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Abstract

^180mTa is a naturally abundant quasistable nuclide and the longest-lived nuclear isomer known to date. It is of interest, among others, for the search for dark matter, for the development of a γ laser, and for astrophysics. So far, its excitation energy has not been measured directly but has been based on an evaluation of available nuclear reaction data. We have determined the excitation energy of this isomer with high accuracy using the Penning-trap mass spectrometer JYFLTRAP. The determined mass difference between the ground and isomeric states of ^180mTa yields an excitation energy of 76.79(55) keV for ^180mTa. This is the first direct measurement of the excitation energy and provides a better accuracy than that of the previous evaluation value, 75.3(14) keV.