US 3075925 A
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ted States 3,075,925 RADIATION SHIELDING COMPOSITION Harold L. Dunegan, Livermore, Califi, assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Dec. 21, 1960, Ser. No. 77,480 3 Claims. (Cl. 252-478) This invention relates to radiation shielding compositions and, more particularly, to a lightweight shielding composition whose mechanical and radiological properties can be varied within wide limits.
The most efilcient absorbers of gamma radiation are elements of high atomic number, since the gamma absorption cross section of a nucleus varies approximately as the fourth power of its atomic number. For reasons of economy and relative ease of fabrication, the two most commonly used gamma shielding materials are lead and tungsten. These elements are used in the form of blocks, sheets, and in combination with other materials such as lead glass, which is utilized in the construction of viewing windows for radioactive hot cells. When employed in this manner, lead and tungsten provide eiiicient gamma shielding. Unfortunately, however, shielding efiiciency is only obtainable at the price of high density, the densities of lead and tungsten being respectively 11.3 and 19.3 grams per cubic centimeter. In addition to being heavy and unwieldy, the lead and tungsten gamma shields of the prior art can only be applied in the form of rigid geometrical shapes, i.e., rods, sheets, blocks, etc. This limitation necessitates that shielding blocks be stacked in staggered rows, so as to eliminate or minimize the probability of radiation leaking through the spaces between adjacent shielding blocks.
Now it has been discovered that lead and tungsten powders can be incorporated into various plastic materials to yield gamma shielding compositions which, for equivalent gamma attenuation, weigh one-third to onehalf as much as the gamma shielding of the prior art. Remarkably low densities are achieved by incorporating air into the novel composition during the mixing of the ingredients thereof. For example, densities of typical tungsten-plastic compositions range from 1.3 to 1.5 grams per cubic centimeter. A further feature of the invention is that the shielding composition can be obtained as a flexible rubbery mass, a hard rigid block, or any gradation therebetween, simply by suitably vary-ing the proportions of the ingredients therein.
Accordingly, it is an object of the invention to provide a lightweight gamma radiation shielding composition.
Another object of the invention is to provide a gamma radiation shielding composition whose physical characteristics can be varied as desired over a wide range.
A further object of the invention is to provide a lightweight plastic gamma radiation shielding composition which can be readily molded into any desired shape.
Other objects and advantages of the invention will become apparent upon consideration of the following detailed description.
As provided by the invention, the shielding composition consists of four basic ingredients; metal powder, plastic resin, plasticizer, and catalyst. Varying proportions of the four ingredients are mixed with a mechanical stirrer, which incorporates air into the mixture as a result of the stirring motion. The thoroughly blended mixture is poured into a suitable mold, and cured in an oven for several hours. After removal from the oven, and cooling to room temperature, the composition is ready for use.
Although lead and tungsten are preferred materials for gamma ray attenuation, other elements of high atomic number, e.g., bismuth and tantalum, can be used. The metal powder particle size is a critical feature of the invention. If the metal particles are too large, they will not distribute themselves uniformly in the plastic matrix, thereby resulting in uneven attenuation of gamma radiation. if, on the other hand, the metal particles are too small, they will pack together or agglomerate, which makes it difiicult to obtain the desired low density in the finished composition. The preferred range of particle size for the metal powder is from 30 to microns average diameter. Particles of this size can be distributed uniformly in the plastic matrix, and at the same time, do not show a substantial tendency to agglomerate.
Another parameter which has a great bearing on the properties of the finished composition is the proportion of metal powder therein. High concentrations of metal tend to produce rather brittle compositions of relatively high density and low tensile strength. More desirable mechanical properties, e.g., higher tensile strength, good resistance to abrasion, and increased flexural strength, are obtained with metal powder concentrations below about 50% by weight. The preferred metal concentration is in the range of 15% to 40% by weight. This concentration range represents the best combination between mechanical and radiological properties in the finished composition.
The plastic resin component of the shielding composition acts as a binding matrix into which the individual metal particles are distributed. It is apparent that the mechanical properties of the plastic play a major role in determining the characteristics of the finished composition. Although the novel composition can be formulated with any one of a number of plastic resins, best results are obtained with polyurethane, epoxy, and polyethylene plastics. It has been found that vinyl and acrylic plastics do not generally produce shielding compositions of high tensile strength and good abrasion resistance. However, for applications where such properties are not critical, vinyls, acrylics and phenolformaldehyde plastics can be employed in the formulation. The preferred range of plastic resin concentration is approximately 55% to 70% by weight.
The flexibility of the final composition is largely determined by the concentration and chemical nature of the plasticizer therein. Plasticizer concentrations of 1% to 15% by Weight provide a continuous sprectrum of flexibilities in the final composition, ranging from tough rigid compositions at low plasticizer concentrations, to com positions which can be easily molded and worked by hand, in the case of higher plasticizer concentrations. The chemical nature of the plasticizer to be used in a particular application depends upon the type of plastic resin being used as the matrix for the metal powder. For formulations with polyurethane, the preferred plasticizers are trichloroethyl phosphate, tricresyl phosphate, castor oil, and other unsaturated vegetables oils and their esters. Preferred plasticizers for shielding compositions including polyethylene are dibutyl phthalate, dioctyl phthalate, and various mixed octyl esters of phthalic acid.
The inclusion of a catalyst in the composition promotes the interaction of the plasticizer with the plastic resin, thereby assuring a uniformly flexible product. Preferred catalysts for use with the invention are benzoyl and lauroyl peroxides, methylene-bis (orthochloro-aniline), and other methylene linked bis-substituted anilines. Catalyst concentrations of 1% to 5% by weight are sufficient to promote adequate dispersion of the plasticizer in the resin.
Further details of the invention are given in the following example, which illustrates the composition and method of manufacture for a typical tungsten-polyurethane shielding material.
The above ingredients were mixed with a mechanical stirrer in the order listed. The stirring was stopped after the whipped-in air caused the blended mixture to assume a frothy appearance. The mixture was poured into a mold and cured at 150 C. for six hours. The cured mixture was cooled to room temperature, and the rubbery tungsten-plastic composition was recovered. The density of the composition was 1.3 grams per cubic centimeter. In an attempt to determine the relative surface hardness of the composition, it was sandblasted for a prolonged period. No visible surface abrasion was observed. The composition was tested for X-ray opacity by exposing a 0.436 inch thick disk of the material to 39 kilovolt X-rays and comparing the resulting photodensity with that recorded with varying thicknesses of lead on exposure to the same rays. The results are shown in Table I, from which it is apparent that a 0.436 inch thickness of the tungsten-polyurethane composition provides the same X-ray (or gamma ray) attenuation as a 0.125 inch thickness of lead. Taking into account the difference in density between the tungstenpdlyurethane (1.3 gms./cc.) and metallic lead (11.3 gins/co), it is apparent that, for equivalent gamma attenuation, the shielding composition of the invention weighs one-third to one-half as much as conventional lead shielding.
Table 1 Absorber thickness Photodensity (arbitrary (in inches): units) 0.03125 (lead) 1.58 0.06250 (lead) 1.43 0.12500 (lead) 1.73 0.43600 (tungsten-polyurethane) 1.70 While there have been described above What may be considered to be preferred embodiments of the invention,
various modifications can be made therein without departing from the spirit and scope of the invention as defined by the following claims.
What is claimed is:
1. As a composition of matter, a gamma radiation shielding material consisting essentially of to by weight of a plastic resin selected from the group consisting of polyurethane, polyethylene, and epox 15% to 40% by Weight of a metal powder selected from the group consisting of lead and tungsten, 1% to 15% by Weight of a resin plasticizer selected from the group consisting of trichloroethyl phosphate, tricresyl phosphate, castor oil, dibutyl phthalate, and dioctyi phthalate, 1% to 5% by Weight of a polymerization catalyst selected from the group consisting of benzoyl peroxide, lauroyl peroxide, and methylene-bis (ortho-chloro-aniline), and sufficient included air such that the composition possesses a density of from 1.3 to 1.5 grams per cubic centimeter.
2. As a composition of matter, a gamma radiation shielding material comprising a mixture of about 60.5% polyurethane plastic, 3.0% methylene-bis (ortho-chloroaniline), 9.2% castor oil and 27.3% powdered tungsten metal, intermixed with air such that the mixture has a density of between 1.3 to 1.5 grams per cubic centimeter.
3. In a process for producing a radiation shielding material, the steps comprising adding tungsten powder to a mixture of polyurethane resin, plasticizer, and catalyst, stirring said tungsten powder-plastic mixture vigorously to incorporate air therein, curing said mixture at elevated temperature, and finally recovering said cured metal-plastic mixture.
References Cited in the file of this patent UNITED STATES PATENTS 2,162,178 Marasco et a1 June 13, 1939 2,256,483 Johnston Sept. 23, 1941 2,845,660 Peiler Aug. 5, 1958 2,858,451 Silversher Oct. 28, 1958 2,961,415 Axelrad Nov. 22, 1960 3,002,843 Stocker Oct. 3, 1961 FOREIGN PATENTS 851,479 Great Britain Oct. 19, 1960 OTHER REFERENCES Hackhs Chemical Dictionary, 1946, page 139, The Blakiston C0., Philadelphia.
Gilrnau: Organic Chemistry, vol. I, 1948, p. 741, John Wiley & Sons, Inc., New York.