Protactinium Protactinium (formerly protoactinium) is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray actinide metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds ...

(₉₁Pa) has no stable

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...

s. The three naturally occurring isotopes allow a standard atomic weight to be given. Thirty

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...

s of protactinium have been characterized, with the most stable being ²³¹Pa with a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ...

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 Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consi ...

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 state A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited states have ...

s, ^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 = 1.17 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 Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

for isotopes of Pa lighter than (and including) the most stable isotope ²³¹Pa is

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an at ...

, except for ²²⁸Pa to ²³⁰Pa, which primarily decay by electron capture to

isotopes of thorium Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, 232Th, is ''relatively'' stable, with a half-life of 1.405×1010 years, considerably longer than the age of the Earth, and even slightly longer than the gene ...

. The primary mode for the heavier isotopes is beta minus (β⁻) decay. The primary

decay product In nuclear physics, a decay product (also known as a daughter product, daughter isotope, radio-daughter, or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps ( ...

s of ²³¹Pa and isotopes of protactinium lighter than and including ²²⁷Pa are

isotopes of actinium Actinium (89Ac) has no stable isotopes and no characteristic terrestrial isotopic composition, thus a standard atomic weight cannot be given. There are 33 known isotopes, from 204Ac to 236Ac, and 7 isomers. Three isotopes are found in nature, 225 ...

and the primary decay products for the heavier isotopes of protactinium are

isotopes of uranium Uranium (92U) is a naturally occurring radioactive element that has no stable isotope. It has two primordial isotopes, uranium-238 and uranium-235, that have long half-lives and are found in appreciable quantity in the Earth's crust. The d ...

List of isotopes

, - , ²¹¹Pa , , style="text-align:right" , 91 , style="text-align:right" , 120 , , 3.8(+4.6−1.4) ms , α , ²⁰⁷Ac , 9/2−# , , , - , ²¹²Pa , , style="text-align:right" , 91 , style="text-align:right" , 121 , 212.02320(8) , 8(5) ms
.1(+61−19) ms, α , ²⁰⁸Ac , 7+# , , , - , ²¹³Pa , , style="text-align:right" , 91 , style="text-align:right" , 122 , 213.02111(8) , 7(3) ms
.3(+40−16) ms, α , ²⁰⁹Ac , 9/2−# , , , - , ²¹⁴Pa , , style="text-align:right" , 91 , style="text-align:right" , 123 , 214.02092(8) , 17(3) ms , α , ²¹⁰Ac , , , , - , ²¹⁵Pa , , style="text-align:right" , 91 , style="text-align:right" , 124 , 215.01919(9) , 14(2) ms , α , ²¹¹Ac , 9/2−# , , , - , rowspan=2, ²¹⁶Pa , rowspan=2, , rowspan=2 style="text-align:right" , 91 , rowspan=2 style="text-align:right" , 125 , rowspan=2, 216.01911(8) , rowspan=2, 105(12) ms , α (80%) , ²¹²Ac , rowspan=2, , rowspan=2, , rowspan=2, , - , β⁺ (20%) , ²¹⁶Th , - , ²¹⁷Pa , , style="text-align:right" , 91 , style="text-align:right" , 126 , 217.01832(6) , 3.48(9) ms , α , ²¹³Ac , 9/2−# , , , - , rowspan=2 style="text-indent:1em" , ^217mPa , rowspan=2, , rowspan=2 colspan="3" style="text-indent:2em" , 1860(7) keV , rowspan=2, 1.08(3) ms , α , ²¹³Ac , rowspan=2, 29/2+# , rowspan=2, , rowspan=2, , - , IT (rare) , ²¹⁷Pa , - , ²¹⁸Pa , , style="text-align:right" , 91 , style="text-align:right" , 127 , 218.020042(26) , 0.113(1) ms , α , ²¹⁴Ac , , , , - , rowspan=2, ²¹⁹Pa , rowspan=2, , rowspan=2 style="text-align:right" , 91 , rowspan=2 style="text-align:right" , 128 , rowspan=2, 219.01988(6) , rowspan=2, 53(10) ns , α , ²¹⁵Ac , rowspan=2, 9/2− , rowspan=2, , rowspan=2, , - , β⁺ (5×10⁻⁹%) , ²¹⁹Th , - , ²²⁰Pa , , style="text-align:right" , 91 , style="text-align:right" , 129 , 220.02188(6) , 780(160) ns , α , ²¹⁶Ac , 1−# , , , - , style="text-indent:1em" , ^220m1Pa , , colspan="3" style="text-indent:2em" , 34(26) keV , 308(+250-99) ns , α , ²¹⁶Ac , , , , - , style="text-indent:1em" , ^220m2Pa , , colspan="3" style="text-indent:2em" , 297(65) keV , 69(+330-30) ns , α , ²¹⁶Ac , , , , - , ²²¹Pa , , style="text-align:right" , 91 , style="text-align:right" , 130 , 221.02188(6) , 4.9(8) μs , α , ²¹⁷Ac , 9/2− , , , - , ²²²Pa , , style="text-align:right" , 91 , style="text-align:right" , 131 , 222.02374(8)# , 3.2(3) ms , α , ²¹⁸Ac , , , , - , rowspan=2, ²²³Pa , rowspan=2, , rowspan=2 style="text-align:right" , 91 , rowspan=2 style="text-align:right" , 132 , rowspan=2, 223.02396(8) , rowspan=2, 5.1(6) ms , α , ²¹⁹Ac , rowspan=2, , rowspan=2, , rowspan=2, , - , β⁺ (.001%) , ²²³Th , - , rowspan=2, ²²⁴Pa , rowspan=2, , rowspan=2 style="text-align:right" , 91 , rowspan=2 style="text-align:right" , 133 , rowspan=2, 224.025626(17) , rowspan=2, 844(19) ms , α (99.9%) , ²²⁰Ac , rowspan=2, 5−# , rowspan=2, , rowspan=2, , - , β⁺ (.1%) , ²²⁴Th , - , ²²⁵Pa , , style="text-align:right" , 91 , style="text-align:right" , 134 , 225.02613(8) , 1.7(2) s , α , ²²¹Ac , 5/2−# , , , - , 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) min , α (74%) , ²²²Ac , rowspan=2, , rowspan=2, , rowspan=2, , - , β⁺ (26%) , ²²⁶Th , - , rowspan=2, ²²⁷Pa , rowspan=2, , rowspan=2 style="text-align:right" , 91 , rowspan=2 style="text-align:right" , 136 , rowspan=2, 227.028805(8) , rowspan=2, 38.3(3) min , α (85%) , ²²³Ac , rowspan=2, (5/2−) , rowspan=2, , rowspan=2, , - , EC (15%) , ²²⁷Th , - , rowspan=2, ²²⁸Pa , rowspan=2, , rowspan=2 style="text-align:right" , 91 , rowspan=2 style="text-align:right" , 137 , rowspan=2, 228.031051(5) , rowspan=2, 22(1) h , β⁺ (98.15%) , ²²⁸Th , rowspan=2, 3+ , rowspan=2, , rowspan=2, , - , α (1.85%) , ²²⁴Ac , - , rowspan=2, ²²⁹Pa , rowspan=2, , rowspan=2 style="text-align:right" , 91 , rowspan=2 style="text-align:right" , 138 , rowspan=2, 229.0320968(30) , rowspan=2, 1.50(5) d , EC (99.52%) , ²²⁹Th , rowspan=2, (5/2+) , rowspan=2, , rowspan=2, , - , α (.48%) , ²²⁵Ac , - , style="text-indent:1em" , ^229mPa , , colspan="3" style="text-indent:2em" , 11.6(3) keV , 420(30) ns , , , 3/2− , , , - , rowspan=3, ²³⁰Pa , rowspan=3, , rowspan=3 style="text-align:right" , 91 , rowspan=3 style="text-align:right" , 139 , rowspan=3, 230.034541(4) , rowspan=3, 17.4(5) d , β⁺ (91.6%) , ²³⁰Th , rowspan=3, (2−) , rowspan=3, , rowspan=3, , - , β⁻ (8.4%) , ²³⁰U , - , α (.00319%) , ²²⁶Ac , - , rowspan=4, ²³¹Pa , rowspan=4, Protoactinium , rowspan=4 style="text-align:right" , 91 , rowspan=4 style="text-align:right" , 140 , rowspan=4, 231.0358840(24) , rowspan=4, 3.276(11)×10⁴ y , α , ²²⁷Ac , rowspan=4, 3/2− , rowspan=4, 1.0000Intermediate

of ²³⁵U , rowspan=4, , - , CD (1.34×10⁻⁹%) , ²⁰⁷Tl
²⁴Ne , - , SF (3×10⁻¹⁰%) , (various) , - , CD (10⁻¹²%) , ²⁰⁸Pb
²³F , - , rowspan=2, ²³²Pa , rowspan=2, , rowspan=2 style="text-align:right" , 91 , rowspan=2 style="text-align:right" , 141 , rowspan=2, 232.038592(8) , rowspan=2, 1.31(2) d , β⁻ , ²³²U , rowspan=2, (2−) , rowspan=2, , rowspan=2, , - , EC (.003%) , ''²³²Th'' , - , ²³³Pa , , style="text-align:right" , 91 , style="text-align:right" , 142 , 233.0402473(23) , 26.975(13) d , β⁻ , ²³³U , 3/2− , TraceIntermediate decay product of ²³⁷Np , , - , rowspan=2, ²³⁴Pa , rowspan=2, Uranium Z , rowspan=2 style="text-align:right" , 91 , rowspan=2 style="text-align:right" , 143 , rowspan=2, 234.043308(5) , rowspan=2, 6.70(5) h , β⁻ , ²³⁴U , rowspan=2, 4+ , rowspan=2, TraceIntermediate decay product of ²³⁸U , rowspan=2, , - , SF (3×10⁻¹⁰%) , (various) , - , rowspan=3 style="text-indent:1em" , ^234mPa , rowspan=3, Uranium X₂
Brevium , rowspan=3 colspan="3" style="text-indent:2em" , 78(3) keV , rowspan=3, 1.17(3) min , β⁻ (99.83%) , ²³⁴U , rowspan=3, (0−) , rowspan=3, Trace , rowspan=3, , - , IT (.16%) , ²³⁴Pa , - , SF (10⁻¹⁰%) , (various) , - , ²³⁵Pa , , style="text-align:right" , 91 , style="text-align:right" , 144 , 235.04544(5) , 24.44(11) min , β⁻ , ''²³⁵U'' , (3/2−) , , , - , rowspan=2, ²³⁶Pa , rowspan=2, , rowspan=2 style="text-align:right" , 91 , rowspan=2 style="text-align:right" , 145 , rowspan=2, 236.04868(21) , rowspan=2, 9.1(1) min , β⁻ , ²³⁶U , rowspan=2, 1(−) , rowspan=2, , rowspan=2, , - , β⁻, SF (6×10⁻⁸%) , (various) , - , ²³⁷Pa , , style="text-align:right" , 91 , style="text-align:right" , 146 , 237.05115(11) , 8.7(2) min , β⁻ , ²³⁷U , (1/2+) , , , - , rowspan=2, ²³⁸Pa , rowspan=2, , rowspan=2 style="text-align:right" , 91 , rowspan=2 style="text-align:right" , 147 , rowspan=2, 238.05450(6) , rowspan=2, 2.27(9) min , β⁻ , ''²³⁸U'' , rowspan=2, (3−)# , rowspan=2, , rowspan=2, , - , β⁻, SF (2.6×10⁻⁶%) , (various) , - , ²³⁹Pa , , style="text-align:right" , 91 , style="text-align:right" , 148 , 239.05726(21)# , 1.8(5) h , β⁻ , ²³⁹U , (3/2)(−#) , , , - , ²⁴⁰Pa , , style="text-align:right" , 91 , style="text-align:right" , 149 , 240.06098(32)# , 2# min , β⁻ , ²⁴⁰U , , ,

Actinides and fission products

Protactinium-230

Protactinium-230 has 139

neutrons The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave ...

and a half-life of 17.4 days. Most of the time (92%), it undergoes beta plus decay to ²³⁰Th, with a minor (8%) beta-minus decay branch leading to ²³⁰U. It also has a very rare (.003%) 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 chain In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay dire ...

s of ²³⁵U, ²³⁸U, or ²³²Th. 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 Targeted alpha-particle therapy (or TAT) is an in-development method of targeted radionuclide therapy of various cancers. It employs radioactive substances which undergo alpha decay to treat diseased tissue at close proximity. It has the potential t ...

(TAT). It can be produced through proton or

deuteron Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one n ...

irradiation of nautral 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 In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay directly ...

, which starts with the primordial isotope

uranium-235 Uranium-235 (235U or U-235) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a nuclear chain reaction. It is the only fissile isotope that exi ...

; the equilibrium concentration in uranium ore is 46.55 ²³¹Pa per million ²³⁵U. In

nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat fr ...

s, it is one of the few long-lived radioactive

actinide The actinide () or actinoid () series encompasses the 15 metallic chemical elements with atomic numbers from 89 to 103, actinium through lawrencium. The actinide series derives its name from the first element in the series, actinium. The info ...

s produced as a byproduct of the projected thorium fuel cycle, as a result of (n,2n) reactions where a fast neutron removes a

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the atomic nucleus, nuclei of atoms. Since protons and ...

from ²³²Th or ²³²U, and can also be destroyed by

neutron capture Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons ...

, though the cross section for this reaction is also low. Pa-231 solution

binding energy: 1759860 keV
beta decay energy: −382 keV spin: 3/2−
mode of decay:

alpha Alpha (uppercase , lowercase ; grc, ἄλφα, ''álpha'', or ell, άλφα, álfa) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter aleph , whi ...

to ²²⁷Ac, also others possible parent nuclides:

beta Beta (, ; uppercase , lowercase , or cursive ; grc, βῆτα, bē̂ta or ell, βήτα, víta) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Modern Greek, it represents the voiced labiod ...

from ²³¹Th, EC from ²³¹U,

from ²³⁵Np.

Protactinium-233

Protactinium-233 is also part of the thorium fuel cycle. It is an intermediate beta decay product between

thorium-233 Thorium (90Th) has seven naturally occurring isotopes but none are stable. One isotope, 232Th, is ''relatively'' stable, with a half-life of 1.405×1010 years, considerably longer than the age of the Earth, and even slightly longer than the ge ...

(produced from natural thorium-232 by neutron capture) and

uranium-233 Uranium-233 (233U or U-233) is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel. It has been used successfully in exp ...

(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 In nuclear science, the decay chain refers to a series of radioactive decays of different radioactive decay products as a sequential series of transformations. It is also known as a "radioactive cascade". Most radioisotopes do not decay direct ...

with a half-life of 6.70 hours. It was discovered by

Otto Hahn Otto Hahn (; 8 March 1879 – 28 July 1968) was a German chemist who was a pioneer in the fields of radioactivity and radiochemistry. He is referred to as the father of nuclear chemistry and father of nuclear fission. Hahn and Lise Meitner ...

in 1921.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.

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.http://hpschapters.org/northcarolina/NSDS/Protactinium.pdf About 99.8% of decays of ²³⁴Th produce this isomer instead of the

ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state. ...

(t_1/2 = 6.70 hours).

References

* Isotope masses from: ** * Isotopic compositions and standard atomic masses from: ** ** * Half-life, spin, and isomer data selected from the following sources. ** ** ** {{Navbox element isotopes Protactinium