Isotopes of Protactinium - Protactinium-231

Protactinium-231

Protactinium-231 is the longest-lived isotope of protactinium, with a half-life of 32,760 years. In nature, it is found in trace amounts as part of the actinium series which starts with the primordial isotope uranium-235; the equilibrium concentration in uranium ore is 46.55 231Pa per million 235U.

In nuclear reactors, it is one of the few long-lived radioactive actinides produced as a byproduct of the projected thorium fuel cycle, as a result of (n,2n) reactions where a fast neutron removes a neutron from 232Th or 232U, and can also be destroyed by neutron capture though the cross section for this reaction is also low.

binding energy: 1759860 keV
beta decay energy: -382 keV

spin: 3/2-
mode of decay: alpha to 227Ac, also others

possible parent nuclides: beta from 231Th, EC from 231U, alpha from 235Np.