Isomeric State of Po-212

An isomer of ${\mathrm{Po}}^{212}$ with a half-life of (45.1\ifmmode\pm\else\textpm\fi{}0.6) sec and an excitation energy of 2.93 MeV has been positively identified by chemical and nuclear spectroscopic techniques. The isomer decays to known states of ${\mathrm{Pb}}^{208}$ by the emission of alpha particles with energies and intensities of (11.65\ifmmode\pm\else\textpm\fi{}0.02)MeV-97%, (9.08\ifmmode\pm\else\textpm\fi{}0.015)MeV-(1.00\ifmmode\pm\else\textpm\fi{}0.04)%, and (8.52\ifmmode\pm\else\textpm\fi{}0.015)MeV-(2.05\ifmmode\pm\else\textpm\fi{}0.09)%. In coincidence with ${\mathrm{Po}}^{212m}$ alpha particles we found gamma rays with energies and intensities of (2.61\ifmmode\pm\else\textpm\fi{}0.02)MeV-(2.6\ifmmode\pm\else\textpm\fi{}0.3)% and (0.57\ifmmode\pm\else\textpm\fi{}0.015)MeV-\ensuremath{\sim}2%. An immense alpha-decay hindrance factor for the 11.65-MeV group of 4\ifmmode\times\else\texttimes\fi{}${10}^{13}$ is explained by spin and parity assignments of 18+ for ${\mathrm{Po}}^{212m}$ in conjunction with a theoretical shell-model analysis made by Glendenning.The alpha-particle energies and intensities of previously known alpha groups of ${\mathrm{Po}}^{211m}$ [(25.5\ifmmode\pm\else\textpm\fi{}0.3)sec] were measured yielding values of (8.87\ifmmode\pm\else\textpm\fi{}0.01)MeV-(7.04\ifmmode\pm\else\textpm\fi{}0.14)%, (7.99\ifmmode\pm\else\textpm\fi{}0.015)MeV-(1.66\ifmmode\pm\else\textpm\fi{}0.03)%, and (7.27\ifmmode\pm\else\textpm\fi{}0.015)MeV-91%. A new alpha group was found with an energy and intensity of (8.30\ifmmode\pm\else\textpm\fi{}0.015) MeV-(0.25\ifmmode\pm\else\textpm\fi{}0.02)%. In coincidence with ${\mathrm{Po}}^{211m}$ alpha particles was a gamma ray of (0.90\ifmmode\pm\else\textpm\fi{}0.015)MeV-(1.65\ifmmode\pm\else\textpm\fi{}0.11)%. These ${\mathrm{Po}}^{211m}$ radiations are all consistent with the known energy level scheme of ${\mathrm{Pb}}^{207}$.