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= Protactinium =
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Introduction
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Protactinium is a chemical element; it has symbol Pa and atomic number
91. It is a dense, radioactive, silvery-gray actinide metal which
readily reacts with oxygen, water vapor, and inorganic acids. It forms
various chemical compounds, in which protactinium is usually present
in the oxidation state +5, but it can also assume +4 and even +3 or +2
states. Concentrations of protactinium in the Earth's crust are
typically a few parts per trillion, but may reach up to a few parts
per million in some uraninite ore deposits. Because of its scarcity,
high radioactivity, and high toxicity, there are currently no uses for
protactinium outside scientific research, and for this purpose,
protactinium is mostly extracted from spent nuclear fuel.
The element was first identified in 1913 by Kazimierz Fajans and
Oswald Helmuth Göhring and named "brevium" because of the short
half-life of the specific isotope studied, 234mPa. A more stable
isotope of protactinium, 231Pa, was discovered in 1917/18 by Lise
Meitner in collaboration with Otto Hahn, and they named the element
protactinium. In 1949, the IUPAC chose the name "protactinium" and
confirmed Hahn and Meitner as its discoverers. The new name meant
"(nuclear) precursor of actinium," suggesting that actinium is a
product of radioactive decay of protactinium. John Arnold Cranston
(working with Frederick Soddy and Ada Hitchins) is also credited with
discovering the most stable isotope in 1915, but he delayed his
announcement due to being called for service in the First World War.
The longest-lived and most abundant (nearly 100%) naturally occurring
isotope of protactinium, 231Pa, has a half-life of 32,760 years and is
a decay product of uranium-235. Much smaller trace amounts of the
short-lived 234Pa and its nuclear isomer 234mPa occur in the decay
chain of uranium-238. 233Pa occurs as a result of the decay of
thorium-233 as part of the chain of events necessary to produce
uranium-233 by neutron irradiation of 232Th. It is an undesired
intermediate product in thorium-based nuclear reactors, and is
therefore removed from the active zone of the reactor during the
breeding process. Ocean science uses the element to understand the
ancient ocean's geography: analysis of the relative concentrations of
various uranium, thorium, and protactinium isotopes in water and
minerals is used in radiometric dating of sediments up to 175,000
years old, and in modeling of various geological processes.
History
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In 1871, Dmitri Mendeleev predicted the existence of an element
between thorium and uranium. The actinide series was unknown at the
time, so Mendeleev positioned uranium below tungsten in group VI, and
thorium below zirconium in group IV, leaving the space below tantalum
in group V empty. Until the general acceptance of the actinide concept
in the late 1940s, periodic tables were published with this structure.
For a long time, chemists searched for eka-tantalum as an element with
similar chemical properties to tantalum, making a discovery of
protactinium nearly impossible. Tantalum's heavier analogue was later
found to be the transuranic element dubnium - although dubnium is more
chemically similar to protactinium, not tantalum.
In 1900, William Crookes isolated protactinium as an intensely
radioactive material from uranium; however, he could not characterize
it as a new chemical element and thus named it uranium X (UX). Crookes
dissolved uranium nitrate in ether, and the residual aqueous phase
contained most of the and . His method was used into the 1950s to
isolate and from uranium compounds. Protactinium was first
identified in 1913, when Kasimir Fajans and Oswald Helmuth Göhring
encountered the isotope 234mPa during their studies of the decay
chains of uranium-238: → → → . They named the new element "brevium"
(from the Latin word 'brevis', meaning brief or short) because of the
short half-life of 1.16 minutes for (uranium X2). In 1917-18, two
groups of scientists, Lise Meitner in collaboration with Otto Hahn of
Germany and Frederick Soddy and John Cranston of Great Britain,
independently discovered another isotope, 231Pa, having a much longer
half-life of 32,760 years. Meitner changed the name "brevium" to
'protactinium' as the new element was part of the decay chain of
uranium-235 as the parent of actinium (from the 'prôtos', meaning
"first, before"). The IUPAC confirmed this naming in 1949. The
discovery of protactinium completed one of the last gaps in early
versions of the periodic table, and brought fame to the involved
scientists.
Aristid von Grosse produced 2 milligrams of Pa2O5 in 1927, and in 1934
first isolated elemental protactinium from 0.1 milligrams of Pa2O5. He
used two different procedures: in the first, protactinium oxide was
irradiated by 35 keV electrons in vacuum. In the other, called the van
Arkel-de Boer process, the oxide was chemically converted to a halide
(chloride, bromide or iodide) and then reduced in a vacuum with an
electrically heated metallic filament:
: 2 PaI5 → 2 Pa + 5 I2
In 1961, the United Kingdom Atomic Energy Authority (UKAEA) produced
127 grams of 99.9% pure protactinium-231 by processing 60 tonnes of
waste material in a 12-stage process, at a cost of about US$500,000.
For many years, this was the world's only significant supply of
protactinium, which was provided to various laboratories for
scientific studies. The Oak Ridge National Laboratory in the US
provided protactinium at a cost of about US$280/gram.
Isotopes
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Thirty radioisotopes of protactinium have been discovered, ranging
from 210Pa to 239Pa. The most stable are 231Pa with a half-life of
32,650 years, 233Pa with a half-life of 26.975 days, and 230Pa with a
half-life of 17.4 days. All other isotopes have half-lives shorter
than 1.6 days, and the majority of these have half-lives less than 1.8
seconds. Protactinium also has six nuclear isomers, with the most
stable being 234mPa (half-life 1.159 minutes).
The primary decay mode for the most stable isotope 231Pa and lighter
isotopes (210Pa to 227Pa) is alpha decay, producing isotopes of
actinium. The primary decay mode for 228Pa to 230Pa is electron
capture or beta plus decay, producing isotopes of thorium, while the
primary decay mode for the heavier isotopes (232Pa to 239Pa) is beta
decay, producing isotopes of uranium.
Nuclear fission
=================
The longest-lived and most abundant isotope, 231Pa, can fission from
fast neutrons exceeding ~1 MeV. 233Pa, the other isotope of
protactinium produced in nuclear reactors, also has a fission
threshold of 1 MeV.
Occurrence
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Protactinium is one of the rarest and most expensive naturally
occurring elements. It is found in the form of two isotopes - 231Pa
and 234Pa, with the isotope 234Pa occurring in two different energy
states. Nearly all natural protactinium is 231Pa. It is an alpha
emitter and is formed by the decay of uranium-235, whereas the
beta-radiating 234Pa is produced as a result of uranium-238 decay.
Nearly all uranium-238 (99.8%) decays first to the shorter-lived
234mPa isomer.
Protactinium occurs in uraninite (pitchblende) at concentrations of
about 0.3-3 parts 231Pa per million parts (ppm) of ore. Whereas the
usual content is closer to 0.3 ppm (e.g. in Jáchymov, Czech Republic),
some ores from the Democratic Republic of the Congo have about 3 ppm.
Protactinium is homogeneously dispersed in most natural materials and
in water, but at much lower concentrations on the order of one part
per trillion, corresponding to a radioactivity of 0.1 picocuries
(pCi)/g. There is about 500 times more protactinium in sandy soil
particles than in water, even when compared to water present in the
same sample of soil. Much higher ratios of 2,000 and above are
measured in loam soils and clays, such as bentonite.
In nuclear reactors
=====================
Two major protactinium isotopes, 231Pa and 233Pa, are produced from
thorium in nuclear reactors; both are undesirable and are usually
removed, thereby adding complexity to the reactor design and
operation. In particular, 232Th, via ('n', 2'n') reactions, produces
231Th, which quickly decays to 231Pa (half-life 25.5 hours). The last
isotope, while not a transuranic waste, has a long half-life of 32,760
years, and is a major contributor to the long-term radiotoxicity of
spent nuclear fuel.
Protactinium-233 is formed upon neutron capture by 232Th. It either
further decays to 233U, or captures another neutron and converts into
the non-fissile 234U. 233Pa has a relatively long half-life of 27 days
and high cross section for neutron capture (the so-called "neutron
poison"). Thus, instead of rapidly decaying to the useful 233U, a
significant fraction of 233Pa converts to non-fissile isotopes and
consumes neutrons, degrading reactor efficiency. To limit the loss of
neutrons, 233Pa is extracted from the active zone of thorium molten
salt reactors during their operation, so that it can only decay into
233U. Extraction of 233Pa is achieved using columns of molten bismuth
with lithium dissolved in it. In short, lithium selectively reduces
protactinium salts to protactinium metal, which is then extracted from
the molten-salt cycle, while the molten bismuth is merely a carrier,
selected due to its low melting point of 271 °C, low vapor pressure,
good solubility for lithium and actinides, and immiscibility with
molten halides.
Preparation
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Before the advent of nuclear reactors, protactinium was separated for
scientific experiments from uranium ores. Since reactors have become
more common, it is mostly produced as an intermediate product of
nuclear fission in thorium fuel cycle reactors as an intermediate in
the production of the fissile 233U:
:^{232}_{90}Th + ^{1}_{0}n -> ^{233}_{90}Th ->[\beta^-][22.3\
\ce{min}] ^{233}_{91}Pa ->[\beta^-][26.967\ \ce{d}] ^{233}_{92}U.
The isotope 231Pa can be prepared by irradiating 230Th with slow
neutrons, converting it to the beta-decaying 231Th; or, by irradiating
232Th with fast neutrons, generating 231Th and 2 neutrons.
Protactinium metal can be prepared by reduction of its fluoride with
calcium, lithium, or barium at a temperature of 1300-1400 °C.
Properties
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Protactinium is an actinide positioned in the periodic table to the
left of uranium and to the right of thorium, and many of its physical
properties are intermediate between its neighboring actinides.
Protactinium is denser and more rigid than thorium, but is lighter
than uranium; its melting point is lower than that of thorium, but
higher than that of uranium. The thermal expansion, electrical, and
thermal conductivities of these three elements are comparable and are
typical of post-transition metals. The estimated shear modulus of
protactinium is similar to that of titanium. Protactinium is a metal
with silvery-gray luster that is preserved for some time in air.
Protactinium easily reacts with oxygen, water vapor, and acids, but
not with alkalis.
At room temperature, protactinium crystallizes in the body-centered
tetragonal structure, which can be regarded as distorted body-centered
cubic lattice; this structure does not change upon compression up to
53 GPa. The structure changes to face-centered cubic ('fcc') upon
cooling from high temperature, at about 1200 °C. The thermal expansion
coefficient of the tetragonal phase between room temperature and 700
°C is 9.9/°C.
Protactinium is paramagnetic and no magnetic transitions are known for
it at any temperature. It becomes superconductive at temperatures
below 1.4 K. Protactinium tetrachloride is paramagnetic at room
temperature, but becomes ferromagnetic when cooled to 182 K.
Protactinium exists in two major oxidation states: +4 and +5, both in
solids and solutions; and the +3 and +2 states, which have been
observed in some solids. As the electron configuration of the neutral
atom is [Rn]5f26d17s2, the +5 oxidation state corresponds to the
low-energy (and thus favored) 5f0 configuration. Both +4 and +5 states
easily form hydroxides in water, with the predominant ions being
Pa(OH)3+, , , and Pa(OH)4, all of which are colorless. Other known
protactinium ions include , , PaF3+, , , , and .
Chemical compounds
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Formula color symmetry space group No Pearson symbol 'a' (pm) 'b' (pm)
'c' (pm) 'Z' density (g/cm3)
Pa silvery-gray tetragonal I4/mmm 139 tI2 392.5 392.5 323.8 2 15.37
PaO rocksalt Fmm 225 cF8 496.1 4 13.44
PaO2 black 'fcc' Fmm 225 cF12 550.5 4 10.47
Pa2O5 white Fmm 225 cF16 547.6 547.6 547.6 4 10.96
Pa2O5 white orthorhombic 692 402 418
PaH3 black cubic Pmn 223 cP32 664.8 664.8 664.8 8 10.58
PaF4 brown-red monoclinic C2/c 15 mS60 2
PaCl4 green-yellow tetragonal I41/amd 141 tI20 837.7 837.7 748.1 4
4.72
PaBr4 brown tetragonal I41/amd 141 tI20 882.4 882.4 795.7
PaCl5 yellow monoclinic C2/c 15 mS24 797 1135 836 4 3.74
PaBr5 red monoclinic P21/c 14 mP24 838.5 1120.5 1214.6 4 4.98
PaOBr3 monoclinic C2 1691.1 387.1 933.4
Pa(PO3)4 orthorhombic 696.9 895.9 1500.9
Pa2P2O7 cubic Pa3 865 865 865
Pa(C8H8)2 golden-yellow monoclinic 709 875 1062
Here, 'a', 'b', and 'c' are lattice constants in picometers, No is the
space group number, and 'Z' is the number of formula units per unit
cell; 'fcc' stands for the face-centered cubic symmetry. Density was
not measured directly but calculated from the lattice parameters.
Oxides and oxygen-containing salts
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Protactinium oxides are known for the metal oxidation states +2, +4,
and +5. The most stable is the white pentoxide Pa2O5, which can be
produced by igniting protactinium(V) hydroxide in air at a temperature
of 500 °C. Its crystal structure is cubic, and the chemical
composition is often non-stoichiometric, described as PaO2.25. Another
phase of this oxide with orthorhombic symmetry has also been reported.
The black dioxide PaO2 is obtained from the pentoxide by reducing it
at 1550 °C with hydrogen. It is not readily soluble in either dilute
or concentrated nitric, hydrochloric, or sulfuric acid, but easily
dissolves in hydrofluoric acid. The dioxide can be converted back to
pentoxide by heating in oxygen-containing atmosphere to 1100 °C. The
monoxide PaO has only been observed as a thin coating on protactinium
metal, but not in an isolated bulk form.
Protactinium forms mixed binary oxides with various metals. With
alkali metals 'A', the crystals have a chemical formula APaO3 and
perovskite structure; A3PaO4 and distorted rock-salt structure; or
A7PaO6, where oxygen atoms form a hexagonal close-packed lattice. In
all of these materials, the protactinium ions are octahedrally
coordinated. The pentoxide Pa2O5 combines with rare-earth metal oxides
R2O3 to form various nonstoichiometric mixed-oxides, also of
perovskite structure.
Protactinium oxides are basic; they easily convert to hydroxides and
can form various salts, such as sulfates, phosphates, nitrates, etc.
The nitrate is usually white but can be brown due to radiolytic
decomposition. Heating the nitrate in air at 400 °C converts it to the
white protactinium pentoxide. The polytrioxophosphate Pa(PO3)4 can be
produced by reacting the difluoride sulfate PaF2SO4 with phosphoric
acid (H3PO4) under an inert atmosphere. Heating the product to about
900 °C eliminates the reaction by-products, which include hydrofluoric
acid, sulfur trioxide, and phosphoric anhydride. Heating it to higher
temperatures in an inert atmosphere decomposes Pa(PO3)4 into the
diphosphate PaP2O7, which is analogous to diphosphates of other
actinides. In the diphosphate, the PO3 groups form pyramids of C2v
symmetry. Heating PaP2O7 in air to 1400 °C decomposes it into the
pentoxides of phosphorus and protactinium.
Halides
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Protactinium(V) fluoride is a white compound that forms tetragonal
crystals, isomorphic to β-UF5. Protactinium(V) chloride forms yellow
crystals where protactinium ions are arranged in pentagonal bipyramids
and coordinated by 7 other ions. The coordination changes to
octahedral in the brown protactinium(V) bromide, but is unknown for
protactinium(V) iodide. The protactinium coordination in all its
tetrahalides is 8, but the arrangement is square antiprismatic in
protactinium(IV) fluoride and dodecahedral in the chloride and
bromide. Brown-colored protactinium(III) iodide has been reported,
where protactinium ions are 8-coordinated in a bicapped trigonal
prismatic arrangement.
Protactinium(V) chloride has a polymeric structure of monoclinic
symmetry. There, within one polymeric chain, all chlorine atoms lie in
one graphite-like plane and form planar pentagons around the
protactinium ions. The 7-coordination of protactinium originates from
the five chlorine atoms and two bonds to protactinium atoms belonging
to the nearby chains. It easily hydrolyzes in water. It melts at 300
°C and sublimates at even lower temperatures.
Protactinium(V) fluoride can be prepared by reacting protactinium
oxide with either bromine pentafluoride or bromine trifluoride at
about 600 °C, and protactinium(IV) fluoride is obtained from the oxide
and a mixture of hydrogen and hydrogen fluoride at 600 °C; a large
excess of hydrogen is required to remove atmospheric oxygen leaks into
the reaction.
Protactinium(V) chloride is prepared by reacting protactinium oxide
with carbon tetrachloride at temperatures of 200-300 °C. The
by-products (such as PaOCl3) are removed by fractional sublimation.
Reduction of protactinium(V) chloride with hydrogen at about 800 °C
yields protactinium(IV) chloride - a yellow-green solid that sublimes
in vacuum at 400 °C. It can also be obtained directly from
protactinium dioxide by treating it with carbon tetrachloride at 400
°C.
Protactinium bromides are produced by the action of aluminium bromide,
hydrogen bromide, carbon tetrabromide, or a mixture of hydrogen
bromide and thionyl bromide on protactinium oxide. They can
alternatively be produced by reacting protactinium pentachloride with
hydrogen bromide or thionyl bromide. Protactinium(V) bromide has two
similar monoclinic forms: one is obtained by sublimation at 400-410
°C, and another by sublimation at a slightly lower temperature of
390-400 °C.
Protactinium iodides can be produced by reacting protactinium metal
with elemental iodine at 600 °C, and by reacting Pa2O5 with AlI3 at
elevated temperatures. Protactinium(III) iodide can be obtained by
heating protactinium(V) iodide in vacuum. As with oxides, protactinium
forms mixed halides with alkali metals. The most remarkable among
these is Na3PaF8, where the protactinium ion is symmetrically
surrounded by 8 F− ions, forming a nearly perfect cube.
More complex protactinium fluorides are also known, such as Pa2F9 and
ternary fluorides of the types MPaF6 (M = Li, Na, K, Rb, Cs or NH4),
M2PaF7 (M = K, Rb, Cs or NH4), and M3PaF8 (M = Li, Na, Rb, Cs), all of
which are white crystalline solids. The MPaF6 formula can be
represented as a combination of MF and PaF5. These compounds can be
obtained by evaporating a hydrofluoric acid solution containing both
complexes. For the small alkali cations like Na, the crystal structure
is tetragonal, whereas it becomes orthorhombic for larger cations K+,
Rb+, Cs+ or NH4+. A similar variation was observed for the M2PaF7
fluorides: namely, the crystal symmetry was dependent on the cation
and differed for Cs2PaF7 and M2PaF7 (M = K, Rb or NH4).
Other inorganic compounds
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Oxyhalides and oxysulfides of protactinium are known. PaOBr3 has a
monoclinic structure composed of double-chain units where protactinium
has coordination 7 and is arranged into pentagonal bipyramids. The
chains are interconnected through oxygen and bromine atoms, and each
oxygen atom is related to three protactinium atoms. PaOS is a
light-yellow, non-volatile solid with a cubic crystal lattice
isostructural to that of other actinide oxysulfides. It is obtained by
reacting protactinium(V) chloride with a mixture of hydrogen sulfide
and carbon disulfide at 900 °C.
In hydrides and nitrides, protactinium has a low oxidation state of
about +3. The hydride is obtained by direct action of hydrogen on the
metal at 250 °C, and the nitride is a product of ammonia and
protactinium tetrachloride or pentachloride. This bright yellow solid
is thermally stable to 800 °C in vacuum. Protactinium carbide (PaC) is
formed by the reduction of protactinium tetrafluoride with barium in a
carbon crucible at a temperature of about 1400 °C. Protactinium forms
borohydrides, which include Pa(BH4)4. It has an unusual polymeric
structure with helical chains, where the protactinium atom has
coordination number of 14 and is surrounded by six BH4− ions.
Organometallic compounds
==========================
Protactinium(IV) forms a tetrahedral complex
tetrakis(cyclopentadienyl)protactinium(IV) (or Pa(C5H5)4) with four
cyclopentadienyl rings, which can be synthesized by reacting
protactinium(IV) chloride with Be(C5H5)2. One ring can be substituted
with a halide atom. Another organometallic complex is the
golden-yellow bis(π-cyclooctatetraene) protactinium, or protactinocene
(Pa(C8H8)2), which is analogous in structure to uranocene. There, the
metal atom is sandwiched between two cyclooctatetraene ligands.
Similar to uranocene, it can be prepared by reacting protactinium
tetrachloride with dipotassium cyclooctatetraenide (K2C8H8) in
tetrahydrofuran.
Applications
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Although protactinium is situated in the periodic table between
uranium and thorium, both of which have numerous applications, there
are currently no uses for protactinium outside scientific research
owing to its scarcity, high radioactivity, and high toxicity.
231Pa arises naturally from the decay of natural 235U, and
artificially in nuclear reactors by the reaction 232Th + n → 231Th +
2n and the subsequent beta decay of 231Th. It was once thought to be
able to support a nuclear chain reaction, which could in principle be
used to build nuclear weapons; the physicist once estimated the
associated critical mass as . However, the possibility of criticality
of 231Pa has since been ruled out.
With the advent of highly sensitive mass spectrometers, an application
of 231Pa as a tracer in geology and paleoceanography has become
possible. In this application, the ratio of 231Pa to 230Th is used for
radiometric dating of sediments which are up to 175,000 years old, and
in modeling of the formation of minerals. In particular, its
evaluation in oceanic sediments helped to reconstruct the movements of
North Atlantic water bodies during the last melting of Ice Age
glaciers. Some of the protactinium-related dating variations rely on
analysis of the relative concentrations of several long-living members
of the uranium decay chain - uranium, protactinium, and thorium, for
example. These elements have 6, 5, and 4 valence electrons, thus
favoring +6, +5, and +4 oxidation states respectively, and display
different physical and chemical properties. Thorium and protactinium,
but not uranium compounds, are poorly soluble in aqueous solutions and
precipitate into sediments; the precipitation rate is faster for
thorium than for protactinium. The concentration analysis for both
protactinium-231 (half-life 32,760 years) and 230Th (half-life 75,380
years) improves measurement accuracy compared to when only one isotope
is measured; this double-isotope method is also weakly sensitive to
inhomogeneities in the spatial distribution of the isotopes and to
variations in their precipitation rate.
Precautions
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Protactinium is both toxic and highly radioactive; thus, it is handled
exclusively in a sealed glove box. Its major isotope 231Pa has a
specific activity of 0.048 Ci per gram and primarily emits
alpha-particles with an energy of 5 MeV, which can be stopped by a
thin layer of any material. However, it slowly decays, with a
half-life of 32,760 years, into 227Ac, which has a specific activity
of 74 Ci per gram, emits both alpha and beta radiation, and has a much
shorter half-life of 22 years. 227Ac, in turn, decays into lighter
isotopes with even shorter half-lives and much greater specific
activities (SA).
!Isotope |231Pa 227Ac 227Th 223Ra 219Rn 215Po 211Pb 211Bi
207Tl
!SA (Ci/g) 0.048 73 3.1 5.2 1.3 3 2.5 4.2 1.9
!Decay |α α, β α α α α β α, β β
!Half-life 33 ka 22 a 19 days 11 days 4 s 1.8 ms 36 min 2.1
min 4.8 min
As protactinium is present in small amounts in most natural products
and materials, it is ingested with food or water and inhaled with air.
Only about 0.05% of ingested protactinium is absorbed into the blood
and the remainder is excreted. From the blood, about 40% of the
protactinium
deposits in the bones, about 15% goes to the liver, 2% to the kidneys,
and the rest leaves the body. The biological half-life of protactinium
is about 50 years in the bones, whereas its biological half-life in
other organs has a fast and slow component. For example, 70% of the
protactinium in the liver has a biological half-life of 10 days, and
the remaining 30% for 60 days. The corresponding values for kidneys
are 20% (10 days) and 80% (60 days). In each affected organ,
protactinium promotes cancer via its radioactivity. The maximum safe
dose of Pa in the human body is 0.03 µCi, which corresponds to 0.5
micrograms of 231Pa. The maximum allowed concentrations of 231Pa in
the air in Germany is .
See also
======================================================================
* Ada Hitchins, who helped Soddy in discovering the element
protactinium
External links
======================================================================
* [
http://www.periodicvideos.com/videos/091.htm Protactinium] at 'The
Periodic Table of Videos' (University of Nottingham)
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=========
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Original Article:
http://en.wikipedia.org/wiki/Protactinium
