Isotopes of protactinium

Actinides and fission products by half-life
Actinides[1] by decay chain Half-life
range (y)
Fission products of 235U by yield[2]
4n 4n+1 4n+2 4n+3
4.5–7% 0.04–1.25% <0.001%
228Ra 4–6 155Euþ
244Cmƒ 241Puƒ 250Cf 227Ac 10–29 90Sr 85Kr 113mCdþ
232Uƒ 238Puƒ№ 243Cmƒ 29–97 137Cs 151Smþ 121mSn
248Bk[3] 249Cfƒ 242mAmƒ 141–351

No fission products
have a half-life
in the range of
100–210 k years ...

241Amƒ 251Cfƒ[4] 430–900
226Ra 247Bk 1.3 k  1.6 k
240Puƒ№ 229Th 246Cmƒ 243Amƒ 4.7 k  7.4 k
245Cmƒ 250Cm 8.3 k  8.5 k
239Puƒ№ 24.1 k
230Th 231Pa 32 k  76 k
236Npƒ 233Uƒ№ 234U 150 k  250 k 99Tc 126Sn
248Cm 242Puƒ 327 k  375 k 79Se
1.53 M 93Zr
237Npƒ№ 2.1 M  6.5 M 135Cs 107Pd
236U 247Cmƒ 15 M  24 M 129I
244Pu 80 M

... nor beyond 15.7 M years[5]

232Th 238U 235Uƒ№ 0.7 G  14.1 G

Legend for superscript symbols
  has thermal neutron capture cross section in the range of 8–50 barns
ƒ  fissile
m  metastable isomer
  naturally occurring radioactive material (NORM)
þ  neutron poison (thermal neutron capture cross section greater than 3k barns)
  range 4–97 y: Medium-lived fission product
  over 200,000 y: Long-lived fission product

Protactinium (Pa) has no stable isotopes. There are three naturally occurring isotopes, allowing a standard mass to be given.

Relative atomic mass: 231.03588(2).

Twenty-nine radioisotopes of protactinium have been characterized, with the most stable being 231Pa with a half-life of 32,760 years, 233Pa with a half-life of 26.967 days, and 230Pa with a half-life of 17.4 days. All of the remaining radioactive isotopes have half-lives that are less than 1.6 days, and the majority of these have half-lives that are less than 1.8 seconds. This element also has 3 meta states, 217mPa (t1/2 1.15 milliseconds), 229mPa (t1/2 420 nanoseconds), and 234mPa (t1/2 1.17 minutes).

The only naturally occurring isotopes are 231Pa, which occurs as an intermediate decay product of 235U, 234Pa and 234mPa, both of which occur as intermediate decay products of 238U. 231Pa makes up nearly all natural protactinium.

The primary decay mode for isotopes of Pa lighter than (and including) the most stable isotope 231Pa is alpha decay, except for 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. The primary decay products of 231Pa and isotopes of protactinium lighter than and including 227Pa are isotopes of actinium and the primary decay products for the heavier isotopes of protactinium are isotopes of uranium.


Protactinium-230 has 139 neutrons and a half-life of 17.4 days. It undergoes beta-minus decay to 230U. It is not found in nature because its half-life is so short and it is not found in the decay chains of 235U, 238U, or 232Th. It has a mass of 230.034541 u.


Transmutation in the thorium fuel cycle
230Th 231Th 232Th 233Th (White actinides: t½<27d)
231Pa 232Pa 233Pa 234Pa (Colored : t½>68y)
231U 232U 233U 234U 235U 236U 237U
(Fission products with t½<90y or t½>200ky) 237Np

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 231Pa per million 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 keV
beta decay energy: −382 keV

spin: 3/2−
mode of decay: alpha to 227Ac, also others

possible parent nuclides: beta from 231Th, EC from 231U, alpha from 235Np.


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.


Z(p) N(n)  
isotopic mass (u)
half-life decay
mode(s)[6][n 1]
isotope(s)[n 2]
(mole fraction)
range of natural
(mole fraction)
excitation energy
212Pa 91 121 212.02320(8) 8(5) ms
[5.1(+61−19) ms]
213Pa 91 122 213.02111(8) 7(3) ms
[5.3(+40−16) ms]
α 209Ac 9/2−#
214Pa 91 123 214.02092(8) 17(3) ms α 210Ac
215Pa 91 124 215.01919(9) 14(2) ms α 211Ac 9/2−#
216Pa 91 125 216.01911(8) 105(12) ms α (80%) 212Ac
β+ (20%) 216Th
217Pa 91 126 217.01832(6) 3.48(9) ms α 213Ac 9/2−#
217mPa 1860(7) keV 1.08(3) ms α 213Ac 29/2+#
IT (rare) 217Pa
218Pa 91 127 218.020042(26) 0.113(1) ms α 214Ac
219Pa 91 128 219.01988(6) 53(10) ns α 215Ac 9/2−
β+ (5×10−9%) 219Th
220Pa 91 129 220.02188(6) 780(160) ns α 216Ac 1−#
221Pa 91 130 221.02188(6) 4.9(8) µs α 217Ac 9/2−
222Pa 91 131 222.02374(8)# 3.2(3) ms α 218Ac
223Pa 91 132 223.02396(8) 5.1(6) ms α 219Ac
β+ (.001%) 223Th
224Pa 91 133 224.025626(17) 844(19) ms α (99.9%) 220Ac 5−#
β+ (.1%) 224Th
225Pa 91 134 225.02613(8) 1.7(2) s α 221Ac 5/2−#
226Pa 91 135 226.027948(12) 1.8(2) min α (74%) 222Ac
β+ (26%) 226Th
227Pa 91 136 227.028805(8) 38.3(3) min α (85%) 223Ac (5/2−)
EC (15%) 227Th
228Pa 91 137 228.031051(5) 22(1) h β+ (98.15%) 228Th 3+
α (1.85%) 224Ac
229Pa 91 138 229.0320968(30) 1.50(5) d EC (99.52%) 229Th (5/2+)
α (.48%) 225Ac
229mPa 11.6(3) keV 420(30) ns 3/2−
230Pa 91 139 230.034541(4) 17.4(5) d β+ (91.6%) 230Th (2−)
β (8.4%) 230U
α (.00319%) 226Ac
231Pa Protoactinium 91 140 231.0358840(24) 3.276(11)×104 y α 227Ac 3/2− 1.0000[n 3]
CD (1.34×10−9%) 207Tl
SF (3×10−10%) (various)
CD (10−12%) 208Pb
232Pa 91 141 232.038592(8) 1.31(2) d β 232U (2−)
EC (.003%) 232Th
233Pa 91 142 233.0402473(23) 26.975(13) d β 233U 3/2−
234Pa Uranium Z 91 143 234.043308(5) 6.70(5) h β 234U 4+ Trace[n 4]
SF (3×10−10%) (various)
234mPa Uranium X2
78(3) keV 1.17(3) min β (99.83%) 234U (0−) Trace[n 4]
IT (.16%) 234Pa
SF (10−10%) (various)
235Pa 91 144 235.04544(5) 24.44(11) min β 235U (3/2−)
236Pa 91 145 236.04868(21) 9.1(1) min β 236U 1(−)
β, SF (6×10−8%) (various)
237Pa 91 146 237.05115(11) 8.7(2) min β 237U (1/2+)
238Pa 91 147 238.05450(6) 2.27(9) min β 238U (3−)#
β, SF (2.6×10−6%) (various)
239Pa 91 148 239.05726(21)# 1.8(5) h β 239U (3/2)(−#)
240Pa 91 149 240.06098(32)# 2# min β 240U
  1. Abbreviations:
    CD: Cluster decay
    EC: Electron capture
    IT: Isomeric transition
    SF: Spontaneous fission
  2. Stable isotopes in bold
  3. Intermediate decay product of 235U
  4. 1 2 Intermediate decay product of 238U



  1. 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 isotopes have half-lives of at least four years (the longest-lived isotope 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.
  2. Specifically from thermal neutron fission of U-235, e.g. in a typical nuclear reactor.
  3. 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. 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 Bk248 with a half-life greater than 9 y. No growth of Cf248 was detected, and a lower limit for the β half-life can be set at about 104 y. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 y."
  4. This is the heaviest isotope with a half-life of at least four years before the "Sea of Instability".
  5. Excluding those "classically stable" isotopes with half-lives significantly in excess of 232Th; e.g., while 113mCd has a half-life of only fourteen years, that of 113Cd is nearly eight quadrillion years.
  6. "Universal Nuclide Chart". nucleonica. (registration required (help)).
Isotopes of thorium Isotopes of protactinium Isotopes of uranium
Table of nuclides
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