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Description
Knowing the half-life of a radioactive isotope as precisely as possible is crucial for many research areas, for example nuclear astrophysics, nuclear energy technology or environmental research. Radionuclides with half-lives of hundreds to millions of years are particularly important, not only because of their significance in the relevant scientific field but also because of their potential impact on humans and the environment. Their half-lives cannot be determined by following the decay but have to be measured by the direct method, requiring the determination of the specific activity and the corresponding number of atoms. The half-lives of a many of these long-lived nuclides are poorly known, with data only based on a small number of measurements that can be very old. Some of these half-lives have large uncertainties, or the results of various studies are discrepant. This situation is largely caused by the low quantity and poor quality of the available samples. Especially for radionuclides that decay via electron capture only (so-called “hard-to-measure-isotopes”), the exact determination of the activity is a big challenge, since a radiochemically pure sample is required. The required level of radiochemical purity can be achieved either by a dedicated production route or by isolation of the wanted isotope using preparative mass separation.
In our present contribution, we focus on two such isotopes, 53Mn and 93Mo. 53Mn is a radionuclide that is mainly produced in freeze-out phases of nuclear statistical equilibria of explosive burning in both thermonuclear and core-collapse supernovae explosions. Although the primordial 53Mn is now extinct, its former presence can be observed by enhancements of its daughter 53Cr in meteorites from asteroids formed in the Early Solar System. The 53Mn/53Cr chronometer could be used for precise dating of events in the Early Solar System on the timescale of 20 Ma [1], given that the half-life is known with low uncertainty.
93Mo is produced by neutron activation of stable 92Mo, and, since molybdenum is a component of the construction materials in nuclear power plants (e.g. in steel, Inconel alloys and cables), it becomes the dominant dose contributor in low and intermediate level radioactive waste after the shorter-lived radionuclides have decayed. It is one of the few long-lived isotopes where no direct experimental determination of half-life existed so far.
The present work reports the determination of the half-lives of 53Mn and 93Mo using a combination of isotope dilution mass spectrometry for the measurement of the numbers of atoms, and liquid scintillation counting for measurement of activity. Low mass resolution multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) was used for 93Mo, and a combination of high mass resolution MC-ICP-MS and thermal ionization mass spectrometry were used for 53Mn.
A radiochemically pure sample of 93Mo was directly produced by proton irradiation of niobium and followed by a chemical separation of molybdenum with a decontamination factor larger than 1.6×1014. The 53Mn sample, chemically separated from irradiated steel samples, underwent a mass separation using the RISIKO laser mass separator facility at the University of Mainz to get rid of 54Mn that would interfere with the activity measurements.
The determination of the activity is particularly challenging in the case of 93Mo since the decay partly leads to the isomer 93mNb. With the help of long-term liquid scintillation counting measurements and a very complicated evaluation method, the decay probability of 93Mo to 93mNb could be determined with unprecedented accuracy in addition to the activity and the half-life value [2].
We will present details on the sample preparation as well as the corresponding activity and mass spectrometry measurements and report on the results [2,3].
[1] Birck, JL., Allègre, C. Manganese—chromium isotope systematics and the development of the early Solar System. Nature 331, 579–584 (1988). https://doi.org/10.1038/331579a0
[2] [2] Kajan, I., Heinitz, S., Kossert, K. et al. First direct determination of the 93Mo half-life. Sci Rep 11, 19788 (2021).
[3] J. Ulrich, PhD thesis 2020, High precision nuclear data of 53Mn for astrophysics and geosciences, University of Berne, Switzerland