US2503075A - X-ray tube energizing circuit - Google Patents

Description

April 4, 1950 H. M. SMITH .XRAY TUBE ENERGIZING CIRCUIT Filed Feb. 15, 1947 AMPLIFIER AMPLIFIER Z2 REACTOR I2 SAMPLE FILTER AMPLIFIER VOLTAGE REGULATOR AC. SOU RC E h, w nm n 3 ,w n m 6M A m 5 Patented Apr. 4, 1950 I X-RAY TUBE ENERGIZING CIRCUIT Henry M. Smith, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application February 15, 1947, Serial No. 728,851

2 Claims.

My invention relates to X-ray tube energizing circuits, and more particularly to an automatically stabilized energizing circuit for an X-ray tube whereby the intensity and the hardness of X-rays produced by such tube remains essentially unchanged for usual variations in voltage of the energizing source.

X-rays have become a valuable means for investigating the internal structure and physical characteristics of otherwise opaque materials. Other similar uses are also known in the art. In conducting these investigations, it is usual to employ means for measuring the intensity of the unabsorbed X-rays emanating from the material being analyzed. When employing such a method, the accuracy of investigation is dependent upon the accuracy with which the intensity and the hardness of the penetrating X-ray beam may be held constant.

An X-ray tube is usually employed to produce the X-ray beams utilized in investigation of materials of the type described. The characteris-.

to-cathode voltage is dependent upon the temperature at which its cathode is maintained. Also,

the equivalent wavelength or hardness of the X-rays produced by a tube energized by an A.-C. voltage is critically dependent upon the magnitude of the anode-to-cathode voltage. If this voltage is increased, the wavelength becomes shorter and the penetrating power of the X-ray becomes greater. This efiect is most pronounced in the long wavelengthregion which region is preferably employed in analyses of materials whenever feasible, such as in analysis of materials of such structure and dimensions which will absorb less than all of such longer wavelength X-rays. Thus, it has been found that a one per cent increase in the X-ray tube anode-to-cathode voltage may, in some instances, increase the amount of unabsorbed X-rays passing through a material under test as much as twenty per cent or more. Similar effects upon unabsorbed X-ray intensity may be caused by changes inthe cathode filament emission. It is desirable, therefore, if accurate measurements are to be obtained, that the X-ray tube be subjected to voltages whose magnitudes are controlled with a high degree of It is another object of my invention to provide regulating means responsive to the intensity of the X-rays produced by an X-ray tube whereby variations in filament emission are minimized.

Another object of my invention is to provide an X-ray tube energizing circuit of such type that the intensity and hardness of the X-rays produced thereby is maintained constant with a high degree of accuracy.

The features of the invention which are believed to be novel and patentable will be pointed out in the claims appended hereto. For a better understanding of the invention, reference is made in the following description to the accompanying drawing in which the single figure thereof is an embodiment of my invention in which an energiz ing circuit for an X-ray tube is disclosed.

Referring now to the drawing, X-ray tube l possessing anode 2 and a filament-type cathode 3 is employed as a source of X-rays whose intensity-and hardness are to be accurately controlled. X-ray tube l is preferably placed within an absorbing shield, partially disclosed at 4, which shield possesses appropriate openings 5 and 6 through which X-ray beams may emanate. Openings 5 and 6 may be made at convenient points on shield 4, such that the emanating X-rays may either be of equal intensity or of intensities of constant ratio with respect to each other.

X-ray tube I may be energized from a suitable A.-C. source I, of the usual type, in which the magnitude of the supply voltage, as expressed in effective or R. M. S. value, may be subjected to both gradual and rapid variations because of changes and disturbances in its connected load other than the X-ray tube under consideration. A voltage regulator 8 of conventional type is preferably used to remove the greater portion of changes in the effective'value of the A.-C. source by connecting it to source 1 through a suitable switch 9, and utilizing its output terminals as a voltage source. Cathode 3 of X-ray tube l is energized from the output terminals of voltage regulator 8 through an isolating transformer l0, whose secondary'll is connected directly across the filament of cathode 3 and whose primary I2 is preferably connected to the output terminals of a tapped auto-transformer [3 with a variable impedance of a type to be described in series therewith. The input terminals of transformer l3 are connected across the output of regulator 8. Tap changing switch M is provided whereby the voltage ratio of autotransformer 13 may be manually adjusted.

"crease.

A high voltage step-up transformer 15 is employed in the following manner to provide the necessary anode-cathode voltage. Anode 2 is connected to ground, and cathode 3 is connected to one side of the high voltage secondary it of transformer l5 by connecting one side of secondary i! of the isolating transformer it to one terminal of secondary It. The remaining terminal of secondary i6 is connected to ground through an anode current measuring device ii and a tapped resistor id in series therewith. The primary [9 of transformer I5 is energized from the output terminals of a tapped auto-transformer Zll through a variable impedance to be described. A manually operated tap changing switch *2! is also provided whereby the ratio of auto-transformer 29 can be adjusted. Auto-transformer 2B is also energized from regulator 8 as is auto-transformer it.

The anode current of X-ray tube 1 for a constant anode-to-cathode voltage is dependent upon the filament emission of cathode 3. Such filament emission can be held constant by properly controlling the voltage impressed across pri* mary l?! of transformer lfl. The voltage across primary l2 can be regulated by placing in series therewith an adjustable impedance such as a saturable reactor 22 including A.-C. windings 22' and a D.-C. winding 22" and regulating the D.-C. excitation supplied thereto.

Since anode current exists in X-ray tubes only when the anode is positive with respect to the cathode, a pulsating D.-C'. voltage appears across resistor it when the X-ray tube is energized by an A.-C. source. A portion of this voltage can be utilized by providing a tap 23 on resistor it. This voltage has a D.-C. component which is positive with respect to ground for a circuit such as disclosed herein and is essentially directly proportional to filament emission at constant anodeto-cathcde voltage. The voltage appearing at tap '23 is impressed upon the input terminals of a direct current amplifier 24 of conventional type whose output terminals are utilized as a source of direct current whose value is inversely proportional to the positive D.-C. component of input voltage. The amplifier may or may not include filtering means to eliminate A.-C. components. In either case the D.-C. saturating current is the same, and A.-C. components, if present, have little effect upon the reactor. Thus, energization of the direct current winding on saturable reactor 22 from amplifier 2d causes the impedance of reactor 2? to increase if the anode current of tube l tends to increase, and correspondingly to decrease if the anode current of tube l tends to de- Thus, for a constant cathode-to-anode voltage, the filament emission is maintained essentially constant without respect to variations in effective value of the A.-C. voltage appearing across the output terminals of auto-transformer I3.

The anode-to-cathode voltage of tube l is maintained constant in the following manner. An X-r ay detector 25, responsive to the intensity of X-rays reaching it, is placed in such position with respect to X-ray tube i that the X-ray beam' emanating through opening 3 impinges upon the detector. Since X-ray detectors, particularly those of a type such as a Geiger-Muller counter or phctotube possessing a fluorescent coating, detect intensity variations, it is desirable to utilize them in a system designed to cause their operation in a region where variations in specimen thickness and in energizing voltage values cause maximum X-ray intensity variation. This can be accomplished by utilizing a filter 26 of X-ray absorbing material, preferably essentially equivalent in total absorption to that of the specimen to be analyzed, and placing it between the X-ray tube 1 and the detector 25. The intensity of unabsorbed rays reaching the detector is then a function of the penetrating power or hardness of the original rays. Detector 25 and filter 26 together thus constitute an X-ray hardness responsive device. The output of detector 25 is amplified and, converted into a direct current whose value varies inversely with the output of detector 25 by utilizing an amplifier 21 of appropriate conventional type. A saturable reactor 28, including A.-C. windings 28' and a D.-C. winding 28'', whose D.-C. winding is energized by the output of amplifier 2?, is employed as a variable impedance in series with primary 19 of transformer [5 across the output terminals of auto-transformer 2E3. Since the impedance of reactor 28 varies inversely with the amount of direct current magnetizing efiect supplied by amplifier 2? to D.-C. winding 28" the voltage across the primary !9 is automatically maintained at that value which causes X-rays of desired intensity to be produced by X-ray tube 2'. Since it is desirable to operate both reactors 22 and 28 normally in such fashion that either an increase or a decrease in their effective impedance is obtainable with equal ease, direct current measuring devices 29 and 30 preferably are provided in series with the D.- C. windings 28 and 22 of reactors 28 and 2?. respectively, whereby normal operation at the desired point on their impedance characteristic may be assured.

Utilization of X-ray tube l as a source of X-rays for inspection apparatus is illustrated by showing a material 3!, to be analyzed, intercepting the X-ray beam emanating through opening 5 of shield l, an X-ray detector 32 subject to the unabsorbed X-rays passing through material 3i, an amplifier 33, and a meter 34 connected to detector 32 through amplifier 33 in such fashion that the intensity of the unabsorbed rays may be directly indicated on meter 34.

The operation and advantages of the circuit shown in the drawing may be more fully described in the following manner. When inspecting a sample 3! it is desirable that the X-ray tube i provide X-rays of such intensity and wavelength that the unabsorbed X-rays impinging upon detector 32 are of such value that the indication of meter 3 is a function only of the thickness and characteristics of sample 3! The volt age applied to X-ray tube l between anode 2 and cathode 3 and the voltage applied across the filament of cathode 3 are made adjustable so that X-rays of appropriate intensity and hardness to produce a conveniently readable m'eter indication may be produced.

When the circuit is energized from source 7 by closing switch 9, a voltage dependent upon the setting of contact switch l4 and upon the value of impedance represented by the saturable reactor 22 is impressed across'the filament of cathode 3. Similarly, a voltage-dependent upon the setting of contact switch 2! and upon the value of impedance represented by saturable reactor 23 is impressed between cathode 3 and anode 2. Adjustment of contact switches M and 2! preferably should be made essentially simultaneously and in a manner to be described.

Since X-ray detectors 25 and 32 are preferably of similar design and are made to operate most efficiently at the same low'X-ray intensityl'evel, a filter 26;w-hose total-absorption of X-raysis essentially equivalent to that of sample 3|, is interposed between X-ray tube l and. the X-ray detector 25. The anode-cathode voltage can then be adjusted by manually positioning contact switch 2| until the unabsorbed X-rays impinging upon detector 25 are of the desired intensity as evidenced by meter 29 while also adjusting contact l4 to obtain or maintain the desired value of anode current as indicated by meter H. The desired X-ray intensity is that value which will cause amplifier '21 to deliver a direct current to the D.-C. winding 28" of reactor 28 to partially saturate reactor 28 to such an extent that decrease or increase of the saturating direct current causes a corresponding increase or decrease of impedance of the A.-C. winding of reactor 28.

If the desired anode current causes amplifier 24 to supply an improper value of D.-C. current to reactor 22, as evidenced by meter 30, the tap 23 on resistor 18 can be changed to correct this condition. Additional adjustment of contact switch l4 may then be required since a change in position of tap 23 changes the impedance represented by the A.-C. winding 22 of reactor 22. In this fashion, X-ray tube I may be caused to operate at the desired intensity and equivalent Wavelength with reactors 22 and 28 normally excited by direct currents of desired value.

Whenever undesirable voltage fluctuations of source 1 are caused by extraneous conditions, voltage regulator 8 compensates for such variations but only to an extent dependent upon its characteristics. across auto-transformers l3 and 20 contains objectionable components whenever disturbances on source I occur.

When the voltage across transformer 20 increases, the anode-to-cathode voltage of X-ray tube I tends to increase correspondingly. Such an increase ofanode-to-cathode voltage materially increases the penetrating power of X-rays produced, and X-ray detector 25 is immediately influenced thereby. Amplifier 21, as controlled by detector 25, correspondingly decreases the D.-C. excitation to reactor 28 which thereby increases its value of impedance which is in series with primary l9 of transformer I5. Such an increase in impedance when the voltage across r transformer 20 tends to rise compensates for such rise, and the voltage across winding i9 is thereby maintained essentially constant regardless of voltage variations across transformer 20. Conversely, when the voltage across transformer 20 decreases, the impedance of reactor 28 decreases. Similarly, the filament emission or anode current, as indicated by meter I1, is maintained essentially constant since the voltage across primary l2 of transformer I0 is essentially isolated from voltage variations across transformer i3 by the automatic change of impedance value of the A.-C. winding 22 of reactor 22 in series therewith. Thus, the intensity and equivalent wavelength of the X-rays produced by X-ray tube l are held constant with a high degree of accuracy, independent of expected disturbances in the X-ray tube energizing source.

Since filters of any desired total X-ray absorption may be substituted for filter 26, it is evident that the X-ray tube may be operated at any degree of intensity and at any equivalent wavelength at or below its maximum rating, while still retaining the automatically controlled features of my invention.

Thus, the voltage impressed "'As'will 'occur 'to' those skilled in the art, varione different arrangements and combinations of the principles described above may be employed without departing from the true spirit and scope of the invention and I, therefore, do not wish to limit my invention to the particular arrangement described. v

What I claim as new and desire to secure by Letters Patent of the United States, is:

value of direct current in the D.-C. winding of said reactor, an X-ray absorbing filter positioned to intercept X-rays emanating from one port of said X-ray tube, an X-ray detector to provide an electric current having a value which varies with variations in the penetration of X-rays through said absorbing filter, and an amplifier' connected in responsive relation to said detector and in energizing relation to the D.-C. winding of said reactor to provide through such winding a value of direct current which varies in inverse relation to values of X-ray penetration detected by said detector, whereby the penetrating power of X-rays produced by said tube is maintained substantially constant.

2. In combination, a two-port X-ray tube having an anode and a cathode, said cathode comprising a filament which is heated bythe passage of electric current therethrough, a resistor, first and second transformers each having a primary and a secondary, the secondary of said first transformer being connected to said filament, the secondary of said second transformer being connected in series with said resistor between the anode and the cathode of said X-ray tube, energizing means to apply alternating current to the primaries of said first and second transformers, first and second saturable reactors each having an A.-C. winding and a D.-C. winding, the A.-C. winding of said first reactor being connected in series between said energizing means and the primary of said first transformer, the A.-C. winding of said second reactor being connected in series between said energizing means and the primary of said second transformer, an X-ray absorbing filter positioned to intercept X-rays emanating from one port of said X-ray tube, an X-ray detector to provide an electric current having a value which varies with variations in the penetration of X-rays through said absorbing filter, a first amplifier connected in voltage-responsive relation across said resistor and in energizing relation to the D.-C. winding of said first reactor to provide through such winding a value of direct current which varies in inverse relation to current through the anode-to-cathode circuit of said X-ray tube, and a secondary amplifier connected in responsive relation to said detector and in energizing relation to the D.-C. winding of said second reactor to provide through such winding a value of direct current which varies in inverse relation to values of X-ray penetration detected by said detector, whereby both the intensity and 7*" 8 tha-penetratingrpowenxsofutxrayspztoducedmby Number Name? Date.- said tube. are maintained=zsubstantially constant. 2,151,602 Kearsley Mar..:21,,1939

I HENRY M. vSMITH. 2,156,074 Westendorp April'25, 1939 2,160,605 Suits May 30, 1939- REF NC C I 5 2,217,939 Bischoff Oct. 15,.1940 The following-references areaof record inthe 2,222,451 Trost' NOV 19, 4 file of this patent: 2,319,373 Weisglass y 1943 V 2,401,289 Morgan et' a1. May 28, 1946: UNITED STATES PATENTS 2,404,905 Garretson July 30, 1946 Nmnber Name Date 10 1,876,437 Wantz Sept. 6,1932 OTHER REFERENCES 2,094,318 Fai a Sept 1937 G. E. Review, vol. 48, No. 3, March 1945. 9

2,097,760, Failla NOV. 2, 1937

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