US 3706689 A
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States Unite ABSTRACT OF THE DISCLOSURE Process for the preparation of a Sr --Y beta source in a substantially radiation hazard free manner comprising, in combination, the steps of providing a radioactive material containing Sr separating at least part of the Sr from the radioactive material to provide a Sr rich material which emits only low energy beta rays; fabricating a source from the low energy beta ray emitting Sr rich material; storing the fabricated source for a period suflicient to permit the Sr therein to become an equilibrium mixture of Sr -Y and utilizing the resulting stored fabricated source as a Sr -Y beta source. This process is useful in fabricating Se -Y beta sources with a minimum of shielding. Such beta sources are in turn useful in medical applications of radioactivity such as radioactive eye applicators and nasopharyngeal applicators.
BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to radioactive beta sources and more particularly to radioactive Sr -Y beta sources and methods for fabricating such sources.
The term source, as used herein, connotes a usable arrangement of radioactive material rather than the radioactive material per se. The term radioactive material refers to elements or isotopes, whether free or chemically combined, which spontaneously emit particles and/ or rays by a process of atomic disintegration. Satisfactory sources should permit utilization of radiation from a radioactive material while preventing contamination of the surroundings through the loss of material.
Heretofore, the fabrication of Sr -Y beta sources has required that the work be done in a glove box with thick lead-filled gloves and plenty of auxiliary shielding, and for large activities, has required that the fabrication be carried out in a hot cell equipped with mechanical manipulators. The necessity of providing large amounts of shielding hinders many of the delicate fabrication procedures such as welding, handling, etc., and in general makes the fabrication unwieldy.
The radiation hazard encountered in the fabrication of Sr Y beta sources can be attributed to the high energy of the yttrium-90 beta particles (Y and their resulting bremsstrahlung. Y beta particles have a maximum energy (E equal to 2.27 million electron volts (mev.) and an average energy (E) equal to 0.91 mev. Bremsstrahlung consist of secondary X-rays emitted when the high energy Y beta particles strike other matter and are slowed down. In general, a radiation hazard is present with Su -Y activities greater than a few millicuries, and it is especially dangerous when activities of one curie or greater are used.
Processes for separating yttrium ions from other ions are well known in the prior art. For example, US. Pat. 3,188,169, issued June 8, 1965, discloses a process for separating weakly adsorbed ions from strongly adsorbed ions in an ion exchange column using perchloric acid as an eluting liquid. This process, however, categor- Patented Dec. 19, 1972 izes both yttrium and strontium as strongly adsorbed ions, and therefore would not be effective in separating one from the other. US. Pat. 3,037,841, issued June 5, 1962, discloses another process for separating yttrium from other rare earth elements. This process consists of running a mixture of the rare earths and yttrium through an ion exchange column in the presence of a complexing agent like ethylenediamine tetraacetic acid. This process is not used, however, to separate yttrium from strontium.
Processes are known, however, in the prior art for separating Y from strontiumbeta particles (Sr For example, ion exchange methods for such a separation are disclosed in Experimental Nuclear Chemistry by Gregory R. Choppin, copyrighted in 1961, at pages 87-89. Another process for separating Y from Sr is disclosed in Analytical Chemistry, November 1967, at pages 1634- 1 639. This method consists of selectively adsorbing ions onto manganese dioxide (MnO None of the prior art references, however, discloses or suggests that these known processes of separating Y from Sr can be successfully applied in the fabrication of Sr -Y beta sources to reduce the radiation hazard.
SUMMARY OF THE INVENTION A method has now been found to reduce the radiation hazard present during the fabrication of Sr -Y beta. sources. This process comprises:
(1) Separating the Y from the Sr -Y matrix to form a Sr rich material;
(2) Fabricating the Sr Y beta source using only the Sr rich material; and
(3) Allowing the Y to grow back to an equilibrium state with the Sr within the beta source.
This process has the advantage of allowing fabrication of the beta source to take place using material rich in Sr which has a maximum energy (B,,,,,, of only 0.54 mev. and an average energy (E) of only 0.2 mev. Because the material contains none or only small amounts of Y very little shielding is required.
In addition, the relative half-lives for Sr and Y result in many further advantages. The half-life of Y is 64 hours and the half-life of Sr is 28 years. An advantage of the half-life of Y is that fabrication of the beta source can be accomplished before a substantial portion of the Y has grown back. For example, according to the equation for radioactive growth and decay in the case of Sr and Y, which are in secular equilibrium, one-half the maximum attainable yield of Y would grow back into the separated parent Sr after 64 hours, after 128 hours, after 192 hours, A after 256 hours, and so forth. Another advantage in the relative half-lives is that within a short time, normally only a couple of weeks, the sources would contain an equilibrium mixture of Sr -Y and could be calibrated.
DESCRIPTION OF THE INVENTION As mentioned above, Sr -Y exists in secular equilib rium. Secular equilibrium means equality of disintegration rate between the parent (Sr and daughter (Y but nonequality in the number of atoms of each present. Secular equilibrium exists when the half-life of the parent is much greater than the half-life of the daughter. In such a case, the daughter activity does not pass through a maximum but reaches a constant value after which the rates of decay and formation are equal.
In practicing this invention, the Sr which emits high energy betas is separated from the 81- which emits only low energy betas before fabrication of the beta source. Separation of the Y from the Sr -Y may be accomplished by many techniques including: (1) Ion exchange chromatography; (2) Selective precipitation; and (3) Solvent extraction. Each of these methods is described in greater detail below.
Ion exchange chromatography involves separating the Y from the Sr by contacting the Sr -Y with a material having selective attractions for the Sr and Y ions. One convenient method for accomplishing this type of separation is to place the Sr -Y in the top of an ion exchange column packed wih an ion exchange resin and then to elute the column with a liquid eluant which will remove either the Sr or Y ions from the resin. Suitable examples of ion exchange separations within the scope of this invention are disclosed by Choppin in Experimental Nuclear Chemistry at pages 87-89.
Ion exchange resins suitable for use with this invention can be organic or inorganic compounds. "High amounts of radioactivity, however, can damage organic resins, and this should be considered in the selection of an appropriate resin for a particular application.
In general, any resin is suitable which exhibits a different tendency to adsorb the divalent strontium cation and the trivalent yttrium anion.
Some examples of suitable cationic resins include: Dowex 50, a sulfonated polystyrene manufactured by Dow Chemical Co.; and Amberlite-CG-lZO, Type 3, manufactured by Rohm and Haas Co. and distributed by Fisher Scientific Co. Some examples of suitable anionic resins include Dowex 1, a polystyrene crosslinked with divinyl benzene, manufactured by Dow Chemical Co.; and Amberlite-IRA-400, manufactured by Rohm and Haas and having a similar composition.
Either the Sr or the Y can be eluted first, depending upon the type of ion exchange resin used and the eluting liquid.
A particularly preferred ion exchange technique for separating the Sr and Y ions consists of selectively adsorbing them on manganese dioxide (MnO- Such a technique is described in Analytical Chemistry, November 1967, at pages 1634-1639.
The choice of a suitable eluant also depends upon whether it is desired to elute the Sr ions or Y ions first. In this regard, pH is an important factor, and must be adjusted to obtain the desired result taking into consideration the ion exchange resin used.
The efiect of pH on the Sr -Y separation can be illustrated by using as an example the separation using manganese dioxide as the ion exchange resin. If the eluant has a high pH (alkaline solution), the resin will retain both ions. When the eluant is neutral, the Sr" ions are removed from the resin while the Y ions are retained. When the pH of the eluant is low (acidic solution), the Sr ions are removed from the resin first, followed by removal of Y ions. By varying the pH of an eluant, it is possible to vary the time required for each of the Sr and Y ions to be removed from the resin, and also to vary the time lag between the end of the Sr removal and the beginning of the Y removal.
It will be obvious to those skilled in the art that many other suitable combinations of ion exchange resins and eluting liquids can be used in this separation.
After the Y is removed from the Sr -Y a relatively pure Sr" rich material is left. This material can be used in fabricating the Sr -Y beta source without the concomitant problem of using large amounts of shielding as is required with the Sr Y Because the beta particles emitted by the Sr are of low energy, little shielding is necessary in working with this material.
Selective precipitation, including coprecipitation techniques, is another suitable method for effecting the separation required in this invention. For example, the Y ions can be separated from a solution of Sr -Y by adding an isotopic or nonisotopic carrier such as nonradioactive yttrium ions or ferric ions followed by coprecipitation of the carrier and Y ions. The coprecipitation can be accomplished by adding a precipitating agent such as ammonium hydroxide or ammonium carbonate to the Sr Y and carrier solution. Sr can be separated from a Sr -Y solution by adding a hold back carrier to prevent Y precipitation followed by addition of a precipitating agent including those mentioned above. Other suitable selective precipitations will be recognized by those skilled in the art.
Solvent extraction is another method which may be used to separate the Y from the Sr. Solvent extraction consists of distributing the desired ions between two immiscible liquid phases, one of which is usually an aqueous phase, the other usually being an organic solvent. Normally, the ions are tied up as complexes in solvent extraction by the use of chelating agents or ion association systems. Some examples of suitable chelating agents include acetylacetone, 8-hydroxy quinoline, dimethylglyoxime, cupferron, dithizone, and thenoyltrifluoroacetone. A suitable ion association agent is lead thenoyltrifluoroacetonate.
Beta sources can be prepared from the separated Sr rich material by many methods, many of which are presently described in the literature. For example, a solution of the Sr ions can be introduced into the pores of a heat fusible porous matrix which can thereafter be sealed and encapsulated. Such a process is described in greater detail in Kivel, US. Pat. 3,364,148, issued Jan. 16, 1968. Any of the other well-known processes for fabricating beta sources from Sr -Y" can be simplified by using the Sr rich material.
After the beta source is fabricated, the Y is allowed to grow back to an equilibrium state with the Sr. Because of the difference in their respective half-lives, the source will effectively reach equilibrium after several Y" halflives (one or two weeks).
After the source has reached equilibrium again, it can be calibrated by any of the well-known methods.
In the preferred embodiment of this process, the Y is separated from the Sr by ion exchange chromatography, particularly in an ion exchange column. Such a separation is preferred because it is economical, relatively easy to carry out, and results in an excellent separation.
The process of this invention is useful in the fabrication of the beta sources, which are in turn particularly useful for medicinal applications of radioactivity. Beta sources are conveniently used as blood irradiators, lymph irradiators, nasopharyngeal applicators and beta therapy sources which can be used in the treatment of eye diseases. Beta sources are also useful in any other application where a supply of beta particles can be effectively used to accomplish an objective.
The following examples illustrate the invention. All parts and percentages are by weight unless otherwise specified.
EXAMPLE 1 Separation of Sr from SI -Y by ion exchange chromatography 81* 'is separated from the Sr -Y using the ion exchange technique and equipment described by Choppin, Experimental Nuclear Chemistry, at pages 87-88.
As a check on the separation, a sample is taken of the material from the first peak and the buildup of Y activity is measured and plotted against time. After 64 hours, the halflife of Y 60% of the maximum activity is reached. This compares favorably with theoretical data.
A further check is made by taking one sample of material from each of the two peaks and performing an absorption study on the samples. Various thicknesses of aluminum shielding material are placed between the samples and a geiger counter which is surrounded by lead to eliminate background activity. A plot is made of the shielding thickness versus the log of the measured activity in counts per minute, and the absorption half-thicknesses of the first and second samples corresponds respectively to the theoretical values for Sr and 6 Beta sources can be constructed from the separated Sr rich material by the method of US. Pat. 3,364,148. The sources are allowed to stand for two weeks before calibration, during which time the Y grows back into secular equilibrium with the Sr EXAMPLE 2 Separation of Sr from Sr -Y by selective precipitation Activity No absorber With absorber Time (hr.) S w ou S oo yno The results are characteristic of Sr and Y absorption patterns and also show the characteristic buildup of Y in the Sr samples.
The separated Sr rich material can be fabricated into beta sources with the use of little shielding. Such sources are allowed to stand for two weeks, after which time the Y has grown back to secular equilibrium with the Sr in the beta source.
EXAMPLE 3 Separation of Sr from Sr-"-Y with M110 In a glove box, three curies of Sr -Y in a nitric acid solution is added to the top of an ion exchange column containing a manganese dioxide resin bed, 2 cm. high and 1 cm. in diameter. The M110 is commercial grade with a grain size of 40100 mesh. Nitric acid (0.1 N) is used to elute the adsorbed Sr and elution is monitored by flowing the eluant past a Jordon Radgun ion chamber detector in a thin walled plastic tube. Eluant is collected in a flask until most of the Sr is eluted.
The Y still adsorbed on the M110 is subsequently eluted with 1.0 N nitric acid and the results monitored as above.
A plot is constructed using the eluted volume in milliliters as the abscissa and the radiation activity expressed as intensity (relative units,/hr. as the ordinate. The resulting curve has two peaks, the first representing the separated Sr and the second representing Y The Sr peak is less pronounced than the Y peak, which is a result of the relative energy levels of the beta particles. Additionally, the Sr peak is much broader than the Y peak because an equilibrium mixture of Sr Y contains a much greater amount of the parent nuclide.
The Sr is transferred from the glove box to a fume hood shielded only by thin plastic, where the delicate manipulations required in beta source fabrication can be performed.
The Sr solution is evaporated and the volume adjusted to give a concentration of 200 mg./ml. of Sr (NO in 0.1 N HNO Portions of the active solution are then loaded into a porous glass matrix to form a beta source as described in Kivel, US. Pat. No. 3,364,148. Fabrication can be accomplished within 24 hours of separation, at which time only 24% of the Y has grown back. After fabrication, equilibrium is reached in several half-lives of the Y What is claimed is:
1. A process for the preparation of a Sr --Y high energy beta source in a substantially radiation hazard free manner comprising, in combination, the steps of,
(a) providing a radioactive material comprising Sr (b) separating at least a part of said Sr from said radioactive material to provide a Sr material substantially free from Y and other high energy emitting materials;
(c) fabricating a source from the resulting separated Sr material during the period of time commencing with the separation of the Sr material from the radioactive material and terminating before substantially any Y has grown back in said separated Sr material by radioactive decay of said separated Sr" material to complete substantially the fabrication of said source in said period when substantially only Sr material is present and is emitting only low energy beta rays; and
(d) permitting the resulting fabricated source to decay for a period at least sufiicient to permit the Sr within said fabricated source to become substantially an equilibruim mixture of Sr and Y which may be utilized as said Sr Y high energy beta source.
2. The process of claim 1 further characterized by said period at least sufficient to permit the Sr within said fabricated source to become substantially an equilibrium mixture being at least about 60 hours.
3. The process of claim 1 further characterized by said radioactive material comprising Sr and Y References Cited UNITED STATES PATENTS 3,154,500 10/1964 Jansen, Jr., et al 23338 3,156,532 11/1964 Doering et a1. 252-301.1X 3,188,169 6/1965 Kraus et a1 23338 3,287,084 11/1966 Tuyl 252301.1 X 3,364,148 1/1968 Kivel et al. 252301.1
CARL D. QUARFORTH, Primary Examiner F. M. GITTES, Assistant Examiner US. Cl. X.R.