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= Berkelium =
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Introduction
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Berkelium is a synthetic chemical element; it has symbol Bk and atomic
number 97. It is a member of the actinide and transuranium element
series. It is named after the city of Berkeley, California, the
location of the Lawrence Berkeley National Laboratory (then the
University of California Radiation Laboratory) where it was discovered
in December 1949. Berkelium was the fifth transuranium element
discovered after neptunium, plutonium, curium and americium.
The major isotope of berkelium, 249Bk, is synthesized in minute
quantities in dedicated high-flux nuclear reactors, mainly at the Oak
Ridge National Laboratory in Tennessee, United States, and at the
Research Institute of Atomic Reactors in Dimitrovgrad, Russia. The
longest-lived and second-most important isotope, 247Bk, can be
synthesized via irradiation of 244Cm with high-energy alpha particles.
Just over one gram of berkelium has been produced in the United States
since 1967. There is no practical application of berkelium outside
scientific research which is mostly directed at the synthesis of
heavier transuranium elements and superheavy elements. A 22-milligram
batch of berkelium-249 was prepared during a 250-day irradiation
period and then purified for a further 90 days at Oak Ridge in 2009.
This sample was used to synthesize the new element tennessine for the
first time in 2009 at the Joint Institute for Nuclear Research,
Russia, after it was bombarded with calcium-48 ions for 150 days. This
was the culmination of the Russia-US collaboration on the synthesis of
the heaviest elements on the periodic table.
Berkelium is a soft, silvery-white, radioactive metal. The
berkelium-249 isotope emits low-energy beta particles and thus is
relatively safe to handle. It decays with a half-life of 330 days to
californium-249, which is a strong emitter of ionizing alpha
particles. This gradual transmutation is an important consideration
when studying the properties of elemental berkelium and its chemical
compounds, since the formation of californium brings not only chemical
contamination, but also free-radical effects and self-heating from the
emitted alpha particles.
Physical
==========
Berkelium is a soft, silvery-white, radioactive actinide metal. In the
periodic table, it is located to the right of the actinide curium, to
the left of the actinide californium and below the lanthanide terbium
with which it shares many similarities in physical and chemical
properties. Its density of 14.78 g/cm3 lies between those of curium
(13.52 g/cm3) and californium (15.1 g/cm3), as does its melting point
of 986 °C, below that of curium (1340 °C) but higher than that of
californium (900 °C). Berkelium is relatively soft and has one of the
lowest bulk moduli among the actinides, at about 20 GPa (2 Pa).
ions shows two sharp fluorescence peaks at 652 nanometers (red light)
and 742 nanometers (deep red - near-infrared) due to internal
transitions at the f-electron shell. The relative intensity of these
peaks depends on the excitation power and temperature of the sample.
This emission can be observed, for example, after dispersing berkelium
ions in a silicate glass, by melting the glass in presence of
berkelium oxide or halide.
Between 70 K and room temperature, berkelium behaves as a Curie-Weiss
paramagnetic material with an effective magnetic moment of 9.69 Bohr
magnetons (μB) and a Curie temperature of 101 K. This magnetic moment
is almost equal to the theoretical value of 9.72 μB calculated within
the simple atomic L-S coupling model. Upon cooling to about 34 K,
berkelium undergoes a transition to an antiferromagnetic state. The
enthalpy of dissolution in hydrochloric acid at standard conditions is
−600 kJ/mol, from which the standard enthalpy of formation (Δf'H'°) of
aqueous ions is obtained as −601 kJ/mol. The standard electrode
potential /Bk is −2.01 V. The ionization potential of a neutral
berkelium atom is 6.23 eV.
Allotropes
============
At ambient conditions, berkelium assumes its most stable α form which
has a hexagonal symmetry, space group 'P63/mmc', lattice parameters of
341 pm and 1107 pm. The crystal has a double-hexagonal close packing
structure with the layer sequence ABAC and so is isotypic (having a
similar structure) with α-lanthanum and α-forms of actinides beyond
curium. This crystal structure changes with pressure and temperature.
When compressed at room temperature to 7 GPa, α-berkelium transforms
to the β modification, which has a face-centered cubic ('fcc')
symmetry and space group 'Fmm'. This transition occurs without change
in volume, but the enthalpy increases by 3.66 kJ/mol. Upon further
compression to 25 GPa, berkelium transforms to an orthorhombic
γ-berkelium structure similar to that of α-uranium. This transition is
accompanied by a 12% volume decrease and delocalization of the
electrons at the 5f electron shell. No further phase transitions are
observed up to 57 GPa.
Upon heating, α-berkelium transforms into another phase with an 'fcc'
lattice (but slightly different from β-berkelium), space group 'Fmm'
and the lattice constant of 500 pm; this 'fcc' structure is equivalent
to the closest packing with the sequence ABC. This phase is metastable
and will gradually revert to the original α-berkelium phase at room
temperature. The temperature of the phase transition is believed to be
quite close to the melting point.
Chemical
==========
Like all actinides, berkelium dissolves in various aqueous inorganic
acids, liberating gaseous hydrogen and converting into the state.
This trivalent oxidation state (+3) is the most stable, especially in
aqueous solutions, but tetravalent (+4), pentavalent (+5), and
possibly divalent (+2) berkelium compounds are also known. The
existence of divalent berkelium salts is uncertain and has only been
reported in mixed lanthanum(III) chloride-strontium chloride melts. A
similar behavior is observed for the lanthanide analogue of berkelium,
terbium. Aqueous solutions of ions are green in most acids. The color
of ions is yellow in hydrochloric acid and orange-yellow in sulfuric
acid. Berkelium does not react rapidly with oxygen at room
temperature, possibly due to the formation of a protective oxide layer
surface. However, it reacts with molten metals, hydrogen, halogens,
chalcogens and pnictogens to form various binary compounds.
In 2025 an organometallic compound containing berkelium was
synthesized from 0.3 mg of berkelium and named berkelocene.
Isotopes
==========
Nineteen isotopes and six nuclear isomers (excited states of an
isotope) of berkelium have been characterized, with mass numbers
ranging from 233 to 253 (except 235 and 237). All of them are
radioactive. The longest half-lives are observed for 247Bk (1,380
years), 248Bk (over 300 years), and 249Bk (330 days); the half-lives
of the other isotopes range from microseconds to several days. The
isotope which is the easiest to synthesize is berkelium-249. This
emits mostly soft β-particles which are inconvenient for detection.
Its alpha radiation is rather weak (1.45%) with respect to the
β-radiation, but is sometimes used to detect this isotope. The second
important berkelium isotope, berkelium-247, is an alpha-emitter, as
are most actinide isotopes.
Occurrence
============
All berkelium isotopes have a half-life far too short to be
primordial. Therefore, any primordial berkelium − that is, berkelium
present on the Earth during its formation − has decayed by now.
On Earth, berkelium is mostly concentrated in certain areas, which
were used for the atmospheric nuclear weapons tests between 1945 and
1980, as well as at the sites of nuclear incidents, such as the
Chernobyl disaster, Three Mile Island accident and 1968 Thule Air Base
B-52 crash. Analysis of the debris at the testing site of the first
United States' first thermonuclear weapon, Ivy Mike, (1 November 1952,
Enewetak Atoll), revealed high concentrations of various actinides,
including berkelium. For reasons of military secrecy, this result was
not published until 1956.
Nuclear reactors produce mostly, among the berkelium isotopes,
berkelium-249. During the storage and before the fuel disposal, most
of it beta decays to californium-249. The latter has a half-life of
351 years, which is relatively long compared to the half-lives of
other isotopes produced in the reactor, and is therefore undesirable
in the disposal products.
The transuranium elements up to fermium, including berkelium, should
have been present in the natural nuclear fission reactor at Oklo, but
no longer do so.
History
======================================================================
Although very small amounts of berkelium were possibly produced in
previous nuclear experiments, it was first intentionally synthesized,
isolated and identified in December 1949 by Glenn T. Seaborg, Albert
Ghiorso, Stanley Gerald Thompson, and Kenneth Street Jr. They used the
60-inch cyclotron at the University of California, Berkeley. Similar
to the nearly simultaneous discovery of americium (element 95) and
curium (element 96) in 1944, the new elements berkelium and
californium (element 98) were both produced in 1949-1950.
The name choice for element 97 followed the previous tradition of the
Californian group to draw an analogy between the newly discovered
actinide and the lanthanide element positioned above it in the
periodic table. Previously, americium was named after a continent as
its analogue europium, and curium honored scientists Marie and Pierre
Curie as the lanthanide above it, gadolinium, was named after the
explorer of the rare-earth elements Johan Gadolin. Thus, the discovery
report by the Berkeley group reads: "It is suggested that element 97
be given the name berkelium (symbol Bk) after the city of Berkeley in
a manner similar to that used in naming its chemical homologue terbium
(atomic number 65) whose name was derived from the town of Ytterby,
Sweden, where the rare earth minerals were first found." This
tradition ended with berkelium, though, as the naming of the next
discovered actinide, californium, was not related to its lanthanide
analogue dysprosium, but after the discovery place.
The most difficult steps in synthesising berkelium were its separation
from the final products and the production of sufficient quantities of
americium for the target material. First, americium (241Am) nitrate
solution was coated on a platinum foil, the solution was evaporated
and the residue converted by annealing to americium dioxide (). This
target was irradiated with 35 MeV alpha particles for 6 hours in the
60-inch cyclotron at the Lawrence Radiation Laboratory, University of
California, Berkeley. The (α,2n) reaction induced by the irradiation
yielded the 243Bk isotope and two free neutrons:
: + → + 2
After the irradiation, the coating was dissolved with nitric acid and
then precipitated as the hydroxide using concentrated aqueous ammonia
solution. The product was centrifugated and re-dissolved in nitric
acid. To separate berkelium from the unreacted americium, this
solution was added to a mixture of aqueous ammonia and ammonium
sulfate and heated in the presence of atmospheric oxygen to convert
all the dissolved americium into the oxidation state +6. Unoxidized
residual americium was precipitated by the addition of hydrofluoric
acid as americium(III) fluoride (). This step yielded a mixture of the
accompanying product curium and the expected element 97 in form of
trifluorides. The mixture was converted to the corresponding
hydroxides by treating it with potassium hydroxide, and after
centrifugation, was dissolved in perchloric acid.
Further separation was carried out in the presence of a citric
acid/ammonium buffer solution in a weakly acidic medium , using ion
exchange at elevated temperature. The chromatographic separation
behavior was unknown for element 97 at the time but was anticipated by
analogy with terbium. The first results were disappointing because no
alpha-particle emission signature could be detected from the elution
product. With further analysis, searching for characteristic X-rays
and conversion electron signals, a berkelium isotope was eventually
detected. Its mass number was uncertain between 243 and 244 in the
initial report, but was later established as 243.
Preparation of isotopes
=========================
Berkelium is produced by bombarding lighter actinides uranium (238U)
or plutonium (239Pu) with neutrons in a nuclear reactor. In a more
common case of uranium fuel, plutonium is produced first by neutron
capture (the so-called (n,γ) reaction or neutron fusion) followed by
beta-decay:
:^{238}_{92}U ->[\ce{(n,\gamma)}] ^{239}_{92}U ->[\beta^-][23.5
\ \ce{min}] ^{239}_{93}Np ->[\beta^-][2.3565 \ \ce{d}]
^{239}_{94}Pu (The times are half-lives.)
Plutonium-239 is further irradiated by a source that has a high
neutron flux, several times higher than a conventional nuclear
reactor, such as the 85-megawatt High Flux Isotope Reactor (HFIR) at
the Oak Ridge National Laboratory in Tennessee, US. The higher flux
promotes fusion reactions involving not one but several neutrons,
converting 239Pu to 244Cm and then to 249Cm:
:
Curium-249 has a short half-life of 64 minutes, and thus its further
conversion to 250Cm has a low probability. Instead, it transforms by
beta-decay into 249Bk:
:^{249}_{96}Cm ->[{\beta^-}][64.15 \ \ce{min}] ^{249}_{97}Bk
->[\beta^-][330 \ \ce{d}] ^{249}_{98}Cf
The thus-produced 249Bk has a long half-life of 330 days and thus can
capture another neutron. However, the product, 250Bk, again has a
relatively short half-life of 3.212 hours and thus does not yield any
heavier berkelium isotopes. It instead decays to the californium
isotope 250Cf:
:^{249}_{97}Bk ->[\ce{(n,\gamma)}] ^{250}_{97}Bk
->[\beta^-][3.212 \ \ce{h}] ^{250}_{98}Cf
Although 247Bk is the most stable isotope of berkelium, its production
in nuclear reactors is very difficult because its potential progenitor
247Cm has never been observed to undergo beta decay. Thus, 249Bk is
the most accessible isotope of berkelium, which still is available
only in small quantities (only 0.66 grams have been produced in the US
over the period 1967-1983) at a high price of the order 185 USD per
microgram. It is the only berkelium isotope available in bulk
quantities, and thus the only berkelium isotope whose properties can
be extensively studied.
The isotope 248Bk was first obtained in 1956 by bombarding a mixture
of curium isotopes with 25 MeV α-particles. Although its direct
detection was hindered by strong signal interference with 245Bk, the
existence of a new isotope was proven by the growth of the decay
product 248Cf which had been previously characterized. The half-life
of 248Bk was estimated as hours, though later 1965 work gave a
half-life in excess of 300 years (which may be due to an isomeric
state). Berkelium-247 was produced during the same year by irradiating
244Cm with alpha-particles:
:
Berkelium-242 was synthesized in 1979 by bombarding 235U with 11B,
238U with 10B, 232Th with 14N or 232Th with 15N. It converts by
electron capture to 242Cm with a half-life of minutes. A search for
an initially suspected isotope 241Bk was then unsuccessful; 241Bk has
since been synthesized.
:
Separation
============
The fact that berkelium readily assumes oxidation state +4 in solids,
and is relatively stable in this state in liquids, greatly assists
separation of berkelium from many other actinides. These are produced
in relatively large amounts during the nuclear synthesis and often
favor the +3 state. This fact was not yet known in the initial
experiments, which used a more complex separation procedure. Various
inorganic oxidation agents can be applied to the solution to convert
it to the +4 state, such as bromates (), bismuthates (), chromates (
and ), silver(I) thiolate (), lead(IV) oxide (), ozone (), or
photochemical oxidation procedures. More recently, it has been
discovered that some organic and bio-inspired molecules, such as the
chelator 3,4,3-LI(1,2-HOPO), can also oxidize Bk(III) and stabilize
Bk(IV) under mild conditions. is then extracted with ion exchange,
extraction chromatography or liquid-liquid extraction using HDEHP
(bis-(2-ethylhexyl) phosphoric acid), amines, tributyl phosphate or
various other reagents. These procedures separate berkelium from most
trivalent actinides and lanthanides, except for the lanthanide cerium
(lanthanides are absent in the irradiation target but are created in
various nuclear fission decay chains).
A more detailed procedure adopted at the Oak Ridge National Laboratory
was as follows: the initial mixture of actinides is processed with ion
exchange using lithium chloride reagent, then precipitated as
hydroxides, filtered and dissolved in nitric acid. It is then treated
with high-pressure elution from cation exchange resins, and the
berkelium phase is oxidized and extracted using one of the procedures
described above. Reduction of the thus-obtained to the +3 oxidation
state yields a solution, which is nearly free from other actinides
(but contains cerium). Berkelium and cerium are then separated with
another round of ion-exchange treatment.
Bulk metal preparation
========================
In order to characterize chemical and physical properties of solid
berkelium and its compounds, a program was initiated in 1952 at the
Material Testing Reactor, Arco, Idaho, US. It resulted in preparation
of an eight-gram plutonium-239 target and in the first production of
macroscopic quantities (0.6 micrograms) of berkelium by Burris B.
Cunningham and Stanley Gerald Thompson in 1958, after a continuous
reactor irradiation of this target for six years. This irradiation
method was and still is the only way of producing weighable amounts of
the element, and most solid-state studies of berkelium have been
conducted on microgram or submicrogram-sized samples.
The world's major irradiation sources are the 85-megawatt High Flux
Isotope Reactor at the Oak Ridge National Laboratory in Tennessee,
USA, and the SM-2 loop reactor at the Research Institute of Atomic
Reactors (NIIAR) in Dimitrovgrad, Russia, which are both dedicated to
the production of transcurium elements (atomic number greater than
96). These facilities have similar power and flux levels, and are
expected to have comparable production capacities for transcurium
elements, although the quantities produced at NIIAR are not publicly
reported. In a "typical processing campaign" at Oak Ridge, tens of
grams of curium are irradiated to produce decigram quantities of
californium, milligram quantities of berkelium-249 and einsteinium,
and picogram quantities of fermium. In total, just over one gram of
berkelium-249 has been produced at Oak Ridge since 1967.
The first berkelium metal sample weighing 1.7 micrograms was prepared
in 1971 by the reduction of fluoride with lithium vapor at 1000 °C;
the fluoride was suspended on a tungsten wire above a tantalum
crucible containing molten lithium. Later, metal samples weighing up
to 0.5 milligrams were obtained with this method.
:
Similar results are obtained with fluoride. Berkelium metal can also
be produced by the reduction of oxide with thorium or lanthanum.
Oxides
========
Two oxides of berkelium are known, with the berkelium oxidation state
of +3 () and +4 (Berkelium(IV) oxide). oxide is a brown solid, while
oxide is a yellow-green solid with a melting point of 1920 °C and is
formed from BkO2 by reduction with molecular hydrogen:
:
Upon heating to 1200 °C, the oxide undergoes a phase change; it
undergoes another phase change at 1750 °C. Such three-phase behavior
is typical for the actinide sesquioxides. oxide, BkO, has been
reported as a brittle gray solid but its exact chemical composition
remains uncertain.
Halides
=========
In halides, berkelium assumes the oxidation states +3 and +4. The +3
state is the most stable, especially in solutions, while the
tetravalent halides and are only known in the solid phase. The
coordination of berkelium atom in its trivalent fluoride and chloride
is tricapped trigonal prismatic, with the coordination number of 9. In
trivalent bromide, it is bicapped trigonal prismatic (coordination 8)
or octahedral (coordination 6), and in the iodide it is octahedral.
Oxidation number F Cl Br I
+4 (yellow) (orange)
+3 (yellow) (green) (yellow-green) (yellow)
fluoride () is a yellow-green ionic solid and is isotypic with
uranium tetrafluoride or zirconium tetrafluoride. fluoride () is also
a yellow-green solid, but it has two crystalline structures. The most
stable phase at low temperatures is isotypic with yttrium(III)
fluoride, while upon heating to between 350 and 600 °C, it transforms
to the structure found in lanthanum trifluoride.
Visible amounts of chloride () were first isolated and characterized
in 1962, and weighed only 3 billionths of a gram. It can be prepared
by introducing hydrogen chloride vapors into an evacuated quartz tube
containing berkelium oxide at a temperature about 500 °C. This green
solid has a melting point of 600 °C, and is isotypic with uranium(III)
chloride. Upon heating to nearly melting point, converts into an
orthorhombic phase.
Two forms of bromide are known: one with berkelium having
coordination 6, and one with coordination 8. The latter is less stable
and transforms to the former phase upon heating to about 350 °C. An
important property of radioactive solids has been studied on these two
crystal forms: the structure of fresh and aged 249BkBr3 samples was
probed by X-ray diffraction over a period longer than 3 years, so that
various fractions of berkelium-249 had beta decayed to
californium-249. No change in structure was observed upon the
249BkBr3--249CfBr3 transformation. However, other differences were
noted for 249BkBr3 and 249CfBr3. For example, the latter could be
reduced with hydrogen to 249CfBr2, but the former could not - this
result was reproduced on individual 249BkBr3 and 249CfBr3 samples, as
well on the samples containing both bromides. The intergrowth of
californium in berkelium occurs at a rate of 0.22% per day and is an
obstacle to studying berkelium properties. Beside a chemical
contamination, 249Cf, being an alpha emitter, brings undesirable
self-damage of the crystal lattice and the resulting self-heating. The
chemical effect however can be avoided by performing measurements as a
function of time and extrapolating the obtained results.
Other inorganic compounds
===========================
The pnictides of berkelium-249 of the type BkX are known for the
elements nitrogen, phosphorus, arsenic and antimony. They crystallize
in the rock-salt structure and are prepared by the reaction of either
hydride () or metallic berkelium with these elements at elevated
temperature (about 600 °C) under high vacuum.
sulfide, , is prepared by either treating berkelium oxide with a
mixture of hydrogen sulfide and carbon disulfide vapors at 1130 °C, or
by directly reacting metallic berkelium with elemental sulfur. These
procedures yield brownish-black crystals.
and hydroxides are both stable in 1 molar solutions of sodium
hydroxide. phosphate () has been prepared as a solid, which shows
strong fluorescence under excitation with a green light. Berkelium
hydrides are produced by reacting metal with hydrogen gas at
temperatures about 250 °C. They are non-stoichiometric with the
nominal formula (0 < 'x' < 1). Several other salts of berkelium
are known, including an oxysulfide (), and hydrated nitrate (),
chloride (), sulfate () and oxalate (). Thermal decomposition at about
600 °C in an argon atmosphere (to avoid oxidation to ) of yields the
crystals of oxysulfate (). This compound is thermally stable to at
least 1000 °C in inert atmosphere.
Organoberkelium compounds
===========================
Berkelium forms a trigonal (η5-C5H5)3Bk metallocene complex with three
cyclopentadienyl rings, which can be synthesized by reacting chloride
with the molten beryllocene () at about 70 °C. It has an amber color
and a density of 2.47 g/cm3. The complex is stable to heating to at
least 250 °C, and sublimates without melting at about 350 °C. The high
radioactivity of berkelium gradually destroys the compound (within a
period of weeks). One cyclopentadienyl ring in (η5-C5H5)3Bk can be
substituted by chlorine to yield . The optical absorption spectra of
this compound are very similar to those of (η5-C5H5)3Bk.
Berkelium also forms berkelocene, an actinocene complex, with
substituted cyclooctatetraenides.
Applications
======================================================================
There is currently no use for any isotope of berkelium outside basic
scientific research. Berkelium-249 is a common target nuclide to
prepare still heavier transuranium elements and superheavy elements,
such as lawrencium, rutherfordium and bohrium. It is also useful as a
source of the isotope californium-249, which is used for studies on
the chemistry of californium in preference to the more radioactive
californium-252 that is produced in neutron bombardment facilities
such as the HFIR.
A 22 milligram batch of berkelium-249 was prepared in a 250-day
irradiation and then purified for 90 days at Oak Ridge in 2009. This
target yielded the first 6 atoms of tennessine at the Joint Institute
for Nuclear Research (JINR), Dubna, Russia, after bombarding it with
calcium ions in the U400 cyclotron for 150 days. This synthesis was a
culmination of the Russia-US collaboration between JINR and Lawrence
Livermore National Laboratory on the synthesis of elements 113 to 118
which was initiated in 1989.
Nuclear fuel cycle
======================================================================
The nuclear fission properties of berkelium are different from those
of the neighboring actinides curium and californium, and they suggest
berkelium to perform poorly as a fuel in a nuclear reactor.
Specifically, berkelium-249 has a moderately large neutron capture
cross section of 710 barns for thermal neutrons, 1200 barns resonance
integral, but very low fission cross section for thermal neutrons. In
a thermal reactor, much of it will therefore be converted to
berkelium-250 which quickly decays to californium-250. In principle,
berkelium-249 can sustain a nuclear chain reaction in a fast breeder
reactor. Its critical mass is relatively high at 192 kg, which can be
reduced with a water or steel reflector but would still exceed the
world production of this isotope.
Berkelium-247 can maintain a chain reaction both in a thermal-neutron
and in a fast-neutron reactor, however, its production is rather
complex and thus the availability is much lower than its critical
mass, which is about 75.7 kg for a bare sphere, 41.2 kg with a water
reflector and 35.2 kg with a steel reflector (30 cm thickness).
Health issues
======================================================================
Little is known about the effects of berkelium on human body, and
analogies with other elements may not be drawn because of different
radiation products (electrons for berkelium and alpha particles,
neutrons, or both for most other actinides). The low energy of
electrons emitted from berkelium-249 (less than 126 keV) hinders its
detection, due to signal interference with other decay processes, but
also makes this isotope relatively harmless to humans as compared to
other actinides. However, berkelium-249 transforms with a half-life of
only 330 days to the strong alpha-emitter californium-249, which is
rather dangerous and has to be handled in a glovebox in a dedicated
laboratory.
Most available berkelium toxicity data originate from research on
animals. Upon ingestion by rats, only about 0.01% of berkelium ends in
the blood stream. From there, about 65% goes to the bones, where it
remains for about 50 years, 25% to the lungs (biological half-life
about 20 years), 0.035% to the testicles or 0.01% to the ovaries where
berkelium stays indefinitely. The balance of about 10% is excreted. In
all these organs berkelium might promote cancer, and in the skeleton,
its radiation can damage red blood cells. The maximum permissible
amount of berkelium-249 in the human skeleton is 0.4 nanograms.
External links
======================================================================
* [
http://www.periodicvideos.com/videos/097.htm Berkelium] at 'The
Periodic Table of Videos' (University of Nottingham)
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Berkelium
