Improved limit on the electron capture decay branch of ¹⁷⁶Lu

Abstract

We have performed searches for the electron-capture decay branches of ¹⁷⁶Lu to the ground state and first excited state of ¹⁷⁶Yb. No evidence of either decay mode was observed. From these measurements we have established upper limits on both of these possible branches that are each >20 times more stringent than the single previously published limit.

Introduction

The long-lived naturally occurring nuclide ¹⁷⁶Lu (J^π=7⁻) β⁻ decays to levels in ¹⁷⁶Hf. This decay is the basis for a dating method increasingly applied to rocks and meteorites. The utility of this dating method has been hampered by uncertainty in the half-life, as recently reviewed by Begemann et al. (2001). To some extent the half-life estimates vary systematically according to the approach used for its determination, with the lowest values (3.50–3.57)×10¹⁰ years, implied from mass spectrometric measurements of Lu- correlated ¹⁷⁶Hf excesses in meteorites whose ages were independently known (Patchett and Tatsumoto, 1980; Blichert-Toft et al., 2002; Bizzarro et al., 2003). A half-life value of (3.73±0.02)×10¹⁰ years obtained from the direct counting experiment of Nir-El and Lavi (1998) agrees with the (3.72±0.03)×10¹⁰ years value determined by Scherer et al. (2001) through age comparisons for terrestrial rocks. However, Browne and Junde (1998) evaluated the results of a large number of direct counting measurements and recommended a higher value of (4.00±0.22)×10¹⁰ years. Moreover, the recent and precise γ–γ coincidence measurement of the ¹⁷⁶Lu half-life of (4.08±0.03)×10¹⁰ years by Grinyer et al. (2003), which agrees very well with the earlier direct γ-ray counting results of (4.08±0.24)×10¹⁰ years by Norman (1980) and (4.05±0.09)×10¹⁰ years by Gehrke et al. (1990), reaffirms a systematic difference of order 15% in the half-life deduced from these two approaches, which must be resolved before the ¹⁷⁶Lu-¹⁷⁶Hf method can be established as a reliable geo- and cosmo-chronometer.

It has been suggested that the discrepancies involving age comparisons could be reconciled if ¹⁷⁶Lu also underwent significant electron capture decay. ¹⁷⁶Lu is unstable with respect to electron-capture decay to ¹⁷⁶Yb. The Q_EC for decay to the ¹⁷⁶Yb ground state is 106.2 keV (Firestone, 1996). Thus, EC decays to both the J^π=0⁺ ground state and J^π=2⁺ 82-keV first excited state of ¹⁷⁶Yb are possible. These EC decay branches would be 7th and 5th forbidden transitions, respectively, and thus are expected to be negligibly small. In fact, there are no observed cases of such highly forbidden transitions (Singh et al., 1998). The decay scheme of ¹⁷⁶Lu is illustrated in Fig. 1. The published limit on the EC decay branch of ¹⁷⁶Lu of < 10% was reported by Arnold (1954) in his study of the decay of ¹⁷⁶Lu. This upper limit was deduced from a search for Yb K X-rays that would be produced by the EC decay of ¹⁷⁶Lu. Because of the recent inconsistencies encountered in using the ¹⁷⁶Lu/¹⁷⁶Hf chronometer, it was felt that a new search for the EC decay of ¹⁷⁶Lu was warranted.