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 ²²⁹Th nucleus. Our phenomenological calculations are based on the available experimental data for the intensities of M 1 and E 2 transitions between excited levels of the ²²⁹Th nucleus in the K^π[N n_ZΛ ] =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 ×10⁶seV³/ω³≤τ ≤2.2 ×10⁶seV³/ω³ , where the transition lifetime τ as a function of transition energy ω 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.