Patent Application
20070141332
1. Field of the Invention
This invention relates to radiation shielding materials. In particular, this invention relates to glass-coated microwire for X-ray protection.
2. Discussion of the Background
X-ray radiation is a powerful radiological tool used for diagnostic procedures and extensive medical therapy. However, living tissue is susceptible to damage from prolonged or high intensity exposure to X-ray radiation. In the field of medical radiology, radiation shields are used for protection from electromagnetic flux and, in particular, X-ray radiation.
X-ray radiation has a wavelength in the range of from 10−5 to about 10−12 cm. X-ray radiation with a wavelength λ<2 Å is often referred to as hard radiation, while X-ray radiation with a wavelength λ>2 Å is referred to as soft radiation. Radiation shields are made from materials that interact actively with X-rays.
When X-rays interact with a material the X-rays are attenuated due to absorption in the material. In addition, the electromagnetic radiation undergoes a change in direction. The X-ray absorption predominates in the long wavelength part of the spectrum, while the change in direction predominates in the short wavelength region. The degree of absorption generally increases with increase in atomic number.
The following materials are usually used for X-ray absorption: lead and its alloys, in particular, with tin, bismuth, indium and antimony. These alloys are known as quick solders in practice. The most applicable compositions of such alloys are presented in Table 1.
U.S. Pat. Nos. 4,619,963 and 4,485,838 describe an X-ray radiation shielding composite sheet material of melt-spun lead fibers of more than 99% purity, containing 50 to 500 ppm tin, of a mean length of 0.5 to 1.3 mm, which are embedded in a synthetic resin. The sheet material can be formed by melt spinning the tin-containing lead fibers to a diameter below 60 microns, cutting the fibers, blending the fibers with a thermoplastic resin, and pressing the blend to form sheet.
U.S. Pat. No. 4,938,233 describes a matrix with a radiation attenuating filler. The attenuating filler generally includes an inorganic salt having a radiopaque cation. The cation is preferably selected from the group consisting of barium, iodine and mixtures thereof, although many other suitable cations exist such as bismuth, uranium and zirconium containing compositions alone or in combination. According to a preferred embodiment, the attenuating filler—barium sulfate—is present in a range of up to about 80% by total weight, most preferably in the range of from about 20% to about 60% with less than 0.5% tincture of iodine.
U.S. Pat. No. 6,310,355 describes a shield for attenuating a flux of electromagnetic radiation, for example, radiation from X-rays. The shield comprises a flexible matrix comprising foam including a radiation attenuating material such as barium sulfate or bismuth. The matrix includes at least one space, which reduces the overall weight of the shield without reducing the attenuating characteristics of the shield. The shield has a transmission attenuation factor of at least 50% primary 100 KVP X-ray beam.
Oxides of elements such as tin, lead, boron, strontium, silicon and zirconium interact with X-ray effectively. One conventional glass for X-ray absorption has the following composition (weight %):
Another conventional glass for X-ray absorption has the following composition (weight %):
U.S. Pat. No. 3,987,330 discloses a glass having a high X-ray absorption. The glass has the following composition (weight %):
Table 2 shows glass compositions, described in various U.S. patents, that efficiently interact with X-ray radiation.
Various alloy compositions are known for the preparation of glass-coated microwires of lead or lead-containing intermetallic compounds. U.S.S.R. Inventor's Certificate No. 440,436 discloses Pb-based alloy for casting of microwire, having a high specific gravity (not less than 90 g/cm3). The alloy has the following content (weight %):
Due to the high specific gravity this alloy may be effective for X-ray protection.
U.S.S.R. Inventor's Certificate No. 383,094 discloses an alloy for microwire casting that is based on a PbTe intermetallic compound and has the following content (weight %):
A disadvantage generally associated with these alloys is the absence of elements having a high affinity with oxygen that can act as deoxidizers. Such elements interact with oxygen and inhibit the formation of an oxide layer on the drop surface during microwire casting processes. If the oxide layer forms, then under rotary agitation by high frequency fields, the oxide particles tend to enter the zone of microwire formation and prevent the manufacture of the microwire.
Furthermore, the '436 alloy has a glass wetting ability that is not sufficient for providing a microwire manufacturing process. This disadvantage limits and sometimes even prevents manufacture of a microwire having a small variation in diameter along the length of the microwire. Moreover, this alloy is not suitable for preparing long continuous microwire lines during a microwire casting process. Thus, the alloy is not well suited for preparing from microwire textiles and composites for X-ray protection.
U.S.S.R. Inventor's Certificate No. 374,243 discloses a glass having a high content of lead oxide. The glass has the following composition (mol. %):
A microwire casting process with this glass is easy tuned and stable. The glass is well suited for preparing microwire whose length is in a range of from 200 to 250 m. However, a disadvantage of the microwire casting process with this glass is an insufficient efficacy of X-ray absorption in comparison with the glass containing strontium oxide.
The present invention provides a glass-coated metal microwire and a casting process that can produce the microwire. At the phase boundary between the metal and glass in the microwire, there is an intensive interaction with X-rays and a high effective absorption of X-rays. As a result, the microwire provides superior protection against X-ray radiation. The casting process can provide a long continuous microwire that has a length of 500 m or more and that has a stable diameter along its length (i.e., a variation in diameter of not more than +/−15%).
In embodiments, the metal is an alloy comprising lead, bismuth, copper and silicon, which additionally contains tin, indium and cerium. The metal alloy can comprise (weight %):
In embodiments, the glass can comprise (mol. %):
In order to understand the invention and to see how it may be carried out in practice, preferred embodiments will now be described, by way of non-limiting examples only, with reference to the accompanying drawing, in which:
The alloy should contain not less than 30-35 weight % of heavy metal for sufficient protection from X-ray radiation. Introduction into the alloy of elements such as bismuth, tin and indium provides the most effective composition. At the same time the optimal total amount of tin and indium is 10-20 weight %, where Sn and In are in a weight ratio of Sn:In, ranging from 1.9 to 2.1, that is preferably 2.
Moreover the content of an element such as In, which is an effective amorphizator, provides the amorphous structure to the microwire core. An amorphous alloy has a greater efficacy for X-ray absorption in comparison with a crystalline structure having the same chemical structure. Amorphization of an alloy can be achieved when the content of In is not less than 4 weight %. On the other hand, when more than 8 weight % of In is in the alloy, the liquidus temperature of the alloy is decreased, which tends to destabilize the microwire casting process.
Copper in amounts of 3-5 weight % is introduced to increase of the liquidus temperature of the alloy (during microwire casting process) up to 480-520° C., which corresponds to the optimal viscosity of a glass based on PbO. When the amount of Cu is less than 3 weight %, the needed increase of liquidus temperature is not achieved. However, when the content of Cu is higher than 5 weight %, the liquidus temperature is increased over the desired value.
Silicon as a surface-active element is introduced to provide good wetting between the glass and metal during the microwire casting process. This effect is observed when the content of Si is 0.6 weight %. If the content of Si is higher than 1.5 weight %, then a self-dependent silicide phase is formed that causes brittleness in the microwire.
The further improvement of the technology for the microwire manufacture providing the prolonged stable casting process and increase of the continuous length of the microwire may be attained due to the drop fining. For these purposes Cerium as a strong oxidant must be introduced in the amount of 0.05-1.2 weight %. Alloying by small amounts of Ce beginning from 0.05% provides the fining of the alloy but without change of the functional microwire properties. Introduction of Ce in amounts more than 1.2% increases the brittleness of the microwire.
A method of manufacturing microwire by casting must offer a minimum alignment time, a prolonged stable casting process, and a small variation (small scatter) in microwire diameter along the length of the microwire. At the same time, it is necessary to provide a microwire having strength, flexibility, and X-ray protection efficiency.
These goals are achieved by the present invention by coating a specific glass composition on a specific alloy. Only under the optimal combination of properties for glass and alloy can the development of the novel microwire of the present invention with a predetermined level of properties be achieved. The development of the optimal glass composition is carried out on a base of low-melting glass, having the content (mol. %):
In the composition in accordance with the present invention, strontium oxide in the range of 12-15 mol. % was additionally introduced. An experiment shows that the most absorption of X-rays is achieved when the molar ratio between PbO:SrO is about 5:1, e.g., in a range of from 4.8 to 5.2.
Moreover, in the initial glass composition Sb2O3 was replaced by K2O because the simultaneous presence of the SrO and Sb2O in the alloy leads to partial crystallization. The presence of K2O in the glass composition is important for providing the manufacturability of the microwire casting process. The result is a conservation of the glass viscosity at the casting temperature (480-520° C.). This effect exists when the amounts of the K2O is not less than 2 mol. % in the glass composition. When more than 8 mol. % of K2O is in the glass content, an efficiency of X-ray protection is decreased. Thus, the glass composition in accordance with the present invention has the following content, mol. %:
According to the invention, the alloy is melted in alundum crucibles by the induction furnace. First lead and tin are added. Then, indium, copper, silicon and cerium are added. The glass is produced in a glass-melting furnace using a simultaneous loading of a predetermined homogeneous charge. After fining fusion, glass tubes of 9-12 mm diameter are produced.
A glass-coated microwire with an amorphous metal core is produced by providing a glass tube containing the desired metal and melting the metal in a high frequency induction field. A metal perform weighing 10 g is put into a tube soldered from one end and the metal is melted in a high frequency induction field at 440 (880) KHz frequency. After a drop of the metal in the molten state is formed the microwire is drawn out by means of a special forming mechanism. At that time the microwire passes through a water stream for cooling and solidifying. As a result, the alloy is cooled by quenching the material from the liquid phase with a cooling rate of up to 106 K/s. Under these conditions an amorphous alloy is provided. The diameter of the microwire ranges from 10-40 μm and the glass insulation is in the range of 2-4 μm. It is possible to provide a continuous microwire casting process using the alloy and glass based on lead oxide according the present invention.
Referring to
A tip of the glass tubing 102 loaded with the rod 104 is introduced into a furnace 106 adapted for softening the glass material making up the tubing 102 and melting the rod 104 in the vicinity of the exit orifice 107, such that a drop 105 of the wire material in the molten state is formed. A glass-coated microwire is then drawn from the heated glass tubing 102 and the drop 105 of the wire material.
According to one embodiment of the invention, the furnace 106 includes at least one high frequency induction coil, e.g. one wind coil. The operation of the furnace 106 is known per se, and will not be expounded in details below. An exemplary furnace that has been shown to be suitable for the manufacturing process of the present invention is the Model HFP 12, manufactured by EFD Induction Gmbh, Germany.
The temperature of the drop is measured by a temperature sensor pointing at the hottest point of the drop and diagrammatically represented by a box 108. An example of the temperature sensor includes, but is not limited to, the Model Omega OS1553-A produced by Omega Engineering Ltd.
The temperature sensor 108 is operable for producing a temperature sensor signal. The temperature sensor 108 is coupled to the controller 109 which is, inter alia, responsive to the temperature sensor signal and capable of providing a control by means of a PID loop for regulating the temperature of the drop 105 for stabilizing and maintaining it at a required magnitude. For example, the temperature of the drop can be maintained in the range of 480° C. to 520° C.
It should be appreciated that one way of regulating the drop temperature is the regulation of the temperature of the furnace 106 by changing the furnace's power consumption. For this purpose, controller 109 is capable of generating a furnace power signal, by means of a PID control loop, to a power supply unit 113 of the furnace 106. For example, when the consumption power increases, the drop temperature should also increase, provided by the condition that the position of the drop 105 does not change with respect to the furnace 106. However, since the furnace includes a high frequency induction coil, the increase of the consumption power leads to the elevation of the drop, due to the levitation effect. Hence, the temperature of the drop depends on many parameters and does not always change in the desired direction when only the consumption power is regulated.
An example of the power supply unit 113 includes, but is not limited to the Mitsubishi AC inverter, Model FR-A540-11k-EC, Mitsubishi, Japan.
According to one embodiment of the invention, the compensation of the levitation effect is accomplished by the regulation of the gas pressure in the tubing 102. Thus, in order to avoid the droplet elevation due to the increase of the consumption power, the negative gas pressure (with respect to the atmospheric pressure) is decreased to a required value calculated by the controller 109.
For this purpose, the system 10 is further provided with a vacuum device identified by reference numeral 120 for evacuating gas from the tubing 102. The vacuum device 120 is coupled to the tubing 102 via a suitable seal able coupling element (not shown) so as to apply negative gas pressure to the inside volume of the tube 102 while allowing passage of the rod 104 there through.
The vacuum device 120 is controllable by a vacuum device signal generated by the controller 109 for providing variable negative pressure to the molten metal drop in the region of contact with the glass. The pressure variation permits the manipulation and control of the molten metal in the interface with the glass in a manner as may be suitable to provide a desirable result.
The microwires of the alloys according to the present invention were investigated. The microwires have the following compositions:
Microwire 1:
Microwire 2
The diameter of the metal was 25 μm and the glass coating was 2-4 μm thick. The X-ray mass attenuation coefficient was measured at different wavelengths. Table 3 presents the results of the experiments.
The test results shows that the X-ray mass attenuation coefficient for the microwire according to the present invention is 1.3-1.5 times more than this coefficient for lead.
The disclosure of a numerical range herein is to be considered the disclosure of every number within that numerical range.
