Absolute standardization and use of 186 Re and 188 Re solutions

The modern trends in use of the radiopharmaceutical products of radiotherapy purposes rely on strong beta- week gamma emitters such as 153 Sm, 177 Lu, 186 Re, 188 Re. All these radionuclides are relatively new in use, and their physico-nuclear parameters and methods of standardization are still unsatisfactorily known. On the other hand, the radioisotope calibrators widely used in Nuclear Medicine laboratories are not calibrated for them. The purposes of the present study were: absolute standardization of 186 Re and 188 Re, calibration of the Radioisotopes Department equipment and calibration of a commercial calibrator. 186 Re + 188 Re mixtures were obtained by the irradiation at the TRIGA-SSR-14 MW reactor of a natural Re target, while 188 Re was extracted from 188 W obtained by the irradiation of a 186 W enriched tungsten target and from a 188 W + 188 Re generator. The radioactive solutions were prepared by chemical processing and then conditioned as standard solutions. 186 Re decays by beta transitions (93.1%) and by electron capture (6.9%), having a half-life of (3.775 ± 0.007) days. The 122.6 and 137.2 keV transitions are strongly converted. 188 Re decays by beta transitions with a half-life of (17.005 ± 0.004) h. The 155.04 keV transition is strongly converted.188 Re has a 'simple triangle' decay scheme, while 186 Re has a 'double triangle' decay scheme. For these radionuclides, in applying the 4π PC - gamma coincidence method-extrapolation variant, a special autotracing method is adopted, in which nonlinear extrapolation curves are usually obtained, with short intervals of linearity. The extrapolation relations such as: NβNγ/NcNo = No[1 + (1-k) (Nγ/Nc-1)] where Nβ,Nγ, Nc are counting rates, No is the activity and (1-k) is the extrapolation curve shape, are experimentally determined. In the case of 186 Re the standardization of the solution was performed 9 days after preparation, and consequently the 188 Re content lowered down to 0.01%. In the case of 188 Re, two series of measurements were performed. In the first case, an initial 10% contamination with 186 Re occurred, which amounted up to a 39% value in the moment of standardization, so that a 186 Re + 188 Re mixture was measured; the activity of 188 Re was calculated from the total activity. In the second case, the 188 Re solution has a radionuclidic purity of 99.9%, so the measurement conditions were chosen to detect 188 Re radiation alone. In conclusion, the study involved both aspects of absolute standardization of solutions and their practical use for the calibration of the existing equipment. (authors)