Half life = 6.02 hours. Decay Mode(s) are Internal Conversion and Beta Decay. 99m-Tc is a 1/2- energy state. The beta transition is 3.70 E-05 in yield to stable 99-Ruthenium. The remaining transitions are IT to the 7/2+ state (99.24%, about 2 keV) and the 9/2+ state of 99-Tc (0.76^, 322 keV), which is a beta emitter with a half life of 200,000 years. It is of note, then that (ignoring the beta decay), the internal conversion to the 7/2+ state accounts for the vast majority of the transitions, and this IT involves M and N shell electrons, making this metastable nucleus particularly susceptible to chemical influences. In fact, 99m-Tc was the first metastable nuclear state to demonstrate observable half-life changes due to chemical environment. The change is of the order of 0.3% when the isotope is in the Tc2S7 compared with the KTcO4 molecule. (1951, see references below).
From the 7/2+ state the nucleus decays promptly to 9/2+ (99-Tc), with 89.1% of the transitions resulting in the familiar 140.5 keV. Thus it is seen that for every 99-Mo decay approximately 85% result in (delayed) 140.5 keV gamma-ray transitions.
Prepared by Douglas J. Wagenaar, Ph.D.
________________________________________________________Douglas J. Wagenaar, Ph.D., firstname.lastname@example.org