US3508057A - X-ray sources and methods of making the same - Google Patents

Description

B. KEISCH 3,568,057

X-RAY SOURCES AND METHODS OF MAKING THE SAME April 21, 1970 Filed Jan. 28, 1965 Fig.|.

United States Patent US. Cl. 250-84 12 Claims ABSTRACT OF THE DISCLOSURE A radioactive source made up of a radioactive material within a sealed container having a thin wall or layer of metal defining a portion of a wall of said container integral with the adjacent portion of said container and capa- Me of passing radiation from said radioactive material.

This invention relates to X-ray sources and methods of making the same and particularly to a method of making point sources of X-rays capable of spontaneous radiation.

During recent years, efforts have been made to develop X-ray sources in which the radiation is emitted spontaneously in the decay of radioisotopes instead of by conventional X-ray tubes. The aim of such efiorts has been to obtain a point source of radiation which has small dimensions and can be operated independently of a source of electric energy. Such a source would be advantageous whenever there was a need for a portable device, as in athletic and military field applications, and also for radiography from within cavities having confining geometries.

Moreover, certain radioisotopes have radiation characteristics which are superior to the X-rays emitted from a conventional tube. These are radioisotopes which emit essentially monoenergetic X-rays of high intensity in the energy range from -25 to 100 kev. At the energy level of -25 kev., silver-based photographic emulsions achieve an optimum sensitivity because of the presence of the socalled critical absorption edge at that energy. As conventional emulsions used for roentgenography are silverbased, it is desirable to select a radioisotope in the indicated energy range.

The selection of an essentially monoenergetic X-ray source of the indicated energy is desirable for two reasons. First, resolution on the film is optimized because the opaque substances are not penetrated to the degree that they would be if higher energy radiations were also present. Second, if the X-rays are being employed in a medical application, the radiations are most efficiently utilized, with a resultant minimization of radiation dose to the individual.

A further consideration in the selection of a suitable X- radiation source arises from the nature of the material being radiographed. Where great penetrating power is needed, as in the radiograph of thick metal welds, a highenergy radiation source is required to penetrate the object. In the case of applications involving materials of low atomic weight, such as biological applications, radiations of about 30 kev. provide the desired contrast between soft (e.g., muscle, viscera, etc.) and hard (e.g., bone) tissue with minimal radiation dose to the individual. The presence of substantially more energetic radiation components markedly reduces the contrast, and hence the quality of the X-radiograph obtained.

Several radioisotopes may be considered to be essentially monoenergetic sources of X-rays in the indicated energy range. Among those which have hitherto been proposed for use as isotopic X-ray sources are iodine-125, promethium-147, ytterbium-l69, and americium-241.

3,508,057 Patented Apr. 21, 1970 While all of these may have utility, I have discovered that no one of them combines the characteristics of high specific activity and freedom from higher-energy radiations as satisfactorily as tellurium-125m, which has not previously been reported as a possible X-ray source. Promethium-147 is not suitable to point source production because of the inefificiency of photon production characteristics of the beta decay process; moreover, the bremsstrahlung radiation forms a continuum up to about 200 kev. and the presence of the higher-energy radiations reduces contrast markedly. Ytterbium-169 has a complex decay scheme characterized by several higher-energy radiations (e.g. 110, 130, 198, 261 kev.) which cause blurring. Americium-241 is available with such low specific activity that the preparation of intense point sources is not feasible. Iodine-125 and tellurium-125m, the decays of which produce essentially similar radiations, mostly tellurium K X-rays, are free from the limitations described above. Their radiations lie closer than any of the others to the absorption edge for silver, their radiations are more nearly monoenergetic than those obtained from promethium-147 and ytterbium-l69 sources, and unlike the others they are capable of being made into intense point sources according to my invention. Tellurium-125m does have a higher-energy peak, at kev., but it accounts for only 0.3 percent of the radiation and does not significantly affect radiograph quality. Iodine-125, though free from this radiation is generally accompanied by a small (about two percent) and not very significant amount of higherenergy radiation from an impurity (iodine-126).

In preparing a source using the selected radionuclide, it is important that the radioactivity be highly concentrated, with the area of the emission kept as small as possible. This is necessary in order that good resolution can be achieved with a minimum quantity of radioactivity being employed. It the area of the source is large, a greater distance must be placed between source and object in order to avoid blurring. The intensity of the source required increases as the square of the distance between source and object. Moreover, a need to place the source at a point remote from the object may destroy its usefulness in certain applications, as in intracavitary uses (e.g., in the mouth for dental roentgengrams) where the dimensions of the cavity are confining.

The source area which is required to achieve a given photon flux is dependent on several factors. In considering these, it should be noted that the radioactive material in the source and the matrix in which such material is contained may absorb some of the radiation. Accordingly, as the thickness of the source or matrix is increased, a greater percentage of the radiation is absorbed. This imposes a limitation on the practical thickness of a source.

The factors which will limit the efficiency of producing a flux within a stated thickness are of two types, those which relate to the production of photons and those which relate to their absorption. An isotope such as promethium-l47 produces photons only as bremsstrahlung accompanying the emission of beta particles. This process is known to be highly ineflicient. Another reason that X-ray production may be limited is because the number of disintegrations occurring is relatively small per unit of mass; i.e. the specific activity is low. The longer the half-life of a material, the lower its maximum possible specific activity will be.

The degree to which X-ray absorption will occur is dependent upon the mass of the source (per unit area) and the atomic number of the material therein. Absorption is more likely to occur where the source consists of a material of high atomic number such as americium than where a negligible mass of iodine or tellurium is incorporated into an inert matrix of material having low atomic number, such as carbon.

It is also desirable that the radioisotope be utilized in a manner which will prevent any movement of the radioactive material from the source itself. The reason for this is that persons handling the source should not become contaminated by radioactive material, which might be taken into the body. Governmental regulations prescribe stringent tolerances covering the amount of radioactive material which may be removed from the surface of an X-ray source which uses a radioisotope as the radiation emitter.

Several attempts have been made to develop an X-ray source having the desired characteristics, but these have met with limited success.

Thus, sources have been described in the literature in which compounds of various radioisotopes are encapsulated. The materials studied have included Pm O AmO and NaI, with Pm-l47, Am-241, and I-125 as the radioisotope respectively. None of these materials operate effectively as a point source, however, owing to low specific activity and resultant self-absorption. For example, in the case of the iodine-125 source, the sodium iodide consists primarily of stable iodine 1-127); this material, having an atomic number of 53, causes serious absorption of the irradiation from the 1-125 and reduces the photon flux.

Attempts have also been made to form a high specific activity source by plating a radioactive isotope such as iodine-125 onto the surface of a noble metal member such as silver, platinum, or gold, which member is placed within a shield. Such sources are of limited value because the radio-activity is on the surface of the source and is not firmly bound, with the result that removable contamination exceeds permissible levels.

Furthermore, attempts to protect the source and to prevent its removal from the supporting material by suitable coverings have failed. The use of thin plastics is not sufficient to prevent source leakage. Metallic coatings are difficult to apply in thin layer and to seal using no organic cements (which Would leak). To produce an all-metallic covering and seal is difficult and heretofore unsuccessful because the application of sufficient heat to effect such a seal (by soldering or welding) tends to volatilize the source material.

I have discovered an article and method of manufacture which elfectively provides a source which is readily contained, easily produced, and which does etfectively act as an approximate point source of X-ray radiation of an energy close to, but slightly above, the absorption edge for silver.

The source container design does effectively permit complete enclosure of the source with a metal without gross attenuation of the X-radiation and without danger of source material volatilization. This is accomplished, first by designing the container so that a window (a section of metal sufficiently thin to allow the passage of X- rays with "only insignificant attenuation) is made integral with the body of the source container and second, by thermally isolating the portion of the container to be sealed by soldering or welding from the portion of the container in which the source is located. Thermal isolation consists of providing only a thin walled section of metal having relatively low heat conductivity for most of the distance between the seal and the source, and filling the intervening space with an insulating material such as powdered aluminum oxide. In addition, the design also easily permits the application of a massive heat sink at the source location during the sealing process.

High specific activity is maintained by taking the radioisotope, and either incorporating it in an absorbent matrix of a material with low atomic number (thereby avoiding the necessity to add stable carrier) or by depositing it on a suitable surface which may be placed within a source container of my design.

In this way several curies of a radioisotope such as tellurium-l25m or iodine-125 may be deposited without difficulty in an area having a diameter less than two millimeters in such manner that less than five percent of the radioactivity is lost by self-absorption. A two hundred millicurie iodinesource of this type with the radioisotope adsorbed into a charcoal grain would provide a photon flux at a distance of one meter of approximately 6x10 photons per square centimeter (after considering approximately 10% absorption by the window of the source holder); this compares with a maximum practical flux from a promethium-l47 source (970 curies) with a diameter of seven millimeters of approximately 5x10 photons per square centimeter per second at one meter. Comparing these two sources, it should be noted that at the given intensities my source would provide better resolution (at the same distance from the object) because of the smaller area, that to achieve only the same degree of resolution as characterizes the promethium-l47 source my source may be brought closer to the object with a corresponding reduction in radioactivity used or exposure time required, and that the flux achieved from a source of my design of a given area may readily be increased simply by using more of the radioactive isotope.

In a preferred practice of my invention, I take a radioactive nuclide such as tellurium-lZSm, iodine-125 or the like, capable of providing a relatively high flux of substantially monoenergetic X-rays from a radiation source of small area (a point source), I deposit said nuclide so as not to absorb a substantial portion of the emitted radiation within its body in a matrix such as a charcoal grain, charcoal-impregnated paper, silica gel or the like having the desired small dimensions capable of such absorption and I place the radioactive material (and its surface or matrix) within a source holder of a metal of low atomic number such as beryllium or aluminum or the like such that the radioactive material is in a fixed position and is directly adjacent to the window section of the source holder, said section being constructed as an integral part of the source holder without any welded, soldered, or cemented joints and being sufilciently thin so as to absorb only a small portion of the emitted radiation, and I fill the remainder of the source holder with a heatinsulating non-volatile, non-combustible, chemically inert material such as powdered aluminum oxide or powdered silica or the like. I then place the source holder in a position so that the end containing the radioactive material is in close thermal contact with a massive heat conduotor which can be designed so as to provide a shield against radioactivity as protection to the worker. In order further to inhibit heat transfer, a portion of the source holder remote from the radioactive material may be reduced in thickness. The open end of the source holder is then sealed, as by crimping and then welding or soldering so as to obtain an all-metal seal. The design of the source holder is such that collimating shielding may be used either inside or outside of the source holder, if desired.

In the foregoing general description, I have set out certain objects, purposes and advantages of my invention. Other objects, purposes and advantages will be apparent from a consideration of the following description and the accompanying drawings in which:

FIGURE 1 is a vertical section through an X-ray source according to my invention;

FIGURE 2 is a section on the line IIII of FIG- URE 1;

FIGURE 3 is a vertical section through a second embodiment of X-ray source according to my invention; and

FIGURE 4 is a vertical section through a third embodiment of X-ray source according to my invention.

Referring to FIGURES 1 and 2 I have illustrated a gold wire 10 (about inch in diameter) having electroplated upon one end thereof tellurium-125m 15. The gold wire 10 lies within an inverted well 14 of lead which serves to hold the wire in a desired fixed position and provides collimating shielding. The lead-wire assembly is then placed within a cylinder of aluminum 12. The cylinder 12 is drilled to form the shell 11 and having a thin film of aluminum at the bottom forming a window 13 approximately 0.005 thick integral with it. The shell 11 is filled with aluminum oxide (Al- 17 to act as a thermal insulator and an aluminum cap 18 is welded in place to prevent the shielding from being opened by unauthorized persons or by accident and to completely surround the radioactive material in a sealed metal container. A portion 19 of the cylinder 12 remote from the gold wire is reduced in thickness prior to the welding operation in order to inhibit heat transfer during welding.

In FIGURE 3 I have illustrated a second embodiment of my invention in which those parts which are identical with parts of FIGURES 1 and 2 bear numbers corresponding to those set forth in FIGURE 1 with the sufiix a. In this embodiment, there is substituted for the leadwire assembly a cylindrical plug of gold having an inverted well 16. The tellurium-125m is electroplated onto the interior bottom surface 20 which has a diameter of approximately of an inch. The remaining surface 21 of the metal is coated with a non-conducting material during the electroplating operation.

FIGURE 4 is a vertical section through an X-ray source according to a third embodiment of my invention in which those parts corresponding with those of FIGURES 1 and 2 bear like numbers with the sufiix b. In the drawing, I have illustrated a charcoal grain 22 (about A inch in diameter) having adsorbed therein iodine-125. The charcoal grain 22 lies within a closely fitting cylinder of beryllium having a thickness of about A inch for the shell 11b and window 13b. The shell 11b is filled with aluminum oxide 17b and a brass disc 23 is soldered in place. Shielding and collimation, if desired, are provided outside the cylinder.

In the foregoing specification, I have set out a preferred practice and embodiment of my invention. It will be understood, however, that this invention may be otherwise embodied within the scope of the following claims.

I claim:

1. An X-ray source comprising a nuclide which emits substantially monoenergetic X-rays having an energy level in the range 20 to 100 thousand electron volts adsorbed onto a member of the group consisting of charcoal grain, charcoal impregnated paper, and a silica gel grain.

2. An X-ray source as claimed in claim 1 wherein the nuclide is a member from the group consisting of iodine- 125 and tellurium-125m.

3. An article of manufacture comprising a radioactive material within a metallurgically sealed container surrounding said radioactive material with a mass and density sufficient to attenuate radiation from said radioactive material in all directions except one and having a thin walled portion forming an integral wall of said container capable of passing radiation from said radioactive material in said one direction.

4. The article of manufacture of claim 3 in which the radioactive material is deposited on a metal surface.

5. The article of manufacture of claim 3 in which the radioactive material is adsorbed onto an adsorbent matrix.

6. The article of manufacture of claim 3 in which a thermal insulating material is employed to fill the container prior to sealing.

7. The article of manufacture of claim 3 in which the radioactive material is a radioisotope which emits substantially monoenergetic X-rays having an energy in the range of 20 to thousand electron volts.

8. The article of manufacture of claim 3 in which the radioactive material has a sufficiently high specific activity and sufficiently low self-absorption cross-section in its matrix to produce per square millimeter of source area a photon flux of more than 1000 photons per second per square centimeter at a distance of one meter.

9. The article of manufacture of claim 3 in which the radioactive material is a member from the group consisting of iodineand tellurium-125m.

10. The article of manufacture of claim 3 in which the metal is a member of the group consisting of aluminum and beryllium.

11. The article of manufacture of claim 5 in which the absorbent matrix is a member from the group consisting of a charcoal grain, charcoal impregnated paper, and a silica gel grain.

12. The method of making a radioactive source comprising the steps of forming a container having a cylindrical well therein surrounded by a mass and density sufficient to attenuate radiation on all sides but one and terminated at said one side by a thin wall integral with said mass, inserting a radioactive material in said well adjacent the thin wall and filling said well with a radiation attenuating mass, forming a metallurgically bonded seal with said mass surrounding the well while holding that portion of the container carrying the radioactive material in thermal contact with a massive heat sink.

References Cited UNITED STATES PATENTS 2,830,190 4/1958 Karp 250-106 2,870,341 1/1959 Pennock 250-106 2,939,961 6/1960 Coleman 250-106 2,964,628 12/1960 Ohmart 250-43.5 3,085,157 4/1963 Ginsburgh et al. 250-106 2,847,581 8/1958 Clark 250-84 X 2,884,538 4/1959 Swift 250-84 X 3,204,103 8/1965 Johnson et al. 250-106 3,230,374 1/1966 Jones et al. 250-106 3,322,951 5/1967 Yavorsky 250-106 3,331,962 7/ 1967 Kuhl.

RALPH G. NILSON, Primary Examiner S. ELBAUM, Assistant Examiner U.S. Cl. X.R. 250-106 UNI ED 'STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,508,057

April 21, 1970 Bernard Keisch It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 68, "absorbent" should read adsorbent Column 6, list of References cited, "Swift" should read Swift, Jr.

Signed and sealed this 22nd day of December 1970.

(SEAL) Attest:

WILLIAM E. SCHUYLER, JR.

Edward M. Fletcher, J r.

Attesting Offioer Commissioner of Patents

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