# Isotopes of protactinium

Protactinium ($_{91}$Pa) 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 $^{211}$Pa to $^{239}$Pa. The most stable isotope is $^{231}$Pa with a half-life of 32,760 years, $^{233}$Pa with a half-life of 26.967 days, and $^{230}$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, $^{217m}$Pa (t$_{1/2}$ 1.15 milliseconds), $^{220m1}$Pa (t$_{1/2}$ = 308 nanoseconds), $^{220m2}$Pa (t$_{1/2}$ = 69 nanoseconds), $^{229m}$Pa (t$_{1/2}$ = 420 nanoseconds), and $^{234m}$Pa (t$_{1/2}$ = 1.17 minutes).
The only naturally occurring isotopes are $^{231}$Pa, $^{234}$Pa and $^{234m}$Pa. The former occurs as an intermediate decay product of $^{235}$U, while the latter two occur as intermediate decay products of $^{238}$U. $^{231}$Pa makes up nearly all natural protactinium.
The primary decay mode for isotopes of Pa lighter than (and including) the most stable isotope $^{231}$Pa is alpha decay, except for $^{228}$Pa to $^{230}$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 $^{231}$Pa and isotopes of protactinium lighter than and including $^{227}$Pa are isotopes of actinium and the primary decay products for the heavier isotopes of protactinium are isotopes of uranium.

## List of isotopes

| Nuclide | Historic name | Z | N | Isotopic mass (Da) | Half-life | Decay mode | Daughter isotope | Spin and parity | Isotopic abundance |
| Isotopic abundance |||
| $^{211}$Pa | | 91 | 120 | 211.023674(75) | 6(3) ms | α | $^{207}$Ac | 9/2− | |
| $^{212}$Pa | | 91 | 121 | 212.023185(94) | 5.8(19) ms | α | $^{208}$Ac | 3+# | |
| $^{213}$Pa | | 91 | 122 | 213.021100(61) | 7.4(24) ms | α | $^{209}$Ac | 9/2− | |
| $^{214}$Pa | | 91 | 123 | 214.020891(87) | 17(3) ms | α | $^{210}$Ac | 7+# | |
| $^{215}$Pa | | 91 | 124 | 215.019114(89) | 14(2) ms | α | $^{211}$Ac | 9/2− | |
| $^{216}$Pa | | 91 | 125 | 216.019135(26) | 105(12) ms | α | $^{212}$Ac | 5+# | |
| $^{217}$Pa | | 91 | 126 | 217.018309(13) | 3.8(2) ms | α | $^{213}$Ac | 9/2− | |
| $^{217m}$Pa | | 1860(7) keV | 1860(7) keV | 1860(7) keV | 1.08(3) ms | α (73%) | $^{213}$Ac | (23/2−) | |
| $^{217m}$Pa | | 1860(7) keV | 1860(7) keV | 1860(7) keV | 1.08(3) ms | IT (27%) | $^{217}$Pa | (23/2−) | |
| $^{218}$Pa | | 91 | 127 | 218.020021(19) | 108(5) μs | α | $^{214}$Ac | 8−# | |
| $^{218m}$Pa | | 81(19) keV | 81(19) keV | 81(19) keV | 150(50) μs | α | $^{214}$Ac | | |
| $^{219}$Pa | | 91 | 128 | 219.019950(75) | 56(9) ns | α | $^{215}$Ac | 9/2− | |
| $^{220}$Pa | | 91 | 129 | 220.021770(16) | 850(60) ns | α | $^{216}$Ac | 1−# | |
| $^{220m1}$Pa | | 26(23) keV | 26(23) keV | 26(23) keV | 410(180) ns | α | $^{216}$Ac | | |
| $^{220m2}$Pa | | 290(50) keV | 290(50) keV | 290(50) keV | 260(210) ns | α | $^{216}$Ac | | |
| $^{221}$Pa | | 91 | 130 | 221.021873(64) | 5.9(17) μs | α | $^{217}$Ac | 9/2− | |
| $^{222}$Pa | | 91 | 131 | 222.023687(93) | 3.8(2) ms | α | $^{218}$Ac | 1−# | |
| $^{223}$Pa | | 91 | 132 | 223.023980(81) | 5.3(3) ms | α | $^{219}$Ac | 9/2− | |
| $^{224}$Pa | | 91 | 133 | 224.0256173(81) | 844(19) ms | α (99.9%) | $^{220}$Ac | (5−) | |
| $^{225}$Pa | | 91 | 134 | 225.026148(88) | 1.71(10) s | α | $^{221}$Ac | 5/2−# | |
| $^{226}$Pa | | 91 | 135 | 226.027948(12) | 1.8(2) min | α (74%) | $^{222}$Ac | 1−# | |
| $^{226}$Pa | | 91 | 135 | 226.027948(12) | 1.8(2) min | β$^+$ (26%) | $^{226}$Th | 1−# | |
| $^{227}$Pa | | 91 | 136 | 227.0288036(78) | 38.3(3) min | α (85%) | $^{223}$Ac | (5/2−) | |
| $^{227}$Pa | | 91 | 136 | 227.0288036(78) | 38.3(3) min | EC (15%) | $^{227}$Th | (5/2−) | |
| $^{228}$Pa | | 91 | 137 | 228.0310508(47) | 22(1) h | β$^+$ (98.15%) | $^{228}$Th | 3+ | |
| $^{228}$Pa | | 91 | 137 | 228.0310508(47) | 22(1) h | α (1.85%) | $^{224}$Ac | 3+ | |
| $^{229}$Pa | | 91 | 138 | 229.0320956(35) | 1.55(4) d | EC (99.51%) | $^{229}$Th | 5/2+ | |
| $^{229}$Pa | | 91 | 138 | 229.0320956(35) | 1.55(4) d | α (0.49%) | $^{225}$Ac | 5/2+ | |
| $^{229m}$Pa | | 12.20(4) keV | 12.20(4) keV | 12.20(4) keV | 420(30) ns | IT | $^{229}$Pa | 3/2− | |
| $^{230}$Pa | | 91 | 139 | 230.0345397(33) | 17.4(5) d | β$^+$ (92.2%) | $^{230}$Th | 2− | |
| $^{230}$Pa | | 91 | 139 | 230.0345397(33) | 17.4(5) d | β$^−$ (7.8%) | $^{230}$U | 2− | |
| $^{230}$Pa | | 91 | 139 | 230.0345397(33) | 17.4(5) d | α (0.0032%) | $^{226}$Ac | 2− | |
| $^{231}$Pa | Protoactinium | 91 | 140 | 231.0358825(19) | 3.265(20)×10$^4$ y | α | $^{227}$Ac | 3/2− | 1.0000 |
| $^{231}$Pa | Protoactinium | 91 | 140 | 231.0358825(19) | 3.265(20)×10$^4$ y | CD (1.34×10$^{−10}$%) | $^{207}$Tl $^{24}$Ne | 3/2− | 1.0000 |
| $^{231}$Pa | Protoactinium | 91 | 140 | 231.0358825(19) | 3.265(20)×10$^4$ y | SF (<3×10$^{−10}$%) | (various) | 3/2− | 1.0000 |
| $^{231}$Pa | Protoactinium | 91 | 140 | 231.0358825(19) | 3.265(20)×10$^4$ y | CD (9.9×10$^{−13}$%) | $^{208}$Pb $^{23}$F | 3/2− | 1.0000 |
| $^{232}$Pa | | 91 | 141 | 232.0385902(82) | 1.32(2) d | β$^−$ | $^{232}$U | (2−) | |
| $^{233}$Pa | | 91 | 142 | 233.0402465(14) | 26.975(13) d | β$^−$ | $^{233}$U | 3/2− | Trace |
| $^{234}$Pa | Uranium Z | 91 | 143 | 234.0433056(44) | 6.70(5) h | β$^−$ | $^{234}$U | 4+ | Trace |
| $^{234}$Pa | Uranium Z | 91 | 143 | 234.0433056(44) | 6.70(5) h | SF (3×10$^{−10}$%) | (various) | 4+ | Trace |
| $^{234m}$Pa | Uranium X$_2$ Brevium | 79(3) keV | 79(3) keV | 79(3) keV | 1.159(11) min | β$^−$ (99.84%) | $^{234}$U | (0−) | Trace |
| $^{234m}$Pa | Uranium X$_2$ Brevium | 79(3) keV | 79(3) keV | 79(3) keV | 1.159(11) min | IT (0.16%) | $^{234}$Pa | (0−) | Trace |
| $^{234m}$Pa | Uranium X$_2$ Brevium | 79(3) keV | 79(3) keV | 79(3) keV | 1.159(11) min | SF (10$^{−10}$%) | (various) | (0−) | Trace |
| $^{235}$Pa | | 91 | 144 | 235.045399(15) | 24.4(2) min | β$^−$ | $^{235}$U | 3/2− | |
| $^{236}$Pa | | 91 | 145 | 236.048668(15) | 9.1(1) min | β$^−$ | $^{236}$U | 1(−) | |
| $^{236}$Pa | | 91 | 145 | 236.048668(15) | 9.1(1) min | β$^−$, SF (6×10$^{−8}$%) | (various) | 1(−) | |
| $^{237}$Pa | | 91 | 146 | 237.051023(14) | 8.7(2) min | β$^−$ | $^{237}$U | 1/2+ | |
| $^{238}$Pa | | 91 | 147 | 238.054637(17) | 2.28(9) min | β$^−$ | $^{238}$U | 3−# | |
| $^{238}$Pa | | 91 | 147 | 238.054637(17) | 2.28(9) min | β$^−$, SF (2.6×10$^{−6}$%) | (various) | 3−# | |
| $^{239}$Pa | | 91 | 148 | 239.05726(21)# | 1.8(5) h | β$^−$ | $^{239}$U | 1/2+# | |
| This table header & footer: view ||||||||||
* ↑ $^m$Pa – Excited nuclear isomer.
* ↑ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
* ↑ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
* 1 2 # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
* ↑ Modes of decay: EC:Electron capture CD:Cluster decay IT:Isomeric transition SF:Spontaneous fission
* ↑ Bold italics symbol as daughter – Daughter product is nearly stable.
* ↑ ( ) spin value – Indicates spin with weak assignment arguments.
* ↑ Order of ground state and isomer is uncertain.
* ↑ Intermediate decay product of $^{235}$U
* ↑ Intermediate decay product of $^{237}$Np
* 1 2 Intermediate decay product of $^{238}$U

## 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 $^{230}$Th, with a minor (8%) beta-minus decay branch leading to $^{230}$U. It also has a very rare (.003%) alpha decay mode leading to $^{226}$Ac. It is not found in nature because its half-life is short and it is not found in the decay chains of $^{235}$U, $^{238}$U, or $^{232}$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

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 $^{231}$Pa per million $^{235}$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 $^{232}$Th or $^{232}$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 $^{227}$Ac, also others
possible parent nuclides: beta from $^{231}$Th, EC from $^{231}$U, alpha from $^{235}$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 $^{234}$Th produce this isomer instead of the ground state (t$_{1/2}$ = 6.70 hours).