Protactinium (Pa) has no stable isotopes. There are three naturally occurring isotopes, allowing a standard mass to be given.
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Relative atomic mass: 231.03588(2).
Twenty-nine radioisotopes of protactinium have been characterized, with the most stable being 231Pa with a half-life of 32,760 years, 233Pa with a half-life of 26.967 days, and 230Pa with a half-life of 17.4 days. All of the remaining radioactive isotopes have half-lives that are less than 1.6 days, and the majority of these have half-lives that are less than 1.8 seconds. This element also has 3 meta states, 217mPa (t1/2 1.15 milliseconds), 229mPa (t1/2 420 nanoseconds), and 234mPa (t1/2 1.17 minutes).
The only naturally occurring isotopes are 231Pa, which occurs as an intermediate decay product of 235U, 234Pa and 234mPa, both of which occur as intermediate decay products of 238U. 231Pa makes up nearly all natural protactinium.
The primary decay mode for isotopes of Pa lighter than (and including) the most stable isotope 231Pa is alpha decay, except for 228Pa to 230Pa, which primarily decay by electron capture to isotopes of thorium. The primary mode for the heavier isotopes is beta minus (β−) decay. The primary decay products of 231Pa and isotopes of protactinium lighter than and including 227Pa are isotopes of actinium and the primary decay products for the heavier isotopes of protactinium are isotopes of uranium.
Protactinium-230
Protactinium-230 has 139 neutrons and a half-life of 17.4 days. It undergoes beta-minus decay to 230U. It is not found in nature because its half-life is so short and it is not found in the decay chains of 235U, 238U, or 232Th. It has a mass of 230.034541 u.
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.
Protactinium-233
Protactinium-233 is also part of the thorium fuel cycle. It is an intermediate beta decay product between thorium-233 (produced from natural thorium-232 by neutron capture) and uranium-233 (the fissile fuel of the thorium cycle). Some thorium-cycle reactor designs try to protect Pa-233 from further neutron capture producing Pa-234 and U-234, which are not useful as fuel.