Patent Number: 044302584
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the calibration of gamma spectrometer systems and, more particularly, to a method of producing liquid equivalent solid gamma ray calibration standards. 2. Discussion of the Prior Art Gamma spectrometer systems using either NaI(Tl) or Ge(Li) detectors are used to perform qualitative and quantitative assays of liquid samples for radioactive material. Before the assays can be performed, the spectrometer system must be calibrated. This means that the relationship between gamma ray energy and analyzer channel number as well as the detector efficiency as a function of gamma ray energy must be known. These calibrations are best accomplished using standard gamma ray sources which emit gamma rays of known energy and have an accurately known gamma ray emission rate. With modern high resolution Ge(Li) gamma ray detectors, it is most economical to calibrate using mixed isotope sources which emit numerous gamma rays of different energies. The energy vs. channel number calibration is very easily performed by fitting a polynomial function to the energy vs. channel number data from a mixed source. In most systems, a linear relationship is sufficiently accurate but a quadratic or a cubic equation can be fit to the data for the most accurate calibration. Unfortunately, the efficiency calibration of a gamma spectrometer system is more complicated than the energy calibration. The detector efficiency varies with the energy of the gamma ray and with the distance of the source to the detector. Variations of the detector efficiency with source distance produce variations in sample counting efficiency with container size and shape. Standards containing known quantities of radioactive material in each size and shape container must be measured in each counting position before quantitative analyses can be performed. Calibration standards must also be approximately the same density as the samples being analyzed to avoid the problem of correcting for differences in gamma ray scattering and absorption. Liquid solutions of radioactive material commonly used as calibration standards suffer from several deficiencies. Liquid sources frequently leak or are spilled on radiation detectors, laboratory areas, or laboratory personnel. These types of accidents with high level radioactive material dissolved in chemically corrosive solutions can be very serious. More difficult to detect, low level-long term leakage from radioactive standard solutions has frequently contaminated radiation detection equipment to the point where it is useless in the measurement of low level radioactivity. In addition to these difficulties in the use and storage of liquid radioactive standards, disposal of expired liquid standards is also a problem. Current radioactive waste disposal regulations prevent most laboratories from directly disposing of the quantities of liquid radioactive material used as standards without large dilution tanks. Liquid radioactive waste generally must be solidified in some manner before disposal. Aside from the question of safety, the preparation and storage of liquid radioactive standards is complicated by the solution chemistry of the elements involved. Mixtures containing widely different chemical species are particularly troublesome. Selective plate-out, precipitation, or volatilization can destroy the homogeneity of a liquid standard and render it useless for calibration purposes. The seven element mixture consisting of Cd, Co, Ce, Hg, Sn, Cs and Y commonly used in gamma ray calibration standards is plagued by problems of this kind. If the solution is weaker than 4 N in HCl, the Sn-113 will precipitate. Maintaining this high acidity for a long period of time in relatively porous plastic containers has been difficult. Also, certain types of plastic have been found to selectively remove tin from these standard solutions. Plate-out frequently occurs on glass surfaces where Y, Ce, and Cs are ion exchanged onto glass container surface, thus destroying the standard. Deposits of dirt or chemical residues from laboratory air can cause selective precipitation of some of the elements in a mixed standard. Consideration of these problems with liquid radioactive standards has led to the development of liquid equivalent solid radioactive sources with the radioactive material uniformly dispersed in a low density solid whose gamma ray attentuation properties closely match those of water. Previous Solid Standards Various approaches to the production of solid gamma ray calibration standards have been attempted in the past few years. Processes for solidification of radioactive waste such as the vinyl ester method of Dow Chemical Company or the urea-formaldehyde process of Chem Nuclear Systems, Inc. proved to be unsatisfactory for standard production. The density and homogeneity of solid radioactive sources produced by these methods are not acceptable for calibration standards. Various gelation agents have also been tested and discarded due to instability in the semi-solid products. Progress toward more satisfactory solid standards has been made with emulsified resin mixtures, solvent extraction systems, and water miscible resins. Solid calibration standards have been prepared using resin-water emulsions. Ashland Chemical Company's water extendible polyester (U.S. Pat. No. 3,256,219) is an example of this type of material. To produce solid gamma ray calibration standards using the emulsion technique, an aqueous solution of radioactive material is mixed with an equal weight of polyester resin to form an emulsion which is solidified by adding a catalyst. This process has several deficiencies when applied to the production of gamma ray calibration standards. Uniformity on the fine scale necessary for calibration standards is difficult to obtain using emulsions. The very high speed mixing necessary to produce an emulsion requires that the material be mixed in something other than the final standard container, thereby greatly increasing the volume of radioactive waste generated in the production of customized radioactive standards. Since the radioactive material in an emulsion is generally in the small droplets of aqueous solution, the same plate-out and precipitation processes which plague liquid solution standards cause problems in emulsion systems during mixing. In general, plate-out problems are greater in the emulsion system because the acidity of the aqueous solutions must be kept at a low level or the hardening catalyst will be destroyed. Mixed calibration standards which require Sn-113 are very difficult to produce by the emulsion method due to the high acidity required to stabilize tin in aqueous solution and the high affinity of tin for various container materials. The high percentage of water required to produce a stable, uniform emulsion introduces additional constraints on calibration standards produced from emulsions. Ashland describes these emulsion-produced solids as "water-filled foams," and states that these solids lose water by evaporation over a long period of time. Data published by Ashland Chemical Company shows the water loss to be 0.51% in three days and over 9% in one year. This protracted loss of weight means that the gamma ray attentutation properties of the material are continuously changing over the life of standards produced from it. Furthermore, the water loss rate is not predictable and may be considerately greater than the above values depending on the size, shape, and outside container of the standard. Due to the presence of many small pockets of water, emulsion type solids may suffer cracking problems when shipped in cold weather or when stored in unheated storage areas. In order to escape the difficulties caused by the presence of free water in emulsion produced solids, solvent extraction systems were investigated. Numerous complexing agents have been developed which are capable of extracting various metal ions from aqueous solutions and stabilizing these ions in water immiscible solvents. Complexing agents can be used to extract certain radioactive metal ions from aqeuous solutions into resin solutions where they may be solidified. Single element solid standards produced by the solvent extraction method show excellent uniformity in the final product when the correct conditions for extraction are achieved. Solids of this type do not contain significant quantities of water and are free from the previously discussed problems of water containing systems. Unfortunately, changing from an aqueous system to a non-aqueous solvent system complicates the solution chemistry. Quantitative extraction of metal ions is rarely observed and the conditions of metal ion concentration and acidity must be strictly controlled. Uncertainty in the actual amount of metal ion extracted is a very serious problem in these types of standards. Additional radioactivity assays must be performed after the extraction on either the aqueous phase or the less stable non-aqueous solution. Either of these procedures introduces additional uncertainty into the calibration on the final standards. These extraction problems become much more severe when the production of standards containing a mixture of elements is considered. The optimum conditions for extraction vary widely from element to element and limit the number of different elements that can be incorporated into the same resin solution. For these reasons, extraction standards using the eight element mixture previously discussed have been very difficult to produce and the accuracy of the final product has been poor. Another approach to the production of solid gamma ray calibration standards uses a water miscible resin to dissolve and solidify the radioactive aqueous solution. This process is better suited to the production of custom made standards than is the emulsion process due to the fact that only gentle stirring is necessary to mix the components. Some standards produced using water miscible resins have been found to be of sufficient uniformity for calibration standards. Having not been designed for calibration standard production, commercially available water miscible resin systems cannot tolerate the high acidity necessary to stabilize aqueous radioactive solutions. High acidity prevents the resin from mixing with the aqueous solution and also destroys the catalyst. If the acidity of the aqueous solution is decreased to accomodate the resin, plate-out of radioactive material occurs before solidification. Plate-out problems limit the type of containers that can be used with the water miscible resin systems. Soft polyethylene and cellulose plastic containers cannot be used due to selective plate-out of some of the components of mixed standards solutions. Containers which have acquired a film of basic residue while sitting in the laboratory cause plate-out of Ce, Y and Cs from standard solutions. Soft glass containers also show a tendency to ion exchange Ce, Y, and Cs onto their surfaces. These plate-out problems greatly increase the cost of preparing standards using the water miscible resin system by causing more standards to be rejected, using more materials, and increasing the quantity of radioactive waste. SUMMARY OF THE INVENTION The above disadvantages of the prior art are overcome by the present invention which is directed to a new approach to the solidification of aqueous radioactive solutions by employing a first solvent which allows an aqueous solution of radioactive material to form a homogenous solution with a water immiscible resin dissolved in a second solvent. The process of the present invention is utilized for either mixed or single isotopes requiring either highly or moderately acidic conditions or requiring basic conditions. With highly acidic solutions of radioactive material, a stabilizing agent is present in the first solvent to complex with metal and hydrogen ions, preventing plate-out. The process requires only a gentle mixing of the ingredients and is generally performed in the final standard container. The resultant solution is normally allowed to sit until cured. The currently preferred components include the first solvent being any alcohol with less than 5 carbon atoms, and preferably being butanol; the stabilizing agent being triisooctylamine (TIOA); the second solvent being styrene; the resin being an unsaturated polyester resin derived from ethylene glycol, maleic anhydride and fumaric acid and the catalyst being methyl ethyl ketone peroxide. The present invention has been utilized to produce solid standards comprising mixed isotope sources such as Cd-109, Co-57, Ce-139, Hg-203, Sn-113, Cs-137, Y-88 and Co-60; Ce-139, Sn-113, Cs-137 and Co-60; Ba-133, Cs-137 and Co-60; Ba-133, Cs-137, Mn-54 and Co-60; Cd-109, Eu-152 and Co-60; and Eu-154, Eu-155 and Sb-125; and single isotopes such as Sn-113, Eu-152, Eu-154, Eu-155, Cd-109, Co-57, Co-58, Co-60, Ce-144, Ce-139, Y-88, Cs-134, Cs-137, Cr-51, Fe-59, Zn-65, Mn-54, Ba-133, Sb-125, Hg-203, Na-22, Sr-85, I-125, I-129, T-131 and Cr-51. DETAILED DESCRIPTION OF THE INVENTION This invention is a process for producing liquid equivalent solid gamma ray calibration standards utilizing a monomer, first solvent which is capable of dissolving an aqueous solution of radioactive material, keeping the aqueous solution dissolved while forming a homogenous solution with a polymerizible resin dissolved in a second monomer solvent and reacting with the resin and second solvent under the influence of a catalyst to produce a polymeric, homogenous solid containing the original radioactive material. The first solvent can contain a stabilizing agent to complex with metal and hydrogen ions to prevent plate-out and allow highly acidic solutions to be solidified. The currently preferred use of this invention employs n-butanol as the first solvent; triisooctylamine (TIOA) as the stabilizing agent; styrene as the second solvent; an unsaturated polyester resin derived from ethylene glycol, maleic anhydride, and fumaric acid as the polymerizible resin, preferably USS Polyesters' MR11048 casting resin; and methyl ethyl ketone peroxide as the catalyst. The first solvent can be any alcohol with less than five carbon atoms. Experiments have been conducted with a series of alcohols including n-amyl alcohol, isoamyl alcohol, and all alcohols with less than five carbon atoms. The first experiments were conducted without radioactive material to determine the ability of each alcohol to mix with an acidic aqueous carrier solution and to form a homogeneous solid with polyester resin. From the first tests, it was concluded that alcohols of less than five carbon atoms could be used to solidify aqueous carrier solution. A second series of experiments with radioactive material was conducted using the mixed gamma standard solution of the present invention which confirmed that alcohols with less than five carbon atoms could be used in the process to produce solid radioactive standards.