Isotopes of protactinium

Protactinium (91Pa) has no stable isotopes. As 231Pa occurs in usable quantity, and comprises virtually all of the element, it defines the standard atomic weight.

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 (t1/2 1.15 milliseconds), 220m1Pa (t1/2 = 308 nanoseconds), 220m2Pa (t1/2 = 69 nanoseconds), 229mPa (t1/2 = 420 nanoseconds), and 234mPa (t1/2 = 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

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.

• Isotope masses from: Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), , Nuclear Physics A, 729: 3–128, Bibcode:, doi:
• Isotopic compositions and standard atomic masses from: de Laeter, John Robert; Böhlke, John Karl; De Bièvre, Paul; Hidaka, Hiroshi; Peiser, H. Steffen; Rosman, Kevin J. R.; Taylor, Philip D. P. (2003). . Pure and Applied Chemistry. 75 (6): 683–800. doi:. Wieser, Michael E. (2006). . Pure and Applied Chemistry. 78 (11): 2051–2066. doi:.
• . International Union of Pure and Applied Chemistry. 19 October 2005.
• Half-life, spin, and isomer data selected from the following sources. Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), , Nuclear Physics A, 729: 3–128, Bibcode:, doi: Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9. National Nuclear Data Center. . Brookhaven National Laboratory.