X-ray target

3,239,706

March 8, 1966

S. R. FARRELL ETAL

X-RAY TARGET Filed April 17, 1961

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INVENTORS

SHERMAN R. FARRELL ROY C. MARKER

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ATTORNEYS

United States Patent Office

3,239,706

Patented Mar. 8, 1966

3,239,706

X-RAY TARGET

Sherman R. Farrell and Roy C. Marker, Orinda, Calif., assignors, by mesne assignments, to High Voltage Engineering Corporation, Burlington, Mass., a corporation of Massachusetts

Filed Apr. 17,1961, Ser. No. 103,580

Claims. (CI. 313—55)

This invention relates to the generation of X-rays by the passage of high energy electrons through dense metal targets, and is particularly directed to a target for this purpose which has a very high ratio of X-ray output per unit of heat dissipated in the target.

X-radsography for industrial or medical applications requires the generation of sharply defined images for the detection of small details in the object being examined. Fundamental to the production of sharp images is the provision of a fine focal spot or source of X-rays. This is frequently accomplished by focussing the high energy electron beam emanating from an electron linear accelerator, or equivalent electron source, to form a small image of the beam on a metallic target, usually of a dense or high atomic number metal such as gold, tungsten, molybdenum, platinum, lead, or the like.

The X-rays are produced in a well known manner as the electrons pass through the solid target material. However, considerable heat is also produced due to the electron beam power dissipated in striking the target and, as a consequence, melting of the target will result if the beam power density exceeds certain limits established by the means employed for cooling the target. The heat dissipated in the target is directly proportional to the target thickness and heretofore relatively thick targets have been considered requisite to the efiicient production of X-rays by penetration of the beam. Accordingly, in order to minimize the beam power density at the target it has been the practice to incline the target surface with respect to the beam or to provide a moving target surface such as a rotating disc. The foregoing solutions to the heating problem in X-ray targets are, however, variously disadvantageous, the inclined face target from the standpoint of the limited resolution attainable at high X-ray outputs, and the rotating target from the standpoint of the dynamics involved and difficulties of sealing movable elements in an evacuated envelope.

We have found a further solution to the heating problem in X-ray targets which does not suffer from the previously noted disadvantages associated with conventional target designs. More specifically we have found that X-rays are still produced efficiently by electrons passing through very thin layers or foils of dense metal and that thicker layers do not produce proportionately increased X-ray outputs.

The thermal energy dissipated in the target foil is, however, severely decreased due to the decreased thickness which permits the electrons to pass completely through the foil and give up the major portion of the beam energy in liquid, gas, or a low density solid disposed in backing relation thereto. Moreover, when the foil is cooled by passing liquid or gases over its surface, the energy dissipation is essentially independent of foil thickness since little thermal conduction occurs to spread out the heat spot beyond the diameter of the beam image.

It is therefore an object of the present invention to provide a thin foil X-ray target whereby the X-ray output per unit of heat dissipated in the target is materially increased.

It is another object of the invention to provide a target of the class described which facilitates the focussing of a relatively greater amount of beam current within a given spot diameter at the target whereby an increased X-ray output is produced without loss of definition.

Still another object of the invention is the provision of a target of the class described which permits an equiva5 lent beam current to be focused to a smaller beam diameter than theretofore permissible resulting in improved resolution without reduction of X-ray output.

Yet another object of the invention is to provide a mechanically static X-ray target which may be used at 10 higher beam power densities than previously possible.

A further object of the invention is the provision of an X-ray target of the class described that facilities removal of the balance of thermal energy in the electron beam at relatively low power density subsequent to passing through 15 the target foil.

A still further object of the invention is to provide an X-ray target that facilitates computation of the average beam voltage without the use of an analyzing magnet.

Further objects and advantages of our invention will ^υ be apparent as the specification progresses, and the new and useful features of our X-ray target will be fully defined in the claims attached hereto.

The prefered form of our invention is illustrated in the accompanying drawing forming part of this applica~'^J tion, in which:

FIGURE 1 is a cross-sectional view taken at a diametric plane through a preferred embodiment of the Xray target as employed with an electron accelerator.

· ^Whi.^{le wc have} shown only the preferred form of our invention, it. should be understood that various changes or .modifications may be made within the scope of the claims attached hereto without departing from the spirit of the invention.

Considering now the invention in some detail and referring lo the illustrated form thereof in the drawing, there is shown an X-ray target 11 secured in vacum sealed relationship to the output end of an electron accelerator 12 and receiving a focused high energy electron _4Q beam 13 emanating from the accelerator. In general the target comprises a thin layer or foil 14 of a dense metal such as gold, molybdenum, platinum or lead, and preferably tungsten.

Unlike previous “thick” targets, the foil 14 presents a 45 relatively. thin depth of material to the beam 13, that is, the foil thickness is preferably in the range of 0.010 in. to 0.015 in. of tungsten so that for beams of suitably high energies, e.g., in excess of 500 kev., the electrons pass completely through the foil. With thin foils of this 5q type, X-ray conversion efficiencies of the order of up to 90% of that possible with “thick” targets are attained. Moreover, target heating is relatively low inasmuch as the beam passes completely through the foil. The major portion of the beam energy is then readily dissipated in 55 the end portion of a cupped absorber block 16 across which the foil is transversely disposed, and in a liquid or gas cooling medium circulated in backing relation to the foil through a coolant channel 17 defined between the foil and end of the block. As indicated in the drawing, 00 the beam is focused to a small diameter spot on the foil 14 and passes completely therethrough, scattering into the cooling medium and the absorber block 16. Very little of the beam energy is dissipated in the foil, the major portion being dissipated in the cooling medium and 05 absorber block.

It may thus be seen that the X-ray output per unit of heat dissipated in the foil target is relatively high compared to conventional thick targets. Hence, more beam current may be focused within a given spot diameter re70 suiting in increased X-ray output. Alternatively, an equivalent beam current may be focussed to a smaller

3,239,706 tron beam of at least 500 kev., a thin foil of dense metal through which a high energy electron beam will pass, completely, said foil having a thickness in the order of .010 inch, means for securing said foil in vacuum sealed 5 relation to the output end of an electron accelerator in receiving relation to a high energy electron beam of at least 500 kev. emanating therefrom, and coolant and absorber means secured in backing relation to the exterior surface of said foil to dissipate the thermal energy of said 10 high energy electron beam, said absorber means being further adapted to permit the passage therethrough of the X-rays generated by the passage of said high energy electron beam through said thin foil.

2. In combination with a particle accelerator adapted 15 to produce an electron beam in excess of 500 kev., an X-ray target comprising a thin foil having a thickness less than .015 inch of high atomic number metal in substantially transverse intercepting relationship with said electron beam, an absorber block secured to said foil 20 and having an end portion in spaced relation to the foil defining a coolant channel in backing relation thereto, said absorber block being adapted to dissipate said electron beam while permitting the passage therethrough of X-rays generated by the passage of said electron beam 25 through said thin foil, means on said block defining inlet and outlet connections in communication with said channel for connection to a source of coolant, and means for securing the distal surface of said foil relative to said channel in vacuum sealed relation to the outlet end 30 of said particle accelerator.

3. An X-ray generator according to claim 2, further defined by said foil being of tungsten and having a coating of inert high atomic number metal at its surface adjacent said coolant channel.

4. An X-ray generator comprising, in combination with an electron accelerator adapted to produce an electron beam of at least 500 kev., a cup-shaped absorber block of low density material, a thin foil vacuum sealed transversely within said absorber block in spaced relation to its 40 closed end said foil being of tungsten and having a thickness of between .010 and .015 inch, coolant inlet and outlet connections carried by said block and communicating with the space between its closed end and said foil, and means for securing the open end of said 45 absorber block in vacuum sealed relation to the output end of a high energy electron accelerator.

³ diameter spot resulting in improved resolution without reducing X-ray output.

In the preferred structure of the target assembly, the absorber block 16 is preferably of generally cylindrical cupped configuration and formed of a low density material such as aluminum or the like. The foil 14 is mounted in vacuum sealed relation transversely across the cylindrical block at a longitudinal position spaced from the cupped end so as to define the coolant channel 17 therebetween. Inlet and outlet connections 18, 19 are provided on the block exterior in communication with the channel 17 to facilitate connection of a coolant source (not shown) thereto whereby water or other liquids or gases may be circulated over the foil surface.

Where the coolant is water or other ionizable material and a tungsten foil 14 is used, a chemically inert surface should be provided on the foil to prevent corrosion and possible premature rupture. To this end, a coating 21 of gold or platinum is electroplated to the coolant side of the foil to form an integral corrosion proof surface which also serves as a useful fraction of the foil in the production of X-rays.

It is particularly important to note that, by virtue of the low density material utilized, X-rays are efficiently transmitted through the absorber block 16 as indicated by the arrow 22, even though the end of the block is thick. Consequently a block having a relatively great end thickness sufficient to completely stop the high velocity electrons still permits the efficient transmission of X-rays, and by virtue of the large volume of absorber material provides a relatively low power density for removal of the thermal energy remaining in the electron beam. The thermal energy is consequently dissipated over a large volume so that the absorber block is maintained in reltively cool condition.

The open end of the absorber block with target foil mounted therein could be directly secured in vacuum sealed relation to the output end of accelerator 12 as by flanged attachment or the like. However, it is preferable that the block be secured to the accelerator by means of an electrically insulated ring 23 which is fused or otherwise vacuum sealed between the block and accelerator vacuum envelope. The block is then insulated from the accelerator structure and can be employed to permit current monitoring of the electron beam 13 striking the foil by connection of an ammeter 24 between the block and ground.

Means 26 can then be inserted in the coolant circuit to compare inlet and outlet water temperatures for monitoring the average beam power. By comparing the beam power reading derived from the means 26 with the beam current reading on the ammeter, the average beam voltage can be deduced, simply, without the use of an analyzing magnet. _rr

The means 26 may consist of any suitable device for ⁰⁰measuring the temperature use of the coolant as it passes through channel 17 and absorbs the thermal energy of the electron beam 13. We prefer to use a conventional calorimeter having temperature sensing probes 26a and 26b in the inlet and outlet connections 18 and 19, respectively. ⁶⁰