US2696563A

US2696563A - Variable current radioactive source

Info

Publication number: US2696563A
Application number: US218780A
Authority: US
Inventors: John H Coleman
Original assignee: Radiation Research Corp
Current assignee: Radiation Research Corp
Priority date: 1951-04-02
Filing date: 1951-04-02
Publication date: 1954-12-07
Anticipated expiration: 1971-12-07

Description

Dec. 7, 1954 J. H. COLEMAN 2,696,563

VARIABLE CURRENT RADIOACTIVE SOURCE Filed April 2, 1951 4 Sheets-Sheet 1 FIG. I.

INVENTOR ATTORNEYS 7 1954 J. H. COLEMAN VARIABLE CURRENT RADIOACTIVE SOURCE 4 Sheets-Sheet 2 Filed April 2, 1951 1 N VE NTOR Jay E BY @fwwwq Mr W flf ATTORNEYS Dec. 7, 1954- Filed April 2, 1951 J. H. COLEMAN VARIABLE CURRENT RADIOACTIVE SOURCE 4 Sheets-Sheet 3 1 14 FIG. 9.

FIG. l0.

I al r /7 INVENTOR Jmv 12 (ole-Man ATTORNEYS Dec.'7, 1954 J. H. COLEMAN 2,696,563

VARIABLE CURRENT RADIOACTIVE SOURCE Filed April 2, 1951 4 Sheets-Sheet 4 PEA 27 INVENTOR BY v I W/Qfl M/m ATTORNEYS United States Pater 2,696,563 Patented Dec. 7, 1954 VARIABLE CURRENT RADIOACTIVE SOURCE John H. Coleman, Palm Beach, Fla assignor to Radiation Research Corporation, West Palm Beach, Fla., a corporation of Florida Application April 2, 1951, Serial No. 218,780

Claims. (Cl. 250-833) 'work. For example, health physicists in nuclear experimental installations require instruments covering a range from cosmic ray background to above tolerance levels. Also ClVll defense workers require a rugged, serviceable instrument for radiological warfare to check radiation levels from daily tolerance to lethal doses. Instruments in the past have required complicated circuitry and have hada mechanical type indication such as an ammeter or quartz fibre electroscope which limits their ruggedness for portable'work.

In these prior instruments, it is well known that certain devices such as ionization chambers and Geiger tubes pass electric current as a function of radiation incident upon the sensitive volume of the device. Ionization chamber current, for example, is directly proportional to the intensity of incident radiation; The current from the radiation sensitive element was utilized in prior in.- struments to determine both rate and dose.

It is known that radiation rate can be measured by determining the potential built up across a resistance by the radiation current. The radiation dose, on the other hand, can be measured by determining the potential built up across a capacitance by the current integrated over a period of time, or total charge.

It is well known that measurement of potential produced by a current source across aresistor or capacitor must be made by a meter which draws a current that is not significant compared withthe current to be measured. For measurement of the small radiation. currents, prior instruments utilized a vacuum tube electrometer with an ammeter in the plate circuit or an electroscope type meter" with a quartz fibre. Both the meter and the fibre are sensitive to mechanical vibrations and have usual mass to torque limitation on the indicating needle.

The present device, on the other hand, utilizes the deflection of an electron beam: tovindicate upon a fluorescent screenthe radiation levels without any mechanical indication. This method not only providesa more rugged instrument but simplifies the circuitry as indicated by a significant reduction in cost over an instrument of ordinary type and having a comparable range.

It is well known that the cathode ray type tubes operate by the application of a. potential to a control element to change the pattern on a fluorescent screen. It: was found that the class of cathode ray tube called electron-ray tube was particularly suited to the present invention due to the low internal current flowing to its control elements. Also, it was found, on removing the conventional tube base to eliminate low resistance leakage paths across the base, that the difiFerent types of electron-ray tubes varied in both magnitude and characteristic of control element current. Thus, certain combinations of a particular electron-ray tube with a particular radiation sensitive element are necessary for a particular radiation range. For example, the 6E5 type tube connected in a balancing circuit covered a range with an ionization chamber from 100 to 100,000 milliroentgens per hour, having a grid current of 10 to 10* amperes at balance. The range can be extended below the 100 milliroentgens per hour value by utilizing a Geiger tube for a radiation sensitive element which provides more current than the ionization chamber, or by utilizing another class of electron-ray tube such as the 6AL7 type tube which draws less grid current under certain operating conditions. To cover the low radiation ranges with a low current radiation sensitive element, such as an ionization chamber, the electrometer amplifier described in my copending application Serial No. 196,553 would be particularly suited. On the other hand, the 6AF6 type tube is more suited to the scintillation detector and Geiger tube at all ranges of radiation due to the large control element current when compared to the other types.

The principal features of the invention disclosed here in are: 1) circuitry providing direct reading, on a fluorescent screen by electrostatic deflection of an electron beam, of radiation rates and doses; (2) fluorescent indication of balance of a calibrated voltage source with potential developed by current from a radiation sensitive element; (3) circuitry for comparison of the deflection on a fluorescent screen by different radiation levels with deflection produced on said screen by a known voltage; (4) method of indication of balance of a known current source with a current from a radiation sensitive element for determination of radiation rates and doses; (5) radioactive variable current source particularly suited to the method outlined under (4); (6) electron-ray type of cathode ray tubes particularly suited for use in the circuits and methods outlined under (1) through (4).

The principal object of the invention is to provide a novel method of/and means for determining the rate and dose of radiation.

Other objects will be apparent from the description of the invention as hereinafter set forth in detail and from the drawings made a part hereof in which briefly:

Fig. 1 is a schematic diagram of the basic circuit for determining radiation rates by deflection of an electron beam on a fluorescent screen.

Fig. 2 is a typical scale on the screen of an electronray tube which is illustrated in Fig. 1.

Fig. 3 is the schematic diagram of a circuit illustrating several additional rate scales by use of a voltage divider in the basic circuit of Fig. 1.

Fig. 4 illustrates a typical scale calibration placed over the screen of another type electron-ray tube.

Fig. 5 is a schematic diagram of a rate circuit similar to Fig. 3 illustrating the use of a third type of electronray tube and another type of radiation sensitive element showing that the basic operation of the circuit is unaffected by these changes.

Fig. 6 is a schematic diagram of a basic rate circuit utilizing a calibrated voltage source to obtain a null balance.

Fig. 7 is a schematic diagram of a basic comparison type circuit. Also illustrated is the basic null circuit of Fig. 6 to show that any of the basic rate circuits (i. e. direct reading, null, or comparison) can be combined in one circuit.

Fig. 8 illustrates a guard ring which is particularly suited to the null circuit of Fig. 6. Also illustrated is a simplified switching arrangement which enables adjusting the zero point on the radiation scale while the instrument is still in a radiation field.

Fig. 9 is a schematic diagram of the basic circuit for a dosimeter with the scale directly on the fluorescent screen.

Fig. 10 is a dosimeter circuit analogous to the ratemeter of Fig. 7 in that both a voltage balancing (null) and voltage comparison circuit are illustrated.

Fig. 11 is a schematic diagram of a dosimeter circuit vith a separate voltage supply to charge the capacitance which is then discharged by radiation current. The basic null type circuit is provided by another voltage supply.

Fig. 12 is a schematic diagram of a combination of a rate meter and dosimeter in a null type circuit.

Fig. 13 is a schematic diagram of a circuit utilizing a variable current source to balance out the current from the radiation sensitive element as determined by the cathode ray tube.

Fig. 14 is a cross-section of a radioactive current source which can be used in the circuit of Fig. 13 where 3 substances of varying density shown in the form of sheets can be moved between insulators to vary the current.

Fig. 15 is a cross-section of a radioactive current source of plane symmetry in which the insulation itself is fabricated with a varying density and can be moved to vary the current output.

Fig. 16 is an axial cross-section of a current supply with cylindrical symmetry in which the output current is varied by moving an absorber adjacent to vacuum insulation.

Fig. 17 is a radial cross-section of a current source with cylindrical symmetry in which the radioactive source is directional and can be rotated between several collectors to vary the distribution of current to the collectors.

Referring to Fig. 1, the radiation sensitive element 1 is sketched symbolically as an ionization chamber with its cathode 3 connected to the negative side of the voltage supply 15. The positive side of the voltage supply 15 is connected to ground. The anode 4 is connected to one side of the resistance 5. The other side of the resistance 5 is connected to ground. The triode grid 9 and the cathode 11 of the electron-ray tube 2, which may be of the 6E5 type, are connected across the resistance 5. A high resistance 6 is connected between the triode plate 7 and the target 8 and a battery 14 is connected between the target 8 and the cathode 11. A heater 12 energized by current from the battery 13 is provided for heating the cathode 11. The ray control electrode is connected to the triode plate 7.

The electron-ray tube 2 is constructed to indicate visually by means of a fluorescent target 8 the effects of a change in controlling voltage applied to the grid 9. This tube is divided into two main parts, that is, the triode which operates as a D. C. amplifier and the electron-ray indicator and both of these are shown as positioned in one envelope although they may be in separate envelopes. The target 8 is operated at a positive voltage; it therefore attracts electrons from the cathode 11 and when electrons strike the target 8 they produce a glow on the fluorescent coating thereof. The target then appears as a ring of light as will be more fully described in connection with Fig. 2.

A ray control electrode 10 is mounted between the cathode 11 and the target 8 and when the potential of this ray control electrode is less positive than the target, electrons flowing to the target 8 are repelled by the electrostatic field of the control electrode and do not reach the target 8 behind the control electrode 10. The control electrode 10 therefore casts a shadow on the screen of the target electrode and the extent of this shadow varies from approximately 100 degrees of the target when the control electrode is much more negative than the target to 0 degrees when the control electrode is approximately at the same potential as the target.

The potential of the control electrode 10 is determined by the voltage on the grid 9 of the triode, and the flow of the triode plate current through the resistor 6 produces a voltage drop which determines the potential of the control electrode 10. When the voltage of the triode grid changes in the positive direction the triode plate current increases and the potential of the control electrode 10 goes down because of the increased voltage drop across the resistor 6, therefore the shadow angle on the fluorescent screen of the target 8 widens.

When the radiation sensitive chamber 1 is exposed to radiation the current I produced therein by radiation Rh in Roentgens per hour in an air type ionization chamber of volume v in cubic centimeters is given by:

terms of radiation, can be found by combining

Equations

1 and 2 to give:

Thus, the shadow angle on the screen of the target 8 can then be calibrated in terms of the radiation rate from Equation 3 and the control grid 9 characteristics for a given resistance 5 and chamber 1 volume v.

Fig. 2 shows typical scale markings 16 on the screen of the target 8 of the 6E5 type electron-ray tube in Roentgens per hour. In this case an increase in negative bias on the grid 9 by radiation current through the radiation sensitive element 1 causes a decrease in shadow angle. The voltage supply 15 can be reversed in polarity from that shown in Fig. l of the drawing so that a negative bias applied to the grid 9 may be decreased by radiation current through the element 1 to cause an increase in shadow angle with radiation as illustrated hereinafter in the description of Fig. 5.

The circuit shown in Fig. 3 is employed for increasing the scale range by the use of

taps

17 and 18 on the grid resistor 5 connected to deflecting

electrodes

21 and 22 of the electron-ray tube 19 of the 6AL7 type. The taps 17 and 18 correspond to resistance points R11 and R18 on the grid resistor 5.

The anode 4 of the radiation sensitive element 1 shown in Fig. 3 is connected to one end of the resistor 5 and to the deflecting electrode 20. The deflecting electrode 21 is connected to the tap 17 of the resistor 5 and the tap 18 of the resistor 5 is connected to the third deflecting electrode 22. The grid 24 which controls the fluorescence is connected to the negative terminal of the battery 26. The negative terminal of this battery is also connected to the lower terminal of the variable cathode resistor 28, to the lower terminal of the resistor 5 and to the positive terminal of the battery 15. The positive terminal of the battery 26 is connected to the target anode 25. The upper terminal of the cathode resistor 28 is connected to the cathode 27 which is also provided with a heater 23. The cathode resistance 28 is made variable so it can be varied to adjust the screen pattern for change in tube characteristics.

The voltage 2 from

taps

17 and 18 applied to the

control electrodes

21 and 22 respectively, of the tube 19 is determined by the usual voltage divider equations from Ohms law (2) to be:

Although only two taps are illustrated, as many taps can be used as there are control elements on the electron-ray tube.

Fig. 4 shows a typical scale 29 in Roentgens per hour which can be placed over the screen of the type of electron-ray tube with a plurality of control elements such as the 6AL7 tube. Calibration is the same as in Fig. 2 where the control grid and cathode are connected across the resistor 5. Calibration for the control electrodes connected to the

taps

17 and 18 is a division of the scale across the resistor 5 as determined by

Equations

4 and 5 and the deflection characteristics of the particular control electrode.

The scale markings on both Fig. 2 and Fig. 4 can be marked on a transparent sheet and placed directly over the screen of the tube or they can be scratched directly in the phosphor during the manufacture of the tube.

Fig. 5 illustrates the use of a different radiation sensitive element consisting of a scintillation counter with a cross-section of a crystal 31 adjacent to the photocathode of a multiplier phototube 30. The dynodes 32 are connected to the appropriate positions on the voltage taps of the resistor 33 that is connected across the voltage supply 20. The anode 34 of the multiplier phototube 30 is connected to the top terminal of the resistance 5 and to the control electrode 20. The control electrode 21 is connected to the tap 17 of the grid resistor 5 and the tap 18 of this resistor is connected to the control electrode 22. A grid bias battery 35 is connected between the lower terminal of the resistor 5 and the lower terminal of the variable cathode resistor 18. In other respects the electrodes of the tube 19 of Fig. 5 are connected the same as the corresponding electrodes of tube 19 of Fig. 3.

The only difference in operation of the rate meter of Fig. 5 from the previous cases described above is that the current, instead of being related to the radiation by Formula 1, is related by some other function.

The circuit shown in Fig. 6 utilizes a variable voltage essence source. having a voltage supply .36; connected across the resistance element 37 of a potentiometer and: having the. positive. side grounded to. furnish a variable negative voltage from the. wiper 38 of the potentiometer. Th moving arm or wiper 38. is connected to the lower end of the resistance 5 that is. grounded. in the embodiment shown in Figs. 1 and 3. The; other side. of the resistance 5 is connected to the anode, 4a and to the control electrode of the triode section of the electron-ray tube. 2. It. will be observed that in this circuit thev positive terminal of the battery 15. is. connected to. the electrode. 3a. of

radiation, sensitive. element 1, which is usually connected to the negative terminal of the source. These connections maybe reversed as described hereinafter.

In the operation of the. circuit shown in Fig. 6., the initial position of the screen pattern is noted. Under radiation incident on the radiation. sensitive element 1,

a positive potential is developed across the resistor 5 changing the. pattern on the screen of the tube 2. The wiper 38 of the potentiometer is then adjusted to provide a sutficiently' negative voltage to the grid 9 to restore the pattern on. the screen of the target electrode 8. to its original position. At this balancing point the voltage from the potentiometer wiper 38., V38, is equal to the voltage across the resistor 5' or:

(6,) eg: Vas

Since Vsa can be read from a low resistance voltmeter or the potentiometer can be calibrated with a dial indication, the radiation current I can be determined from the Equation 2 for a particular value Rg of the resistor 5 Reversal of polarity of the supply which is connected between the cathode 3a of the radiation sensitive element 1 and ground, causes a negative voltage to develop across the resistor 5 and consequently requires a positive balancing voltage from the potentiometer wiper 38'. A positive voltage can be obtained simply by reversing the polarity on the voltage supply 36.

When the tube 2 is of the 6E5 type illustrated, however, it was found that zero shadow angle, obtained by an initial negative bias of around fourvolts, was the most convenient balancing point to reproduce and had the lowest internal control electrode current between the grid 9' and cathode 11. This initial negative bias can be obtained by employing a cathode resistor 28 which can be adjusted to raise the cathode potential above that of the grid which is initially at ground potential. Another method for obtaining the initial negative bias is to use a part of the negative balancing voltage from v the source 36, with a proper correction on the scale of the target 8, to lower the potential of the grid 9, below that of the cathode 11 which is fixed at ground potential.

The circuit shown in Fig. 7 illustrates the use of a comparison pattern on the screen of an electron-ray tube 19 of a type similar to that shown in Fig. 3. A calibrated voltage source 39 is provided for this circuit to determine the deflection on the screen of the target electrode produced by the radiation current. The variable voltage supply 39 consists of a voltage source 40, with positive side grounded, and a potentiometer having the resistance 41 thereof connected across the source 40. The moving arm or wiper 42 is connected to a control electrode 21 and the screen pattern controlled thereby is positioned adjacent to the pattern controlled by the control electrode 20, the voltage of which is the potential across the resistor 5 that is developed by the radiation current of the element 1. Thus, when the two patterns are lined other source altogether.

The third control electrode 22 of the tube. 19 shown in Fig. 7 is biased negative by voltage supply 43 to collect positive ions which would normally be collected by :the

other control electrodes

20 and 21. Thus, a larger resistance. 5. can be used to lower the range of the meter Without; a. spurious signal from the control element current.

The circuit shown in Fig. 8; illustrates the use of a guard ring 44. for reducing leakage current on the control element which would give a spurious deflection 0n the screen oi the target 8 in addition to the. deflection produced by the radiation current flowing through the resistor 5. The basic balancing circuit shown in Fig. 6 is particularly suited to. the use of a guard ring 44 of metallic material on any insulating supports, such as. the insulator 45, of the electrode 4a of the radiation sensitive element 1 since the potential of this: electrode is always. the same at balance.

In the circuit shown in Fig. 8 the voltage supply 46 which is employed with its positive. terminal connected to the target 8 is also adapted to be connected to the electrode 3a of the radiation sensitive element 1 when the polarity and magnitude thereof are proper. A switch 47' having

contacts

48 and 49 is used to connect the electrode either to the positive side of the source 46 or to ground this electrode. In addition this switch is used to disconnect the radiation sensitive element 1 from its supply 46 for determining the zero scale point while operating in. a radiation field. Furthermore, a grounding switch adapted to be connected across the resistor 5 can be used, if insulated for current leakage, in any of the herein above described circuits. A switch at 47 connected as shown in Fig. 8, however, has the advantage of correcting for any current leakage from the control element. In the null balancing circuit, this advantage is particularly evident since the control element leakage current is the same for any radiation reading since the control element potential is restored to its initial value at balance; consequently, an initial zero correction by a bias from an element such as the cathode resistor 28 is valid for all readings.

The circuit shown in Fig. 9 provides a basic dosimeter circuit employing a capacitor 50 to collect the total charge, Q, from the radiation current from the radiation sensitive element 1, integrated over a period of time. The potential egc developed across the capacitor 50v is given by:

This potential, when applied to the control electrode 9 of the triode of the electron-ray tube 2, such as the 6E5 shown, causes a deflection on the fluorescent screen of the target 8 similar to the deflection caused by the potential developed across the resistance 5 in the ratcmeter circuit of Fig. 1. Again using the deflection characteristics of the particular tube 2 or tube 19, the calibration can be made in Roentgens on the screen of the target of the tube instead of Roentgens per hour as in the circuit shown in Fig. 2 fora particular value of the capacitor 50, when the charge Q from the radiation sensitive element 1 is known in terms of total radiation in Roentgens.

The circuit shown in Fig. 10 provides for a comparison voltage supply 51 consisting of the battery SZ'and potentiometer having the resistance 53 thereof connected across the battery 52'. The wiper 54 of the potentiometer is connected to one control electrode 21 of the tube 19 to determine the deflection of the screen pattern of another control electrode 20 which is connected to the capacitor 50. The control electrode 22 is connected to the lower terminal of the capacitor 50 and to the upper terminal of the capacitor 55. The lower terminal of the capacitor 55 is connected to the Wiper 38 of the potentiometer that is connected to the battery 36.

The variable voltage supply consisting of the battery 36 and potentiometer associated therewith illustrated in Fig. 10 is used to provide a balancing circuit for a dosimeter similar to the rate meter of Fig. 6.

The capacitance voltage divider employing the capacitors 50; and 55 illustrated in Fig. 10 is similar to the resistance tap for the rate meter shown in Fig. 3. The reduced voltage from tap 56 is connected to the third control electrode 22 in the GAL7 type tube 19 to provide a scale, with a higher radiation range.

The circuit shown in Fig. 11 illustrates the use of a charging potential from the voltage source 5.6;: to charge up the capacitor 50 when the

contacts

57 and 58 are connected by the switch 59. The positive charge from the radia i n sensitive element 1 then discharges the negative potential on the capacitor 50 when he radiation sensitive element 1 is subjected to radiation. An advantage in this arrangement is that the voltage supply 15 can be eliminated since the potential across the capacitor 50 can serve as the supply for the radiation sensitive element 1. Thus it is possible to carry only the radiation sensitive element 1 and charged capacitor 50 into the radiation field. The measurement of dose can be made by reconnecting the circuit shown in Fig. 11 with the switch 59 connected to the contact 60.

The circuit shown in Fig. 12 illustrates the combination of the basic rate meter and dosimeter balancing circuit with the switch 59 functioning to connect the desired meter. The switch 59 should remain on the contact 60, that is the dosimeter setting, for the majority of time since the capacitor 50 does not charge while the switch 59 is on the contact 61 connected to the resistor 5.

A high internal resistance current source 62 is employed in the circuit shown in Fig. 13 to balance out the radiation current from the radiation sensitive element 1 as indicated by the position of the pattern on the screen of the tube 2. In this case, since the current from the radiation sensitive element 1 is known by relationships, such as Equation 1 for ionization chambers, the current source 62 can be calibrated directly in Roentgens per hour. As in the previously described balancing circuits the tube characteristics do not enter in the calibration. An additional feature obtained by using a current balancing source is that the resistance 5 can be eliminated.

Referring to Fig. 14, radioactive particles from the emitter 63, formed from radioactive material 64 attached to an electrode 65 in the form of a sheet, penetrate the insulation 66 and impinge upon collector 67 which is in the form of a conducting sheet such as aluminum. The insulation 66 fabricated from dielectrics such as sheets of polystyrene or amber provides for a cavity in which an absorber 68 can be inserted and be insulated from the

electrodes

65 and 67. The absorber 68 is fabricated from substances of varying density such as a low density absorber 69, for example aluminum, and a high density absorber 70, for example platinum, in the form of mov-' able sheets. Other materials can also be used for fabricating the absorber shown in this and the other embodiments of the invention. A dielectric material or a conductor, as examples, will work satisfactorily. Other materials will be apparent to those in the art and the only restriction is that the absorber material meets the requirements set forth in this specification. Thus in operation, the current between the

electrodes

65 and 67 will be determined by the number of particles absorbed in 68 and, consequently, by the position of the absorber 68 between the

electrodes

65 and 67. For example, when the high density absorber 70 is positioned between the

electrodes

65 and 67, more particles will be absorbed than when the low density absorber 69 is positioned therebetween. Intermediate positions give an intermediate current output through the load connected to the

electrodes

65 and 67.

Referring to Fig. 15 the insulation 66a itself is movable to bring varying absorption densities between the

electrodes

65 and 67. Illustrated is a low density insulator 71 such as polystyrene and a high density insulator 72 such mica which can be moved as a unit by any of the usual mechanical devices, attached to the connection 73. In any case, the position of the absorber can then be calibrated in terms of current. When the radioactive particle emitter 63 is connected in place of the current source 62 in Fig. 13 and when the current from the radiation sensitive element 1 is known as a function of radiation rate, the position of the absorber can be calibrated in terms of radiation.

In the devices of both Fig. 14 and Fig. 15 some back leakage current will result from induced conductivity in the insulation by the radioactive particles themselves. This leakage would limit the potential to which a variable current supply could attain. At the balancing point of the circuit of Fig. 13, however, there is no potential difference across the source 62; consequently, internal leakage by induced conductivity is negligible.

Referring to Fig. 16 the solid insulation 66 of Fig. 14 is replaced by the vacuum type insulation 74 through which the particles pass from the emitter 75 to the collector 76 which are shown in the form of concentric cylinders. The vacuum in the space 74 can be maintained by exhausting the region between the collector 76 and electrode 77, as shown, or it can be maintained by the collector 76 completely surrounding the emitter 75. The radioactive material is positioned inside the electrode 77 on a sup' port 78 in such a manner that an absorber 79 can be lowered between the emitter and the collector 76.

Insulation 80, in the form of an annular disc between

electrodes

76 and 77, maintains one wall of the vacuum chamber as illustrated. There are several other geometries which can be employed; however, only the one example of the invention employing vacuum insulation with an absorber between the radioactive material 75 and collector 76 is given for simplicity.

Referring to Fig. 17, the emitter electrode 81 is constructed in the form of a cylinder which can be rotated on its axis to vary the distribution of the radioactive particles from the material 82 between the two halves of the collector 83 which is in the form of a split cylinder. These two halves of the collector 83 are insulated from each other by insulation 85. A dense absorber 84 is used to provide directional emission from the electrode 81. A high density material 84, such as tungsten, provides a more efficient emitter than a low density material due to back scattering of the radioactive particles from a dense material 84 which would be absorbed in a low density material. Any of the current sources described above which utilize a solid or vacuum dielectric may be provided with an arrangement in which the position of a directional emitter 81 may be changed with respect to a collector 83 to vary the current through a load.

The devices shown in Figs. 14, 15, 16 and 17 may be employed in place of the current source 62 in the circuit shown in Fig. 13 by connecting the

electrodes

65 and 67 of Figs. 14 and 15 to the

terminals

86 and 87, respectively, of Fig. 13. Likewise, the emitter electrode and collector electrode of Fig. 16 may be connected to the

terminals

86 and 87 of Fig. 13, respectively, and the halves of split cylinder 83 of Fig. 17 may be connected to the

terminals

86 and 87 of Fig. 13, respectively.

While I have described various embodiments of this invention in detail it is of course understood that I do not desire to limit this invention to the exact details described and illustrated except in so far as they are defined by the claims.

What I claim is as follows:

1. A variable current supply comprising a source of radioactive radiation, a collecting member positioned in spaced relation with respect to said source, absorbing means positioned between said source and said collecting member for absorbing said radiation and means for varying the absorbing density of said absorbing means to control the internal capacitance and current to obtain desired output electrical characteristics of charging rate and output current.

2. A variable current supply comprising a source of radioactive radiation, a collecting member positioned adjacent to said source, means for insulating said source from said collector and means for variably absorbing the radioactive radiation positioned between said source and said collecting member to control the internal capacitance, current and resistance to obtain desired output electrical characteristics of charging rate, output current and maximum voltage.

3. A variable current supply comprising a source of radioactive radiation, a collecting member positioned adjacent to said source, means for insulating said source from said collector and a movable member for variably absorbing the-radioactive radiation, said member being adapted to be positioned between said source and said collecting member to control the internal capacitance and current to obtain desired output electrical characteristics of charging rate and output current.

4. A variable current supply comprising a source of radioactive radiation, a collecting member positioned adjacent to said source, means for insulating said source from said collector, said means consisting of insulation material having difierent radiation absorption properties in different parts thereof and means for moving said means with respect to said member to control the internal capacitance and current to obtain desired output electrical characteristics of charging rate and output current.

5. A variable current supply comprising a source of radioactive radiation, a collecting member positioned adjacent to said source, means for insulating said source from said collector, said means consisting of layers of insulation material, and means for variably absorbing the radioactive radiation positioned between layers of said insulation material to control the internal capacitance and current to obtain desired output electrical characteristics of charging rate and output current.

6. A variable current supply comprising a source of radioactive radiation, means for controlling the intensity of said radiation in different directions, collecting means positioned in spaced relation with respect to said source, and absorbing means positioned between said source and said collecting means for absorbing said radiation to control the internal capacitance and current to obtain desired output electrical characteristics of charging rate and output current.

7. A radiation meter comprising a radiation-sensitive element having an anode and a cathode, a source of current supply connected with said element in a series circuit, an electron-ray potential indicating tube connected across said series circuit, a radioactive current source characterized by high internal resistance connected across said series circuit, and means for adjusting said last-named source to balance out the potential developed across said series circuit.

8. A radiation meter in accordance with claim 7, in which said radioactive current source comprises a source of radioactivity, a collecting member positioned in spaced relation to said source, absorbing means positioned between said source and said collecting member for absorbing said radiation, and means for varying the absorbing density of said absorbing means to control the internal capacitance and current to obtain desired output electrical characteristics of charging rate and output current.

9. A radiation meter in accordance with claim 8, in which said absorbing means comprises an element having ditferent degrees of absorbtivity from point to point, and which said varying means comprises means for moving said element.

10. Apparatus for indicating rate and dose of radiation, comprising a radiation sensitive element having an output current that is a function of the incident radiation, means for converting said output current into electrical potential, an indicating electron discharge device connected to said means to indicate variations in said potential, a radioactive current source connected to said converting means and means for variably absorbing the radioactive current to control the internal capacitance and current to obtain desired output electrical characteristics of charging rate and output current.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,318,140 Clark May 4, 1943 2,334,473 Carlson Nov. 16, 1943 2,467,812 Clapp Apr. 19, 1949 2,513,356 Perlow et a1 July 4, 1950 2,513,818 Roop July 4, 1950 2,537,388 Wooldridge Jan. 9, 1951 2,543,039 McKay Feb. 27, 1951 2,547,173 Rittner Apr. 3, 1951 2,552,050 Linder May 8, 1951 2,556,768 McKibben June 12, 1951 OTHER REFERENCES A New Electronic Battery from The Electrician, vol. 10, October 31, 1924, page 497.