US2686884A - Space charge controlled X-ray tube - Google Patents

Description

Aug. 17, 1954 2. J. ATLEE 2,636,884

SPACE CHARGE CONTROLLED X-RAY TUBE Filed May 1, 1950 2 Sheets-Sheet l 1 I I 1 1 1 I I INVENTOR.

. fllee air Aug. 17, 1954 z. J. ATLEE 2,686,884 SPACE CHARGE CONTROLLED X-RAY TUBE Filed May 1, 1950 2 Sheets-Sheet 2 INVENTOR.

26d J. @166 Ki wzm shape can be Patented Aug. 17, 1954 SPACE CHARGE CONTROLLED X-RAY TUBE Zed J. Atlee, Chicago, Ill.,

assignor to Dunlee Corporation, Chicago, 111., a, corporation of Illinois Application May 1, 1950, Serial No. 159,226

7 Claims.

The present invention relates to X-ray tubes having means incorporated therewith for controlling electron emission.

The X-ray tube, as it is generallyknown today, i

is an emission limited device, that is, the electron emission of the filament, and hence generation of X-rays by the tube, is controlled entirely by the temperature of the filament. The successful operation of such tubes depends largely upon the stable emission of electrons from the filament and for this reason pure tungsten has generally been considered the ideal filament material even though it is recognized as a relatively poor electron emitter. In a high vacuum, and at the high operating temperatures required, the electron flow from the filament to the anode is relatively stable. Obviously, however, any one tube is inherently relatively inflexible and is adaptable only to a limited range of applications, necessitating the provision of a multiplicity of tubes, each having a relatively narrow range of use.

Rather than being an emission limited device, as in the case of X-ray tubes as heretofore manufactured, thetube of the present invention is a space charge limited device. than X-ray tubes have heretofore been provided employing space charge control of current flow, no one has heretofore conceived the feasibility of providing for space charge control of current flow from the filament to the anode in an X-ray tube. I have discovered that it is not only practicable to provide a space charge limited X-ray tube, but that by the provision of one or more suitably arranged control electrodes in an X-ray tube, many new and unexpected results are accomplished. Electron emitting materials which are more efficient than tungsten, such as thoriated tungsten, or barium coated wire, requiring lower filament temperatures, may be employed to great advantage.

With an X-ray tube constructed in accordance with the present invention, the current Wave accurately controlled so as to increase the X-ray output of the tube per milliampere of current. The efficiency of the tube is thereby greatly increased, with a material reduction in heating of the tube parts. By application of a separate biasing potential to the control electrode, not only may the current wave shape be accurately controlled, but the duration of current fiow, and hence timing of exposures, may also be controlled within precise limits.

It is a general object of the present invention, therefore, to provide a new and improved X-ray tube in which the current flow from the filament to the anode may be controlled by space charge.

While tubes other A further object is to provide an X-ray tube in which the wave shape of current flow may be controlled to increase the efficiency of the tube.

Another object of the invention is to provide an X-ray tube which may advantageously employ relatively low work function materials for the filament at higher gas pressures than heretofore considered possible.

Another object is to provide an X-ray tube having means incorporated therein whereby the X-ray output of the tube may be accurately timed independently of the filament.

A more specific object of the present invention is to provide a new and improved X-ray tube having incorporated therein a novel control electrode arrangement for controlling the electron beam flowing from the cathode to the anode.

Another specific object of the present invention is to provide a new and improved X-ray tube having a control grid arrangement in combination with the cathode structure.

A more specific object of the present invention is to provide a new and improved X-ray tube having a cathode filament arranged within a focusing cup of a cathode head structure, the cathode head being insulated With respect to the filament and a grid structure mounted across the outer end of the focusing cup whereby the cathode electron beam may be accurately controlled.

In accordance with one form of the present invention, an X-ray tube is provided having an anode and a cooperating cathode structure spaced therefrom and arranged within an evacuated envelope. The cathode structure includes a usual filament arranged within a focusing cup but electrically insulated therefrom, and a control grid arranged over the open end of the focusing cup, between the filament and the anode. The filament is preferably of a thoriated tungsten material, or of a barium coated wire, whereby an excess of electron emission may readily be obtained with a low filament temperature. Stability of electron emission from the filament is of relatively minor importance for the reason that current fiow from the filament to the anode may be controlled by control of the biasing potential applied to the control grid. The control grid may be mounted upon and electrically connected to the focusing cup or it may be insulated therefrom and separate leads brought out for both the cup and the grid. Many additional advantages which may be derived from the tube of the present invention will be pointed out hereinafter.

For a consideration of what is believed novel and inventive, attention is directed to the following description taken in connection with the accompanying drawings while the features of novelty will be pointed out with greater particularity in the appended claims.

In the drawings, Fig. 1 is a longitudinal sectional view taken through an X-ray tube constructed in accordance with one modification of the present invention;

Fig. 2 is an enlarged longitudinal sectional view illustrating the details of construction of the cathode structure incorporated in the tube shown in Fig. 1;

Fig. 3 is an end view, with certain parts broken away, of the cathode structure shown in Fig. 2;

Fig. 4 is a longitudinal sectional view taken through a cathode structure constructed in accordance with a further modification of the present invention;

Fig. 5 is a top view of the structure shown in Fig. 4; and

Figs. 6 and (are explanatory curves. 1

To illustrate the invention, the drawing shows an X-ray generator or tube 1 i comprising spaced anode and cathode electrodes i3 and i5, sealed in an enclosing, preferably glass, envelope ll.

The anode It may be of any suitable or preferred construction and is suitably mounted in the envelope ii and sealed therein at one end of the envelope bya glass-to-metal seal it through which a portion '26] of the anode extends outwardly of the envelope to .provide for cooling the anode.

The cathode i5 is mounted in the envelope at the end remote from the seal [9, the cathode being connected within the envelope to conductors 2| and 22 extending outwardly of the envelope for the purpose of energizing the oathode from a suitable source of cathode energizing current outwardly of the envelope. The anode and cathode are supported in coaxial spaced apart alignment in the envelope and have facing heads.

It should be understood that X-ray generators function to produce X-rays in response to the activation of the anode as an X-ray source of impingement on the head of the anode of electrons emitted by and from the cathode. I'he cathode head facing the anode is formed with a cavity or cup 25 in which is disposed an electron emitting filament 2i. nected with the conductors 2! and 22 and may be constituted as an electron emitting element when energized by the passage therethrough of current supplied through the conductors 2i and 22. By applying an electrical potential between the filament 2! and the facing head of the anode, as by connecting one of the conductors 2 l and 22 and outwardly extending portions it of the anode with a suitable source of electric potential, electrons emitted by the filament 2"! may be caused to travel from the filament as an electron stream 29 and impinge upon a portion of the head of the anode forming a target area 3!. The cup 25 serves to focus the electrons comprising the stream 29 uponthe target zone 3!.

It will be noted that the head of the anode may be inclined with respect to the direction of the electron beam 29 in order that Y-rays generated at the target area 35 may be transmitted thence as an X-ray beam 33; directed outwardly of the envelope through a portion of the lateral walls thereof, said portion preferably being of reduced thickness opposite the head of the anode, thereby forming an X-ray window 35 in the envelope. During the operation of the X-ray The filament 27! is coni 4 generator a considerable amount of heat is developed in the anode which may be dissipated outwardly of the envelope by radiation from the anode through the envelope walls and by conduction through the anode and its outwardly extending portion 26.

The cathode head block 45, which may be of solid steel, defining the elongated focusing cup 25 in the upper surface thereof is suitably secured into the upper end of a steel cylindrical sleeve All, the lower end of which is fitted telescopically over the end of the re-entrant cylindrical glass stem portion 48 of the envelope H and. suitably secured thereto such as by means of a plurality of turns it of wire. The upper surface of the head block 45 is provided with a plurality of relatively closely spaced machined grooves Eli} and within each of which is arranged a length of fine wire and which may have a diameter, for example, of .012 inch. These wires are preferably spaced approximately four to five diameters apart. It is necessary that the wires be fine in order that the focal spot upon the anode be not broken up into a series of small focal spots which would be unusable and also that they be spaced relatively closely together, to produce a good focal spot. The wires 5! define a grid element of uniform mesh over the efiective cross section of the electron path between the cathode and anode elements. These wires should be of some poor work function material such as platinum or platinum covered molybdenum wire so that they themselves will not act as electron emitters when subjected to inverse electron bombardment. These grid wires iii are not fixedly secured within the grooves 56 but are retained in place by means of the steel cap 53 fitted over the upper end of the block (it. The cap $3 is provided with a fiange be which is adapted to fit snugly around the upper end portion of the block 45 and may, if desired, be fixedly secured in place. The grid wires 5! retained in place by the cap 53 are thus free to expand in the longitudinal direction under variations in temperature.

It will be observed that the lead-in wires 2| and 22 supporting the cathode filament 2? at opposite ends extend through openings 5% in the head block 45 and in a centered and spaced relation from the side walls of such openings so as to be electrically insulated therefrom. Lead-in wire 5'? suitably connected to the head block 45 is brought out through the pinch stem it together with the filament lead-in wires 2i and 22. The wire 57 extends through the opening in the disc 58 in spaced relation therewith, while the filament is supported at its opposite ends by the lead-in wires which are mounted within glass beads 59 in spaced apart openings in the disc "58.

By connecting a biasing potential between the cathode filament and the head block 45 a high space charge may be created and which may be employed for effectively limiting the electron beam emitted from the flowing to the anode. While not essential to the operation of a tube of the type described, it is preferred that the cathode filament be of a low work function material such as thoriated tungsten or barium oxide coated wire. It has been considered impractical, heretofore, to employ a thoriated tungsten filament, and impossible to use a barium oxide coated filament in an X-ray tube for the reason that such filaments have an inherently erratic on unstable electron emission characteristic, and, moreover, it is extremely cathode filament and which difficult to manufacture X-raytubes with such filaments and which will be uniform from tube to tube. In the operation of a conventional X-ray tube where X-ray generation is controlled solely by the cathode temperature, unstable or nonuniform characteristics are extremely undesirable. By control of the space charge by means of the grid as described, a thoriated tungsten or barium oxide coated filament may be used and the filament current adjusted so as to give on an excess of electron emission, the excess of may be eifectively blocked by the space charge. I

In the prior Patent 2,348,184, issuedto Atlee and dated May 9, 1944,is described an X-ray tube having a filament of thoriated tungsten and mention is made therein of some of the problems and limitations of employment of such a filament. For example, one requirement considered therein as being essential to the operation of the tube was a vacuum of the order of 0.02 micron of mercury or lower. A tube constructed in accordance with the present invention, and employing a thoriated tungsten filament, operates successfully at much higher residual gas pressures, even as high as approximately 1.0 micron of mercury without resulting in the destruction of the thoriated filament surface. In order to be obvious, therefore,that inasmuch as the maximum value of the current wave I03 is several times greater than the maximum value of the current wave I02, the eifective X-ray output of the tube will be very substantially greater than inthe case of a conventional tube while the heating of the tube will be materially less.

Expressed in another way, the peak current in aconventional tube is approximately three times the average current when connected in a halfwave circuit. In a tube of the present invention, employing a thoriated filament, the peak current will be of the order of six times the average current value, when connected in a half-wave circuit, and with the grid connected to filament potential. Thus for the same average current value, the *X-ray output of the present tube will obtain a semblance ofstability of thefilament in the tube as described in that patent, low vacuum was essential. tube uniformity of the rate of electron emission from the filament is relatively immaterial since an extremely the current fio-w between the filament and anode is controlled by space charge.

By operation of the X-ray tube filament with an excess of emission a more stable X-ray generation characteristic can be obtained through controlof the electron beam from the cathode to the anode thereby completely eliminating fluctuations which may be caused by line voltage variations influencing filament temperature. Also, the cathode filament is effectively shielded by the grid from inverse electron bombardment as well as from stray ions.

Referring now to Fig; 6, the curve I M illus' trates a sine wave of voltage which may be ap plied across the filament and anode of an X-ray tube. The dotted curve I92 illustrates the current fiow between the filament and the anode in a conventional emission limited tube and from Which it will be noted that the current builds up gradually until peakfiow occurs as the applied voltage Wavetreaches maximum value. X-ray generation of thetube is a function of the peak Value of the current flow and over an appreciable interval represented by the end portions I Ma and I021) of the curve I532, the current flow through the tube serves no useful function, it being below the threshold required for effec tive X-ray generation from the anode. The, current flow during such periods serves only to heat up the anode andthe envelope and which is undesirable. This is typical of all prior conventional tubes. I I I In the case of tubes constructed in accordance with the present invention, creation of a high space charge by the grid causes a peaking of the current wave as indicated by the curve we in Fig. 6. The average value, or the milliampere readi g, for the current waves I02 and IE3 are the same but the instantaneous current values in the case of curve I03 are a minimum on opposite sides of the crest of thevoltage wave and a maximum at the instant of such crest. It will With the present.

be approximately to per cent of that of a conventional tube. Thus by operating the tube self-rectified in analternating current circuit, the X-ray output will be the equivalent to the output of a conventional tube when operated on constant potential and at the same current readmg. I I

In the operation of the tube described with reference to Fig. 6, it was assumed that the grid was connectedto filament potential. By biasing the grid with an independent biasing potential, the wave shape may be controlled Within wide limits. For example, as illustrated in the curve of Fig.-7, the current fiow from the filamentto the anode may be caused to rise abruptly from zero to maximum and permitted tofiow for a predetermined interval and then suddenly chopped ofi again as indicated by the curve I04. The curve I04 is, in this instance, shown with relation to a sine wave I05 of an applied alternatingcurrentpotential.

By applying a constant potential across the filament and anode, the timing between successive pulsesofX-rays may readily be varied as desired. Thus, for example, the pulses may be of rectangular wave form, of 1 to 5 micro-seconds in width and with a recurrence frequency of sev- & eral hundred per second.

The ease with which the X-ray output of the present tube may be controlled, or timed, is an important feature of the present invention. For controlling the duration of exposures by tubes of the prior art complicated external timingapparatus is required, most of which leave much to be desired in the way of ease of adjustment and accuracyof performance. By applying a separate biasing potential to the grid, the grid may be made sufficiently negative to block all fiow of electrons from the filament to the anode and by simply controlling the biasing potential the duration of the X-ray exposure can be conveniently and precisely regulated.

As mentioned in my earlier Patent 2,340,363, the shape and configuration of the focal spot of the anode target may be considerably influenced by applying a biasing potential to the focusing cup of the cathode structure. It is important, in order to prevent undue distortion ordisplacement of the focal spot, that the grid wires 5I should extend at right angles with respect to the longitudinal direction of the filament. I have discovered that the grid arrangement as described has no effect upon the length of the focal spot throughout any normal range of either positive or negative potentials and that the length of the focal spot is controlled solely by the length of the filament itself.

the application of a biasing potential to the focusing cup element tends to decrease the width of the focal spot upon the anode target. This characteristic also holds true for the instant X-ray not desired, the grid element may be insulated from the focusing cup element so that the biasing potential applied to the grid will not also be ap plied to the focusing cup. In Figs. 4 and 5 is shown a further modification of the invention illustrating one typical manner in which this might be accomplished.

Referring particularly to Fig. 4, the grid element is shown as consisting of a pair of discs 6?! and lil secured together as by means of the rolledover peripheral edge portion 52 of disc 59. The surface of one of the disc elements is provided with a plurality of parallel spaced apart grooves and within each of which is arranged a grid wire 83, the various grid wires being retained in place by virtue of the clamped together relation of the discs Gil and Bi. uniformly apart in a manner as previously described in connection with Fig. 3. The grid element may be supported in any suitable manner in an insulated relation at the upper end of the cathode structure. As shown, in the drawings, the focusing cup defining block 64 is provided with a pair of diametrically opposed openings 65 through which the support wires '66 and 61 for the grid element extend. The lower ends of the wires 68 and Bl are suitably anchored by means of insuiating glass beads 58 within a supporting block Ill. The wire 61 is continued out through the pinch seal 12 by means of which external electrical connection may be made to the grid ele ment. The focusing cup block 54 is supported, as in the previously described modification, upon the upper end of a cylindrical sleeve 13, the lower end of which is fitted telescopically upon the reentrant cylindrical glass stem portion 14 and secured in place by means of a plurality of turns of wire 16. The cathode filament 18 is mounted in the bottom of the focusing cup '19 as previously described and supported by means of the disc block 82 through which the lead-in wires 84 and 85 extend in an insulated manner. The lead-in wires 84 and 85 are secured within cooperating openings provided in the block 82 by means of insulating glass beads 88. Electrical connection may be made to the focusing cup element 54 through the wire 90 which extends through cooperating openings 9| and 92 in the disc blocks 8?. and ill, respectively.

In the case of the X-ray tube cathode structure as shown in Fig. 4, a biasing potential may be applied to the grid element independently of the focusing cup element so that the width of the focal spot upon the anode target will not be decreased upon application of potential to the grid. The width of the focal spot on the target element may be independently controlled, if desired, by applying a different biasing potential to the focusing cup element 64 through the external connection 90. Alternatively, the focusing cup element 64 may be connected directly to one of the cathode filament lead-in wires 84 or $5 to make the focusing cup of the same potential as These wires 53 may be spaced parent to one skilled in the art.

the cathode. It is thus possible to apply independent bias, either positive or negative, to the focusing cup and grid elements entirely independently of each other depending upon the particular focal spot characteristic desired and application madeof the tube.

In the above description of the structure shown in Fig. 4, it was first assumed that the grid wires 83 of the grid element were spaced uniformly apart as shown in Fig. 3. According to a further alternative of the present invention, the grid wires '63 may be spaced relatively close together at the opposite ends of the cathode filament 18 as shown at and spaced relatively widely apart above the center of the cathode filament as shown at 96. In both this modification, as well as in the modification of Fig. 3, the grid wires are substantially symmetrically arranged'as regards the longitudinal axis of the tube, or the axis of the electron beam so that the center of the anode focal spot will not be deflected upon variations in biasing potential. Such substantial symmetry of the grid mesh relative to the electron beam axis is important. With the arrangement of the grid wires as shown in Fig. 5 the tube may be given a variable focus characteristic. As described above, and in connection with the modification shown in Figs. 2 and 3, the length of the cathode spot upon the anode target is determined solely by the length of the cathode filament. Also, as described, the width of thefocal spot may be controlled by adjustment of the biasing potential applied to the focusing cup element 64. With the grid structure as shown in Fig. 5, the length of the focal spot upon the target element may be regulated by adjustment of the biasing potential applied to such grid element. Control over the length of the focal spot has long been desired but heretofore no one has discovered any means whereby this might be accomplished. With this structure, the electron beam flowing from the cathode filament to the anode can readily be decreased in width, that is, in the direction longitudinally of the cathode filament, progressively toward the center of the grid as the negative voltage on the grid is increased. The control supports for the grid element and for the focusing cup element may be linked together in the external circuit to vary the focal spot size as the load varies.

It will be further obvious to those skilled in the art that the grid elements disclosed may be employed for timing the exposure made by the tube, thus eliminating much complicated external support apparatus presently used for controlling X-ray exposure. It is to be understood that by application of a sufficient negative potential to the grid electrodes that the electron fiow from the cathode filament to the anode may be completely cut off. With a very simple external timing switch the length of time of X-ray generation of the tube may be conveniently and accurately controlled within precise limits.

Having described the invention in what are considered to be certain preferred embodiments thereof, it is desired that it be understood that the specific details shown are merely illustrative and that the invention may be carried out in many other ways and which will be readily ap- It is desired, therefore, that it be undertsood that in the following claims I intend to cover all such alternative embodiments as fall within the true spirit and scope of the present invention.

I claim:

1. A space charge controlled X-ray tube comprising an envelope, cooperating spaced apart anode andcathode elements mounted within said envelope, a focusing cup defining element, said cathode element comprising an elongated filament extending at right angles to the common axis of said anode and cathode elements, means supporting said filament within said focusing cup in an electrically insulated relation with respect thereto, a grid element mounted within said envelope adjacent said focusing cup and electrically insulated therefrom, said-grid element extending between said anode and cathode elements,said grid element comprising a plurality of relatively fine wires extending parallel with each other in a relatively closely spaced apart relation, said wires being supported in a plane at right angles to said axis, said wires further extending in a direction at right angles to the longitudinal direction of said cathode filament, said wires being spaced relatively closely together adjacent opposite ends of said filament and relatively far apart toward the center of said filament, and means for connecting separate biasing potentials to said focusing cup and said grid element.

2. A space charge controlled X-ray tube comprising an envelope, cooperating spaced apart anode and cathode elements mounted on a common axis Within said envelope, said cathode element comprising an elongated filament extending at right angles to said axis, a grid element mounted within said envelope adjacent said cathode element and electrically insulated therefrom, said grid element comprising a plurality of relatively fine wires extending parallel with each other in a relatively closely spaced apart relation, means supporting said grid wires in a plane at right angles to said axis, said wires further extending in a direction at right angles to the longitudinal direction of said cathode filament, said grid wires being spaced relatively closely together adjacent opposite ends of said filament and progressively farther apart toward the center of said filament, and an electrical conductor connected to said grid element and extending externally of said envelope.

3. A space charge controlled X-ray tube comprising an envelope, cooperating spaced apart anode and cathode elements arranged within said envelope, means supporting said elements in a coaxial relation within said envelope, said cathode element consisting of an elongated filament made of a material having a high electron emitting characteristic and selected from the class consisting of thoriated tungsten and barium coated wire, said filament extending transversely of the axis of said element, electrical conductors connected to said cathode filament and extending externally of said envelope, said filament being adapted to be energized so as to produce an excess of electron emission, a grid element mounted adjacent said cathode element and between said cathode element and said anode element, said grid element consisting essentially of a plurality of relatively closely spaced fine wires extending substantially in a plane at right angles to said axis, and in a direction at substantially right angles to the longitudinal direction of said filament, an electrical conductor connected to said grid element and extending externally of said envelope, said grid element being adapted for limiting electron flow from said cathode to said anode upon energization with a biasing potential.

, a relatively erratic high electron 4. A space charge controlled X-ray tube comprising an envelope, cooperating spaced apart anode and cathode elements arranged within said envelope, said cathode element consisting of an elongated filament made of a material having emitting characteristic and selected fromthe class consisting of thoriated tungstenand barium coated. wire, said filament being adapted to be normally energized so as to produce an excess of electron emission, a space charge control element mounted adjacent said cathode element and extending between said cathode element and said anode element, said control element consisting essentially of a plurality of relatively closely spaced apart fine wires insulated, from said anode and cathode elements, said wires extending at right angles to the longitudinal direction of said filament, and means for connecting a biasing potential to said control element for limiting and stabilizing electron fiow from said cathode filament to said anode to an amount less than the total electron emission of said filament.

5. A space charge controlled X-raytube comprising an envelope, cooperating spaced apart anode and cathode elements arranged within said envelope, a focusing cup defining element surrounding said cathode element and facing said anode element, said cup element being adapted for focusing electrons emitted by said. cathode element upon said anode element, saidl cathode element being electrically insulated within said envelope from said focusing cup element, the peripheral face surface of said cup element being substantially plane, a plurality of parallel spaced apart grooves in said face surface, a wire positioned in each of said grooves and extending across the open end of said cup, said wires defining a grid element spaced from said cathode element, means for retaining said wires in position, and a conductor connected to said cup element and extending externally of said envelope.

6. A space charge controlled X-ray tube comprising an envelope, cooperating spaced apart anode and cathode elements arranged within said envelope along a common axis, said cathode element comprising an elongated filament extending at right angles with respect to said axis,

sisting of a plurality of relatively fine parallel wires of poor electron emitting metal selected from the class consisting of platinum and molybdenum, an electrical conductor connected to said grid element and extending externally of said tube.

'7. A space charge controlled X-ray tube comprising an envelope, cooperating spaced apart anode and cathode elements arranged within said envelope along a common axis, said cathode element comprising an elongated filament extending at right angles with respect to said axis, a grid element mounted within said envelope and electrically insulated from said anode and cathode elements, said grid element being arranged adjacent said cathode element across the electron path between said anode and said cathode elements, said grid element being substantially plane and mounted at right angles to said axis, said grid element having a width at least as great as the length of said cathode filament, said grid element being of uniform configuration throughout its full extent in the direction at right angles to the longitudinal direction of said filament, an electrical conductor connected to said grid element and extending externally of said tube.

References Cited in the file of this patent UNITED STATES PATENTS Name Date Number 12 Name Date Morrison Dec. 28, 1926 Daumann July 9, 1929 Edwards et a1. Apr. 25, 1933 Strigel et a1. June 6, 1933 Bouwers Feb. 27, 1934 Moran Apr. 24, 1934 Morrison Mar. 31, 1936 Applebaum July 21, 1936 Heintz Aug. 31, 1937 Alfter Feb. 15, 1938 Bouwers Sept. 13, 1938 Atlee et a1 May 9, 1944 Quittner July 5, 1949

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