US 2760077 A
Description (OCR text may contain errors)
1956 R. LONGINI SPIRAL X-RAY IMAGE INTENSIFIER Filed March 29, 1952 INVENTOR Richard L. Longini ATTO R N EY WITNESSES:
United States. Patent 2,760,077 SPIRAL X-RAY IMAGE'NTENSIFIER Richard=L.=Longini, Pittsburgh, Pa., assignor toWesting house Electric Corporation, East Bittsburgh, Pa.,.a. cor:
My invention relates'to X ray devices and in particular, relates to an improved form of B'uckygrid for X-ray apparatus.
The images appearing on the fluorescent 'screens of X-ray apparatus normally are mo'duced by X-rays traveling in straight linesfrom the focal spot on the anticathtode of the X-ray' tube. Actually, however, there is incid'en'tuponthe screen a certain-amountof-st'ray X-radiation emanating from other points than this focal spot-g thisbeing' scattered radiation from atoms= lying inthe path of the main stream of X-rays; For example, the object being observed intervenes between the focal spot and the'fluorescentscreen, and each atomof'such-a body becomes apotential source of secondary X rayswhich scatter, some of these striking the fluorescent'screerr from adirectionout of line with rays coming from thefocal spot. The glass wall of an enclosure around the fluorescent scree'n isi another source of such raysi Thez fluores- 6 nt screen intlle' X'ra'y image-intensifier'sliown in- Mason and Col'tmanU. S. Patent 215 233132 is aspecific' instance of this; such scattered X-ra'di at ion blurs" the image pro du'ce'd by the primary X-rays from the focal spot and so its elimination is desirable; In general, a device known as a Buck-y gri may be used for this purpose. A Bucky gr id may be described as a network'ofi spaced thin plates of material positioned with their edges toward the focal spot so that X-rays: emanating fronr the-focal spot find free passage between the plates, but rays emanating from other directions strike and are absorbed by'the faces oflthe plates;
My present invention resides in an arrangement in which the absorbingplatehas the form of'a spiral wound up frorn a thin ribbon of heavy metat faced withsa. substance of low atomic weight, the ribbon being, so formed that the face of the ribbon: is at all points paral'leltothe path: of X-rays from the focalspot.
One object of my invention isr accordingly torprovide a new and improved arrangement for reducing: stray radiationin X-ray apparatus.
Another object isto provide a new and improved method of forming adirection: filter for X-rayapparatus.
Still another object is toprovide an improved form of X-ray image intensifier.
Another object is torprovide a Bucky grid capable of being-enclosed in a vacuum tube and of being. baked at sufficiently high temperature to' processthe tube.
Other objects will become apparent upon reading the following description and claims in which:
Figure 1 is a schematic showing of. an'X-ray image intensifier of the type described in the above-mentioned Mason and Colt-man patent which isprovi'ded with a stray radiation absorber embodying the principles of my invention;
Figure 2 is a detailed View in section of the radiation absorber of Figure 1 at anintermediate point in itsprocess of manufacture; and r Figure 3 is a plan view of the same absorber.
Referring in detail to "the drawings, a conventional .lCc
2. X-ray'tub'e 1 projects it-s rays'through abody 2 into-in cide'nce with" a' fluorescentscreen 3 of an imageintensifier tube- 4 of th'etypeshown in the above-mentioned Mason and Co'ltman'Patent 2523113 2. The screen 3-is shownfas having-the form-of a watch glass, though a planescr'e'en maybe use d; Between the screen 3 and-the endwall of the tube dis the stray-radiation absorber 5' shown in more detail'inFigures'2 and3;
'Fhescreen 3": hasa l'aye'r'of fluorescent material closely spaced from alayer of photoelectric material. X rays projectedfioni the focalpoint of tube 1 produce a picture of body 2 on the fluorescent screen 3, and the'light rays from the latter in turn produce a corresponding electron image at the surface ofthephotoelectric layer. Suitable e'l'ectrical potentials accelerate the electron image int o: impact with an el'ect'ron phosphor' screen 6' at the other end of tube 4 to produce a light picture for viewing at greatly enhanced brightness by an observer.
As'pr-eviousl'y pointed out, theprirnaryX-rays from the fiocusof tube" I generatesecondary- X-rays' which scatter in! all directions" from atoms such as those in" the glass end of tube 4,or those} composing body 2; Thusffor example, an Xray projected fromfocal point Fthrougli point 7 ofbody- 2 would normally register an image of point Ton screen d 'at pointfl. However, aeertaiii num berof X-rays traversing the direction 1 -7 woulid fb'e absorbed by an atom at point 9 in the end wall of tub 64 or'point 7' of th'e composing body; toscat-ter'X-raysinall sorts of directions such, fol-"example, as9-11". The latter would thus produce light at'point 1'1 superposedon', and confusing the image of point 12 ofbody 2 produced also at'p'oint 1 1' by theprimary ray F1-2 '-1 1;;.
The element- 5 is provided to 'perm'i't primary='X rays such as 7-8 and- 12--1"l-' fronrfocal point to pass through to: screen 3*whiie absorbing; such rays as 9 1 1 or 7--1ll It comprises a thin ribbon '15 ofmaterial having atoms of high atomicjweight, andso-great absorp tivepower for X-rays, wound into a spiralwitl'r its, turns separated by spacing material 16 of low atomib we'igh t andhence' ofl'i'ttl'e' X-ray 'absorbingpower. The metals gold; tungsten, tantalum, and molybdenum are ofsuitabl e absorptive power for ribbon 15. The thinedges of 'the ribbon E5 face the X-ray' sourcel and the ribbon isdis posed so that its faces coincide substantially with radial lines radiatingf'roni the focal spot of'tube 1. The spacesj between turns of the ribbon are large relative" to the thicknessof the ribbonbut small relative to its width; Preferably the edges of the-ribbon adiace nt screen '3ris parallel and close to the back'face of thelatter; It will be seen that most primary rays like F7 and-F-IZ radiatlng from the focahspot will pass through materiaf lffi between turns of ribbon 1'5, but scattered secondary rays like 9-41, being of non-radial direction, will strike one point or another on the face of ribbon 15 and be absorbed bgtore reaching screen 3 and confusing the primary image t ere.
In order to remove boththe 9'-1Z1 and the 7 *1'1 type scattered rays it is necessary to enclose the Bucky grid" in the envelope of the tube. The conventional grid needs only to remove '711 type scattered radiation. Because this new grid must be placed in the envelope, it mustbe composed ofrefractory materials to withstan'd'thef prob essmg to which components of high-vacuum apparatus must be subjected. i l
A structure in which the spiralled ribbon is tilted so that its surface lies in radial lines from thefocal "spot and which is composed only of refractory materials-may be formed by several methods, some of which'willnow be described. A plastic material contaihing onlyfatqms of low atomic weight is'first produced, For .egrample, carbon powder of particle size passing a 300 mesh-p inch screen may be mixed with the niinimumhuantity of polyvinyl alcohol and ethyl borate which will make a plastic or dough-like mixture. Instead of ethyl borate, other similar substances such as borax, or other glassy low atomic weight material such as a phosphate, may be used. Still another alternative would be to use a plastic mix of magnesium oxide with a silicone binder. A coating, say 0.03 inch thick, is then formed from the dough on one side of the metal-absorbing ribbon to be used. This may be tantalum, tungsten, molybdenum, or other high atomic weight metal, say A; inch wide by 0.005 inch thick. An Archimedean spiral is then wound up to the desired diameter from this compound ribbon. A ring of metal is then put around the spiral to hold it together. After further forming to tilt the ribbon sides to conform to radial lines from the focal spot as described below, the spiral is baked at high temperature in a suitable atmosphere to reduce the dough to nearly pure carbon held in a porous matrix held together by a glassy binder, which also holds the separating layer to the metal ribbon. The stray-radiation absorber is then complete and ready for installation in front of the screen 3.
Where the screen 3 has a plane back surface, rolling the ribbon into a plane Archimedean spiral will insure that the front edge of the ribbon conforms to the slope of the screen. Where the screen 3 has a Watch glass form, the required shaping of the unit may be attained by pressing the spiral, before baking, into a mold such as 17 in Fig. 2, or by winding the ribbon into a spiral while maintaining its edge in contact with a spherical guide surface.
Formation of the ribbon so that its side faces comprise lines radiating from a common focal center may be effected in several ways. One of these would, for instance, be to roll the ribbon of dough-like carbonaceous material thinner on one edge than the other, then using the thin edge as that positioned to face the X-ray focus. For many cases it would suflice to make the thin edge about 99 per cent of that of the thicker edge. When wound into a spiral and baked out as described above with a heavy metal ribbon of uniform thickness, the turns of the heavy metal ribbon will be radial to a common focus. Another method would be to use a heavy metal ribbon which was rolled thinner on the edge which is intended to face the X-ray focus than on the other edge. When rolled into a spiral as above described with a carbonaceous ribbon which is either uniform or tapered in thickness, the turns will have faces radiating from a common focus; however, in view of its thinness, the taper of the metal ribbon will usually have to be greater than the one per cent mentioned in the preceding method.
A third method which may be used either with ribbons of uniform thickness or, as a supplement, with the tapered ribbons of the method just described, is to expose the edge of the coiled spiral which is to be adjacent the screen 3 to vapors which will cause the plastic material to swell. To facilitate this, the screen 18 may be provided to align the concave face of the unit in Figure 2. For example, water vapor will cause polyvinyl alcohol to take on a permanent swelling. Silicon tetrachloride, various silicones, and other substances which would leave non-volatile residues of low atomic weight are other examples of substances suitable to cause swelling. Still another way would be to use ordinary rubber as the binder between the carbon particles, and to use silicon tetrachloride vapor to cause swelling on the side of the filter remote from the X-ray source.
It will be noted that the substancess mentioned above as components of the material spacing the turns of heavy metal are all of low atomic weight, chlorinebeing of the largest weight.
I claim as my invention:
1. An X-ray direction-filter comprising a substantially spiral ribbon containing atoms of high atomic Weight, the spaces between its turns containing carbon particles bound with a substance having no atom of atomic weight substantially greater than that of chlorine, and the broad faces of said ribbon radiating from a common center.
2. An X-ray direction-filter comprising a substantially spiral ribbon containing atoms of high atomic weight, the spaces between its turns containing carbon particles mixed with a boracic binder, and the broad faces of said ribbon radiating substantially from a common center.
3. In combination with a source of radiation a vacuum-tight enclosure containing a screen and a direction filter traversed by radiation projected to said screen through the Wall of said enclosure in diverging rays from said source, said screen and said source lying on opposite sides of said filter, said radiation filter comprising a convolute ribbon containing atoms of high atomic weight. the spaces between its convolutions being substantially free of atoms of high atomic weight, and the broad faces of said ribbon radiating substantially from said source.
4. A vacuum-tight enclosure containing a screen and a direction filter for radiation projected to said screen from a central point, said radiation filter comprising a convolute ribbon containing atoms of high atomic weight, the spaces between its convolutions containing carbon particles mixed with a substance having no atom of atomic weight substantially greater than that of chlorine, and the broad faces of said ribbon being radial to said central point.
5. A vacuum-tight enclosure containing a fluorescent screen and a direction filter for radiation projected to said screen from a central point outside said enclosure, said radiation filter comprising a convolute ribbon containing atoms having a high absorptive effect on said radiation, the spaces between its convolutions being substantially free of atoms having a hi' h absorptive effect on said radiation, and the broad sides of said ribbon radiating substantially from said central point.
6. The method of making a radiation direction filter which comprises winding into a convolute a composite ribbon made up of a first layer of material which is highly absorptive to said radiation faced on one side with a second layer of material which is substantially less absorptive to said radiation, exposing one edge of said composite ribbon to contact with a substance which causes expansion of said second layer, and then hardening said composite ribbon in the conformation so obtained.
7. The method of making a radiation direction filter which comprises winding into a spiral a composite ribbon made up of a first layer containing atoms of high atomic weight faced with a second layer of heat-hardening material which contains substantially no atoms of high atomic weight, exposing one edge of said composite ribbon to contact with a substance which causes expansion of said second layer, and then heating said composite ribbon to heat harden said second layer.
8. The method of making an X-ray direction filter which comprises winding into a spiral a composite ribbon having a first layer of metal drawn from the group which consists of gold, tungsten, tantalum and molybdenum faced on one side with a second layer of heat-hardening material containing substantially no atoms of atomic weight greater than that of chlorine,. exposing one edge of said composite ribbon to contact with a substance that expands said heat-hardening material, and then heating said ribbon to harden said second layer.
9. The method of making a radiation direction filter which comprises winding into a spiral a composite ribbon of a first layer of material having a high atomic weight faced on one side with a second layer of material containing substantially no substance of atomic weight substantially greater than that of chlorine, the last said material being heat-hardening, one edge of one of said layers being thicker than the opposite edge, and heating said ribbon to harden it.
10. The method of making an X-ray direction filter which comprises winding into a spiral a composite ribbon having a first layer of metal drawing from the group which consists of gold, tungsten, tantalum and molybdenum and a second layer of carbon particles in a binder of polyvinyl alcohol and ethyl borate, exposing one edge of said ribbon to contact with water, and then baking said spiral to hardness.
11. An X-ray image intensifier comprising a vacuumtight enclosure, a fluorescent screen therein, means positioned adjacent said screen for producing an electronimage corresponding to a light-image generated on said screen, means positioned on the opposite side of said electron-image producing means with respect to said screen for producing a second light-image by impact of the electrons within said electron-image and an assembly of members inside said enclosure, said members positioned adjacent to said screen and on the opposite side of said screen with respect to said electron-image producing means, said members containing atoms of high atomic weight and being separated in directions parallel to the surfaces of the said screen by intervals containing only atoms of low atomic weight, said intervals being small relative to the length of said members in the direction normal to such screen.
12. An X-ray image intensifier comprising a vacuumtight enclosure, a fluorescent screen therein, means positioned adjacent said screen for producing an electronimage corresponding to a light-image generated on said screen, means positioned on the opposite side of said electron-image producing means with respect to said screen for producing a second light-image by impact of the electrons of said electron-image, and an assembly of members within said envelope positioned adjacent to said screen and on the opposite side of said screen with respect to said electron-image producing means, said members containing atoms which efiectively absorb X-rays separated in directions parallel to the surface of said screen by inter vals which are small relative to the length of said members in a direction normal to the surface of said screen, said intervals containing only matter which is an inefiective absorber of X-rays.
13. In combination with a source of radiation, an enclosed device containing means to intensify an image produced by projecting said radiation through the walls of said device on a receiving surface within said device, said radiation being capable of generating secondary radiation in passing through said walls and an assembly of members inside said device containing atoms of high atomic weight and positioned between said source and said receiving surface, said members being separated in directions normal to the path of projection of said radiation by intervals which contain only atoms of low atomic weight, said intervals being small relative to the length of said members in the direction in which said energy is projected.
References Cited in the file of this patent UNITED STATES PATENTS Collins et al. May 26, 1953