Protactinium (₉₁Pa) has no stableisotopes. As²³¹Pa occurs in usable quantity, and comprises virtually all of the element, it defines the standard atomic weight.

Thirtyradioisotopes of protactinium have been characterized, ranging from²¹⁰Pa to²³⁹Pa. The most stable isotopes are²³¹Pa with ahalf-life of 32,700 years,²³³Pa with a half-life of 26.975 days, and²³⁰Pa with a half-life of 17.4 days. All of the remainingradioactive 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 fivemeta 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 are²³¹Pa,²³³Pa,²³⁴Pa, and^234mPa. The first occurs as an intermediate decay product of²³⁵U, the second of (rare)²³⁷Np, and the last two as intermediate decay products of²³⁸U.²³¹Pa dominates solely because of its longer life.

The primarydecay mode for isotopes of Pa lighter than (and including) the most stable isotope²³¹Pa isalpha decay toisotopes of actinium, except²²⁸Pa to²³⁰Pa, which primarily decay by electron capture toisotopes of thorium. The primary mode for the heavier isotopes isbeta minus (β⁻) decay toisotopes of uranium.

Protactinium-230 has 139neutrons and a half-life of 17.4 days. Most of the time (92%), it undergoes beta plus decay to²³⁰Th, with a smaller (8%) beta-minus decay branch leading to²³⁰U. It also has a very rare (0.0032%) alpha decay mode leading to²²⁶Ac. It is not found in nature because its half-life is short and it is not found in thedecay chains of²³⁵U,²³⁸U, or²³²Th.

Protactinium-230 is of interest as a progenitor of uranium-230, an isotope that has been considered for use intargeted alpha-particle therapy (TAT). It can be produced through proton ordeuteron irradiation of natural thorium.^[7]

Protactinium-231 is the longest-lived isotope of protactinium,^[8] with a half-life of 32,760 years.^[9][10] In nature, it is found in trace amounts as part of theactinium series,^[10][11] which starts with theprimordial isotopeuranium-235;^[12] theequilibrium concentration in uranium ore is 46.5 atoms of²³¹Pa per million of²³⁵U. Innuclear reactors, it is one of the few long-lived radioactiveactinides produced as a byproduct of the projectedthorium fuel cycle,^[13] as a result of (n,2n) reactions where afast neutron removes aneutron from²³²Th or²³²U, and can also be destroyed byneutron capture, though thecross section for this reaction is also low.

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

spin: 3/2−
mode of decay:alpha to²²⁷Ac, also others

possible parent nuclides:beta from²³¹Th,EC from²³¹U,alpha from²³⁵Np.

Protactinium-233 is also part of the thorium fuel cycle. It is an intermediate beta decay product betweenthorium-233 (produced from natural thorium-232 by neutron capture) anduranium-233 (the fissile fuel of the thorium cycle).^[14] 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.

Protactinium-234 is a member of theuranium series with a half-life of 6.70 hours. It was discovered byOtto Hahn in 1921.^[15]

Protactinium-234m is a member of the uranium series with a half-life of 1.17 minutes. It was discovered in 1913 byKazimierz Fajans andOswald Helmuth Göhring, who named itbrevium for its short half-life.^[16] It is now believed that all decays of the parentthorium-234 produce this isomer^[17] 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 callednuclear isomerism.^[18]

• Isotope masses from:
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean;Wapstra, Aaldert Hendrik (2003),"The NUBASE evaluation of nuclear and decay properties",Nuclear Physics A,729:3–128,Bibcode:2003NuPhA.729....3A,doi:10.1016/j.nuclphysa.2003.11.001
• 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)."Atomic weights of the elements. Review 2000 (IUPAC Technical Report)".Pure and Applied Chemistry.75 (6):683–800.doi:10.1351/pac200375060683.
  • Wieser, Michael E. (2006)."Atomic weights of the elements 2005 (IUPAC Technical Report)".Pure and Applied Chemistry.78 (11):2051–2066.doi:10.1351/pac200678112051.
• "News & Notices: Standard Atomic Weights Revised".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),"The NUBASE evaluation of nuclear and decay properties",Nuclear Physics A,729:3–128,Bibcode:2003NuPhA.729....3A,doi:10.1016/j.nuclphysa.2003.11.001
  • 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."NuDat 3.0 database".Brookhaven National Laboratory.

• Isotopes of protactinium
• Protactinium
• Lists of isotopes by element

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