Radiative lifetime and energy of the low-energy isomeric level in 229Th

We estimate the range of the radiative lifetime and energy of the anomalous, low-energy 3/2(+)(7.8 +/- 0.5 eV) state in the Th-229 nucleus. Our phenomenological calculations are based on the available experimental data for the intensities of M1 and E2 transitions between excited levels of the 229Th nucleus in the K-pi[N n (Z) Lambda] = 5/2(+)[633] and 3/2+[631] rotational bands. We also discuss the influence of certain branching coefficients, which affect the currently accepted measured energy of the isomeric state. From this work, we establish a favored region, 0.66 x 10(6)s eV(3) / omega(3) <= tau <= 2.2 x 10(6)s eV(3) / omega(3), where the transition lifetime tau as a function of transition energy omega should lie at roughly the 95% confidence level. Together with the result of Beck et al. [LLNL-PROC-415170 (2009)], we establish a favored area where transition lifetime and energy should lie at roughly the 90% confidence level. We also suggest new nuclear physics measurements, which would significantly reduce the ambiguity in the present data.