First identification of ²³¹Pa nuclide is generally accredited to L. Meitner, Zeitschrift fur Elektrochemie und angewandte physikalische Chemie 24, 11-12, 169 (1918), starting with pitchblende without uranium or radium, and isolating both the mother isotope ²³¹Pa and its α-daughter ²²⁷Ac. 2013Fr03 mention the identification of ²³¹Pa, as a parent of ²²⁷Ac, to O. Hahn and L. Meitner, Z. Physik 19, 208 (1918), where half-life was estimated to be between 1200 y and 180,000 y; and also to an earlier indirect evidence of ²³¹Pa in a paper by F. Soddy and J.α. Cranston, Proc. Roy. Soc. (London), A94, 384 (1918), where half-life was estimated to be >10,000 y. 1931Cu01 evaluation adopted half-life of 32,000 y from 1930Gr01, with the citation of the following earlier references: α.v. Grosse, Ber. Chem. Ges. 61, 233 (1928); Naturwiss. 15, 766 (1927); Nature 120, 621 (1927); O. Hahn and α.v. Grosse, Z. Physik 48, 600 (1928), T_1/2=20,200 y; O. Hahn and E. Walling, Naturwiss. 15, 803 (1928); E. Walling, Dissertation (Berlin, 1928), T_1/2=20.760 y. Most current half-life measurement of 32,570 y 130 is reported by 2020Je01, a result from collaboration of independent measurements at six different laboratories

Application of ²³¹Pa as described in n.E. Holden et al., Pure Appl. Chem. 90, 1833 (2018), "²³¹Pa is a natural radiogenic isotope produced by α decay of ²³⁵U to ²³¹Th, followed by β emission to form ²³¹Pa. Although its behavior in the environment as a transient member of the U-series decay chain may be complex, measurements and modeling of ²³¹Pa in relation to the isotopes of uranium and thorium have been used in a variety of geochronologic applications on time scales of 103 to 105 years. Studies include movement of water masses and particles in the oceans, rates of magma melting and movement beneath volcanoes, and ages of carbonate mineral deposits, including corals, in relation to climate change ".

1966Cr15:

Theoretical calculations: consult the NSR database (www.nndc.bnl.gov/nsr) for 115 primary references, 97 dealing with half-lives of α and cluster decays and other aspects of radioactive decays, and 19 with nuclear structure calculations. Some of these references are listed in ’document’ records in this dataset, which can be accessed through on-line ENSDF database at www.nndc.bnl.gov/ensdf/

Levels: ²³¹Pa presents K=1/2 rotational bands with opposite parity and decoupling parameters with very close absolute values, as well as enhanced E1 γ-ray transitions between parity-doublet band levels. These are typical features of nuclei with large stable octupole deformations. However, ²³¹Pa has a small β₃ deformation parameter of 0.02. Consequently, the characteristic features mentioned before have been explained by using large octupole-phonon components in the low-lying states. Extensive calculations of level energies and B(E1) reduced transition probabilities using the quasiparticle-plus-phonon model (QPM) have produced acceptable agreement between theory and experiment (1996Le01).

Levels: The even-parity states in ²³¹Pa are members of rotational bands that are strongly mixed by the Coriolis interaction. This mixing affects level energies as well as α, β, and γ-ray transition probabilities. Comprehensive Coriolis calculations have explained distortions in rotational band level spacings (which includes the unusual inversion of the 84- (5/2⁺), 102- (3/2⁺), and 101-keV (7/2⁺) states in the 3/2[651] rotational band). These calculations also produced values for γ-ray, β^- (²³¹Th β^- decay), and α-particle (²³⁵Np α decay) transition probabilities between these states (1975Ho14, 1973Br12). The agreement between experimental results and calculated values have confirmed the rotational band assignments presented here. Band assignments are from Coulomb excitation, (p,2nγ), (α,t), and ²³¹U ε decay (2003Wu03,1996Le01,1975Er01,1973Br12)

Gammas: Gamma-ray transition probabilities are from Coulomb excitation, unless otherwise indicated

Q-value: S(2n)=12615.1 28, S(2p)=11844 12 (2021Wa16)