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Protactinium (₉₁Pa) has no stable isotopes. The four naturally occurring isotopes allow a standard atomic weight to be given.

Twenty-nine radioisotopes of protactinium have been characterized, ranging from Pa to Pa. The most stable isotope is Pa with a half-life of 32,760 years, Pa with a half-life of 26.967 days, and Pa 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, Pa (t_1/2 1.15 milliseconds), Pa (t_1/2 = 308 nanoseconds), Pa (t_1/2 = 69 nanoseconds), Pa (t_1/2 = 420 nanoseconds), and Pa (t_1/2 = 1.17 minutes).

The only naturally occurring isotopes are Pa, Pa and Pa. The former occurs as an intermediate decay product of U, while the latter two occur as intermediate decay products of U. Pa makes up nearly all natural protactinium.

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

List of isotopes

1. Pa – Excited nuclear isomer.
2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. ^ # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
5. Modes of decay:

   CD:	Cluster decay
   EC:	Electron capture
   IT:	Isomeric transition
   SF:	Spontaneous fission

6. Bold italics symbol as daughter – Daughter product is nearly stable.
7. ( ) spin value – Indicates spin with weak assignment arguments.
8. Theoretically capable of β decay to Th
9. Intermediate decay product of U
10. Intermediate decay product of Np
11. ^ Intermediate decay product of U

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 Th, with a minor (8%) beta-minus decay branch leading to U. It also has a very rare (.003%) alpha decay mode leading to Ac. It is not found in nature because its half-life is short and it is not found in the decay chains of U, U, or Th. It has a mass of 230.034541 u.

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

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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 Pa per million U. 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 Th or U, 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 Ac, also others

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

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.

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. About 99.8% of decays of Th produce this isomer instead of the ground state (t_1/2 = 6.70 hours).

References

1. ^ Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
2. "Standard Atomic Weights: Protactinium". CIAAW. 2017.
3. Prohaska, Thomas; Irrgeher, Johanna; Benefield, Jacqueline; Böhlke, John K.; Chesson, Lesley A.; Coplen, Tyler B.; Ding, Tiping; Dunn, Philip J. H.; Gröning, Manfred; Holden, Norman E.; Meijer, Harro A. J. (2022-05-04). "Standard atomic weights of the elements 2021 (IUPAC Technical Report)". Pure and Applied Chemistry. doi:10.1515/pac-2019-0603. ISSN 1365-3075.
4. Auranen, K (3 September 2020). "Exploring the boundaries of the nuclear landscape: α-decay properties of 211Pa". Physical Review C. 102 (34305): 034305. Bibcode:2020PhRvC.102c4305A. doi:10.1103/PhysRevC.102.034305. S2CID 225343089. Retrieved 17 September 2020.
5. https://www.nndc.bnl.gov/ensnds/219/Pa/adopted.pdf, NNDC Chart of Nuclides, Adopted Levels for Pa.
6. ^ Huang, T.H.; et al. (2018). "Identification of the new isotope Np" (pdf). Physical Review C. 98 (4): 044302. Bibcode:2018PhRvC..98d4302H. doi:10.1103/PhysRevC.98.044302. S2CID 125251822.
7. Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
8. Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
9. Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
   "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk with a half-life greater than 9 . No growth of Cf was detected, and a lower limit for the β half-life can be set at about 10 . No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 ."
10. This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
11. Excluding those "classically stable" nuclides with half-lives significantly in excess of Th; e.g., while Cd has a half-life of only fourteen years, that of Cd is eight quadrillion years.
12. Audi, G.; Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S. (2017). "The NUBASE2016 evaluation of nuclear properties" (PDF). Chinese Physics C. 41 (3): 030001. Bibcode:2017ChPhC..41c0001A. doi:10.1088/1674-1137/41/3/030001.
13. Mastren, T.; Stein, B.W.; Parker, T.G.; Radchenko, V.; Copping, R.; Owens, A.; Wyant, L.E.; Brugh, M.; Kozimor, S.A.; Noriter, F.M.; Birnbaum, E.R.; John, K.D.; Fassbender, M.E. (2018). "Separation of protactinium employing sulfur-based extraction chromatographic resins". Analytical Chemistry. 90 (11): 7012–7017. doi:10.1021/acs.analchem.8b01380. ISSN 0003-2700. OSTI 1440455. PMID 29757620.
14. Fry, C., and M. Thoennessen. "Discovery of the Actinium, Thorium, Protactinium, and Uranium Isotopes." January 14, 2012. Accessed May 20, 2018. https://people.nscl.msu.edu/~thoennes/2009/ac-th-pa-u-adndt.pdf.
15. ^ "Human Health Fact Sheet - Protactinium" (PDF). Argonne National Laboratory (ANL). November 2001. Retrieved 17 October 2023.

• 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
  • National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
  • 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.

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Table of nuclides Categories: Isotopes Tables of nuclides Metastable isotopes Isotopes by element

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