Level Structure ofI131

The levels of ${\mathrm{I}}^{131}$ excited in the decay of the 30-h activity of ${\mathrm{Te}}^{131m}$ have been studied. Scintillation spectrometers were used for recording the gamma spectrum and gamma-gamma coincidences. The internalconversion spectrum with a double-focusing spectrometer showed $K$ and $L$ lines corresponding to gamma rays of energies 81, 102, and 150 keV and $K$ lines corresponding to 200-, 241-, 336-, 452-, 775-, and 854-keV gamma rays. $K$ conversion lines of 775-, 786-, 797-, 831-, 854-, 869-, 1127-, and 1206-keV gamma rays were observed in the external-conversion spectrum. The relative intensity of the gamma rays was found by analyzing the gamma spectrum and also from the external-conversion spectrum. By use of these relative intensities and the intensities of the internal-conversion lines, the conversion coefficients and the possible multipolarities of the gamma transitions were ascertained. In particular, the 200- and 241-keV transitions were found to be of $E1\mathrm{}$ type, indicating the presence of odd-parity states in ${\mathrm{I}}^{131}$. These two transitions were also found to be highly retarded and the 1829-keV level from which they arise was found to have a half-life of 5.9±0.2 mμsec from delayed-coincidence measurements using a time-to-amplitude conversion technique. From these measurements it is concluded that ${\mathrm{Te}}^{131m}$ decays by beta transitions mainly to the 2012-, 1981-, 1965-, 1931-, and 1902-keV levels of ${\mathrm{I}}^{131}$. These levels de-excite by transitions to the levels at 1829, 1929, 1583, 1340, 1145, 1065, 797, 775, 602, and 150 keV. From the beta spectrum studied with an intermediateimage spectrometer, it was concluded that the beta transition to the ground state has an end-point energy of 2460±15 keV and a relative intensity of 6%, while the isomeric transition takes place with a relative intensity of 18%.