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Muon production of 92,94Nb in the Earth's crust

Naturevolume 266pages712713 (1977) | Download Citation



ACTIVITIES due to long-lived 92Nb (t1/2 1.7×108 yr) and to 94Nb (t1/2 = 2.03×104 yr) in a refined bar of terrestrial niobium were discovered by Apt et al.1. They speculated that the 92Nb activity could have applications to chronologies of nucleosynthesis and to cosmogony. As an example, they calculated that their observed activity level would result if the isotopic ratio 4.7×109 yr ago were 92Nb/93Nb = 10−3 and if the halflife were 1.58×108 yr. More definite conclusions were difficult because the concentration expected today depends so strongly on the poorly known halflife, which they estimated to be 1.7×108 yr from a reactor irradiation. This sensitivity can be illustrated by noting that a 5% error in halflife corresponds after 4.7×109 yr to an error of a factor 2.74 in the anticipated activity. Quantitative calculations from galactic nucleosynthesis must therefore await a more precise measurement of the halflife. Implications of primordial 92Nb have recently been enlarged with the proposal2 to use enrichments δ(92Zr) in the isotopic composition of Zr due to extinct 92Nb to measure absolute time differences in the formation of meteorites and planets. If an initial 92Zr/94Zr ratio for each body can be obtained, differences between those initial values could be translated into time differences in the formation of the respective objects. We argue here that both projects are probably doomed to disappointment. It seems to us unlikely that the near equality in the two measured activities is a coincidence. They are respectively 0.058±0.035 d.p.m. per kg (Nb) for 92Nb and 0.32±0.03 d.p.m. per kg (Nb) for 94Nb. Such a result would be expected, for example, if both nuclei were created terrestrially over geologic time in the ratio of their activities, in this case in a production ratio p(92Nb)/p(94Nb) = 0.18±0.11. We suggest that nuclear reactions following the capture of stopped cosmic-ray muons is the likely source, and we will show that both the magnitude of the activities and their relative strengths are consistent with this idea. Similar ideas have been developed by others, most notably to explain Xe anomalies in Te ores3.

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    Apt, K. E., Knight, J. D., Camp, D. C. & Perkins, R. W. Geochim. cosmochim. Acta. 38, 1485–1488 (1974).

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    Morgan, J. A. thesis, Rice Univ., Houston (1976).

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  1. Department of Space Physics and Astronomy, Rice University, Houston, Texas, 77001

    •  & JOHN A. MORGAN


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