Caesium-137 (137 55 Cs, Cs-137), cesium-137, or radiocaesium, is a radioactive isotope of caesium which is formed as one of the more common fission products by the ...
Caesium-137
From Wikipedia, the free encyclopedia
Caesium-137 (
137
55Cs, Cs-137),
cesium-137, or
radiocaesium, is a
radioactive isotope of
caesium which is formed as one of the more common
fission products by the
nuclear fission of
uranium-235 and other
fissionable isotopes in
nuclear reactors and
nuclear weapons.
It is among the most problematic of the short-to-medium-lifetime
fission products because it easily moves and spreads in nature due to
the high water solubility of caesium's most common
chemical compounds, which are
salts.
Decay
Caesium-137 has a
half-life of about 30.17 years.
[1] About 95 percent
decays by
beta emission to a
metastable nuclear isomer of barium:
barium-137m (
137mBa,
Ba-137m). The remainder directly populates the ground state of
barium-137, which is stable. Ba-137m has a half-life of about 153
seconds, and is responsible for all of the emissions of
gamma rays in samples of caesium-137. One gram of caesium-137 has an
activity of 3.215
terabecquerel (TBq).
[3]
The main
photon peak of Ba-137m is 662 keV.
[4]
Uses
Caesium-137 has a number of practical uses. In small amounts, it is used to calibrate radiation-detection equipment.
[5] In medicine, it is used in
radiation therapy.
[5] In industry, it is used in
flow meters, thickness gauges,
[5] moisture-density gauges (For density readings,Americium-241/Beryillium providing the moisture reading
[6] and in gamma ray
well logging devices.
[6]
Caesium-137 is not widely used for
industrial radiography
because it is quite chemically reactive, and hence, difficult to
handle. The salts of caesium are also soluble in water, and this
complicates the safe handling of caesium.
Cobalt-60,
60
27Co, is preferred for radiography, since it is chemically a rather nonreactive metal offering higher energy gamma-ray
photons.
As a man-made isotope it has been used to date wine and detect counterfeits
[7] and as a relative-dating material for assessing the age of sedimentation occurring after 1954.
[8]
Health risk of radioactive caesium
Caesium-137 reacts with water producing a water-soluble compound (
caesium hydroxide). The biological behavior of caesium is similar to that of
potassium and
rubidium.
After entering the body, caesium gets more or less uniformly
distributed throughout the body, with the highest concentrations in
soft tissue.
[9]:114 The
biological half-life of caesium is rather short at about 70 days.
[10] A 1972 experiment showed that when dogs are subjected to a
whole body burden of 3800
μCi/kg (140 MBq/kg, or approximately 44 μg/kg) of caesium-137 (and 950 to 1400
rads), they die within thirty-three days, while animals with half of that burden all survived for a year.
[11]
Accidental ingestion of caesium-137 can be treated with
Prussian blue, which binds to it chemically and reduces the biological half-life to 30 days.
[12]
Radioactive caesium in the environment
Small amounts of
caesium-134 and caesium-137 were released into the environment during nearly all
nuclear weapon tests and some
nuclear accidents, most notably the
Chernobyl disaster and the
Fukushima Daiichi disaster.
As of 2005, caesium-137 is the principal source of radiation in the
zone of alienation around the
Chernobyl nuclear power plant. Together with
caesium-134,
iodine-131, and
strontium-90,
caesium-137 was among the isotopes distributed by the reactor explosion
that constitute the greatest risk to health. The mean contamination of
caesium-137 in Germany following the Chernobyl disaster was 2000 to
4000 Bq/m
2. This corresponds to a contamination of 1 mg/km
2
of caesium-137, totaling about 500 grams deposited over all of Germany.
In Scandinavia, some reindeer and sheep exceeded the Norwegian legal
limit (3000 Bq/kg) 26 years after Chernobyl.
[13]
In April 2011, elevated levels of caesium-137 were also being found in the environment after the
Fukushima Daiichi nuclear disasters in Japan. In July 2011, meat from 11 cows shipped to Tokyo from
Fukushima Prefecture was found to have 1,530 to 3,200
becquerels per kilogram of Cs-137, considerably exceeding the Japanese legal limit of 500 becquerels per kilogram at that time.
[14]
In March 2013, the Japanese utility that owns the tsunami-damaged
nuclear power plant said that it had detected a record 740,000
becquerels per kilogram of radioactive caesium in a fish caught close to
the plant. That is 7,400 times the government limit for safe human
consumption.
[15]
Caesium-137 is reported to be the major health concern in Fukushima.
The government is under pressure to clean up radioactivity from
Fukushima from as much land as possible so that some of the 110,000
people can return. A number of techniques are being considered that will
be able to strip out 80 to 95% of the caesium from contaminated soil
and other materials efficiently and without destroying the organic
material in the soil. These include hydrothermal blasting. The caesium
precipitated with ferric
ferricyanide (
Prussian blue) would be the only waste requiring special burial sites.
[16] The aim is to get annual exposure from the contaminated environment down to 1
millisievert
(mSv) above background. The most contaminated area where radiation
doses are greater than 50 mSv/year must remain off limits, but some
areas that are currently less than 5 mSv/year may be decontaminated
allowing 22,000 residents to return.
Caesium-137 in the environment is
anthropogenic
(human-made). Unlike most other radioisotopes, caesium-137 is not
produced from the same element's nonradioactive isotopes but as a
byproduct of the nuclear fission of much heavier elements,
[17] meaning that until the building of the first artificial
nuclear reactor, the
Chicago Pile-1, in late 1942, it had not occurred on Earth in significant amounts for approximately
1.7 billion years.
By observing the characteristic gamma rays emitted by this isotope, it
is possible to determine whether the contents of a given sealed
container were made before or after the first
atomic bomb explosion (
Trinity test,
July 16, 1945), which spread some of it into the atmosphere, quickly
distributing trace amounts of it around the globe. This procedure has
been used by researchers to check the authenticity of certain rare
wines, most notably the purported "
Jefferson bottles".
[18] It is also possible to date soils and sediments, given the short life of Cs137 across the Earth's entire surface.
Incidents and accidents
Caesium-137 gamma sources have been involved in several radiological
accidents and incidents. Perhaps the best-known case is the
Goiânia accident of 1987, in which an improperly disposed of radiation therapy system from an abandoned clinic in the city of
Goiânia,
Brazil, was scavenged from a junkyard, and the glowing
caesium salt sold to curious, uneducated buyers. This led to four deaths and several serious injuries from radiation exposure.
[19]
Caesium gamma-ray sources that have been encased in metallic housings
can be mixed-in with scrap metal on its way to smelters, resulting in
production of steel contaminated with radioactivity.
[20]
The
Kramatorsk radiological accident
happened in 1989 when a small capsule containing highly radioactive
caesium-137 was found inside the concrete wall of an apartment building
in
Kramatorsk,
Ukrainian SSR.
It is believed that the capsule, originally a part of a measurement
device, was lost in the late 1970s and ended up mixed with gravel used
to construct the building in 1980. Over 9 years, two families lived in
the apartment. By the time the capsule was discovered, 6 residents of
the building had died from leukemia and 17 more had received varying
doses of radiation.
One notable example was the
Acerinox accident of 1998, when the
Spanish recycling company
Acerinox accidentally melted down a mass of radioactive caesium-137 that came from a gamma-ray generator.
[21]
In 2009, a Chinese cement company (in
Tongchuan,
Shaanxi Province) was demolishing an old, unused
cement plant
and did not follow standards for handling radioactive materials. This
caused some caesium-137 from a measuring instrument to be included with
eight truckloads of
scrap metal on its way to a
steel mill, where the radioactive caesium was melted down into the steel.
[22]
See also
References
Cs-137 Decay Scheme
Caesium-137 has a half-life of about 30.17 years.[1] About 95 percent decays by beta emission to a metastable nuclear isomer of barium: barium-137m (137mBa, Ba-137m). The remainder directly populates the ground state of barium-137, which is stable. Ba-137m has a half-life of about 153 seconds, and is responsible for all of the emissions of gamma rays in samples of caesium-137. One gram of caesium-137 has an activity of 3.215 terabecquerel (TBq).[3]Cs-137 γ-spectrum
The ten highest deposits of caesium-137 from U.S. nuclear testing at the Nevada Test Site. Test explosions "Simon" and "Harry" were both from Operation Upshot-Knothole in 1953, while the test explosions "George" and "How" were from Operation Tumbler-Snapper in 1952
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