US2640953A

US2640953A - Radiation current source

Info

Publication number: US2640953A
Application number: US166489A
Authority: US
Inventors: Harald H Rossi
Original assignee: US Atomic Energy Commission (AEC)
Current assignee: US Atomic Energy Commission (AEC)
Priority date: 1950-06-06
Filing date: 1950-06-06
Publication date: 1953-06-02
Anticipated expiration: 1970-06-02

Description

June 2, 1953 H. H. ROSSI 2,640,953

RADIATION CURRENT SOURCE Filed June 6, 1950 INVENTOR. HARALD H. ROSSI Patented June 2, 1953 the United States of America as represented by the United States Atomic Energy Commission Application June 6, 1950,=Serial No. 166,489

"2 Claims.

The present invention relates to a variable source of minute currents. In the measurement of extremely small currents in the order of amperes asecondary sourceof very small current is often required. Accurate measurements of :such -.currents can be made, for example, with a sensitive elect-rometer by balancing a known current against an unknown current so 'that the electrometer does not deflect. It is'also sometimes desirable to have a source of current the absolute value of which is not known but which remains constant or which may be varied reproduceably. The present invention provides a source or current which remains constant or which may be varied reproduceably over extended periods-of time. :By calibrating the source such as is provided according to the present invention with sources of 'knownintensity a source of known current intensity maybe produced.

.It-isaccordingly an objectof the present invention to provide a variable source of .minute currents which may be varied reproduceably.

It is another object of the present invention to provide an apparatus which can supply minute currents over a relatively wide range.

It is a further object of the present invention .to provide a method of (producing minute currents with high accuracyand reproducibility.

Other objects and advantages of the present invention will be part obvious and in part anointed out hereinafter.

,In ne of its broader aspects the objects of the present invention may be achieved .by .pro-

viding an apparatus comprising a container for :enclosinga gasat a known pressure, a source of charged particle radiation disposed in .said .con-

tainer, .aplurality-of ion collector plates in said containenat leastoneof said plates being adaptredto bemoved relatively to .an at least partially shielded ion collector plate, said ion collector plate .being insulated from said movable .plate and from said container, and an .electrical conductor communicating between said collector plate and the container exterior through an insulated port.

The subject apparatus is described with reference to theaccompanying .drawing, wherein:

B'i ure .1 .is a transversesectional view of the current source of the present invention taken aldngiline l--.I oiFigureZ, and

fieure 2 is a vertical section of the current source of Figure -1 taken on the line "22 of Figu e "1.

It "will be understood however that the .embodiment disclosed herein is given primarily for illustrative purposes and is not intended to be understood as limiting 'thescope of the present invention. Therefore substantial variations and substitutionsmaybemade in the elementsuof the disclosed invention without departing from the soopethereof.

carrying out the method of the present invention in one of its broader aspects, gas in a container is ionized by emanations from a radioactive source, a potential is impressed on electrodes in thecontainer to cause the ions to move, a collector electrode is interposed in the path of the moving ions, and a movable shield is disposed adjacent said collector electrode to collect at leasta portion of the ions which move toward said collector electrode.

An apparatus for carrying out this method is described with reference to the drawings.

The apparatus of this particular embodiment comprises generally a radioactive source 12 within acontainer 1.0. The source is ,kept at a high potential and the container at ground potential. The radiation from the source I? causes ionizationof gas in the container [0. Due to the high potential .betweensource l2 and container is ions are caused to move in generally radial di rections within the container. In so moving they impinge on collector plates I6, l8 and 12 causing a charge to accumulate on the collector plates. If the charge is removed as fast as it is collected a relatively constant current will flow from the collector plates.

The apparatus is described in greater detail with reference to the Figures 1 and 2. A container l0 encloses an ionization chamber. The container consists ofa-cylindrical tube 54 which forms the outer wall of the container and of a top .plate 56 and a bottom plate .58 which form the 'topand bottom of the container. Within the container two

insulator rings

22 and 24 are po- .sitioned .at the bottom and top of the container respectively. These insulated rings support a plurality ofion collector plates I8 and 12. These collector plates are in the form of radial segments of tubes extending around an arc of about 120 degrees.

Plates [8 and 12 are positioned in generally concentric relation to the tube 54 and the source of charged particle radiation 12. The shield plate 16 isalso in the form of a radial segment of a tube extending around an arc of about 120.

.It isdisposed in the container on the concave side of ,

plates

18 and 12 but is eccentric in its relationtothe tube54 and source I2. The outer longtltudinal edge of the plate I6 is preferabl bevelled to provide a blade edge at its outer surface. The plate l'6 is attached toand supported at its upper end by'flange plate 66 and at itslower end by .a similarplate 64. The flangeplates 6,4 and 66 are rotatably supported in'the chamber by'their respective tubular extensions 61 and 65. Tube :65 extends through upper plate 56 and rests against its inner surface at shoulder '69. The

end oftube 6'l=thrusts against the inner surface of bottom-plate 58'and is held imposition-by an insulator l4 extending *through the tube core. The lower flange plate 64, the-shield plate It and as follows.

the upper flange plate 66 maybe rotated as a unit. The handle 2|] is attached to the externally extending portion of tube 65 in order to provide means for rotating the shield plate I6 within the chamber. The handle 20 is held to the tube 65 by the set screw 2 I. Thus by rotation of the handle 20 the

flange plates

64 and 66 are rotated and the shield plate I6 may be disposed at any desired position within the chamber. The bladed edge of shield plate I6 is brought close to the concave surface of collector plate I8 as the handle 20 is rotated. This arrangement is preferred in order to minimize the change in capacitance between the members It and I8 as the shield plate I6 is shifted. The change in capacitance is minimized with this arrangement because as is apparent from Figure l the bladed edge of the shield plate is relatively close to the collector plate I8 and thus carries most of the capacitance existing between the two plates. As the shield plate It is shifted in its position with relation to plate I8 the bladed edge remains in close proximity to the plate It and since it carries most of the capacitance little change in capacitance between the members results from shifting of the plates.

Ionization of the gas in container I is produced upon radiation from a radioactive source I2. This source emitting preferably beta radiation is positioned in a generally axial position with respect to the cylindrical chamber wall 54. A foil of silver containing approximately ten milligrams of radium may be used and the foil may be enclosed as a strip within an axially extending glass tube I2. .sealed at one end and imbedded at the other into 'a metal cup 50 by a sealing composition or wax 52. The exterior surface of the glass tube is preferably made conducting by coating with graphite or a similar conducting composition. It is desirable to employ an hermetically sealed container such as the glass tube l2 in order to prevent the radon gas, emitted from the radium source, from contaminating the chamber enclosed within the container 54. The electrically conducting external surface of the tube I2 is electrically connected to a source of potential through the metal cup 59 and rod I. R/od 5| is insulated from the bottom plate 58 of the container Ill and from lower flange plate 64 by the insulator I4. The insulator sleeve I4 is threaded at its externally extending portion to receive the locknut 60. By tightening the nut 60 against the outer surface of the plate 58, a shoulder 59 on the insulator sleeve I4 is urged into contact with the inner surface of the plate 58. A second locknut 62 may be threaded onto the externally extending portion of the rod 5| to retain it in position in the insulator I4. By removal of the

nuts

50 and 62, the tube I2 and rod 52 may be removed through the port 68 and replaced as desired by a source of greater or lesser intensity. The internal end of the insulator I4 is fitted axially into the hole in the lower flange plate 64 thereby providing an axis about which the plate may be rotated.

The operation of the current source is generally The high potential is applied to the conducting surface of the tube I2. The container I0 is connected to ground. This grounds the shield plate I6 which is electrically connected to the container II]. The collector plate I8 is not grounded but is insulated from the container II] by the upper and

lower insulators

24 and 22 respectively. Theradioactive source emits beta ti lesnin al ld ect q eerieral yre i lly fr The tube is preferably 4 the tube I2. Ionization of the gas within the container IU' provides a source 'ofcharged particles which can carry a charge between conducting surface of tube I2 and the container wall 54. The high voltage applied between the conducting surface of the rod I2 and the container wall 54 induces radial motion of the ions and thus causes a charge to collect on plate I8. Charge collected on plate I8 may be removed as a minute current through externally communicating conductor 26. The position of the shield plate I6 determines how many of the charged particles can impinge on the plate member I8 because the plate I6 can be located between the plate I8 and the source I2 where it intercepts ions accelerated toward plate I8. Thus, when a large number of positively charged particles are formed in the region ambient to the source I2 and a high potential' is applied to the source tube I2 while the container It is grounded, positive ions will proceed generally radially from the source I2 toward the container wall 54. If the plate It is positioned to shield part of the surface of the plate I8 from the radially moving ions, a portion of the ions will impinge on the plate I5 and another portion will impinge on the plate I8. The number of ions impinging on plate I8 may be changed by changing the portion of plate I8 shielded by plate IE. It is possible to change the ion current in still a third way. If the position of the plate I6 and the intensity of the radiation from the source I2 are kept constant and the container It is hermetically sealed by plugging port 68, an ion current will flow to the plate member I8 which is roughly proportional to the pressure of the gas enclosed within the container I0.

It is possible to provide minute currents over a very wide intensity range by presetting three parameters. That is, the intensity of the ion current may be changed by changing (1) the intensity of the radiation source, (2) pressure of the gas in the container I0 and (3) the position of the plate i6 relative to the plate I8. By providing an hermetically sealed chamber within the container I B and a source of known intensity,

adjustment of pressure, and then to vary the current entirely by rotating the handle 20. The handle position for various source currents may be calibrated so that by adjusting the position of the handle 20 a certain current will flow. This is true if the voltage developed on the collector I8 is small with respect to the voltage impressed between tube I2 and the container I0.

As previously stated. the embodiment disclosed is illustrative of the present invention. The in= vention may be embodied in many alternate devices without departing from the scope thereof. For example, a rod 'of strontium metal or other high energy beta ray emitter having a long half life might be used. Sucha rod might be prepared, forv example, by inserting it into a nuclear reactor for a sufiicient length of time to give it the desired activity. This type of .rod source does not emit radon.

The configurationof the disclosed embodiment, wherein a movable plate is positioned eccentrically to the tubular arrangement of the apparatus, is used principally because there is a danger .of a-troublesome capacitance beingdeveloped between the two plate members I6 'and lfit v The knifing of one edge of the plate l6 helps to minimize the effects of a capacitance development as explained above. The arrangement disclosed is satisfactory for producing a constant current over an extended period of time. However, changing of the current delivered by shifting the vane l6 may result in changing the capacitance between the plate members. A change in the capacitance results in the production of a pulse current or current kick. It is possible to avoid this latter result by attaching a second chamber having exactly the dimensions of the first but containing a source l2 which is not radioactive. The second chamber is attached so that the shifting of its plates compensates for the change in capacitance between the plates of the first chamber. Such a chamber may be positioned, as indicated in phantom view at the top of Figure 2, as a mirror image of the apparatus described in the embodiment. In this compensating arrangement the movement of the calibrated handle 2ll2ll results in the rotation of a plate H5 in the radioactive portion of the apparatus and a plate 16' in the non-radioactive mirror image of the apparatus. External brackets 10' may be used to join the upper and lower chambers 10 and H], to prevent their relative rotation when the handle 20 is rotated.

It is also possible to provide a greater range of available current intensities from such a source by inclusion of auxiliary plate members such as 1'2 having shorter lengths than plate I8 but disposed in the same circle as plate 18 within the container ID. The shield plate 16 may be used to partially shield the auxiliary plates or the collector plate It alternately. A greater range of currents is provided because the shorter collector plates such as 12 collect only a fraction of the number of ions which the longer plate IE will collect for the same angular exposure. That is. if, half the plate I8 is shielded and half exposed it will collect a certain amount of ions. If half of a plate such as 12 is shielded and half is exposed and the plate is only one-tenth the length of plate I8 approximately one-tenth of the number of ions will be collected on the half of plate 12 which is exposed as would be collected on onehalf of the plate 18 which is exposed. Thus, if the auxiliary plate 12 is approximately one-tenth the length of the plate l8 the number of ions collected for a given angle of exposure is reduced by a factor of ten. A separate insulated lead M is necessary for each of the plates and the plates must be insulated from each other where it is desired to collect ion current from the plates individually.

It is apparent from the foregoing that the radiation current source provided according to the present invention is an extremely useful and simple device for supplying minute currents. The apparatus may also be considered a variable resistance since it is possible to change the current flowing without changing the potential drop in the apparatus. The accuracy and reproducibility of the current supplied by the present apparatus under constant pressure and temperature conditions has been demonstrated to be better than 1 percent.

Since many embodiments might be made of the present invention and since many changes might be made in the embodiment described, it is to be understood that the foregoing description is to be interpreted as illustrative only and not in a limiting sense.

I claim:

1. Apparatus for supplying minute currents over a wide range which comprises in combination a substantially cylindrical, gas filled, hermetically scalable chamber, a source of charged particle radiation axially disposed in said chamber, a first arcuate electrode fixedly mounted within said chamber and electrically insulated therefrom, said first electrode extending substantially the length of said chamber, a second electrically insulated arcuate electrode fixedly mounted within said chamber and diametrically opposed to said first electrode, said second electrode being substantially shorter than the length of said cham her, a third arcuate electrode rotatably mounted within said chamber and electrically connected thereto, said third electrode being rotatable between said radioactive source and said first and second electrodes, means external to said chamber for rotating said third electrode, and means for impressing a high potential between said radioactive source and said container whereby radiation emitted from said source produces ions in said chamber which are collected by said first and second electrodes and are intercepted by means of said third electrode.

2. Apparatus for supplying minute currents over a Wide range which comprises in combination a substantially cylindrical, gas filled, hermetically scalable chamber, a cylindrical radioactive source holder axially disposed in said chamber, said source holder having an electrically conductive coating, a radioactive source mounted within said source holder, a first cylindrical tubular segment fixedly mounted within said chamber and electrically insulated therefrom, said first segment extending substantially the length of said chamber, a second electrically insulated cylindrical tubular segment fixedly mounted within said chamber and diametrically opposed to said first segment, said second segment being substantially shorter than the length of the chamber, a third cylindrical tubular segment mounted within said chamber electrically connected thereto and rotatable about an axis parallel to said radioactive source holder, said third segment being closer to said source holder than said other segments, means for rotating said third segment external to said chamber, and means for impressing a high potential between the electrically conductive coating on said radioactive source holder and said chamber whereby radiation emitted from said source produces ions in said chamber which are collected by said first and second segments and are intercepted by means of said third segment.

HARALD H. ROSSI.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date Re. 22,982 Foulkes Mar. 9, 1948 2,494,641 Anderson Jan. 1'7, 1950 2,497,213 Downing Feb. 14, 1950 2,517,120 Linder Aug. 1, 1950 OTHER REFERENCES The Electrician, October 31, 1924, page 497.

Potential Nuclear Monokinetic Electron Sources, Pool, Journal of Applied Physics, vol. 15, October 1944, pages 716- 717, Atomic Energy Digest.

Nuclear Electrostatic Generator, Linder, Physilcsallgeview, vol. 71, #2, pages 129, 130, January