Radiative lifetime and energy of the low-energy isomeric level in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mmultiscripts><mml:mi mathvariant="normal">Th</mml:mi><mml:mprescripts/><mml:none/><mml:mn>229</mml:mn></mml:mmultiscripts></mml:mrow></mml:math>

We estimate the range of the radiative lifetime and energy of the anomalous, low-energy $3/{2}^{+}(7.8\ifmmode\pm\else\textpm\fi{}0.5$ eV) state in the $^{229}\mathrm{Th}$ nucleus. Our phenomenological calculations are based on the available experimental data for the intensities of $M1$ and $E2$ transitions between excited levels of the $^{229}\mathrm{Th}$ nucleus in the ${K}^{\ensuremath{\pi}}[N{n}_{Z}\mathrm{\ensuremath{\Lambda}}]=5/{2}^{+}[633]$ 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, $0.66\ifmmode\times\else\texttimes\fi{}{10}^{6}{\text{s}\phantom{\rule{4.pt}{0ex}}\text{eV}}^{3}/{\ensuremath{\omega}}^{3}\ensuremath{\le}\ensuremath{\tau}\ensuremath{\le}2.2\ifmmode\times\else\texttimes\fi{}{10}^{6}{\text{s}\phantom{\rule{4.pt}{0ex}}\text{eV}}^{3}/{\ensuremath{\omega}}^{3}$, where the transition lifetime $\ensuremath{\tau}$ as a function of transition energy $\ensuremath{\omega}$ 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.