Long-lived radionuclides half-life determination

Determination of Half-life of Long-lived Radionuclides... 

A major topic in isotope mass spectrometry is the determination of the half-lives of long-lived radionuclides. De Bievre and Verbruggen34 determined the half-life of 241 Pu for 3-decay in the isobaric radionuclide 241 Am on material from Oak Ridge that had initially been about 93% isotopically enriched. Due to the isobaric interference of 241 Pu and 241 Am radionuclides during mass spectrometric measurements by TIMS, Am had to be removed by chemical separation immediately (less than 48 h) prior to measurements as described in reference 34. On the basis of all the measurements performed over an extended period of more than 20 years and after considering the possible effects of systematic errors during these measurements, a half-life for the 3 decay of 241 Pu of (ti/2 = 14.290 0.006 a) was reported.34... 

Extinct radionuclides have a huge advantage in precision over long-lived radionuclides. For example, a change in a factor of two in the abundance of requires only one half-life, 15.7 Ma, while the same length of time will cause a change of less than 1% in the abundance of since it is only about 1% of a half-life. On the other hand, since the extinct radionuclide is, by definition, completely decayed away, it is necessary to determine its abundance at some particular time to determine ages, a problem for all extinct radionuclides. 

The process of determining a permanent disposal solution involves social, political, economic, and technical issues that compete and coalesce to determine the specific end state. Characteristics that make permanent disposal of TRU and HLW complex include contamination with long-lived radionuclides (e.g., plutonium-239 has a half-life of approximately 24,000 years) and high concentrations of radioachvity. While less contentious, LLRW disposal also presents challenges, as discussed in Section 16.6. The discussion in this section is primarily applicable to TRU and HLW disposal. 

However, the limitation of particle-counting techniques is that the half-life of the analyte isotope has a significant impact on the method s detection limit. This means that to get meaningful data in a realistic amount of time, they are better suited for the determination of short-lived radioisotopes. They have been successfully applied to the quantitation of long-lived radionuclides, but unfortunately require a combination of extremely long counting times and large amounts of sample to achieve low levels of quantitation. 

When an element has more than one radioisotope, determinations and data analysis are generally more complex because the isotopes may differ in half-life, especially when a series is involved, e.g., radium, thorium, polonium, radon, actinium, protactinium, and uranium. One possibility is to make measurements after the decay of the short-lived radionuclides, but this may require long waiting times. In favorable cases, it is more convenient to measure the activity of decay products (e.g., radon, thoron ( Rn), actinon ( Rn)), or correct the measurements of the short-lived radioisotopes after determination of the isotopic composition. 

---