Isotopes of protactinium

Thirty radioisotopes of protactinium have been characterized, ranging from ²¹⁰Pa to ²³⁹Pa. The most stable isotopes are ²³¹Pa with a half-life of 32,700 years, ²³³Pa with a half-life of 26.975 days, and ²³⁰Pa with a half-life of 17.4 days. All of the remaining radioactive isotopes have half-lives less than 1.6 days, and the majority of these have half-lives less than 1.8 seconds. This element also has five meta states, ^217mPa (t_1/2 1.15 milliseconds), ^220m1Pa (t_1/2 = 308 nanoseconds), ^220m2Pa (t_1/2 = 69 nanoseconds), ^229mPa (t_1/2 = 420 nanoseconds), and ^234mPa (t_1/2 = 1.16 minutes).

The only naturally occurring isotopes are ²³¹Pa, ²³³Pa, ²³⁴Pa, and ^234mPa. The first occurs as an intermediate decay product of ²³⁵U, the second of (rare) ²³⁷Np, and the last two as intermediate decay products of ²³⁸U. ²³¹Pa dominates solely because of its longer life.

The primary decay mode for protactinium isotopes lighter than (and including) the most stable isotope ²³¹Pa is alpha decay to isotopes of actinium, except 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 to isotopes of uranium.

List of isotopes

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Actinides and fission products

Protactinium-230

Protactinium-230 has 139 neutrons and a half-life of 17.4 days. Most of the time (92%), it undergoes beta plus decay to ²³⁰Th, with a smaller (8%) beta-minus decay branch leading to ²³⁰U. It also has a very rare (0.0032%) alpha decay mode leading to ²²⁶Ac. It is not found in nature because its half-life is short and it is not found in the decay chains of ²³⁵U, ²³⁸U, or ²³²Th.

Protactinium-230 is of interest as a progenitor of uranium-230, an isotope that has been considered for use in targeted alpha-particle therapy (TAT). It can be produced through proton or deuteron irradiation of natural thorium.

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 46.5 atoms of ²³¹Pa per million of 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 ²³³Pa from further neutron capture producing ²³⁴Pa and ²³⁴U, which are not useful as fuel.

Protactinium-234

Protactinium-234 is a member of the uranium series with a half-life of 6.70 hours. It was discovered by Otto Hahn in 1921.

Protactinium-234m

Protactinium-234m is a member of the uranium series with a half-life of 1.17 minutes. It was discovered in 1913 by Kazimierz Fajans and Oswald Helmuth Göhring, who named it brevium for its short half-life. It is now believed that all decays of the parent thorium-234 produce this isomer and the ground state is observed because of (invisible) IT decay. Protactinium-234m has the same mass (same number of protons and neutrons) as protactinium-234, the difference merely visible in their non-identical half-life, with protactinium-234m having a noticeably shorter lifespan. This phenomenon is called nuclear isomerism.