US3433955A - X-ray generator with emission control arrangement within the focusing cup - Google Patents

Description

March 18. 1969 J. T. PERRY 3,433,955

X-RAY GENERATOR WITH EMISSION CONTROL ARRANGEMENT WITHIN THE FOGUSING CUP Filed Sept. 26. 1966 JOHN 7. PERRY //VVENTOR B) BUCKHOR/V, BL ORE, K LAROU/S 7' 8 SPAR/(MAN ATTORNEYS United States Patent 3,433,955 X-RAY GENERATOR WITH EMISSION CONTROL ARRANGEMENT WITHIN THE FOCUSING CUP John T. Perry, Melrose Park, Ill., assignor, by mesne assignments, to Picker Corporation, White Plains, N.Y., a corporation of New York Filed Sept. 26, 1966, Ser. No. 582,027 US. Cl. 250--90 Int. Cl. H01j 35/16 7 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to an X-ray generator and more particularly to an X-ray generator having a new and improved structure for controlling the electron emission of the cathode thereof.

The quantity of X-r-ays generated at the anode of an X-ray generator is a function both of the electrical potential between the anode and cathode and which accelerates the electrons from the cathode to the anode, and also is a function of the rate at which electrons are emitted by the cathode; thus, by controlling the emission of the electrons from the cathode to the anode the quantity of X-rays generated may be controlled. Control of the quantity of X-rays may be desired, for example, to provide pulses of X-rays of specified predetermined duration, or it may be desired to provide X-rays of varying quantity over predetermined periods.

In the so-called line focus type of X-ray generator in which electrons strike the anode in a rectangular area, it is desired that the electrons impinge upon such area as uniformly as possible and that the area be as sharply defined as possible. For the purpose of controlling the emission of electrons from the cathode to the anode in an X-ray generator, it has been proposed heretofore to provide an electrically biased grid between the cathode and the anode. While the prior structures were adequate for generators having a relatively low amount of electron fiow between the cathode and the anode, they have inherent limitations where substantial electron flow is desired. For example, in prior structures the focal spot size will vary with the grid bias; grid currents become large when positive bias is applied to the grid, and the plate current is extremely sensitive to changes in the plate voltage. Also, the total emission can be controlled only by changes in filament temperature unless significant changes in positive bias on the grid are used.

Accordingly, it is a principal object of the present invention to provide an X-ray generator having a grid therein to control electron flow from the cathode to the anode but which is not subject to the disadvantages mentioned above.

More particularly, it is an object of the present invention to provide a grid structure in an X-ray generator which can be varied in potential without appreciably changing the focal spot seize.

Another object is to provide a grid structure which will draw a minimum of current at positive potential.

3,433,955 Patented Mar. 18, 1969 Still another object is to provide an X-ray generator having a grid structure permitting the passage of substantial electron flow.

A further object is to provide an X-ray generator enabling passage of large currents with a minimum value of positive grid bias.

Still further objects and advantages of the invention will become more apparent hereinafter.

In accordance with the present invention an X-ray generator is provided having an anode and a facing cathode means comprising a planar faced metal body having an electron focusing cup formed therein. An electron generating filament is mounted in the cup and a grid comprising a plurality of parallel wires is disposed forwardly of the filament but closely adjacent thereto and rearwardly of the planar face of the cathode.

The grid is insulated from the remainder of the cathode structure and is connected to a source of potential so that an electrical bias may be applied thereto to control the emission of the electrons from the filament to the anode. However, because the grid is positioned within the focusing cup, that is, the focusing cup extends forwardly of the grid towards the anode, the focusing cup structure may exert its electrostatic focusing effect upon the electrons passing through the grid whereupon the dimension of the focal spot upon the anode will not change even though the potential upon the grid may be varied, and because of the close proximity of the grid to the filament control over electron flow may be achieved with a minimum change of grid bias.

For more detailed description of the invention reference is made to the accompanying drawings wherein:

FIG. 1 is a side elevation of an X-ray generator constructed in accordance with the invention;

FIG. 2 is an enlarged end elevation of the cathode taken along line 22 of FIG. 1;

FIG. 3 is a sectional view taken along line 33 of FIG. 2;

FIG. 4 is a sectional view taken along line 4-4 of FIG. 3; and

FIG. 5 is a fragmentary elevation of the anode target element as viewed along line 55 of FIG. 1.

The illustrated generator comprises an envelope 10 having a cathode structure 12 mounted in one end thereof and an anode 14 in the opposite end thereof. The anode is shown as being of the rotating type having the rotating target element 16 preferably formed of tungsten and which is provided with a frusto-conical target surface 18. The target element 16 is mounted on a stem 20 extending from a rotor 22 suitably supported on the envelope in a conventional manner and which rotor is adapted to be driven by a magnetic field generated by coils (not shown) positioned exteriorally of the envelope It The cathode structure 12 is of the double focus type and, referring to FIGS 2 and 3, comprises a head 30 including a metal body 32, which may be formed of nickel or other suitable material, and which body is provided with two planar faces 34, 36, disposed at a large obtuse angle with respect to one another. The body 32 is formed of two portions including a base portion 38 and a face defining portion 40, screws 42, or other suitable means, being provided to secure the two portions rigidly together.

Formed in the body 32 are a pair of elongated focusing cup recesses 44, 46, the recess 44 being longer than the recess 46. An elongate electron generating filament 48 is mounted in the recess 44 and a similar but shorter filament 50 is mounted in the recess 46. Suitable leads 52 are provided to connect the filaments 48, 50 to a source of electrical energy. The recesses 44, 46 are arranged to focus electrons from the filaments 48, 50 upon a pair of rectangular anode focal spots 51, 53, respectively (FIG.

5). Control of the emission from the smalled filament 50 upon the focal spot 53 is obtained solely by controlling the bias between the cathode and the anode. However, suitable control over the emission from the larger filament 48 upon the focal spot 51 cannot be obtained in such manner, and the improved means of the invention for obtaining such control will now be described.

Referring to FIG. 3 it will be noted that the recess 44 is relatively deep and extends entirely through the face defining portion 40 into the base portion 38. It will be further noted that the filament 48 is mounted rearwardly with respect to the planar face 34 of the parting plane 54 which extends through the recess 44 between the portions 38, 40.

Mounted forwardly of the filament 48 but rearwardly of the front face 34 is a grid means comprising a plurality of parallel wires 64 extending latterally of the recess 44 in the direction at right angles to th longitudinal axis of the filament 48.

The wires 64 are platinum covered molybdenum or other suitable construction and are supported by a pair of support rods 66, 68 mounted one on each of the opposite sides of the recess 44, the rods having slots 70 for receiving the ends of the wires 64 which are preferably spot welded to the rods. The rods 66, 68 are supported in insulated relation from the cathode body 32, the opposite ends of the rods being of reduced diameter and encompassed by tubular insulators 72 which are seated within semicircular recesses 58 formed in the face 56 of the body base portion 38. The wires 64 are spaced a short distance forwardly of the face 56 of the base portion 38 and the rear surface of the base portion 40 is cut away as indicated at 74 so as to provide clearance between the wires 64 and the base portion 40. The cut away portion 74 is in addition provided with shallow arcuate grooves 76 for receiving the insulators 72. The rod 68 is connected to a lead 78 extending through an opening 80 in the body 32 and outwardly of the envelope for connection to a suitable source of electrical potential.

The filaments 48, 50 are connected to a conventional control system so that they may be selectively operated. When the shorter filament 50 is operated control of the emission from the cathode to the anode is achieved by biasing the entire cathode structure. However, when the longer filament 48 is operated emission is controlled by applying a bias to the grid 64. Because the grid is positioned a substantial distance within the focusing cup 44, electrons passing the grid 64 are subjected to the focusing effect of the electrostatic field within the cup arising from the potential or bias between the anode and cathode and which field is not materially affected by changes in the potential upon the grid. Accordingly, the size of the focal spot 51 produced by electrons emitted from the filament 48 does not change materially as the potential upon the grid changes. Furthermore, because the grid wires are of limited total area, as the grid is swung positive the grid current remains relatively small because of the small collecting area. Another significant advantage of the structure of the invention is that control of the electron emission from the cathode to the anode may be made with relatively small changes in grid voltage while enabling high forward current. For example, in a typical generator having 150 kv. plate voltage cut-off is obtained at 2,500 volts. In the same generator the electron emission expressed as tube current is in excess of 400 milliamperes at 85 kv. with +500 volts bias applied to the grid.

Having illustrated and described a preferred embodiment of the invention it should be apparent to those skilled in the art that it permits modification in arrangement and detail. I claim all such modifications as come within the scope and purview of the appended claims.

I claim:

1. An X-ray generator comprising cooperating spaced apart anode and cathode means,

said cathode means comprising a planar faced metal body having an electron focusing cup formed therein, means for applying an electrical potential between said anode and cathode means,

an electron generating filament mounted in said cup,

grid means disposed between said filament and said body face and insulated from said body whereby electrons flowing through said grid means are subjected within said focusing cup to electrostatic focusing from the field of the potential between said anode and cathode means after passing said grid means,

and means for connecting said grid means to a source of electrical potential.

2. The X-ray generator of claim 1 wherein said body comprises a base portion and a face portion, said grid means being positioned between said portions.

3. The X-ray generator of claim 1 wherein said focusing cup and said filament are elongated and said grid means comprises a plurality of laterally spaced apart grid wires extending at right angles to the longitudinal axis of said filament.

4. The X-ray generator of claim 2 wherein said grid means comprises grid wires fixedly secured between a pair of parallel rods spaced one on each side of said filament,

said rods being rigidly clamped between said body portions but insulated therefrom.

5. The X-ray generator of claim 4 wherein said rods are located within recesses provided between said body portions and outside said electron focusing cup, said grid wires extending into said cup from said rods and across said cup.

6. The X-ray generator of claim 1 wherein said means for connecting said grid means to a source of electrical potential comprises means for connecting said grid means to a controllable source of grid bias potential different [from the potential of the cathode means.

7. An X-ray generator comprising cooperating spaced apart anode and cathode means,

said cathode means including a metal body having a pair of elongated, longitudinally parallel electron focusing cup recesses in the surface thereof facing said anode means,

an elongated, filamentary cathode element mounted longitudinally one in each of said cups, said cups being arranged to focus the electrons emitted by the respective cathode clement upon said anode means in longitudinally parallel anode focal spots,

and a grid means disposed within one of said recesses bet-ween the filament therein and the opening of said one recess and in insulated relation with respect to said body.

References Cited UNITED STATES PATENTS WILLIAM F. LINDQUIST, Primary Examiner.

US. Cl. X.R. 250-99; 31357

Images