The gamma-ray spectrum emitted in the decay of ${\mathrm{Co}}^{57}$ has been measured with a high-resolution Ge(Li) detector. In addition to the well-known gamma transitions of 136.4, 122.0, and 14.4 keV, the following weak transitions with energies given in keV (intensity per ${10}^5$ ${\mathrm{Co}}^{57}$ decays) were observed: 229.8±1.0 (0.5), 339.7±0.4 (4.2), 352.5±0.4 (3.2), 366.8±0.5 (0.6), 570.0±0.4 (13.2), 692.1±0.3 (137), and 706.4±0.4 (5.7). A unique decay scheme is established on the basis of these energy measurements and gamma-gamma coincidence measurements employing NaI(Tl) scintillation detectors. The excited states of ${\mathrm{Fe}}^{57}$ at 366.8 and 706.4 keV have spin and parity $\frac{3}{2}$- and $\frac{5}{2}$-, respectively. The total internal-conversion coefficient for the 14-keV transition was measured by two independent methods: (1) A gamma-gamma coincidence measurement in a calibrated geometry yielded a value of 9.0±0.5. (2) A measurement of the Mössbauer absorption cross section of 14-keV gamma rays in iron yielded a value of 8.9±0.6. The good agreement between the results of these two independent methods resolves previously reported discrepancies. A more accurate determination of the half-life of the 14.4-keV first excited state by a gamma-gamma delayed-coincidence measurement gave a result of (9.8±0.1)×${10}^{-8\mathrm{}}$ sec.

• Received 3 March 1965

DOI:https://doi.org/10.1103/PhysRev.139.B295