Listing description
Polonium (pronounced /pɵˈloʊniəm/,
po-LOH-nee-əm)
is a chemical element with the symbol Po and atomic
number 84, discovered in 1898 by Marie
Skłodowska-Curie and Pierre Curie. A rare and highly radioactive metalloid,[1]
polonium is chemically similar to bismuth[2]
and tellurium,
and it occurs in uranium ores.
Polonium has been studied for possible use in heating spacecraft.
It is unstable; all isotopes of polonium
are radioactive.
Detailed descrtption
Characteristics
Solid state form
Polonium is a
radioactive element that exists in two metallic allotropes. The alpha form has a simple
cubic crystal
structure with an edge length of 335.2 picometres; the beta form is rhombohedral. The structure of polonium has been
characterized by X-ray diffraction
and electron
diffraction.
210Po (in common with 238Pu) has the ability to
become airborne with ease:
if a sample is heated in air to 55 °C (131 °F), 50% of it is vaporized in 45
hours, even though the melting point of polonium is 254 °C (489 °F) and its
boiling point is 962 °C (1763 °F). More than one hypothesis exists for how
polonium does this; one suggestion is that small clusters of polonium atoms are
spalled off by the alpha decay.
Detection
Gamma counting
By means of
radiometric methods such as gamma
spectroscopy
(or a method using a chemical separation followed by an activity measurement with a
non-energy-dispersive counter), it is possible to measure the concentrations of
radioisotopes and to distinguish one from another.
In practice, background noise would be present and depending on the detector,
the line width would be larger which would make it harder to identify and
measure the isotope. In biological/medical work it is
common to use the natural 40K present in all tissues/body fluids as a check
of the equipment and as an internal standard.
Alpha counting
The best way to
test for (and measure) many alpha emitters is to use alpha-particle
spectroscopy
as it is common to place a drop of the test solution on a metal disk which is
then dried out to give a uniform coating on the disk. This is then used as the
test sample. If the thickness of the layer formed on the disk is too thick then
the lines of the spectrum are broadened, this is because some of the energy of
the alpha
particles is lost during
their movement through the layer of active material. An alternative method is
to use internal liquid scintillation where the sample is mixed with a
scintillation cocktail. When the light emitted is then counted, some machines
will record the amount of light energy per radioactive decay event.
Occurrence and production
Polonium is a
very rare element in nature because of the short half-life of all its isotopes. It is found in uranium ores at about 100 micrograms per metric ton (1 part in 1010), which is
approximately 0.2% of the abundance of radium. The amounts in the Earth's crust
are not harmful. Polonium has been found in tobacco smoke from tobacco leaves grown with phosphate fertilizers.
Neutron capture
Synthesis by
(n,γ) reaction
In 1934 an
experiment showed that when natural 209Bi is bombarded with neutrons, 210Bi is created, which
then decays to 210Po via β decay. The final purification is done
pyrochemically followed by liquid-liquid extraction techniques. Polonium may
now be made in milligram amounts in this procedure which uses high neutron
fluxes found in nuclear
reactors. Only about
100 grams are produced each year, practically all of it in Russia, making
polonium exceedingly rare.
Proton capture
Synthesis by
(p,n) and (p,2n) reactions
It has been found
that the longer-lived isotopes of polonium can be formed by proton bombardment of bismuth using a cyclotron. Other more neutron rich isotopes can
be formed by the irradiation of platinum with carbon nuclei.
Applications
When it is mixed
or alloyed with beryllium, polonium can be a neutron source: beryllium releases a neutron upon absorption of an alpha particle that is supplied by 210Po.
It has been used in this capacity as a neutron
trigger or initiator
for nuclear
weapons. However, a license
is needed to own and operate this form of neutron source. Other uses include
the following.
- Devices that
eliminate static charges in textile mills and
other places. However, beta
particle
sources are more commonly used and are less dangerous. A non-radioactive
alternative is to use a high-voltage DC power supply to ionise air
positively or negatively as required.
- 210Po can be
used as an atomic heat source to power radioisotope thermoelectric generators via thermoelectric materials.
- Because of
its very high toxicity, polonium can be used as a poison (see, for
example, Alexander Litvinenko poisoning).
- Polonium is
also used to eliminate dust on film.
Toxicity
Overview
By mass,
polonium-210 is around 250,000 times more toxic than hydrogen cyanide (the actual LD50 for 210Po is about 1 microgram for an 80 kg person (see below)
compared with about 250 milligrams for hydrogen cyanide). The main
hazard is its intense radioactivity (as an alpha emitter), which makes it very
difficult to handle safely: one gram of Po will self-heat to a temperature of
around 500 °C (932 °F). Even in microgram amounts, handling 210Po is
extremely dangerous, requiring specialized equipment and strict handling
procedures. Alpha particles emitted by polonium will damage organic tissue
easily if polonium is ingested, inhaled, or absorbed, although they do not
penetrate the epidermis and hence are not hazardous if the
polonium is outside the body.
Acute effects
The median lethal
dose (LD50) for acute radiation exposure is
generally about 4.5 Sv. The committed effective dose equivalent 210Po is 0.51 µSv/Bq if ingested, and 2.5 µSv/Bq if
inhaled. Since 210Po has an activity of 166 TBq per gram (4,500
Ci/g)[35] (1 gram produces 166×1012
decays per second), a fatal 4.5 Sv (J/kg) dose can be caused by ingesting
8.8 MBq (238 microcuries, µCi), about 50 nanograms (ng), or inhaling 1.8 MBq
(48 µCi), about 10 ng. One gram of 210Po could thus in
theory poison 20 million people of whom 10 million would die. The actual
toxicity of 210Po is lower than these estimates, because radiation
exposure that is spread out over several weeks (the biological
half-life of polonium in
humans is 30 to 50 days) is somewhat less damaging than an instantaneous dose.
It has been estimated that a median lethal dose of 210Po is 0.015 GBq (0.4 mCi),
or 0.089 micrograms, still an extremely small amount.
Long term (chronic) effects
In addition to
the acute effects, radiation exposure (both internal and external) carries a
long-term risk of death from cancer of 5–10% per Sv. The general population is
exposed to small amounts of polonium as a radon daughter in indoor air; the isotopes 214Po
and 218Po are thought to cause the majority of the estimated
15,000-22,000 lung cancer deaths in the US every year that have been attributed
to indoor radon. Tobacco
smoking causes
additional exposure to polonium.
Regulatory exposure limits
The maximum
allowable body burden for ingested 210Po is only 1.1 kBq
(30 nCi), which is equivalent to a particle massing only 6.8 picograms.
The maximum permissible workplace concentration of airborne 210Po is
about 10 Bq/m3 (3 × 10−10 µCi/cm³). The target organs for
polonium in humans are the spleen and liver. As the spleen (150 g) and the liver (1.3 to
3 kg) are much smaller than the rest of the body, if the polonium is
concentrated in these vital organs, it is a greater threat to life than the
dose which would be suffered (on average) by the whole body if it were spread
evenly throughout the body, in the same way as caesium or tritium (as T2O).
210Po is widely used in industry, and
readily available with little regulation or restriction. In the US, a tracking
system run by the Nuclear Regulatory Commission will be implemented in 2007 to
register purchases of more than 16 curies (590 GBq) of polonium 210
(enough to make up 5,000 lethal doses). The IAEA "is said to be
considering tighter regulations... There is talk that it might tighten the
polonium reporting requirement by a factor of 10, to 1.6 curies (59
GBq)."
Treatment
It has been
suggested that chelation
agents such as British
Anti-Lewisite (dimercaprol) can be used to decontaminate humans.
In one experiment, rats were given a fatal dose of 1.45 MBq/kg (8.7 ng/kg)
of 210Po; all untreated rats were dead after 44 days, but 90% of the
rats treated with the chelation agent HOEtTTC remained alive after 5 months.
Commercial products containing polonium
No nuclear
authority has asserted that a commercial
product was a likely
source for the poisoning of Litvinenko. However, as Prof. Peter D. Zimmerman
says, "Polonium 210 is surprisingly common. ...Polonium sources with about
10 percent of a lethal dose are readily available—even in a product sold on
Amazon.com."
Potentially
lethal amounts of polonium are present in anti-static brushes sold to
photographers Many of the devices are available by mail order. General Electric markets a static eliminator module with 500 µCi (20 MBq), roughly
2.5 times the lethal dose of 210Po if 100%-ingested, for US$79;
Staticmaster sells replacement units with the same amount (500 µCi) of 210Po
for US$36. In USA, the devices with no more than 500 µCi of (sealed) 210Po
per unit can be bought in any amount under a "general license" which
means that a buyer need not be registered by any authorities: the general
license "is effective without the filing of an application with the
Commission or the issuance of a licensing document to a particular
person."
PRICE
$12,900/KG OR $5,863.63/IB
For more information:
mobile: +2348039721941
contact person: emeaba uche
e-mail: emeabau@yahoo.com
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