======================================================================
= Californium =
======================================================================
Introduction
======================================================================
Californium is a synthetic chemical element; it has symbol Cf and
atomic number 98. It was first synthesized in 1950 at Lawrence
Berkeley National Laboratory (then the University of California
Radiation Laboratory) by bombarding curium with alpha particles
(helium-4 ions). It is an actinide element, the sixth transuranium
element to be synthesized, and has the second-highest atomic mass of
all elements that have been produced in amounts large enough to see
with the naked eye (after einsteinium). It was named after the
university and the U.S. state of California.
Two crystalline forms exist at normal pressure: one above and one
below 900 C. A third form exists at high pressure. Californium slowly
tarnishes in air at room temperature. Californium compounds are
dominated by the +3 oxidation state. The most stable of californium's
twenty known isotopes is californium-251, with a half-life of 898
years. This short half-life means the element is not found in
significant quantities in the Earth's crust. (252)Cf, with a half-life
of about 2.645 years, is the most common isotope used and is produced
at Oak Ridge National Laboratory (ORNL) in the United States and
Research Institute of Atomic Reactors in Russia.
Californium is one of the few transuranium elements with practical
uses. Most of these applications exploit the fact that certain
isotopes of californium emit neutrons. For example, californium can be
used to help start up nuclear reactors, and it is used as a source of
neutrons when studying materials using neutron diffraction and neutron
spectroscopy. It can also be used in nuclear synthesis of higher mass
elements; oganesson (element 118) was synthesized by bombarding
californium-249 atoms with calcium-48 ions. Users of californium must
take into account radiological concerns and the element's ability to
disrupt the formation of red blood cells by bioaccumulating in
skeletal tissue.
Physical properties
=====================
Californium is a silvery-white actinide metal with a melting point of
900 ± and an estimated boiling point of 1743 K. The pure metal is
malleable and is easily cut with a knife. Californium metal starts to
vaporize above 300 C when exposed to a vacuum. Below 51 K californium
metal is either ferromagnetic or ferrimagnetic (it acts like a
magnet), between 48 and 66 K it is antiferromagnetic (an intermediate
state), and above 160 K it is paramagnetic (external magnetic fields
can make it magnetic). It forms alloys with lanthanide metals but
little is known about the resulting materials.
The element has two crystalline forms at standard atmospheric
pressure: a double-hexagonal close-packed form dubbed alpha (α) and a
face-centered cubic form designated beta (β). The α form exists below
600-800 °C with a density of 15.10 g/cm3 and the β form exists above
600-800 °C with a density of 8.74 g/cm(3). At 48 GPa of pressure the β
form changes into an orthorhombic crystal system due to delocalization
of the atom's 5f electrons, which frees them to bond.
The bulk modulus of a material is a measure of its resistance to
uniform pressure. Californium's bulk modulus is , which is similar to
trivalent lanthanide metals but smaller than more familiar metals,
such as aluminium (70 GPa).
Chemical properties and compounds
===================================
Representative californium compounds
state !! compound !! formula !! color !!
+2 californium(II) bromide CfBr yellow |
+2 californium(II) iodide CfI dark violet |
+3 californium(III) oxide CfO yellow-green |
+3 californium(III) fluoride CfF bright green |
+3 californium(III) chloride CfCl emerald green |
+3 californium(III) bromide CfBr yellowish green |
+3 californium(III) iodide CfI lemon yellow |
+3 |californium(III) polyborate |Cf[BO(OH)] |pale green |
+4 californium(IV) oxide CfO black brown |
+4 californium(IV) fluoride CfF green |
Californium exhibits oxidation states of 4, 3, or 2. It typically
forms eight or nine bonds to surrounding atoms or ions. Its chemical
properties are predicted to be similar to other primarily 3+ valence
actinide elements and the element dysprosium, which is the lanthanide
above californium in the periodic table. Compounds in the +4 oxidation
state are strong oxidizing agents and those in the +2 state are strong
reducing agents.
The element slowly tarnishes in air at room temperature, with the rate
increasing when moisture is added. Californium reacts when heated with
hydrogen, nitrogen, or a chalcogen (oxygen family element); reactions
with dry hydrogen and aqueous mineral acids are rapid.
Californium is only water-soluble as the californium(III) cation.
Attempts to reduce or oxidize the +3 ion in solution have failed. The
element forms a water-soluble chloride, nitrate, perchlorate, and
sulfate and is precipitated as a fluoride, oxalate, or hydroxide.
Californium is the heaviest actinide to exhibit covalent properties,
as is observed in the californium borate.
Isotopes
==========
Twenty isotopes of californium are known (mass number ranging from 237
to 256); the most stable are (251)Cf with half-life 898 years, (249)Cf
with half-life 351 years, (250)Cf at 13.08 years, and (252)Cf at 2.645
years. All other isotopes have half-life shorter than a year, and most
of these have half-lives less than 20 minutes.
(249)Cf is formed by beta decay of berkelium-249, and most other
californium isotopes are made by subjecting berkelium to intense
neutron radiation in a nuclear reactor. Though californium-251 has the
longest half-life, its production yield is only 10% due to its
tendency to collect neutrons (high neutron capture) and its tendency
to interact with other particles (high neutron cross section).
(252)Cf is a very strong neutron emitter, which makes it extremely
radioactive and harmful. (252)Cf, 96.9% of the time, alpha decays to
curium-248; the other 3.1% of decays are spontaneous fission. One
microgram (μg) of (252)Cf emits 2.3 million neutrons per second, an
average of 3.7 neutrons per spontaneous fission. Most other isotopes
of californium, alpha decay to curium (atomic number 96).
History
======================================================================
Californium was first made at University of California Radiation
Laboratory, Berkeley, by physics researchers Stanley Gerald Thompson,
Kenneth Street Jr., Albert Ghiorso, and Glenn T. Seaborg, about
February 9, 1950. It was the sixth transuranium element to be
discovered; the team announced its discovery on March 17, 1950.
To produce californium, a microgram-size target of curium-242 () was
bombarded with 35 MeV alpha particles () in the 60 in cyclotron at
Berkeley, which produced californium-245 () plus one free neutron ().
: + → +
To identify and separate out the element, ion exchange and adsorsion
methods were undertaken. Only about 5,000 atoms of californium were
produced in this experiment, and these atoms had a half-life of 44
minutes.
The discoverers named the new element after the university and the
state. This was a break from the convention used for elements 95 to
97, which drew inspiration from how the elements directly above them
in the periodic table were named. However, the element directly above
element 98 in the periodic table, dysprosium, has a name that means
"hard to get at", so the researchers decided to set aside the informal
naming convention. They added that "the best we can do is to point out
[that] ... searchers a century ago found it difficult to get to
California".
Weighable amounts of californium were first produced by the
irradiation of plutonium targets at Materials Testing Reactor at
National Reactor Testing Station, eastern Idaho; these findings were
reported in 1954. The high spontaneous fission rate of californium-252
was observed in these samples. The first experiment with californium
in concentrated form occurred in 1958. The isotopes (249)Cf to (252)Cf
were isolated that same year from a sample of plutonium-239 that had
been irradiated with neutrons in a nuclear reactor for five years. Two
years later, in 1960, Burris Cunningham and James Wallman of Lawrence
Radiation Laboratory of the University of California created the first
californium compounds--californium trichloride, californium(III)
oxychloride, and californium oxide--by treating californium with steam
and hydrochloric acid.
The High Flux Isotope Reactor (HFIR) at ORNL in Oak Ridge, Tennessee,
started producing small batches of californium in the 1960s. By 1995,
HFIR nominally produced 500 mg of californium annually. Plutonium
supplied by the United Kingdom to the United States under the 1958
US-UK Mutual Defence Agreement was used for making californium.
The Atomic Energy Commission sold (252)Cf to industrial and academic
customers in the early 1970s for $10/microgram, and an average of 150
mg of (252)Cf were shipped each year from 1970 to 1990. Californium
metal was first prepared in 1974 by Haire and Baybarz, who reduced
californium(III) oxide with lanthanum metal to obtain microgram
amounts of sub-micrometer thick films.
Occurrence
======================================================================
Traces of californium can be found near facilities that use the
element in mineral prospecting and in medical treatments. The element
is fairly insoluble in water, but it adheres well to ordinary soil;
and concentrations of it in the soil can be 500 times higher than in
the water surrounding the soil particles.
Nuclear fallout from atmospheric nuclear weapons testing prior to 1980
contributed a small amount of californium to the environment.
Californium-249, -252, -253, and -254 have been observed in the
radioactive dust collected from the air after a nuclear explosion.
Californium is not a major radionuclide at United States Department of
Energy legacy sites since it was not produced in large quantities.
Californium was once believed to be produced in supernovas, as their
decay matches the 60-day half-life of (254)Cf. However, subsequent
studies failed to demonstrate any californium spectra, and supernova
light curves are now thought to follow the decay of nickel-56.
The transuranic elements up to fermium, including californium, should
have been present in the natural nuclear fission reactor at Oklo, but
no longer do so.
Production
======================================================================
Californium is produced in nuclear reactors and particle accelerators.
Californium-250 is made by bombarding berkelium-249 ((249)Bk) with
neutrons, forming berkelium-250 ((250)Bk) via neutron capture (n,γ)
which, in turn, quickly beta decays (β(−)) to californium-250
((250)Cf) in the following reaction:
:(n,γ) → + β(−)
Bombardment of (250)Cf with neutrons produces (251)Cf and (252)Cf.
Prolonged irradiation of americium, curium, and plutonium with
neutrons produces milligram amounts of (252)Cf and microgram amounts
of (249)Cf. As of 2006, curium isotopes 244 to 248 are irradiated by
neutrons in special reactors to produce mainly californium-252 with
lesser amounts of isotopes 249 to 255.
Microgram quantities of (252)Cf are available for commercial use
through the U.S. Nuclear Regulatory Commission. Only two sites produce
(252)Cf: Oak Ridge National Laboratory in the U.S., and the Research
Institute of Atomic Reactors in Dimitrovgrad, Russia. As of 2003, the
two sites produce 0.25 grams and 0.025 grams of (252)Cf per year,
respectively.
Three californium isotopes with significant half-lives are produced,
requiring a total of 15 neutron captures by uranium-238 without
nuclear fission or alpha decay occurring during the process. (253)Cf
is at the end of a production chain that starts with uranium-238, and
includes several isotopes of plutonium, americium, curium, and
berkelium, and the californium isotopes 249 to 253 (see diagram).
Neutron source
================
has a number of specialized uses as a strong neutron emitter; it
produces 139 million neutrons per microgram per minute. This property
makes it useful as a startup neutron source for some nuclear reactors
and as a portable (non-reactor based) neutron source for neutron
activation analysis to detect trace amounts of elements in samples.
Neutrons from californium are used as a treatment of certain cervical
and brain cancers where other radiation therapy is ineffective. It has
been used in educational applications since 1969 when Georgia
Institute of Technology got a loan of 119 μg of (252)Cf from the
Savannah River Site. It is also used with online elemental coal
analyzers and bulk material analyzers in the coal and cement
industries.
Neutron penetration into materials makes californium useful in
detection instruments such as fuel rod scanners; neutron radiography
of aircraft and weapons components to detect corrosion, bad welds,
cracks and trapped moisture; and in portable metal detectors. Neutron
moisture gauges use (252)Cf to find water and petroleum layers in oil
wells, as a portable neutron source for gold and silver prospecting
for on-the-spot analysis, and to detect ground water movement. The
main uses of (252)Cf in 1982 were, reactor start-up (48.3%), fuel rod
scanning (25.3%), and activation analysis (19.4%). By 1994, most
(252)Cf was used in neutron radiography (77.4%), with fuel rod
scanning (12.1%) and reactor start-up (6.9%) as important but
secondary uses. In 2021, fast neutrons from (252)Cf were used for
wireless data transmission.
Superheavy element production
===============================
In October 2006, researchers announced that three atoms of oganesson
(element 118) had been identified at Joint Institute for Nuclear
Research in Dubna, Russia, from bombarding (249)Cf with calcium-48,
making it the heaviest element ever made. The target contained about
10 mg of (249)Cf deposited on a titanium foil of 32 cm(2) area.
Californium has also been used to produce other transuranic elements;
for example, lawrencium was first synthesized in 1961 by bombarding
californium with boron nuclei.
Hypothetical nuclear weapons
==============================
has a very small calculated critical mass of about 5 kg, high
lethality, and a relatively short period of toxic environmental
irradiation. The low critical mass of californium led to some
exaggerated claims about possible uses for the element.
Precautions
======================================================================
Californium that bioaccumulates in skeletal tissue releases radiation
that disrupts the body's ability to form red blood cells. The element
plays no natural biological role in any organism due to its intense
radioactivity and low concentration in the environment.
Californium can enter the body from ingesting contaminated food or
drinks or by breathing air with suspended particles of the element.
Once in the body, only 0.05% of the californium will reach the
bloodstream. About 65% of that californium will be deposited in the
skeleton, 25% in the liver, and the rest in other organs, or excreted,
mainly in urine. Half of the californium deposited in the skeleton and
liver are gone in 50 and 20 years, respectively. Californium in the
skeleton adheres to bone surfaces before slowly migrating throughout
the bone.
The element is most dangerous if taken into the body. In addition,
californium-249 and californium-251 can cause tissue damage
externally, through gamma ray emission. Ionizing radiation emitted by
californium on bone and in the liver can cause cancer.
External links
======================================================================
* [
http://www.periodicvideos.com/videos/098.htm Californium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
http://www.nuclearweaponarchive.org/Nwfaq/Nfaq6.html#nfaq6.2
NuclearWeaponArchive.org - Californium]
*
[
http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@rel+@na+californium,radioactive
Hazardous Substances Databank - Californium, Radioactive]
License
=========
All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Californium
