US2834899A - Radioactive resistor - Google Patents

Description

1958 v. E. RAGOSINE 2,834,899 RADIOACTIVE RESISTOR Filed April 29, 1955 "nunuuunnnnund F/GJ IN V EN TOR.

VICTOR E. 5AQ05/NE United RADIOACTIVE RESISTOR Victor E. Ragosine, Williamstown, Mass., assignor to Sprague Electric Company, North Adams, Mass, :1 corporation of Massachusetts Application April 29, 1955, Serial No. 504,966 2 Claims. (Cl. 31354) This invention relates to radioactive apparatus and particularly to devices utilizing radioactivity suitable for use as resistances of high ohmic value.

The measurement of small currents of the order of 10 amps. or less is most conveniently performed by allowing the current to flow through a resistor of high ohmic value. The resultant potential drop may then be measured by means of an electrometer of high input impedance. Important applications of this method exist in nuclear instrumentation, particularly in instruments using an ionization chamber as the primary sensing element. Many survey instruments of this type are in use by the military and civil defense agencies. It can readily be seen, therefore, that it is important that the high megohm resistor used in such applications be temperature insensitive to the temperatures encountered in field use.

Conventional resistors of the deposited carbon type commonly used in these applications are not stable with regard to temperature. Radioactive resistors consisting essentially of a gas ionized by a quantity of radioactive material and confined between electrodes, such as those to be found in the prior art, are stable with respect to temperature. A

These radioactive resistors, however, exhibit self-generating electromotive forces and thus require a plurality of electrodes to obtain approximate compensation of these electromotive forces. Self-generated electromotive forces will exist in such resistors as a result of contact potential difference between electrodes of different material and as a result of preferential charge collection due to the location of the radioactive material on one of the electrodes. These may be partially compensated by the introduction of auxiliary electrodes.

Further, the range over which these resistors exhibit a linear current-voltage relationship is limited to, at most, several volts. This is a severe limitation when devices of low voltage sensitivity such as quartz fiber electrometers are used to measure the potential.

As indicated above, prior radioactive resistors have comprised a plurality of electrodes contained in an insulating gas-tight envelope. One or more of the electrodes have been coated with a radioactive material to provide ionization in the gas which is at a pressure of the order of magnitude of an atmosphere. With the electrode configurations and gas pressures utilized, an applied voltage between two electrodes will result in an increasing collection of the primary ionization until all primary ions are collected. .Further increases in applied voltage result in no further increase in current over a considerable range. Saturation ion collection occurs at a low voltage of the order of to volts. Increasing the voltage mate- .rially causes a catastrophic breakdown of the gas.

It is an object of this invention to overcome the fore- ;going and related disadvantages of the prior art. It is a further object of the present invention to provide a high megohm resistor of the radioactive type having a linear difference between electrodes 231,834,899 Patented May 13, 1958 current-voltage relationship over a much greater range of voltage than hitherto possible.

It is an additional object of this invention to provide a high megohm resistor of the radioactive type having no self-generating electromotive forces so as to avoid compensation therefor.

It is a still, further object of this invention to provide a high megohm resistor of the radioactive type with a maximum current not limited by the saturation ion current.

In the drawings:

Fig. 1 is a longitudinal cross-section of one embodiment of the invention;

Fig. 2 is a cross-section in a 1 taken along line A-A;

Fig. 3 is a longitudinal cross-section of another embodiment of the invention;

Fig. 4 is a cross-section in a plane orthogonal to Fig. 3 taken along line B-B; and

Fig. 5 is a typical current-voltage curve obtained with devices of the general type of the invention.

In Fig. l the-numbers 11 and 12 are thin conductor lead wires of tungsten or some similar material having a thermal coefficient of expansion substantially that of the glass envelope 15. The wires are vacuum sealed tothe glass envelope 15 with the glass coated with a silicone compound or similar material to reduce surface electrical leakage and to prevent large increases in surface leakage with increases in atmospheric humidity. The electrodes 13 and 14 are of a material having a small coeificient of secondary emission, such as aluminum or carbon. The volume 16 defined by the envelope 15 is occupied by a radioactive gas mixture at reduced pressure. In a preferred form of the invention electrodes 13 and 14 would be circular with an area of approximately one square centimeter and of a conductor such as aluminum or carbon.

Looking now at Fig. 2, 11 refers to the lead wire which is terminated by electrode 13, both confined within the envelope 15. In this particular view the electrode 13 is of pressed carbon particles presenting a roughened surface which assists in trapping the limited amount-of secondary emission which occurs. The bonding of the electrode to the lead is mechanically obtained by pressing of the former to the latter.

Figs. 3 and 4 show another embodiment of this invention. As in Fig. 1, lead wires 33 and 34 are sealed to the glass envelope 30. The electrodes 31 and 32, in contrast to Fig. 1, are of a cylindrical configuration, the former positioned concentrically about the latter. The outer electrode 31 is electrically connected to and supported by lead wire 36 which is sealed to the envelope 30. Also envelope 30 serves to support electrode 31 which need not be restricted to a foil but can be a deposited conductor, the latter requiring additional process steps as masking, etc. but having the advantage of a non-planar surface. The inner electrode 32 is of the same material as electrode 31, is supported by lead wires 33 and 34 and can be either a tube or rod. In Fig. 4 the inner electrode 32 is shown supported by lead wire 33 and the outer cylindrical electrode 31 is supported by the envelope 30 and terminated by lead wire 36 with the envelope 30 containing the radioactive gas mixture 35.

In the construction of this device the electrodes are to be of the same material so as to prevent the self-generation of electromotive forces such as are found with the present radioactive resistors due to the contact potential of different material. The

plane orthogonal to Fig.

small coefiicient of second- By a small coefficient of secondary emisthose materials which emit a secondary electron current equal to the smallest possible fraction of the primary electron current and include aluminum, carbon, lithium, beryllium, barium, titanium, zirconium. Of these conductors suitable as electrode materials alumimum and carbon are preferred, the latter being particularly advantageous as it has a roughened surface when in pressed form, which assists in trapping any electrons which are emitted through secondary emission during operation of the device. The configuration of the electrode is preferably of a cylindrical geometry, however this is not to exclude electrode plates since they have proved to be satisfactory in the operation of the device of the invention. Where it is desired to utilize a greater surface area of the electrodes in a limited volume a corrugated structure could be utilized with the corrugations of the two electrodes cooperating so as to allow limited separation. The spacing of the electrodes should be between from about 1 millimeter to about 2 centimeters with a preferred spacing ranging between about 1 and 1 /2 millimeters. Although this device is operated at a low pressure in contrast to the high gaseous pressure devices of the prior art, it is desirable to operate with as high a pressure as possible consistent with gas multiplication. For this reason the preferred range of spacing of the electrodes tends toward the minimum separation of the electrodes.

The radioactive gas mixture which is utilized in the device of the invention consists essentially of at least one radioactive gas admixed with electronegative gases. Representative of useful radioactive gases are the beta emitters such as tritium and methane or other gases which are tagged with radioactive carbon 14. The present limitation upon the utilization of the latter gases containing carbon 14 is'the higher cost of material. The radioactive gas, as indicated above, is admixed with an electronegative gas of which oxygen, air and water vapor are representative. The ratio of radioactive gas to the electronegative gas is from about 4 to l to about 1 to 4 by weight. Preferably, one would use equal amounts by weight of the two gases. The electronegative gas is used for admixture with the radioactive gas rather than electropositive types, for with ionization the electrons will attach themselves to the gas molecules forming negative ions and prevent preferential concentration of the electrons at one of the electrodes. As discussed above, this device uses the gaseous mixture at a low pressure, which, for purposes of this invention, is limited to from about 500 microns of mercury to 1 /2 centimeters of mercury. The separation of the electrodes will determine to a great extent the pressure which one should use in this resistor for it is desired to operate at as high a pressure as is possible, consistent with gas multiplication. The final characteristic of the gaseous mixture is its radioactivity which should be within the range of from about 5 to about 100 microcures, although the determination of any particular activity within this range will be defined mostly by the resistance range in which one desires to operate, for with the higher level of activity the lower the resistance range and conversely with the higher resistance range the lower amounts of radioactivity will be used.

As indicated in the foregoing, the radioactive gas mixture and electrodes are confined within an enclosure. This enclosure should have extremely low gas transmission characteristics, should readily outgas any gas which might be occluded in its surfaces, should be an excellent insulator and form a hermetically sealed device when properly sealed to the lead wires. Vitreous materials are generally suitable and glass is preferred. Typical glasses include both the hard and soft glasses, the former being generally borosilicates having a silica content of 72% and upward and a temperature working range of from 1000 to 1300" F., and the latter a soda-potash-lead and soda-potash-lime having a silica content of from 57 to 72% and a temperature working range of roughly 800 to 1000" F. For the hard glasses the lead wire which is sealed to the glass should be of such metals as tungsten, molybdenum or Kovar (trade name for an alloy consisting of about 54% by weight of iron, 28% by weight of nickel, and 18% by weight of cobalt), as these metals have substantially the same temperature coetlicient as the hard glasses. For the soft glasses those metals having substantially the same temperature coetficient include chrome-iron, cold rolled steel, platinum, and copper sheathed nickel-iron alloys. 'It is essential that if the device is to have a long operational life that the gas transmission through the enclosure be infinitesimally small, that is, it should have a hermetic seal even against moisture penetration, for otherwise its operational characteristics would change as the pressure of the gas confined within the enclosure varied; In the processing of this device the enclosure should be cleaned thoroughly and thereafter outgased so as to remove any occluded surface gases This outgassing is simply accomplished by exposing the glass to reduced pressure followed by heating within the reduced pressure to approximately 500 C. and maintained there for upwards of 30 minutes. If desired to reduce the humidity penetration to substantially 0, the inside of the enclosure can be coated with silicone resin fully polymerized and degassed which further has the advantage of reducing the leakage current along the interior surface of the enclosure. If it is further desired to decrease the leakage resistance of the enclosure, numerous corrugations can be imposed in the enclosure which greatly increases the leakage path, thus favorably reducing the leakage current. The sealing of the lead wire to the enclosure is readily accomplished by techniques well-known to the glassrto-metal sealing art and need not be elaborated on.

With a mixture of the radioactive gas and the electronegative gas, for example, tritium and air, one will obtain a self-ionizing gas with a uniform ionization density. Then with appropriate electrode separation and appro priate gas pressure, which separation and pressure has previously been discussed to some length, a small applied potential (in the order of .1 of avolt) will result in the collection of a saturation ion current. In contrast to the devices presently known wherein pressures, in theorder of an atmosphere or higher are utilized, the device of the present invention does not exhibit anyplateau or con stant current level with increasing voltage. Apparently because of low pressure in the small electrode separation, the increasing potential imposed on the electrodes will result in gaseous ions acquiring sufficient energy be tween collisions to produce secondary ionization. The current will increase linearly with applied potential; and a typical current voltage relationship is shown inthe curve 41 of Fig. 5. As the phenomena taking place in the gas during operation is quite complexand the theory is not fully understood, it is impossible to drawa simple rigorous mathematical relationship for the electricalresistance. An approximation which appears satisfactory, however, exists in the form lost) l e" where i is the current, V is the appliedpotential, s is the total activity of the radioactive gas in curies, v isthe volume of gas, p is the pressure and k and a are experimentally determined constants.

The current voltage of relationship curve of Fig.v 5 was obtained by the device of the invention using parallel circular plate electrodes, each having an area ofone squarecentimeter and a separation of one millimeter. The radioactive gaseous mixture consists; of 25%; by weight of tritium and 75% by weight of air in'which the tritium had an activity of 20 microcuries. The pressure of the radioactive gaseous mixture was 0.25 centimeter of mercury. The values of k and a were determined 6.1 x 10- and 0.9 respectively.

Although the. radioactive resistor of vthe. .-invention---is useful in many applications where it is desired to measure small currents, which currents are in the neighborhood of 1O amperes, it is particularly useful when incorporated in a circuit in parallel with an electrorneter or some other device which measures small incremental changes of voltage. The parallel resistor and electrometer are series connected with a generator of currents of that magnitude, for example, a radiation detection instrument such as an ionization chamber.

As many apparently widely dilferent embodiments of this invention may be made without departing from the spirit and scope hereof, it is to be understood that the invention is not limited to the specific embodiments hereof except as defined in the appended claims.

What is claimed is:

1. A resistor comprising a sealed glass envelope, at least two uniformly spaced electrodes of low secondary emission material positioned within said envelope, said envelope filled with a radioactive gaseous mixture consisting essentially of tritium and an electronegative gas selected from the group consisting of oxygen, air and Water vapor, having a pressure of from 500 microns to 1 /2 centimeters of mercury, said electrodes having an electrode spacing of about 1 to 1.5 millimeters, so constructed and arranged that the current and voltage across the electrodes vary linearly in an inter-related manner to enable either to function as the measure of the other.

2. In a resistor as claimed in claim 1 electrodes of pressed carbon particles presenting a roughened surface.

References Cited in the file of this patent UNITED STATES PATENTS 2,354,786 Wall Aug. 1, 1944 2,449,961 Treece et a1. Sept. 21, 1948 2,576,100 Brown Nov. 27, 1951 2,617,088 Cohen Nov. 4, 1952

Claims (1)

1. A RESISTOR COMPRISING A SEALED GLASS ENVELOPE, A LEAST TWO UNIFORMLY SPACED ELECTRODES OF LOW SECONDARY EMISSION MATERIAL POSITIONED WITHIN SAID ENVELOPE, SAID ENVELOPE FILLED WITH A RADIOACTIVE GASEOUS MIXTURE CONSISTING ESSENTIALLY OF TRITIUM AND AN ELECTRONEGATIVE GAS SELECTED FROM THE GROUP CONSISTING OF OXYGEN, AIR AND WATER VAPOR, HAVING A PRESSURE OF FROM 500 MICRONS TO 11/2 CENTIMETERS OF MERCURY, SAID ELECTRODES HAVING AN ELECTRODE SPACING OF ABOUT 1 TO 1.5 MILLIMETERS, SO CONSTRUCTED AND ARRANGED THAT THE CURRENT AND VOLTAGE ACROSS THE ELECTRODES VARY LINEARLY IN AN INTER-RELATED MANNER TO ENABEL EITHER TO FUNCTION AS THE MEASURE OF THE OTHER.

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