US3514607A - Composite shields against low energy x-rays - Google Patents

Description

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nited States Patent 3,514,607 Patented May 26, 1970 US. Cl. 250---108 Claims ABSTRACT OF THE DISCLOSURE An invention is described which makes use of the discovery that the absorption of low energy X-ray photons in a specified element when combined with lead is greater than the sum of the absorptions measured separately for this element and for lead. The X-ray fields for which this synergistic effect occurs are those generated with X-ray tube voltages up to 130 kvp. and comprise those commonly used in medical X-ray examinations and light industrial radiography. The special elements for which this effect occurs include tin, antimony, iodine and barium. The effect is optimized when lead and one of these elements are arranged in layers with the lead layer next to zone to be protected against radiation, and when the amount by weight of the special element exceeds that of lead. The invention specifically relates to flexible or rigid radiation:shields which may take the form of protective garmentsor structural radiation barriers for the express purpose of absorbing X-rays generated at kilovoltages below 130 kvp.

This application is a continuation-in-part of application Ser. No. 484,893, filed Sept. 3, 1965.

This invention relates to radiation shielding and provides improved materials resistant to penetration by X-rays of commonly encountered energies.

Lead or compounds of lead are in general use as protective materials against X-rays and gamma rays. The advantage of lead stems from its high atomic number, its high density, its easy availability and its low cost. It has, however, been known that narrow beams of low energy X-rays within welLdefined bands of photon energy are attenuated more readily by certain chemical elements than by lead per gram of absorber. Specifically, the total mass absorption coefiicients of these elements, which include tin, iodine and barium, exceed that of lead for wavelengths longer than 0.141 A. (i.e., energies lower than 88.23 kev.) and shorter than the critical absorption wavelength for the K electron shell in those elements. For example, for monochromatic X-rays ranging from 29 kev. to 88 kev., the total mass absorption coefiicient for tin is nearly twice that for lead. Usually, a large fraction of the X-ray photons generated by machines operating up to about 130 kvp. has energies lower than 88.23 kev., the average photon energy being about 0.4 of the peak energy. Thus for machines operating in the 50 to 130 kvp. range, the mean photon energy will usually lie in the range from kev. to 50 kev.

However, the above known facts have not previously led to the use of these elements in protective barriers against X-rays. Indeed simple experiments to compare the transmission of broad beams of X-rays in the above voltage range through these elements and through lead when prepared in sheets with equal weights per unit area have shown that these elements are not in fact more efficient than lead. When these elements are used alone with X-ray spectra generated by voltages from 50 to 130 kvp.,

they are clearly inferior to lead. There are two reasons for this inferiority: (a) these elements by themselves generate secondary characteristic X-rays which are readily transmitted through the element, adding greatly to the intensity of the attenuated primary X-ray beam; (b) these elements are quite inferior to lead for photon energies below their K-shell levels.

I have discovered that the addition of an element of high atomic number, e.g., lead, to a shield consisting principally of one of the aforementioned elements of intermediate atomic number, will markedly increase the absorption of radiation by the shield. The eifect of this addition is much greater than the effect which would be predicted by adding the separate absorption produced by the separate materials. In other words there is a synergistic etfect: the total absorption is greater than the sum of the parts. Thus, using iodine as an example, a double layer of material consisting of a particular arrangement of iodine and lead will absorb X-ray beams generated at kvp. more completely than either a layer of iodine or a layer of lead with the same weight per unit area. It is important to recognize that the principle discovered will allow weight reductions in radiation shielding against low energy X-rays and that the benefits realizable depend on the particular ratios and arrangement of the absorbing elements in the shield.

The benefits of the above-mentioned principle are realized optimally when the majority of the weight of the absorber per unit area comprises one or more of the elements tin, antimony, iodine and barium (which are called primary absorbers) combined with a minor amount of lead or another element of high atomic number such as bismuth or mercury (which are called secondary absorbers). The exact ratio of the primary and secondary absorbers for optimum absorption will depend on the kilovoltage at which the X-ray generator is operated and the filtration of the X-ray beam, i.e., on the energy spectrum of the X-ray beam. However, for most purposes it has been found that a satisfactory ratio of primary to secondary weight per unit area is 70%:3070, the useful range lying between 50%:50% and :10%.

In addition to the weight ratio, the relative arrangement of the primary and secondary absorbers is significant in optimizing the absorption for a given weight of absorber. The best arrangement is to locate the primary absorbing layer on the outside of the shield, i.e., facing the incident radiation, with the secondary absorber next to the shielded area. Such an arrangement has the disadvantage in a portable or movable shield, of its absorption being dependent on its orientation, i.e., if the primary absorber is placed next to the shielded area, the radiation level in that area will be markedly increased. An alternative arrangement whereby the primary absorber is sandwiched between two layers of secondary absorber has the practical advantage of independence of orientation and is almost as good a shield. Thirdly, it has been found that a uniform mixture of the primary and secondary absorbers provides a shield with a Weight advantage over lead, although the benefits are less marked than in the above two arrangements. In all cases the majority Weight should comprise the primary absorber. Multiple interspersed layers of primary and secondary absorbers are not precluded in the above arrangements, providing the layer next to the shielded area incorporates the secondary absorber.

This invention accordingly provides new materials for use as X-ray absorbers, particularly in the form of flexible or rigid sheets, wherein elements chosen according to the above principles are bonded into vinyl, rubber, polyethylene or other plastic materials. Such materials have an advantage over existing absorptive materials since they provide a given degree of protection against X-rays in the above category for a smaller weight of absorber. Thus where the weight of the absorber is a principal consideration, as for example in protective garments for X-ray workers, or in portable shields against low energy X-rays, these materials may represent a substantial improvement over existing materials.

Such improved materials are not restricted to only two principal absorbing elements. The secondary absorber (lead) may be combined with two or more of the elements cited in Table I, either as a uniform mixture or in separate layers. In a multiple layer arrangement the lead layer should be remote from the incident radiation and the other layers preferably arranged in order of atomic numher with the highest Z element in the first layer which receives radiation. Typical examples of twophase arrangements are: barium-Head, iodine-l-lead, antimony-l-lead, tin-Head. Typical examples of three-phase arrangements are: barium+iodine+lead, barium-l-tin-l-lead, iodine-ftin+lead, in that order.

The elements used may be in elementary (metallic) form or in the form of simple compounds. However the weight advantage over existing protective materials will disappear if the proportion by weight of the element in the compound is too low, e.g., if, in the case of barium, the sulfate is employed. Nevertheless, the use of such compounds is also envisaged in this invention since the synergistic effect continues to operate and there are significant economic advantages to employing the compounds of barium under these circumstances. Therefore, the oxides, fluorides, hydroxides, sulfides, and iodides of tin and barium and sodium iodide are all possible primary absorbers where weight considerations are important, while the carbonate and sulfate of barium may be used as the primary absorber where economic considerations are important. The latter considerations are usually more significant in permanent structural radiation barriers.

The elements and compounds thereof included in this specification as principal ingredients of the primary absorber are given in Table I.

TABLE I Element: Chemical form Tin Metal, oxide, fluoride, hydroxide, sulfide or iodide. Antimony Metal, oxide, fluoride, hydroxide, sulfide or iodide. Iodine lodides of sodium, magnesium,

aluminum and lead. Barium Oxide, sulfide, sulfate, carbonate, hydroxide or iodide.

The improved materials are prepared in flexible form 'by incorporating the finely-divided element or compound thereof into a plastic or rubber material. Such materials include polyvinyl chloride, polyethylene, polybutylene, polypropylene, polyurethane, and the natural or synthetic rubbers. The proportion of the plastic or rubber should be kept to a minimum consistent with mechanical strength and flexibility, in order to minimize the overall weight of the material for a given degree of radiation attenuation. The minimum content of carrier which has been found to yield materials with acceptable mechanical strength is 16% by weight. The carrier material after being loaded and mixed with the absorbing material or materials may be moulded or extruded into thin layers by presently accepted methods. The improved materials are prepared in rigid form by incorporation into plastic carriers, such as polyethylene or acrylics, or into building materials such as Masonite or wall board, or in the form of metal sheets in layers or as alloys with the specified ratios of primary to secondary absorber.

Experimental tests have shown that the ratio of primary to secondary absorber is fairly critical; for example, for the absorption of lightly filtered X-rays generated at 90 kvp. material containing parts of tin to 2 parts of lead by weight is more effective than material containing 4 parts of tin and 3 parts of lead by weight. The latter arrangement is, however, more economical.

The following examples provide flexible or rigid materials equivalent to about 0.25 mm. of lead in absorbing X-rays generated at kilovoltages up to kvp. and have particular merit from the standpoint of chemical stability and economy. These examples are included as examples of the large number of variations possible in the type and amount of absorbers and of plastic or rubber carrier, and, therefore, should not be interpreted in a limiting sense.

EXAMPLE I A double layer was assembled, consisting of an outside layer sheeted out from a uniform mixture of tin and polyvinyl chloride containing 4 oz./sq. ft. of powdered tin metal and 0.75 oz./sq. ft. of polyvinyl chloride and a second layer sheeted out from a uniform mixture of lead monoxide, powdered lead and polyvinyl chloride containing 1.5 oz./sq. ft. of lead monoxide (PbO), 1.5 oz./ sq. ft. of powdered lead metal, and 0.6 oz./ sq. ft. of polyvinyl chloride. The total weight of the 2-layer assembly was 8.35 oz./ sq. ft. for equivalence to about M1 mm. of lead and 16.7 oz./ sq. ft. for equivalence to about /2 mm. of lead.

EXAMPLE IA A double layer was assembled, consisting of an outside layer sheeted out from a uniform mixture of tin and polyvinyl chloride containing 4.8 oz./sq. ft. of powdered tin metal and 0.9 oz./ sq. ft. of polyvinyl chloride and a second layer sheeted out from a uniform mixture of powdered lead and polyvinyl chloride containing 2.7 02/ sq. ft. of powdered lead metal, and 0.5 oz./ sq. ft. of polyvinyl chloride. The total weight of the 2-layer assembly was 8.9 oz./sq. ft. for equivalence to about mm. of lead and 17.8 oz./sq. ft. for equivalence to about /2 mm. of lead.

EXAMPLE II A single layer was made up containing 4.2 oz./sq. ft. of powdered tin metal, 3.3 oz./sq. ft. of lead monoxide (PbO) and 1.4 oz./sq. ft. of polyvinyl chloride in a uniform mixture. The total weight is 8.9 oz./sq. ft. for equivalence to about Mi mm. of lead and 17.8 oz./ sq. ft. for equivalence to about /2 mm. of lead.

EXAMPLE IIA A single layer was made up containing 5 oz./sq. ft. of powdered tin metal, 3 oz./sq. ft. of powdered lead metal and 1.4 oz./sq. ft. of polyvinyl chloride in a uniform mixture. The total weight was 9.4 oz./ sq. ft. for equivalence to about A: mm. of lead and 18.8 oz./sq. ft. for equivalence to about A mm. of lead.

EXAMPLE III Three layers were assembled, the outside layer containing 2 oz./sq. ft. of barium oxide and 0.6 oz./sq. ft. of polyvinyl chloride, the middle layer containing 2.2 02/ sq. ft. of powdered tin metal and 0.4 oz./sq. ft. of polyvinyl chloride, and the inside layer containing 1.5 oz./ sq. ft. of lead monoxide (PbO) and 1.5 oz./sq. ft. of powdered lead metal uniformly mixed with 0.6 oz./ sq. ft. of polyvinyl chloride. The total weight for the 3-layer assembly was 8.8 oz./sq. ft.

EXAMPLE IIIA Three layers were assembled, the outside layer containing 3 oz./sq. ft. of barium oxide and 0.6 oz./sq. ft. polyvinyl chloride, the middle layer containing 2.3 02/ sq. ft. of powdered tin metal and 0.5 oz./sq. ft. of polyvinyl chloride and the inside layer containing 2 02/ sq. ft. of powdered lead metal uniformly mixed with 0.4 oz./ sq. ft. of polyvinyl chloride. The total weight for the 3- layer assembly was 8.8 oz./sq. ft. for equivalence to about mm. of lead.

EXAMPLE IV A, double layer was assembled, the outer layer containing 4 oz./sq. ft. of barium oxide and 0.75 oz./sq. ft. of polyethylene; the second layer containing 3.5 oz./sq. ft. of lead monoxide (PbO) and 0.7 oz./ sq. ft. of polyethylene. The total weight is 8.95 oz./ sq. ft.

EXAMPLE IVA A double layer was assembled, the outer layer containing 4 oz./sq. ft. of barium oxide and 0.75 oz./sq. ft. of polyethylene; the second layer containing 3.3 oz./sq. ft. of powdered lead metal and 0.6 oz./sq. ft. of polyethylene. The total weight is 8.65 02/ sq. ft. for equivalence to 4mm. of lead. V

EXAMPLE V Three layers were assembled, the two outer layers each containing 1.6 oz./sq. ft. of powdered lead and 0.3 02/ sq. ft. of polyvinyl chloride and the inner (sandwiched) layer containing 4.3 0z./sq. ft. of powdered tin and 0.8 oz./sq; ft. of polyvinyl chloride. The total weight is 8.9 oz./sq. ft. This arrangement has the advantage of equal performance when irradiated from either side.

EXAMPLE VI A single rigid layer was made up containing 7.2 oz./ sq. ft. of barium carbonate, 2.1 oz./ sq. ft. of lead monoxide (PbO) and 3.1 oz./sq. ft. of polymethylmethacrylate, in a uniform mixture. The total weight was 12.4 02/ sq. ft. for equivalence to about mm. of lead.

In all cases the two layers or several layers may be cemented integrally together or may be attached only at the edges.

In the sheet material of this invention shielding of low energy X-rays comparable to that provided by a quarter millimeter of lead (about 9.4 oz./sq. ft.) is obtained by using only about 7.0 to 7.5 oz./ sq. ft. of the preferred mixture of the specified primary elements together with lead. When compounds of these elements are used the weight per unit area will be greater. Frequently, a lesser shielding effect will be required in which correspondingly lesser amounts of lead and the specified primary element will be suitable: for instance, it is contemplated that densities as low as 3 oz./sq. ft. may frequently be satisfactory, as for instance in photographic and X-ray film packaging, and other applications where the radiation to be shielded is of low intensity and only a small amount of shielding is required.

Although the sheet material of this invention is useful generally for low energy X-ray shielding, it is known that a particularly advantageous use is in the garments, such as aprons and gloves, worn by persons working near X-ray machines. In forming such from the multilayer sheets described above, the layer containing the lead should be placed innermost, nearest the wearer. Garments of this material are preferably formed with hydrocarbon plastics such as polyethylene, polypropylene and other polyolefins as the plastic sheet material, because of their lightness, flexibility and durability. It will be appreciated that such garments are substantially lighter in weight than those currently formed containing lead as the only shielding material. Garments according to this invention are from 20 to 30% lighter than those currently available.

Having thus disclosed my invention and described in detail preferred embodiments thereof, I claim and desire to secure by Letters Patent:

1. X-ray shielding material comprising a sheet containing the components lead and at least one element selected from the group consisting of tin, antimony, iodine and barium, the combined amount of said components being at least 3 oz./sq. ft. of sheet area, the amount of said element being between 50% and of the total weight of said element plus lead.

2. X -ray shielding material as defined by claim 1 wherein said sheet comprises an integral'layer containing lead and an integral layer containing said element.

3. X-ray shielding material as defined by claim 1 wherein said sheet comprises three integral layers, the two outer layers containing lead and the intermediate layer containing said element.

4. X-ray shielding material as defined by claim 1 wherein said sheet comprises three integral layers, including an inner layer containing lead, and intermediate layer containing one of said elements, and an outer layer containing another of said elements of atomic number higher than that of said one element.

5. Xray shielding material as defined by claim 1 wherein said components are contained in a sheet of plastic.

6. X-ray shielding material as defined by claim 5 wherein said sheet comprises an integral layer containing lead and an integral layer containing said element.

7. X-ray shielding material as defined by claim 5 wherein said sheet comprises three integral layers, the two outer layers containing lead and the intermediate layer contaning said element.

8. X-ray shielding material as defined by claim 5 wherein said sheet comprises three integral layers, including an inner layer containing lead, an intermediate layer containing one of said elements, and an outer layer containing another of said elements of atomic number higher than that of said one element.

9. X-ray shielding material as defined by claim 5 in the form of a garment of radiation shielding material.

10. A garment of radiation shielding material as defined by claim 9 wherein said sheet comprises an integral layer containing lead being adjacent to the wearer.

References Cited UNITED STATES PATENTS 2,441,945 5/1948 Frolich et al. 2,928,948 3/ 1960 Silversher. 3,093,829 6/1963 Maine.

RALPH G. NILSON, Primary Examiner M. J. FROME, Assistant Examiner US. Cl. X.R. 252-478