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New Determination of the Half-life of Be-10 - A Contribution to Research on Climate History

04.01.2010

For research on climate history, the radioactive isotope 10Be (beryllium-10) plays an important role. This isotope is generated by cosmic radiation in the Earth's atmosphere and is washed out by rainfall so that it can also attain the ice in the polar caps. The quantity of generated 10Be depends on the intensity of the cosmic radiation which, in turn, depends on the behaviour of the Sun - especially of its magnetic fields.

In this way, the analysis of the quantity of 10Be in ice cores from Greenland or the Antarctic supplies precious information on the solar activity and the climate in the past. The procedure is comparable with the analysis of the - likewise cosmogenic - isotope 14C (carbon-14) in a tree’s annual rings. Due to the longer half-life of 10Be it is, however, possible to cover a longer period of time.

Another application is the investigation of the melting of glaciers. In the ice-free stone, 10Be can be found, too. Since the ice more or less acts as a shield, it can be determined by means of the quantity of 10Be how long the stone has been free of ice.

An essential pre-condition for reliable applications of such and other methods is the accurate knowledge of the half-life of 10Be. The published values have great uncertainties and exhibit large discrepancies.

In two independent experiments in which several institutes were involved, its half-life was now determined with a considerably lower uncertainty. In an experiment [1], the number of 10Be nuclei was determined by means of a mass-spectrometric method, the so-called Multi-Collector Inductive Coupled Plasma Mass Spectrometry (MC-ICP-MS). The activity was measured at PTB by means of liquid scintillation counting, whereby the efficiency was determined by means of an efficiency-tracing method. For the beta emitter 10Be, whose spectrum had previously been checked at PTB [2], it is possible to achieve small uncertainties with this method. In the second experiment [3], the activity was measured in Munich by means of liquid scintillation counting, whereby here, too, the efficiency-tracing method was used. In Munich, however, the number of 10Be nuclei was determined for the first time by means of another method: the Heavy Ion Elastic Recoil Detection Method (HI-ERD).

The half-lives determined in the two experiments, T½ = (1.386 ± 0.016) million years [1] and T½ = (1.388 ± 0.018) million years [3], are in excellent agreement. As the uncertainties of the efficiency computation for the liquid scintillation counting are very low, the independent results can be considered as being uncorrelated. By combining the two values, the authors of the two studies have determined a new value for the half-life of 10Be amounting to T½ = (1.387 ± 0.012) million years. The relative uncertainty of this value is thus smaller by more than a factor 4 than the smallest relative uncertainty achieved so far for this half-life.

Literature

  1. Chmeleff, J., v. Blanckenburg, F., Kossert, K., Jakob, D.:
    Determination of the 10Be half-life by multicollector ICP-MS and liquid scintillation counting.
    In: Nuclear Instruments & Methods B, in press, doi:10.1016/j.nimb.2009-09-012.
  2. Grau Carles, A.; Kossert, K.:
    Measurement of the shape-factor functions of the long-lived radionuclides 87Rb, 40K and 10Be.
    In: Nuclear Instruments and Methods A 572 (2007). 760-767.
  3. Korschinek, G., Bergmaier, A., Faestermann, T., Gerstmann, U.C., Knie, K., Rugel, G., Wallner, A., Dillmann, I., Dollinger, G., Lierse von Gostomski, Ch., Kossert, K., Maiti, M., Poutivtsev, M., Remmert, A.:
    A new value for the half-life of 10Be by heavy ion elastic recoil detection and liquid scintillation counting.
    In: Nuclear Instruments and Methods in Physics Research Section B, in press, doi:10.1016/j.nimb.2009.09.020.