Lead-208m is an excited nuclear state of the nuclide 208Pb, characterized by a defined excitation energy (4895.23 keV). Its spin and parity quantum numbers differ from those of the ground state. Simultaneously, this metastable nuclear isomer possesses an independent, experimentally measurable lifetime: decay occurs via an isomeric transition to Pb-208 in the ground state with a half-life of approximately 0.535 microseconds.
See also: List of individual Lead isotopes (and general data sources).
General data
Name of the isotope:Lead-208m; Pb-208mSymbol:208mPb or 208m82PbMass number A:208 (= number of nucleons)Atomic number Z:82 (= number of protons)Neutrons N:126Nucleon pairing (Z - N):even - evenNuclear ratio (N/Z ratio):1.5365853658537 (= neutron-proton ratio)Neutron excess (N-Z):44Excitation energy:4895.23(5) keVIsotopic mass:207.976653(8) u (atomic weight of Lead-208m)Nuclide mass:207.9316746 u (calculated nuclear mass without electrons)Mass excess:-21.74759 MeVMass defect:1.75677904 u (per nucleus)Nuclear binding energy:1636.42913509 MeV (per nucleus) 7.86744776 MeV (average binding energy per nucleon)Half-life:0.535(35) μsDecay constant λ:1295602.2066541 s-1Specific activity α:3.751526310714 × 10+27 Bq g-1 1.013926029922 × 10+17 Ci g-1Spin and parity: (nuclear angular momentum)10+Year of discovery:1998
The following table shows the atomic nuclei that are isotonic (same neutron number N = 126) and isobaric (same nucleon number A = 208) with Lead-208m. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.
