at reactor temperature. In the collection process, x-rays are produced that may have to be shielded prior to reactor installation, although less than 1% of the electron energy is transformed into x-radiation that escapes to the environment. There is a small competing electron-capture reaction that is insignificant. The half-life of Tl.sub.204 is 3.77 years, which is sufficient to provide adequate fabrication time, shelf-life and operational life. In its elemental form, thallium is a bluish metal with a specific gravity of 11.85 and a melting point higher than 303.degree. C. All of its properties are adequate for the applications proposed herein. In accordance with an alternative preferred embodiment of the present invention, the cell shown in FIGS. 2A and 2B may be replicated many times and connected together ("sandwiched") to provide adequate current for conversion to voltage in practical applications. FIG. 3 is a schematic representation of this configuration for five unit cells. However, the number of cells sandwiched together can be more or less than five. The preferred embodiments of the .beta.-battery of the invention have a very thin layer of low-density ceramic electroplated on every emitter surface, which is used as a substrate. Then, the ceramic surfaces are metallized and then electroplated with a metal having suitable electrical conductivity. The metal electroplated cells are then bonded together to form a multi-cell array, an example of which is seen in FIG. 3. In this array, the metallic collectors 26 separate each unit cell and form a bus to which electrical lead 36 is connected. The electrical leads 34 are connected to a bus 38. The feed-through ceramics and leads are also deposited by electrodeposition. Processes and techniques similar to those used in semiconductor device fabrication are available for manufacture of the device, with adequate care taken to shield emitter x-rays during fabrication. The amount of current density j generated can be estimated from the following formula, which takes account of source decay and self-absorption : EQU j=N(.rho..Fourier./A.tau..mu.)e.sup.-t/.tau. [1-e.sup.-.mu.l ]amp/cm.sup.2 where l is the emitter thickness; .rho. is the emitter density; A is the emitter mass number; .tau. is the emitter mean-life (1.44t.sub.1/2); .Fourier. is the Faraday constant (96487 coulomb/gm-mole); .mu. is the electron absorption coefficient of the emitter/insulator; and N is the number of cells; Using the appropriate numbers for Tl.sup.204, it turns out that about 100 cells, each with emitter diameter 10 cm and emitter thickness 0.5 mm, would produce 3.6 mA of current initially. If a dropping resistor of 3.35 k.OMEGA. were used to generate voltage, a peak voltage of 12 volts and initial steady power of 474 mW would result. If regulated to a maximum of 4 volts, the source would have a life of about 3 years. The total source package would be about 3.5 cm thick and 11 cm in outer diameter. The spectrum of collected electrons is expected to look like the curve shown in FIG. 4, where E.sub.max is the maximum .beta.-decay energy (763 keV for Tl.sup.204) and p(E) is the probability per unit energy of an electron emission event. It is a characteristic of this type of emission that the shape of the spectrum is unchanged by the absorption process for thin emitters. The most probable electron energy is about 150 keV, and the area under the curve is unity. The x-rays are due primarily to the electrons around this energy. A practical battery design requires thin cells with relatively large surface areas and efficient heat dissipation to assure adequate cooling of the collectors. The thickness of the thallium layers is limited by self-absorption of electrons inside the .beta.-emitting material. The thallium layers can be deposited, by sputtering or electroplating, on both sides of a substrate material of sufficient thickness to assure structural integrity, For this example, the circuit to be driven by this source would have to dissipate about 200 mW in order that the battery have adequate life. Carefully designed, simple operational amplifier circuits can meet this requirement. Leakage currents would have to be virtually eliminated in the source design. The preferred embodiments have been disclosed for the purpose of illustration only. Variations and modifications of those embodiments will be readily apparent to persons skilled in the art of battery design. For example, certain applications can be foreseen that require low DC currents directly, without conversion to voltage, such as cathodic protection of metals subject to corrosion. An .alpha.-emitting material (e.g., Am.sup.241) may also be used in a manner entirely analogous to .beta.-emitting material. All such variations and modifications are intended to be encompassed by the claims appended hereto.