US2559526A - Anode target for high-voltage highvacuum uniform-field acceleration tube - Google Patents

Description

y 3, 1951 R. J. VAN DE GRAAFF ETAL 2,559,526

. ANODE TARGET FOR HIGH-VOLTAGE HIGH-VACUUM UNIFORM-FIELD ACCELERATION TUBE 2 Sheets-Sheet 1 Original Filed Sept. 18. 1945 ll? Iii 18 Jive/@211 July 3, 1951 R J VAN DE GRAAFF ETAL ANODE T'AR'GET FOR HIGH-VOLTAGE HIGH-VACUUM UNIFORM-FIELD ACCELERATION TUBE Original Filed Sept. 18. 1945 2 Sheets-Sheet 2 Patented July 3, 1951 UNITED STATES PATENT 2,559,526 ANODE TARGET FOR HIGH-VOLTAGE men- VACUUM UNIFORM- TUBE FIELD ACCELERATION Robert J. Van de Graafl, Belmont, and William Weber Buechner, Arlington, Mass., assignors to Research Corporation, notation of New York Original application Se New York, N. Y., a corptember 18, 1945, Serial Divided and this application December 20, 1949, Serial No. 133,972

3 Claims. (01. 313330) 1 This application is a division of our co-pending application Ser. No. 617,036, filed September 18, 1945, now Patent 2,517,260.

This invention relates to anode targets for high-voltage high-vacuum uniform-field acceleration tubes, and constitutes an entity or structure patentable per se or apart from the remainder of the apparatus for generating an accurately focused beam of charged particles; either electrons or ions, disclosed in our said copending application Ser. No. 617,036, with which,

however, it is advantageously and preferably used. Our invention provides and constitutes an anode target which, when subjected to bombardment by a concentrated beam of such swift particles, melts in part, but still remains mechanically strong and does not disappear as by evaporation, so that the concentrated beam of swift particles always bombards the same material and does not bore through the target and so eventually bombard the target support.

In order fully to set forth the operation as well as the construction of the said anode target, we will describe sufiiciently the apparatus in which it is used and of which it forms a part.

The invention makes possible an improvement in high-voltage radiography and is applicable to high-voltage vacuum tubes. The invention is useful in such fields as nuclear physics, cancer therapy, radiography, high-voltage X-rays, the rectification of high-voltage currents, the production of cathode rays and the acceleration of electrons for high-voltage electric microscopes.

The high-voltage vacuum tube herein disclosed and in which our anode target invention is used and of whichit forms a part, is a constant potential X-ray tube of the order of two million volts suitable for operation either sealed oil or with a continuously operated vacuum pump.

Inasmuch as it is appropriate and desirable to disclose substantially the entire apparatus for a complete understanding of our invention and the operation and advantages thereof, it is to be noted that such apparatus comprises a highvoltage tube of insulating material having electrodes adapted to be connected to a suitable source of high potential, such as a high voltage electrostatic generator. At one end of the tube, bein the upper end as shown in the drawings, is located means providing an emitting source, which means in the present disclosure is a filament having a plane emitting surface of relatively minute area. As will be subsequently described in detail, the wall of the tube comprises electrode rings or ring-like disks or centrallyopen metallic diaphragms arranged along the tube, spaced by insulation such as glass, and connected in suitable manner to the corresponding electrodes in a high-voltage generator in such a way that the potential gradient down the tube is uniform, and in such a way that equal steps in the voltage between successive" electrodes are provided. Thus, there is provided in the tube a substantially uniform electrostatic field.

The focusing of the electron beam by the herein disclosed apparatus and by the use of a substantially uniform electrostatic field, is less sensitive to variations in the potential applied to the various tube electrodes than is the case in tubes employing non-uniform electrostatic fields.

One advantage resulting from the use of a substantially uniform electrostatic field, in combination with a magnetic lens in a high-Voltage vacuum tube, is that thereby extremely fine focusing is obtained of a high-speed electron beam of the order of two million volts. Moreover, it appears that the construction referred to, one embodiment of which is herein disclosed, being simpler, is more reliable than prior constructions. The employment of a substantially uniform electrostatic field is ,materially associated with breaking up the voltage along the tube into very small divisions, which is also desirable from the point of view of insulating very high voltage.

With a uniform electrostatic field substantially the entire cross section of the high voltage vacuum tube can be used for the acceleration of the chargedparticles, whereasin a non-uniform electrostatic field, as heretofore generally employed, the region which is usable for focusing is usually very close to the axis and is most effective only for the paraxial rays. Thus, with the use of a uniform electrostatic field the electrons may be suitably accelerated through a region whose cross sectional diameter is relatively large when compared with the length of the high the electrons is done in a uniform electric field, there is made a full and direct use of the longitudinal component of the electric field, with a minimum interplay or even presence of the transverse component of the electric field, a component in itself useless for the acceleration of the ions in the desired direction.

Heretofore in attempting to focus the electron beam, means have been used constituting a relatively complicated guidance or compulsion. It appears that, both in theory and in practice, if the electrons are permitted to fall in or be accelerated by a simple uniform electrostatic field, the result is more satisfactory than the result obtained with more complicated means, the elements whereof require a certain definite, simultaneous adjustment relative to each other.

In some prior high voltage tubes a part only of the tube had a substantially uniform electrostatic field, but in all such cases known to us the part of the field of such tubes that are nonuniform in character was actually the part that was the most important of all as regards directing the motions of the charged particles. Thus, where in prior instances, a uniform electrostatic field was created in part of a high voltage tube, it was not primarily for the purpose of focusing a beam, but mainly to simplify other features of the construction. An instance thereof is shown in the patent to Trump, No. 2,182,185, above referred to.

In certain other tubes of the prior art the very beginning of the path of the electrons or ions was not in a uniform field and was actually sharply distorted, so that there resulted an initial spontaneous breakdown creating a localized source of ionization by virtue of the fact that the electrostatic field was extremely non-uniform in character. Also in such instances in the prior art, tubes were made for operation with impulses where the voltage was on for periods of the order of only a few microseconds each, and in order to pass sufficient average currents they had to have high instantaneous currents.

The momentary breakdown in the tube afforded extremely high instantaneous currents, so high that the accompanying space charge would tend to distort, during the moment of actual operation of the tube, the uniformity of the electric field in regions which had been uniform just previous to the discharge.

In the apparatus herein disclosed, in order to provide a path for the electrons or charged particles through the high voltage tube, preferably a two-inch diameter hole is cut out of or is otherwise provided at the center of each of the metallic ring-like electrode disks or diaphragms provided along the extent of the tube. Since such successive electrode ring or diaphragm is more and more positive from the filament toward the target, the electrons or negatively charged particles are attracted down the tube and strike the target with an energy corresponding to the full generator voltage. The conditions are reversed when positive ions are to be accelerated. In their passage down the tube, they tend to follow the lines of electric force, and in the high-voltage vacuum tube herein disclosed, the lines of force are straight lines. Consequently when the electrons or charged particles reach the bottom of -the tube and strike the target, they are all traveling in parallel paths and all have the same energy. Such a result could hardly be secured where an alternating current device, such as a transformer, is used for the voltage source, be-

cause in such case the electrons usually have all energies ranging from some indeterminate low value up to that corresponding to the peak of the alternating current wave. There is thus an essential difference between direct current equipment where all the electrons striking the target have the full generator energy, and all alternating current equipment where only a few of the electrons have the full rated peak voltage, the remainder being of lower energy.

The structure herein disclosed is equally well suited for the acceleration of either positive ions or negatively charged particles. This follows since the manner of construction and the use make the tube completely symmetric. Thus, it is possible to accelerate charges in either direction through the tube without the necessity of having to change the arrangement of potentials on the electrodes. The electrons are emitted at the negative end of the tube and are accelerated toward the electron-collecting target, while at the same time positive ions are to be produced at the positive end of the tube and accelerated toward the region of the cathode.

Other things being equal, the diameter of the beam of charged particles after passing through the tube is proportional to the size of the source of the charged particles. When the apparatus is used as an X-ray tube for radiography, the definition in the radiograph depends critically upon the spot size, and hence it is very desirable that the effective portion of the filament be as small as possible. As more fully'set forth in the description of the drawings, the focused spot upon the target can be smaller than 0.01 of an inch in diameter.

To obtain radiographs of thick sections having good definition, the size of the focal spot must be very small so that the X-rays will be emanating from a point source. Thick metallic sections of objects requiring on the order of two million volt X-rays present new geometric problems making essential the use of such size of focal spot. As stated, the high voltage tube operates in conjunction with an electrostatic generator producing a potential of the order of two million volts. The use of such constant potential has been found necessary in order to obtain and to maintain the extremely fine focusing referred to and to provide optimum conditions for heat dissipation at the focal point. As subsequently set forth in detail, the target, upon which the electron beam is focused and to which the present invention is particularly directed, is a thick disk of gold used in association with high pressure water cooling. The use of such a relatively thick target disk permits operation with the target spot in molten condition without, however, melting entirely through the disk. It becomes possible as a result to make full use of the high intensity, sharply concentrated, electron beam and thus to obtain X-ray pictures of greatly improved quality.

The invention will be better understood in detail by reference to the following description when taken in connection with the accompanying illustration of one specific embodiment thereof, while the invention will be more particularly pointed out in the appended claims.

In the drawings:

Fig. l is a vertical or longitudinal, central, cross section of a high-voltage vacuum tube wherein the anode target herein claimed is used and wherewith such anode target cooperates;

Fig. 2 is a transverse or cross section upon the line 2-2 of Fig. 1;

Fig. 3 is a detail invertical central section through the lower end of the filament and the surrounding guard ring;

Fig. 4 is a view similar to Fig. 3, but on a larger scale and representing only a portion of the guard ring; and

Fig. is a broken-away detail in side elevation of a portion of the high-voltage vacuum tube shown in Fig. 1, with a diagrammatic indication of the connections between the electrode rings of the tube and corresponding electrodes of an electrostatic generator.

Referring to the drawings, there is shown a high-voltage vacuum tube consisting of a column of glass rings and of metal electrode rings or ring-like diaphragms or disks suitably welded together in alternation throughout the column, a part only of which is shown in a manner not herein necessary to disclose in detail. When the apparatus is used as an X-ray tube, the electron beam is controlled and compelled to strike the target at a point of exceedingly small diameter.

This configuration of electric field is also well suited for the acceleration and focusing of ion beams.

Fig. 1, the glass rings are respectively indicated at l, and the metal electrode rings, centrally open *diaphragms or disks at 2. The said metal rings 2 or the like are electrode rings and lie accurately placed in planes perpendicular to the axis of the tube, and they are placed at equal distances apart, as, for example,vone-third of an inch in the present disclosure. represented as broken away because of the necessity of presenting a view of the complete tube in a single figure. While obviously the invention is 'not limited to any particular size or proportion of parts, it is pointed out that in the illustrated embodiment of the high-voltage tube the distance in the actual structure from the horizontal line 3' to the horizontal line 4 is fifty-seven inches, the diameter of the opening in each ring 2 is two inches, and the outside diameter of the tube or column is three inches. As clearly shown in Fig. 3, the outer edge of each of the metal electrode rings 2 is substantially coterminous with the outer edge of the glass rings I.

In the simplified form .of high-voltage tube, represented in Fig. 1, the distance from the line 3 to the top of the dome-like glass insulation is about six inches. As stated, however, these dimensions may be varied as found suitable, and the scope of the invention is in no wise restricted by this recitation of dimensions.

In the plane of the top metal electrode ring 2, a metal disk 5 is provided which substantially fills the opening inside said topmost metal ring 2, which disk 5 is maintained at the same potential as the top metal electrode 2. This insures that the electric field immediately below the region of the disk 5 is uniform. The glass insula-.

In Fig. 1, the tube or column is tion which holds the metal disks 2 in correct relative alignment and which consists of the glass rings I, may have on its inner surface an uncontrolled distribution of electric charge which would tend to distort in a random and uncontrolled manner the uniformity of the electrostatic field within the main region of the tube. However, the disturbing charges'is reduced to a negligible degree by the shielding effect of the metal rings 2, which extend influence of these inward from the glass wall composed of the glass rings I toward the axis of the tube to a sufilcient extent to produce the desired shielding. The fact that the gap between adjacent metal rings 2 is relatively small, being in the present disclosure one-third of an inch less the thickness of one disk, the actual structure having the other proportions above specified, makes it possible to obtain the desired shielding efiect with only a relatively narrow region or portion of each metal ring extending inwardly beyond the inner surface of the glass wall composed of the multiplicity of glass rings I.

In the present disclosure the amount that each metal ring must project inward from the glass wall of the tube must be approximately the same as the length of the gap between next adjacent metal rings 2 all along the glass wall ofv the tube. Thus, the fact that in the present disclosure the gap between the next adjacent metal rings 2 is small is in itself advantageous, inasmuch as it reduces the amount that each metal ring 2 must extend inward beyond the inner surface of the glass wall.

In order to obtain a uniform electrostatic field, it is essential that the metal rings 2 be close together. The fact that they are placed close together makes it possible to insulate a high voltage per unit length of the tube.

Moreover, the fact that the metal rings 2 are close together makes it possible to use more of the internal space in the tube for the beam of charged particles.

Certain metal rings 2 of the tube or column, which are indicated at 2a in Fig. 5, are connected to corresponding electrodes of the generating apparatus which may take the form of a highvoltage electrostatic generator, as indicated in the diagrammatic part of Fig. 5 in such a way that the voltage between the successive electrodes of the tube is the same.

In Fig. 5 a few of the generator electrodes are represented at 2b, and a portion of the resistors at 20. As shown, every third electrode ring 2 of the. tube is connected to a corresponding electrode of the generator, which generator electrodes are an inch apart. Each of the metal electrode rings 2 in Fig. 5, as well as in Fig. 1, has its outer edge substantially coterminous with the outer edge of each of the glass rings I.

As has been stated in the foregoing, the acceleration of an electron beam in a'uniform field has many basic advantages as contrasted with the more usual methods of acceleration in strongly non-uniform electric fields. However,

it may be desirable while still using a substan-' tially uniform electric field for acceleration to modify it or warp it slightly, for example, in dealing with certain practical situations which would not arise in an entirely ideal case. In order toovercome the spreading effect, due to the space charge of a positive ion beam, it might be desirable to have the top part of the accelerating electric field slightly converging. This condition could be realized simply by having the voltage difference from electrode to electrode constantin the lower and middle portion of the tube, but with this voltage difference slightly decreasing as the very top of the tube is approached.

Referring to the use of the apparatus as an X-ray tube, the filament of the tube from which emanates the electron beam is indicated at I0 in Fig. 1, and is shown in detail in Figs. 3 and 4. The said filament is composed of tungsten, and is of a hairpin type. It has the apex of the bend ground ofi, as indicated at II in Figs. 3 and 4, in order to provide a plane emitting surface II of relatively minute area. The diameter of the filament in the unreduced portion thereof is 0.010, and at the ground-off portion of the apex of the bend it is desirably less than one-half such thickness, thereby insuring an intense heat at said ground-off portion when the apparatus is in use, being the plane emitting surface of the electrons. The cross section of the filament being the least at the ground-01f portion, the resistance is the greatest at that area.

The filament I has placed in conjunction therewith and encircling the same, a guard ring l2, shown enlarged and in part in Fig. 4, which has a plane lower surface lying exactly in the same plane as the emitting plane of the filament. The said guard ring l2 has therein a central through-opening l2a, which is approximately 0.040 of an inch in diameter and within which the apex of the bend, constituting the plane emitting surface I I, is symmetrically positioned.

The filament and the surrounding guard ring are usually maintained at approximately the same potential. However, by making the potential of the guard ring substantially more negative than that of the filament, the grid action of the guard ring can be used to reduce, or even entirely out off, the electron stream. For this purpose there are shown in Fig. 4 wires la and lib leading respectively from the filament l0 and from the guard ring l2 to the positive and negative sides of a battery B. Also the over-all focusing properties of the tube as a whole may be affected by providing relatively small voltage differences between the filament and the surrounding guard ring. Thus, although the filament and the guard ring have been generally operated at the same potential, there are some occasions when it is desirable to operate the filament and guard ring at somewhat different potentials.

By reason of the plane emitting surface ll of the filament l0 and of the uniform field within the tube column, the beam of electrons proceeds in a substantially straight line along the tube or column from the point of emission, as indicated at l3, resulting in a beam whose cross section in the region near the top of the tube corresponds closely to the size and shape of the emitting plane, and wherein the energy of the individual charged particles is substantially identical. Such a beam may readily be focused by a relatively weak magnetic field on an extremely concentrated spot, as by an electric magnet H, the arrangement constituting a magnetic lens, the magnetic lines of force whereof are indicated at Ma.

Where the apparatus is used for generating X-rays, as for high voltage radiography, the electron beam is focused on a target which is a thick metal disk 15 of gold, used in association with a high pressure water cooling jacket, indicated at Hi, and provided with a water inlet I1 and water outlet la. The target I5 is a gold disk one-quarter of an inch in thickness.

With the usual construction for X-ray targets, a high voltage beam of electrons of great concentration would melt locally the target employed in such construction, and thus cause leakage of the cooling water in the vacuum of the X-ray tube, or cause cracking of the tungsten target and impair its usefulness. This would prevent further use of any such device until repaired. However, with a thick target of material such as gold, which has a high melting point and high heat conductivity, the molten region is small, and since it does not extend entirely through the target, 'no leak is caused. The surface tension of the liquid gold tends to keep the gold from flowing away. It is observed in practice that the 8 vapor pressure of the liquid gold is so low, under the operating conditions that the thinning of the target due to evaporation is negligible.

Although tungsten has generally been used as a standard material for targets, experience with the gold targets herein disclosed indicates certain advantages. Gold has a high heat conductivity and also chemical and physical properties such that it can be repeatedly melted and allowed to freeze without appreciable oxidation or change in physical structure.

Since the efficiency of X-ray production rises rapidly with the atomic number of the target material, x-ray tube anodes are commonly made of the heavy metals. Tungsten has been the traditional material for this purpose, primarily because of its very high melting point. In the usual low-voltage tube, the target is quite often allowed to operate at white heat, and, when cooling is necessary, the target is usually embedded in a massive disk of copper that may be cooled either by water or by an air blast. In such low-voltage tubes, the penetration of the electrons into the target material is so slight that the energy is delivered essentially to the target surface, and radiation from the surface plays a very large part in dissipating the heat energy so generated.

Such tungsten targets may also work satisfactorily for tubes operating in the million-volt range if the focal-spot size is large enough so that the power density on the target is not excessive. However, accelerating tubes of the uniform-field type can deliver an electron beam so concentrated that the power density on the target is suflicient to melt any material. With these high-energy densities, it is essential that the heat be transferred as rapidly as possible from the focal point to the surrounding unbombarded target material. This requires a target material having a high conductivity rather than a high melting point. Gold has twice the thermal conductivity of tungsten, and, in addition, has a higher atomic number. Moreover, its other physical properties, such as ease of soldering, malleability, and so forth, give it many advantages over tungsten for this particular application,

Unfortunately, those materials that are most suitable for the efficient production of X-rays are also the best X-ray absorbers. For low-voltage tubes, the X rays produced are not sufficiently penetrating to pass through the target structure; hence, the radiation is commonly brought out through the side of the X-ray tube. This is not a serious limitation, since, at these low voltages, the X-radiation has a spatial distribution that is essentially symmetric about the position where the electron stream strikes the target. This is not the case for high-voltage X-ray tubes, since here the radiation is produced primarily in the direction of the electron beam. For this reason and also because it is generally more convenient from the point of view of construction, the radiation from such high-voltage tubes is allowed to pass directly through the target structure. To reduce the absorption of the radiation in the target, it has been customary to make the target as thin as possible. There are numerous references in the literature to thin targets which are cooled on the side away from the vacuum by a stream of water or air. Such thin targets have been found to be unsatisfactory when used with concentrated electron beams, such as those produced by the uniform-field accelerating tube herein disclosed. The high current densities employed on the targets herein disclosed are sufficient to melt the material, with the result that the pressure of the cooling medium was sufficient to force a hole through the target, thus permitting the cooling medium to enter the highvacuum tube. In an attempt to prevent this, previous workers had made the targets even thinner in an attempt to bring the cooling medium closer to the region where the heat was being produced. Such attempts were not successful, and our investigations and experimentsindicated that the best hope of success was to make the targets so thick that even if local melting occurred in the region of the focal spot. there was still sufii'cient metal between this focal region and the cooling medium so as to prevent puncture. At first sight. it would appear that this additional target material would involve a'serious reduction in the beam intensity, but our work on the problem of the eflicient utilization of this radiation in the problems of radiography and therapy showed that the additional filtration provided by the thick targets was actually beneficial. and experiments have shown that, were it not for the filtration provided by the thick target, it would be necessary to put additional absorbing material in the path of the radiation proceeding from the tube.

We have in accordance with our invention provided in a high-voltage high-vacuum tube for generating an [accurately focused beam of charged particles 'of great concentration upon a minute area, a thick anode target composed wholly of a metal having a high atomic number, the said target, being sufiiciently thick so that even though under the action of the highly concentrated beam of charged particles impinging on it. the target material becomes molten at the point of impact, sufficient solid target material still remains surrounding the molten region to prevent mechanical failure, and to which the molten material adheres by reason of surface tension. The said anode target is too thick to permit the passage of an accurately focused beam of charged particles therethrough, it having a thickness on the order of one-quarter of an inch, and

I hence is not of a thickness sufficient to prevent the passage of high-energy X-rays. The said thick, high atomic'number, metal-anode target under the action consequent upon the passage of the charged particles therethrough becomes molten at the said minute point of impact, but the surface of the said metal-anode target there retains its position by reason of the high surface tension on the high atomic number metal while molten and because of the said thickness of said metal-anode target. Thus the said metal-anode target is prevented from melting through under the impact of said focused beam of charged particles.

As already stated, we construct the said anode target of gold.

The present invention comprehends a highvoltage vacuum tube adapted to the acceleration and focusing of charged particles, and in the case of electrons this beam is extremely concentrated. The disclosure includes charged particle accelerating means providing a uniform accelerating field. thus reducing to a minimum the dispersion of the charged particles throughout their travel. Therefore, a large number of accelerating sections are provided, the number used in present of said electrodes of which each such group is In fact, investigations composed is directly electrically connected to a correponding electrode of a high-voltage electrostatic generator, so that the voltage between the successive disks of the tube isthe same.

Having thus described one illustrative embodiment of the invention, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims. We claim: 1. In a high-voltage high-vacuum tube for generating an accurately focused beam of charged particles of great concentration upon a minute area on the order of 0.01 of an inch in diameter, a thick anode target composed wholly of a metal having a high atomic-number, said anode target being on theorder of one-quarter of an inch thick so that, even though under the action of the highly concentrated beam of charged particles impinging on it, the anode target material becomes molten at the point of impact, sufficient solid target material still remains surrounding the-molten region to prevent mechanical failure, and to which the molten material adheres by reason of surface tension, the said anode target being thus .too thick to permit the passage of an accurately focused beam of charged particles therethrough, but not too thick to prevent the passage of X-rays therethrough.

2. For the production of high-voltage radiographs of very high quality, to be taken at relatively high speed through heavy objects,.a highvacuum acceleration tube in association with a high-voltage generator of the order of a million or more volts for generating an accurately. focused beam of electrons upon a minute area approximately of the order of 0.01 inch in diameter, such beam having a very greatpower density throughout its cross-sectional area, the said acceleration tube having a target anode composed of gold and having athickness on the. order of one-quarter of an inch, and having a water-cooling jacket, the said target being usable at temperatures farabove the melting pointlof gold,

said target material dfgOld becoming molten at the point of impact; but remaining sufficiently solid surrounding the molten region to prevent mechanical failure, such as through penetration, the said gold anode target being therefore too thick to prevent the passage of the accurately focused beam of charged particles, but not of a thickness to prevent the passage of high-voltage X-rays.

3. For the production of high-voltage radiographs of very high quality, to be taken at great L ticles upon a minute area on the order of 0.01

of an inch in diameter as a minimumsuch beam practice for two million volts'being approximately 180, thereby providing uniform accelerating steps of 12,000 volts each. Thus, in such embodiment having a verygreat power density throughout its cross-sectional area, the said acceleration tube having a target anode composed of gold and having a thickness on the order of one-quarter of an inch, and having a high-pressure water-cooling jacket, the said target being usable at temperatures far above the melting point of gold, said target material of gold becoming molten at the point of impact, but remaining sufliciently solid surrounding the molten region to prevent mechanical failure such as through penetration, the said gold anode target being therefore too thick to prevent the passage of the accurately focused beam of charged particles, but not of a thickness to prevent the passage of high-voltage X-rays.

ROBERT J. VAN m: GRAAFF.

WILLIAM WEBER BUECHNER.

REFERENCES crrEn The following references are of record in the flle of this patent:

UNITED STATES PATENTS Number

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