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 (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 isotopes of Pa 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

|-id=Protactinium-210 | ²¹⁰Pa | | style="text-align:right" | 91 | style="text-align:right" | 119 | | | α | ²⁰⁶Ac | 3+ | |-id=Protactinium-211 | ²¹¹Pa | | style="text-align:right" | 91 | style="text-align:right" | 120 | 211.023674(75) | 6(3)&nbsp;ms | α | ²⁰⁷Ac | 9/2− | |-id=Protactinium-212 | ²¹²Pa | | style="text-align:right" | 91 | style="text-align:right" | 121 | 212.023185(94) | 5.8(19)&nbsp;ms | α | ²⁰⁸Ac | 3+# | |-id=Protactinium-213 | ²¹³Pa | | style="text-align:right" | 91 | style="text-align:right" | 122 | 213.021100(61) | 7.4(24)&nbsp;ms | α | ²⁰⁹Ac | 9/2− | |-id=Protactinium-214 | ²¹⁴Pa | | style="text-align:right" | 91 | style="text-align:right" | 123 | 214.020891(87) | 17(3)&nbsp;ms | α | ²¹⁰Ac | 7+# | |-id=Protactinium-215 | ²¹⁵Pa | | style="text-align:right" | 91 | style="text-align:right" | 124 | 215.019114(89) | 14(2)&nbsp;ms | α | ²¹¹Ac | 9/2− | |-id=Protactinium-216 | ²¹⁶Pa | | style="text-align:right" | 91 | style="text-align:right" | 125 | 216.019135(26) | 105(12)&nbsp;ms | α | ²¹²Ac | 5+# | |-id=Protactinium-217 | ²¹⁷Pa | | style="text-align:right" | 91 | style="text-align:right" | 126 | 217.018309(13) | 3.8(2)&nbsp;ms | α | ²¹³Ac | 9/2− | |-id=Protactinium-217m | rowspan=2 style="text-indent:1em" | ^217mPa | rowspan=2| | rowspan=2 colspan="3" style="text-indent:2em" | 1860(7)&nbsp;keV | rowspan=2|1.08(3)&nbsp;ms | α (73%) | ²¹³Ac | rowspan=2|(23/2−) | rowspan=2| |- | IT (27%) | ²¹⁷Pa |-id=Protactinium-218 | ²¹⁸Pa | | style="text-align:right" | 91 | style="text-align:right" | 127 | 218.020021(19) | 108(5)&nbsp;μs | α | ²¹⁴Ac | 8−# | |-id=Protactinium-218m | style="text-indent:1em" | ^218mPa | | colspan="3" style="text-indent:2em" | 81(19)&nbsp;keV | 150(50)&nbsp;μs | α | ²¹⁴Ac | | |-id=Protactinium-219 | ²¹⁹Pa | | style="text-align:right" | 91 | style="text-align:right" | 128 | 219.019950(75) | 56(9)&nbsp;ns | α | ²¹⁵Ac | 9/2− | |-id=Protactinium-220 | ²²⁰Pa | | style="text-align:right" | 91 | style="text-align:right" | 129 | 220.021770(16) | 850(60)&nbsp;ns | α | ²¹⁶Ac | 1−# | |-id=Protactinium-220m1 | style="text-indent:1em" | ^220m1Pa | | colspan="3" style="text-indent:2em" | 26(23)&nbsp;keV | 410(180)&nbsp;ns | α | ²¹⁶Ac | | |-id=Protactinium-220m2 | style="text-indent:1em" | ^220m2Pa | | colspan="3" style="text-indent:2em" | 290(50)&nbsp;keV | 260(210)&nbsp;ns | α | ²¹⁶Ac | | |-id=Protactinium-221 | ²²¹Pa | | style="text-align:right" | 91 | style="text-align:right" | 130 | 221.021873(64) | 5.9(17)&nbsp;μs | α | ²¹⁷Ac | 9/2− | |-id=Protactinium-222 | ²²²Pa | | style="text-align:right" | 91 | style="text-align:right" | 131 | 222.023687(93) | 3.8(2)&nbsp;ms | α | ²¹⁸Ac | 1−# | |-id=Protactinium-223 | ²²³Pa | | style="text-align:right" | 91 | style="text-align:right" | 132 | 223.023980(81) | 5.3(3)&nbsp;ms | α | ²¹⁹Ac | 9/2− | |-id=Protactinium-224 | ²²⁴Pa | | style="text-align:right" | 91 | style="text-align:right" | 133 | 224.0256173(81) | 844(19)&nbsp;ms | α | ²²⁰Ac | (5−) | |-id=Protactinium-225 | ²²⁵Pa | | style="text-align:right" | 91 | style="text-align:right" | 134 | 225.026148(88) | 1.71(10)&nbsp;s | α | ²²¹Ac | 5/2−# | |-id=Protactinium-226 | rowspan=2|²²⁶Pa | rowspan=2| | rowspan=2 style="text-align:right" | 91 | rowspan=2 style="text-align:right" | 135 | rowspan=2|226.027948(12) | rowspan=2|1.8(2)&nbsp;min | α (74%) | ²²²Ac | rowspan=2|1−# | rowspan=2| |- | β⁺ (26%) | ²²⁶Th |-id=Protactinium-227 | rowspan=2|²²⁷Pa | rowspan=2| | rowspan=2 style="text-align:right" | 91 | rowspan=2 style="text-align:right" | 136 | rowspan=2|227.0288036(78) | rowspan=2|38.3(3)&nbsp;min | α (85%) | ²²³Ac | rowspan=2|(5/2−) | rowspan=2| |- | EC (15%) | ²²⁷Th |-id=Protactinium-228 | rowspan=2|²²⁸Pa | rowspan=2| | rowspan=2 style="text-align:right" | 91 | rowspan=2 style="text-align:right" | 137 | rowspan=2|228.0310508(47) | rowspan=2|22(1)&nbsp;h | β⁺ (98.15%) | ²²⁸Th | rowspan=2|3+ | rowspan=2| |- | α (1.85%) | ²²⁴Ac |-id=Protactinium-229 | rowspan=2|²²⁹Pa | rowspan=2| | rowspan=2 style="text-align:right" | 91 | rowspan=2 style="text-align:right" | 138 | rowspan=2|229.0320956(35) | rowspan=2|1.55(4)&nbsp;d | EC (99.51%) | ²²⁹Th | rowspan=2|5/2+ | rowspan=2| |- | α (0.49%) | ²²⁵Ac |-id=Protactinium-229m | style="text-indent:1em" | ^229mPa | | colspan="3" style="text-indent:2em" | 12.20(4)&nbsp;keV | 420(30)&nbsp;ns | IT | ²²⁹Pa | 3/2− | |- | rowspan=3|²³⁰Pa | rowspan=3| | rowspan=3 style="text-align:right" | 91 | rowspan=3 style="text-align:right" | 139 | rowspan=3|230.0345397(33) | rowspan=3|17.4(5)&nbsp;d | β⁺ (92.2%) | ²³⁰Th | rowspan=3|2− | rowspan=3| |- | β^− (7.8%) | ²³⁰U |- | α (0.0032%) | ²²⁶Ac |- | rowspan=4|²³¹Pa | rowspan=4|Protoactinium<br/>Protactinium | rowspan=4 style="text-align:right" | 91 | rowspan=4 style="text-align:right" | 140 | rowspan=4|231.0358825(19) | rowspan=4|3.265(20)×10⁴&nbsp;y | α | ²²⁷Ac | rowspan=4|3/2− | rowspan=4|1.0000 |- | CD (1.34×10^{−9}%) | ²⁰⁷Tl<br />²⁴Ne |- | SF (<3×10^{−10}%) | (various) |- | CD (~10^{−12}%) | ²⁰⁸Pb<br />²³F |-id=Protactinium-232 | ²³²Pa | | style="text-align:right" | 91 | style="text-align:right" | 141 | 232.0385902(82) | 1.32(2)&nbsp;d | β^− | ²³²U | (2−) | |- | ²³³Pa | | style="text-align:right" | 91 | style="text-align:right" | 142 | 233.0402465(14) | 26.975(13)&nbsp;d | β^− | ²³³U | 3/2− | Trace |- | ²³⁴Pa | Uranium Z | style="text-align:right" | 91 | style="text-align:right" | 143 | 234.0433056(44) | 6.70(5)&nbsp;h | β^− | ²³⁴U | 4+ | Trace |- | rowspan=2 style="text-indent:1em" | ^234mPa | rowspan=2|Uranium X₂<br />Brevium | rowspan=2 colspan="3" style="text-indent:2em" | 79(3)&nbsp;keV | rowspan=2|1.159(11)&nbsp;min | β^− (99.84%) | ²³⁴U | rowspan=2|(0−) | rowspan=2|Trace |- | IT (0.16%) | ²³⁴Pa |-id=Protactinium-235 | ²³⁵Pa | | style="text-align:right" | 91 | style="text-align:right" | 144 | 235.045399(15) | 24.4(2)&nbsp;min | β^− | ²³⁵U | 3/2− | |-id=Protactinium-236 | rowspan=2|²³⁶Pa | rowspan=2| | rowspan=2 style="text-align:right" | 91 | rowspan=2 style="text-align:right" | 145 | rowspan=2|236.048668(15) | rowspan=2|9.1(1)&nbsp;min | β^− | ²³⁶U | rowspan=2|1(−) | rowspan=2| |- | β^−, SF (6×10^{−8}%) | (various) |-id=Protactinium-237 | ²³⁷Pa | | style="text-align:right" | 91 | style="text-align:right" | 146 | 237.051023(14) | 8.7(2)&nbsp;min | β^− | ²³⁷U | 1/2+ | |-id=Protactinium-238 | rowspan=2|²³⁸Pa | rowspan=2| | rowspan=2 style="text-align:right" | 91 | rowspan=2 style="text-align:right" | 147 | rowspan=2|238.054637(17) | rowspan=2|2.28(9)&nbsp;min | β^− | ²³⁸U | rowspan=2|3−# | rowspan=2| |- | β^−, SF (2.6×10^{−6}%) | (various) |-id=Protactinium-239 | ²³⁹Pa | | style="text-align:right" | 91 | style="text-align:right" | 148 | 239.05726(21)# | 1.8(5)&nbsp;h | β^− | ²³⁹U | 1/2+# |

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&nbsp;keV<br /> beta decay energy: −382&nbsp;keV

spin: 3/2−<br /> 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 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. 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.

References

• Isotope masses from:
• Isotopic compositions and standard atomic masses from:
• Half-life, spin, and isomer data selected from the following sources.