Patent Number: 044302584
Section: description

The following are examples of various solid calibration standards produced in accordance with the present invention: EXAMPLE 1 Mixed Gamma Ray Standards Requiring Highly Acidic Conditions This process begins with a calibrated aqueous solution containing Cd-109, Co-57, Ce-139, Hg-203, Sn-113, Cs-137, Y-88, and Co-60 or a mixture of only Ce-139, Sn-113, Cs-137, and Co-60 together with about 10 to 200 parts per million of each stable element in 4 N HCl. One volume of the aqueous solution is added to 10 volumes of a first solvent, n-butanol, containing about 2.5% TIOA by volume and stirred until a clear, first solution is obtained. To this first solution, 100 volumes of 50% resin by volume in styrene, the second solvent is added and the mixture is stirred until a clear, second solution is again obtained. The order of addition of reagents is essential or the aqueous solution will not mix with the resin to form a clear solution. An appropriate amount of hardening catalyst (methyl ethyl ketone peroxide) is added and mixed with the second solution. The second solution is allowed to sit until cured, which generally takes about 24 hours. Cooling may be necessary during curing depending on the shape of the standard being cast. In the above example, the strength of the HCl can range from 3-4 N. The ratio of aqueous solution to first solvent (1:10) is the maximum that can be used for 4 N HCl solutions. Any lower ratio can be used if desired. After addition of the mixed radionuclide solution to the first solvent, water may be added as long as the ratio of aqueous solution to first solvent does not exceed 1 to 10. The amount of resin solution can be cut up to 50%, but the mechanical strength of the final product will be decreased. The amount of the resin to second solvent can range from 40-60%; if the strength is below 40%, the resin usually doesn't polymerize. The TIOA stabilizing agent is necessary in this example to complex with the hydrogen ions in the highly acidic solution and to allow it to mix with the co-solvent. The concentration of TIOA can vary from 1% up to 10% by volume without adverse effects. The exact amount of hardening catalyst required depends on the acidity of the aqueous phase, the ratio of first solvent to resin solution, and the size and shape of the standard being cast. The quantity of catalyst is generally determined by tests on nonradioactive solutions prior to making standards. The range of concentration of catalyst runs from 0.3% for 4 liter Marinelli beaker standards to 5% for 20 ml scintillation vials. The large, complicated shaped containers generally require cooling during curing to prevent internal cracking. Room temperatures should be maintained between 60.degree. F. and 90.degree. F. for best results. EXAMPLE 2 Sn-113 Single Isotope Standards Requiring Highly Acidic Conditions Following the procedure outlined in Example 1, single isotope standards of Sn-113 can be produced. An aqueous solution (4 N HCl) consisting of a calibrated quantity of Sn-113 with 10 to 200 parts per million stable tin can be solidified using the materials and proportions given above. TIOA stabilizing agent is necessary to complex with the hydrogen ions in this system, also. EXAMPLE 3 Long-Lived Mixed Gamma Ray Calibration Standards Requiring Moderate Acidity Four different mixtures of long lived radioisotopes can be used with moderately acid solutions of 0.1 N HCl to 1.0 N HCl. These mixtures are (1) Ba-133, Cs-137, and Co-60; (2) Ba-133, Cs-137, Mn-54, and Co-60; (3) Cd-109, Eu-152, and Co-60; and (4) Eu-154, Eu-155, and Sb-125. The procedure of Example 1 can still be used to solidify these solutions; however, the concentration of TIOA and of hardening catalyst may be reduced due to the lower acidity. TIOA concentration may vary from 0.1% to 10% by volume for these mixtures. The amount of catalyst needed is generally lower than the amount required for highly acid solutions, but it is best to determine the exact amount by experimentation with similar non-radioactive solutions. EXAMPLE 4 Single Isotope Standards Requiring Moderately Acid Conditions Most of the radionuclides of interest in calibration standards fall into this category. Solutions of the following radioisotopes may be solidified using the procedure outlined in Example 3: 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, and Sr-85. Generally, carrier or stable element concentrations are kept below 100 parts per million but higher concentrations do not interfere, except in the case of manganese. Manganese concentrations of 500 ppm and above interfere with the polymerization process. EXAMPLE 5 Single Isotope Standards Requiring Reducing Conditions in a Basic Medium Isotopes of iodine require reducing conditions in basic aqueous solutions to prevent the formation and loss of volatile iodine compounds. Solutions of I-125, I-129 or I-131 are kept in NaOH or KOH solutions with a sulfite reducing agent. In these solutions; the hydroxide ion concentration varies from 0.01 N to 0.2 N and the sulfite reducing agent concentration is of the order of 10 to 100 parts per million. Stable iodine is generally added to I-125 and I-131 solutions in concentrations between 1 and 100 parts per million. These iodine solutions may be solidified using the procedure outlined in Example 1. When working with basic solutions, the TIOA stabilizing agent is not required. However, TIOA does not interfere with the process and may be used without any adverse effects. The presence of a small amount of reducing agent does not interfere with the solidification process. EXAMLE 6 Single Isotope Standards Requiring Basic Conditions Standards may be prepared using Cr-51 as the chromate in a weakly basic solution. The general procedure outlined in Example 1 is used to solidify Cr-51 solutions when the chromate concentration is less than 200 parts per million and the hydroxide concentration is less than 0.2 normal. TIOA is generally used when solidifying these solutions, but it is not necessary. There are several advantages to the solid standards prepared by the present invention. For instance, custom made solid radioactive calibration standards can be prepared in any of the common counting containers in use in the nuclear industry. Also, the number of standards rejected due to plate-out or precipitation has been reduced to a negligible level, well below 10% of the number rejected when using other processes. The process requires only gentle stirring with a hand held rod or a laboratory mixer. Standards can be mixed in the final container without loss of material, thereby eliminating additional weighing errors and reducing the volume of radioactive waste generated in the production process. Glass stirring rods can be used instead of the expensive Teflon-coated metal rods required for high speed mixing in the emulsion process. Using glass stirrers is also advantageous, because Teflon-coated metal stirrers frequently experience plate-out of radioactive material on metal surfaces exposed by abrasion. The solid standards of the present invention have the high degree of homogeneity required for calibration standards. After curing, the physical dimensions are constant and the gamma ray attentuation properties show no variation with time. Also, the solid standard material of the present invention is not subject to the freeze-thaw damage of emulsion solids, thereby making winter shipment possible. Further, the solidification process of the present invention incorporates all of the initial aqueous solutions in the final solid. In contrast with the prior art extraction process, there is no aqueous phase remaining and no additional radioactivity assays are required. Finally, the present solidification process has been able to produce homogenous solid standards using a wide variety of radioactive elements. Mixed standards using the eight element mixture of Cd, Co, Ce, Hg, Sn, Cs, and Y are easily produced with no plate-out or precipitation problems. Standards using radioactive iodine have been prepared with no volatilization loss of iodine and have exhibited an exceptional stability which far exceeds other forms of radioactive iodine standards.