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Determination of long half-lives


Figure: Is the half‑life of silicon‑32 correct? In collaboration with the Paul Scherrer Institute (PSI) and other partners, PTB will pursue this question and very precisely determine other long half‑lives as well.

Department 6.1, Radioactivity, has in recent years already determined the half‑lives of numerous long‑lived radionuclides. In this process, the activity is usually combined with the number of nuclei. While PTB is responsible for the accurate determination of activity, the number of atomic nuclei of a radioactive isotope is in most cases determined by means of mass spectrometry conducted by external partners. A prerequisite for all such half‑life determinations, however, is the availability of a suitable base material, which must, in addition to sufficient activity, possess high radionuclide purity and a suitable chemical structure. The necessary radiochemical expertise in this field is available at the Paul Scherrer Institute (PSI) in Villigen, Switzerland. For this reason, PTB has for several years now been working successfully with PSI and other partners to determine a number of relevant long half‑lives with a high degree of accuracy.

This collaboration has now enabled the first‑ever determination of the half‑life of molybdenum‑93 by means of a direct method [1]. Long‑term measurements with liquid scintillation spectrometers and a rather complex analysis of the data have also made it possible to determine the probabilities of two competing electron‑capture branches.

The determination of the half‑life of manganese‑53 [2] is also nearly complete and the results are now being prepared for publication. A particular challenge with manganese‑53 was a contamination by manganese‑54, which cannot be removed chemically. Scientists at the University of Mainz were finally able to remove the manganese‑54 by means of mass separation, in the process producing a unique material found nowhere else in the world. At PTB, the activity measurements were carried out with special liquid scintillation counters for the application of the TDCR method [3].

The SINCHRON project funded by the Swiss National Science Foundation (SNSF) is now focused on determining the half‑life of silicon‑32. The success of the chemical work was already demonstrated back in 2020. The activity measurements, here being done with two established liquid scintillation counting methods, are also promising. A discrepancy of approximately one percent that was observed between the different methods during initial measurements has now been eliminated by improving the chemical preparation of the material. It is assumed that the previously observed discrepancy was caused by radioactive contamination with tritium. A confirmation of this thesis is, however, still pending.

A number of further radionuclides are also on the agenda: The half‑lives of terbium‑157 and lanthanum‑137 are to be measured as accurately as possible within the scope of the ERAWAST III project being sponsored by SNSF and DFG. Both radionuclides are electron‑capture nuclides with relatively high atomic numbers. The rearrangement processes in the atomic shell that occur as a consequence of electron capture are very complex and activity determinations of these radionuclides are considered extremely challenging. As was the case with the above‑mentioned nuclides, liquid scintillation counting will here again play a central role in the determination of activity.

The half‑life of potassium‑40 has already been determined once at PTB [4]. With enriched materials, however, new findings on the decay data and thus also on the half‑life have been obtained. These measurements were carried out by means of mass spectrometry at the Australian National University. A detailed publication describing the measurement methods, the new decay data and the half‑life is currently being prepared. Long half‑lives are needed in different fields, depending on the radionuclide. Some radionuclides are important in the treatment of radioactive waste, while others facilitate important studies in the geosciences, including climate research. The very long‑lived primordial potassium‑40 is one of the most important naturally occurring radionuclides in geochronology.


[1]    Kajan, I., Heinitz, S., Kossert, K., Sprung, P., Dressler, R., Schumann, D.: The first direct determination of the 93Mo half-life, Scientific Reports 11:19788, https://doi.org/10.1038/s41598-021-99253-5.

[2]    Ulrich, J., Cassette, P. Dressler, R., Kneip, N., Kossert, K., Mougeot, X., Schumann, D., Sprung, P., Studer, D., Wendt, K.: 53Mn half-life determination. Paul-Scherrer Institut, Laboratory of Radiochemistry, Annual Report 2020 (https://www.psi.ch/de/lrc).

[3]    Broda, R., Cassette, P., Kossert, K.: Radionuclide Metrology using Liquid Scintillation Counting. Metrologia 44 (2007) S36-S52.

[4]    Kossert, K., Günther, E.: LSC measurements of the half-life of 40K. Applied Radiation and Isotopes 60 (2004) 459-464.


Opens local program for sending emailK. Kossert, Department 6.1, 6.14

Opens local program for sending emailD. Schumann, Paul-Scherrer Institut, Laboratory of Radiochemistry