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=                               Curium                               =
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                            Introduction
======================================================================
Curium is a synthetic chemical element; it has symbol Cm and atomic
number 96. This transuranic actinide element was named after eminent
scientists Marie and Pierre Curie, both known for their research on
radioactivity. Curium was first intentionally made by the team of
Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso in 1944, using
the cyclotron at Berkeley. They bombarded the newly discovered element
plutonium (the isotope 239Pu) with alpha particles. This was then sent
to the Metallurgical Laboratory at University of Chicago where a tiny
sample of curium was eventually separated and identified. The
discovery was kept secret until after the end of World War II. The
news was released to the public in November 1947. Most curium is
produced by bombarding uranium or plutonium with neutrons in nuclear
reactors - one tonne of spent nuclear fuel contains ~20 grams of
curium.

Curium is a hard, dense, silvery metal with a high melting and boiling
point for an actinide. It is paramagnetic at ambient conditions, but
becomes antiferromagnetic upon cooling, and other magnetic transitions
are also seen in many curium compounds. In compounds, curium usually
has valence +3 and sometimes +4; the +3 valence is predominant in
solutions. Curium readily oxidizes, and its oxides are a dominant form
of this element. It forms strongly fluorescent complexes with various
organic compounds. If it gets into the human body, curium accumulates
in bones, lungs, and liver, where it promotes cancer.

All known isotopes of curium are radioactive and have small critical
mass for a nuclear chain reaction. The most stable isotope, 247Cm, has
a half-life of 15.6 million years; the longest-lived curium isotopes
predominantly emit alpha particles. Radioisotope thermoelectric
generators can use the heat from this process, but this is hindered by
the rarity and high cost of curium. Curium is used in making heavier
actinides and the 238Pu radionuclide for power sources in artificial
cardiac pacemakers and RTGs for spacecraft. It served as the α-source
in the alpha particle X-ray spectrometers of several space probes,
including the 'Sojourner', 'Spirit', 'Opportunity', and 'Curiosity'
Mars rovers and the Philae lander on comet 67P/Churyumov-Gerasimenko,
to analyze the composition and structure of the surface.


                              History
======================================================================
Curium was chemically identified at the Metallurgical Laboratory (now
Argonne National Laboratory), University of Chicago. It was the third
transuranium element to be discovered even though it is the fourth in
the series - the lighter element americium was still unknown.

The sample was prepared as follows: first plutonium nitrate solution
was coated on a platinum foil of ~0.5 cm2 area, the solution was
evaporated and the residue was converted into plutonium(IV) oxide
(PuO2) by annealing. Following cyclotron irradiation of the oxide, the
coating was dissolved with nitric acid and then precipitated as the
hydroxide using concentrated aqueous ammonia solution. The residue was
dissolved in perchloric acid, and further separation was done by ion
exchange to yield a certain isotope of curium. The separation of
curium and americium was so painstaking that the Berkeley group
initially called those elements 'pandemonium' (from Greek for 'all
demons' or 'hell') and 'delirium' (from Latin for 'madness').

Curium-242 was made in July-August 1944 by bombarding 239Pu with
α-particles to produce curium with the release of a neutron:
: ^{239}_{94}Pu + ^{4}_{2}He -> ^{242}_{96}Cm + ^{1}_{0}n

Curium-242 was unambiguously identified by the characteristic energy
of the α-particles emitted during the decay:
: ^{242}_{96}Cm -> ^{238}_{94}Pu + ^{4}_{2}He
The half-life of this alpha decay was first measured as 5 months (150
days) and then corrected to 162.8 days.

Another isotope 240Cm was produced in a similar reaction in March
1945:
: ^{239}_{94}Pu + ^{4}_{2}He -> ^{240}_{96}Cm + 3^{1}_{0}n
The α-decay half-life of 240Cm was determined as 26.8 days and later
revised to 30.4 days.

The discovery of curium and americium in 1944 was closely related to
the Manhattan Project, so the results were confidential and
declassified only in 1945. Seaborg leaked the synthesis of the
elements 95 and 96 on the U.S. radio show for children, the 'Quiz
Kids', five days before the official presentation at an American
Chemical Society meeting on November 11, 1945, when one listener asked
if any new transuranic element beside plutonium and neptunium had been
discovered during the war. The discovery of curium (242Cm and 240Cm),
its production, and its compounds was later patented listing only
Seaborg as the inventor.


The element was named after Marie Curie and her husband Pierre Curie,
who are known for discovering radium and for their work in
radioactivity. It followed the example of gadolinium, a lanthanide
element above curium in the periodic table, which was named after the
explorer of rare-earth elements Johan Gadolin:
:: As the name for the element of atomic number 96 we should like to
propose "curium", with symbol Cm. The evidence indicates that element
96 contains seven 5f electrons and is thus analogous to the element
gadolinium, with its seven 4f electrons in the regular rare earth
series. On this basis element 96 is named after the Curies in a manner
analogous to the naming of gadolinium, in which the chemist Gadolin
was honored.

The first curium samples were barely visible, and were identified by
their radioactivity. Louis Werner and Isadore Perlman made the first
substantial sample of 30 μg curium-242 hydroxide at University of
California, Berkeley in 1947 by bombarding americium-241 with
neutrons. Macroscopic amounts of curium(III) fluoride were obtained in
1950 by W. W. T. Crane, J. C. Wallmann and B. B. Cunningham. Its
magnetic susceptibility was very close to that of GdF3 providing the
first experimental evidence for the +3 valence of curium in its
compounds. Curium metal was produced only in 1950 by reduction of CmF3
with barium.


Physical
==========
A synthetic, radioactive element, curium is a hard, dense metal with a
silvery-white appearance and physical and chemical properties
resembling gadolinium. Its melting point of 1344 °C is significantly
higher than that of the previous elements neptunium (637 °C),
plutonium (639 °C) and americium (1176 °C). In comparison, gadolinium
melts at 1312 °C. Curium boils at 3556 °C. With a density of 13.52
g/cm3, curium is lighter than neptunium (20.45 g/cm3) and plutonium
(19.8 g/cm3), but heavier than most other metals. Of two crystalline
forms of curium, α-Cm is more stable at ambient conditions. It has a
hexagonal symmetry, space group P63/mmc, lattice parameters 'a' = 365
pm and 'c' = 1182 pm, and four formula units per unit cell. The
crystal consists of double-hexagonal close packing with the layer
sequence ABAC and so is isotypic with α-lanthanum. At pressure >23
GPa, at room temperature, α-Cm becomes β-Cm, which has face-centered
cubic symmetry, space group Fmm and lattice constant 'a' = 493 pm. On
further compression to 43 GPa, curium becomes an orthorhombic γ-Cm
structure similar to α-uranium, with no further transitions observed
up to 52 GPa. These three curium phases are also called Cm I, II and
III.

Curium has peculiar magnetic properties. Its neighbor element
americium shows no deviation from Curie-Weiss paramagnetism in the
entire temperature range, but α-Cm transforms to an antiferromagnetic
state upon cooling to 65-52 K, and β-Cm exhibits a ferrimagnetic
transition at ~205 K. Curium pnictides show ferromagnetic transitions
upon cooling: 244CmN and 244CmAs at 109 K, 248CmP at 73 K and 248CmSb
at 162 K. The lanthanide analog of curium, gadolinium, and its
pnictides, also show magnetic transitions upon cooling, but the
transition character is somewhat different: Gd and GdN become
ferromagnetic, and GdP, GdAs and GdSb show antiferromagnetic ordering.

In accordance with magnetic data, electrical resistivity of curium
increases with temperature - about twice between 4 and 60 K - and then
is nearly constant up to room temperature. There is a significant
increase in resistivity over time (~) due to self-damage of the
crystal lattice by alpha decay. This makes uncertain the true
resistivity of curium (~). Curium's resistivity is similar to that of
gadolinium, and the actinides plutonium and neptunium, but
significantly higher than that of americium, uranium, polonium and
thorium.

Under ultraviolet illumination, curium(III) ions show strong and
stable yellow-orange fluorescence with a maximum in the range of
590-640 nm depending on their environment. The fluorescence originates
from the transitions from the first excited state 6D7/2 and the ground
state 8S7/2. Analysis of this fluorescence allows monitoring
interactions between Cm(III) ions in organic and inorganic complexes.


Chemical
==========
Curium ion in solution almost always has a +3 oxidation state, the
most stable oxidation state for curium. A +4 oxidation state is seen
mainly in a few solid phases, such as CmO2 and CmF4. Aqueous
curium(IV) is only known in the presence of strong oxidizers such as
potassium persulfate, and is easily reduced to curium(III) by
radiolysis and even by water itself. Chemical behavior of curium is
different from the actinides thorium and uranium, and is similar to
americium and many lanthanides. In aqueous solution, the Cm3+ ion is
colorless to pale green; Cm4+ ion is pale yellow. The optical
absorption of Cm3+ ion contains three sharp peaks at 375.4, 381.2 and
396.5 nm and their strength can be directly converted into the
concentration of the ions. The +6 oxidation state has only been
reported once in solution in 1978, as the curyl ion (): this was
prepared from beta decay of americium-242 in the americium(V) ion .
Failure to get Cm(VI) from oxidation of Cm(III) and Cm(IV) may be due
to the high Cm4+/Cm3+ ionization potential and the instability of
Cm(V).

Curium ions are hard Lewis acids and thus form most stable complexes
with hard bases. The bonding is mostly ionic, with a small covalent
component. Curium in its complexes commonly exhibits a 9-fold
coordination environment, with a tricapped trigonal prismatic
molecular geometry.


Isotopes
==========
About 19 radioisotopes and 7 nuclear isomers, 233Cm to 251Cm, are
known; none are stable. The longest half-lives are 15.6 million years
(247Cm) and 348,000 years (248Cm). Other long-lived ones are 245Cm
(8500 years), 250Cm (8300 years) and 246Cm (4760 years). Curium-250 is
unusual: it mostly (~86%) decays by spontaneous fission. The most
commonly used isotopes are 242Cm and 244Cm with the half-lives 162.8
days and 18.11 years, respectively.

!colspan="7"| Thermal neutron cross sections (barns)
242Cm   243Cm   244Cm   245Cm   246Cm   247Cm
|Fission        5       617     1.04    2145    0.14    81.90
|Capture        16      130     15.20   369     1.22    57
|C/F ratio      3.20    0.21    14.62   0.17    8.71    0.70
!colspan="7"| LEU spent nuclear fuel 20 years after 53 MWd/kg burnup
|colspan="2" |3 common isotopes         51      3700    390
!colspan="7"| Fast-neutron reactor MOX fuel (avg 5 samples, burnup
66-120 GWd/t)
|colspan="2" |Total curium 3.09%        27.64%  70.16%  2.166%  0.0376%
0.000928%
Class = "wikitable"
Isotope 242Cm   243Cm   244Cm   245Cm   246Cm   247Cm   248Cm   250Cm
|Critical mass, kg       25      7.5    33      6.8     39      7       40.4    23.5


All isotopes ranging from 242Cm to 248Cm, as well as 250Cm, undergo a
self-sustaining nuclear chain reaction and thus in principle can be a
nuclear fuel in a reactor. As in most transuranic elements, nuclear
fission cross section is especially high for the odd-mass curium
isotopes 243Cm, 245Cm and 247Cm. These can be used in thermal-neutron
reactors, whereas a mixture of curium isotopes is only suitable for
fast breeder reactors since the even-mass isotopes are not fissile in
a thermal reactor and accumulate as burn-up increases. The mixed-oxide
(MOX) fuel, which is to be used in power reactors, should contain
little or no curium because neutron activation of 248Cm will create
californium. Californium is a strong neutron emitter, and would
pollute the back end of the fuel cycle and increase the dose to
reactor personnel. Hence, if minor actinides are to be used as fuel in
a thermal neutron reactor, the curium should be excluded from the fuel
or placed in special fuel rods where it is the only actinide present.

The adjacent table lists the critical masses for curium isotopes for a
sphere, without moderator or reflector. With a metal reflector (30 cm
of steel), the critical masses of the odd isotopes are about 3-4 kg.
When using water (thickness ~20-30 cm) as the reflector, the critical
mass can be as small as 59 grams for 245Cm, 155 grams for 243Cm and
1550 grams for 247Cm. There is significant uncertainty in these
critical mass values. While it is usually on the order of 20%, the
values for 242Cm and 246Cm were listed as large as 371 kg and 70.1 kg,
respectively, by some research groups.

Curium is not currently used as nuclear fuel due to its low
availability and high price. 245Cm and 247Cm have very small critical
mass and so could be used in tactical nuclear weapons, but none are
known to have been made. Curium-243 is not suitable for such, due to
its short half-life and strong α emission, which would cause excessive
heat. Curium-247 would be highly suitable due to its long half-life,
which is 647 times longer than plutonium-239 (used in many existing
nuclear weapons).


Occurrence
============
The longest-lived isotope, 247Cm, has half-life 15.6 million years; so
any primordial curium, that is, present on Earth when it formed,
should have decayed by now. Its past presence as an extinct
radionuclide is detectable as an excess of its primordial, long-lived
daughter 235U. Traces of 242Cm may occur naturally in uranium minerals
due to neutron capture and beta decay (238U → 239Pu → 240Pu → 241Am →
242Cm), though the quantities would be tiny and this has not been
confirmed: even with "extremely generous" estimates for neutron
absorption possibilities, the quantity of 242Cm present in 1 × 108 kg
of 18% uranium pitchblende would not even be one atom. Traces of 247Cm
are also probably brought to Earth in cosmic rays, but this also has
not been confirmed. There is also the possibility of 244Cm being
produced as the double beta decay daughter of natural 244Pu.

Curium is made artificially in small amounts for research purposes. It
also occurs as one of the waste products in spent nuclear fuel. Curium
is present in nature in some areas used for nuclear weapons testing.
Analysis of the debris at the test site of the United States' first
thermonuclear weapon, Ivy Mike (1 November 1952, Enewetak Atoll),
besides einsteinium, fermium, plutonium and americium also revealed
isotopes of berkelium, californium and curium, in particular 245Cm,
246Cm and smaller quantities of 247Cm, 248Cm and 249Cm.

Atmospheric curium compounds are poorly soluble in common solvents and
mostly adhere to soil particles. Soil analysis revealed about 4,000
times higher concentration of curium at the sandy soil particles than
in water present in the soil pores. An even higher ratio of about
18,000 was measured in loam soils.

The transuranic elements up to fermium, including curium, should have
been present in the natural nuclear fission reactor at Oklo, but any
quantities produced then would have long since decayed away.


Isotope preparation
=====================
Curium is made in small amounts in nuclear reactors, and by now only
kilograms of 242Cm and 244Cm have been accumulated, and grams or even
milligrams for heavier isotopes. Hence the high price of curium, which
has been quoted at 160-185 USD per milligram, with a more recent
estimate at US$2,000/g for 242Cm and US$170/g for 244Cm. In nuclear
reactors, curium is formed from 238U in a series of nuclear reactions.
In the first chain, 238U captures a neutron and converts into 239U,
which via β− decay transforms into 239Np and 239Pu.

{{NumBlk|:|^{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).|}}

Further neutron capture followed by β−-decay gives americium (241Am)
which further becomes 242Cm:
{{NumBlk|:|^{239}_{94}Pu->[\ce{2(n,\gamma)}] ^{241}_{94}Pu
->[\beta^-][14.35\ \ce{yr}] {^{241}_{95}Am} ->[\ce{(n,\gamma)}]
^{242}_{95}Am ->[\beta^-][16.02 \ce{h}] ^{242}_{96}Cm.|}}

For research purposes, curium is obtained by irradiating not uranium
but plutonium, which is available in large amounts from spent nuclear
fuel. A much higher neutron flux is used for the irradiation that
results in a different reaction chain and formation of 244Cm:
{{NumBlk|:|^{239}_{94}Pu ->[\ce{4(n,\gamma)}] ^{243}_{94}Pu
->[\beta^-][4.956\ \ce{h}] ^{243}_{95}Am ->[(\ce n,\gamma)]
^{244}_{95}Am ->[\beta^-][10.1 \ce{h}] ^{244}_{96}Cm
->[\alpha][18.11\ \ce{yr}] ^{240}_{94}Pu|}}

Curium-244 alpha decays to 240Pu, but it also absorbs neutrons, hence
a small amount of heavier curium isotopes. Of those, 247Cm and 248Cm
are popular in scientific research due to their long half-lives. But
the production rate of 247Cm in thermal neutron reactors is low
because it is prone to fission due to thermal neutrons. Synthesis of
250Cm by neutron capture is unlikely due to the short half-life of the
intermediate 249Cm (64 min), which β− decays to the berkelium isotope
249Bk.

{{NumBlk|:||}}

The above cascade of (n,γ) reactions gives a mix of different curium
isotopes. Their post-synthesis separation is cumbersome, so a
selective synthesis is desired. Curium-248 is favored for research
purposes due to its long half-life. The most efficient way to prepare
this isotope is by α-decay of the californium isotope 252Cf, which is
available in relatively large amounts due to its long half-life (2.65
years). About 35-50 mg of 248Cm is produced thus, per year. The
associated reaction produces 248Cm with isotopic purity of 97%.

{{NumBlk|:||}}

Another isotope, 245Cm, can be obtained for research, from α-decay of
249Cf; the latter isotope is produced in small amounts from β−-decay
of 249Bk.
{{NumBlk|:|
^{249}_{97}Bk ->[\beta^-][330\ \ce{d}] ^{249}_{98}Cf
->[\alpha][351\ \ce{yr}] ^{245}_{96}Cm
|}}


Metal preparation
===================
Most synthesis routines yield a mix of actinide isotopes as oxides,
from which a given isotope of curium needs to be separated. An example
procedure could be to dissolve spent reactor fuel (e.g. MOX fuel) in
nitric acid, and remove the bulk of the uranium and plutonium using a
PUREX (Plutonium - URanium EXtraction) type extraction with tributyl
phosphate in a hydrocarbon. The lanthanides and the remaining
actinides are then separated from the aqueous residue (raffinate) by a
diamide-based extraction to give, after stripping, a mixture of
trivalent actinides and lanthanides. A curium compound is then
selectively extracted using multi-step chromatographic and
centrifugation techniques with an appropriate reagent. 'Bis'-triazinyl
bipyridine complex has been recently proposed as such reagent which is
highly selective to curium. Separation of curium from the very
chemically similar americium can also be done by treating a slurry of
their hydroxides in aqueous sodium bicarbonate with ozone at elevated
temperature. Both americium and curium are present in solutions mostly
in the +3 valence state; americium oxidizes to soluble Am(IV)
complexes, but curium stays unchanged and so can be isolated by
repeated centrifugation.

Metallic curium is obtained by reduction of its compounds. Initially,
curium(III) fluoride was used for this purpose. The reaction was done
in an environment free of water and oxygen, in an apparatus made of
tantalum and tungsten, using elemental barium or lithium as reducing
agents.
:

Another possibility is reduction of curium(IV) oxide using a
magnesium-zinc alloy in a melt of magnesium chloride and magnesium
fluoride.


Oxides
========
Curium readily reacts with oxygen forming mostly Cm2O3 and CmO2
oxides, but the divalent oxide CmO is also known. Black CmO2 can be
obtained by burning curium oxalate (), nitrate (), or hydroxide in
pure oxygen. Upon heating to 600-650 °C in vacuum (about 0.01 Pa), it
transforms into the whitish Cm2O3:
: 4CmO2 ->[\Delta T] 2Cm2O3 + O2.

Or, Cm2O3 can be obtained by reducing CmO2 with molecular hydrogen:
: 2CmO2 + H2 -> Cm2O3 + H2O

Also, a number of ternary oxides of the type M(II)CmO3 are known,
where M stands for a divalent metal, such as barium.

Thermal oxidation of trace quantities of curium hydride (CmH2-3) has
been reported to give a volatile form of CmO2 and the volatile
trioxide CmO3, one of two known examples of the very rare +6 state for
curium. Another observed species was reported to behave similar to a
supposed plutonium tetroxide and was tentatively characterized as
CmO4, with curium in the extremely rare +8 state; but new experiments
seem to indicate that CmO4 does not exist, and have cast doubt on the
existence of PuO4 as well.


Halides
=========
The colorless curium(III) fluoride (CmF3) can be made by adding
fluoride ions into curium(III)-containing solutions. The brown
tetravalent curium(IV) fluoride (CmF4) on the other hand is only
obtained by reacting curium(III) fluoride with molecular fluorine:
:

A series of ternary fluorides are known of the form A7Cm6F31 (A =
alkali metal).

The colorless curium(III) chloride (CmCl3) is made by reacting curium
hydroxide (Cm(OH)3) with anhydrous hydrogen chloride gas. It can be
further turned into other halides such as curium(III) bromide
(colorless to light green) and curium(III) iodide (colorless), by
reacting it with the ammonia salt of the corresponding halide at
temperatures of ~400-450 °C:
:

Or, one can heat curium oxide to ~600°C with the corresponding acid
(such as hydrobromic for curium bromide). Vapor phase hydrolysis of
curium(III) chloride gives curium oxychloride:
:


Chalcogenides and pnictides
=============================
Sulfides, selenides and tellurides of curium have been obtained by
treating curium with gaseous sulfur, selenium or tellurium in vacuum
at elevated temperature. Curium pnictides of the type CmX are known
for nitrogen, phosphorus, arsenic and antimony. They can be prepared
by reacting either curium(III) hydride (CmH3) or metallic curium with
these elements at elevated temperature.


Organocurium compounds and biological aspects
===============================================
Organometallic complexes analogous to uranocene are known also for
other actinides, such as thorium, protactinium, neptunium, plutonium
and americium. Molecular orbital theory predicts a stable "curocene"
complex (η8-C8H8)2Cm, but it has not been reported experimentally yet.

Formation of the complexes of the type  (BTP =
2,6-di(1,2,4-triazin-3-yl)pyridine), in solutions containing
n-C3H7-BTP and Cm3+ ions has been confirmed by EXAFS. Some of these
BTP-type complexes selectively interact with curium and thus are
useful for separating it from lanthanides and another actinides.
Dissolved Cm3+ ions bind with many organic compounds, such as
hydroxamic acid, urea, fluorescein and adenosine triphosphate. Many of
these compounds are related to biological activity of various
microorganisms. The resulting complexes show strong yellow-orange
emission under UV light excitation, which is convenient not only for
their detection, but also for studying interactions between the Cm3+
ion and the ligands via changes in the half-life (of the order ~0.1
ms) and spectrum of the fluorescence.

There are a few reports on biosorption of Cm3+ by bacteria and
archaea, and in the laboratory both americium and curium were found to
support the growth of methylotrophs.


Radionuclides
===============
Curium is one of the most radioactive isolable elements. Its two most
common isotopes 242Cm and 244Cm are strong alpha emitters (energy 6
MeV); they have fairly short half-lives, 162.8 days and 18.1 years,
and give as much as 120 W/g and 3 W/g of heat, respectively.
Therefore, curium can be used in its common oxide form in radioisotope
thermoelectric generators like those in spacecraft. This application
has been studied for the 244Cm isotope, while 242Cm was abandoned due
to its prohibitive price, around 2000 USD/g. 243Cm with a ~30-year
half-life and good energy yield of ~1.6 W/g could be a suitable fuel,
but it gives significant amounts of harmful gamma and beta rays from
radioactive decay products. As an α-emitter, 244Cm needs much less
radiation shielding, but it has a high spontaneous fission rate, and
thus a lot of neutron and gamma radiation. Compared to a competing
thermoelectric generator isotope such as 238Pu, 244Cm emits 500 times
more neutrons, and its higher gamma emission requires a shield that is
20 times thicker--2 in of lead for a 1 kW source, compared to 0.1 in
for 238Pu. Therefore, this use of curium is currently considered
impractical.

A more promising use of 242Cm is for making 238Pu, a better
radioisotope for thermoelectric generators such as in heart
pacemakers. The alternate routes to 238Pu use the (n,γ) reaction of
237Np, or deuteron bombardment of uranium, though both reactions
always produce 236Pu as an undesired by-product since the latter
decays to 232U with strong gamma
emission.[http://www.kronenberg.kernchemie.de/ Kronenberg, Andreas],
[http://www.kernenergie-wissen.de/pu-batterien.html
Plutonium-Batterien]  (in German)

Curium is a common starting material for making higher transuranic
and superheavy elements. Thus, bombarding 248Cm with neon (22Ne),
magnesium (26Mg), or calcium (48Ca) yields isotopes of seaborgium
(265Sg), hassium (269Hs and 270Hs), and livermorium (292Lv, 293Lv, and
possibly 294Lv). Californium was discovered when a microgram-sized
target of curium-242 was irradiated with 35 MeV alpha particles using
the 60 in cyclotron at Berkeley:
: +  →  +
Only about 5,000 atoms of californium were produced in this
experiment.

The odd-mass curium isotopes 243Cm, 245Cm, and 247Cm are all highly
fissile and can release additional energy in a thermal spectrum
nuclear reactor. All curium isotopes are fissionable in fast-neutron
reactors. This is one of the motives for minor actinide separation and
transmutation in the nuclear fuel cycle, helping to reduce the
long-term radiotoxicity of used, or spent nuclear fuel.


X-ray spectrometer
====================
The most practical application of 244Cm--though rather limited in
total volume--is as α-particle source in alpha particle X-ray
spectrometers (APXS). These instruments were installed on the
Sojourner, Mars, Mars 96, Mars Exploration Rovers and Philae comet
lander, as well as the Mars Science Laboratory to analyze the
composition and structure of the rocks on the surface of planet Mars.
APXS was also used in the Surveyor 5-7 moon probes but with a 242Cm
source.

An elaborate APXS setup has a sensor head containing six curium
sources with a total decay rate of several tens of millicuries
(roughly one gigabecquerel). The sources are collimated on a sample,
and the energy spectra of the alpha particles and protons scattered
from the sample are analyzed (proton analysis is done only in some
spectrometers). These spectra contain quantitative information on all
major elements in the sample except for hydrogen, helium and lithium.


                               Safety
======================================================================
Due to its radioactivity, curium and its compounds must be handled in
appropriate labs under special arrangements. While curium itself
mostly emits α-particles which are absorbed by thin layers of common
materials, some of its decay products emit significant fractions of
beta and gamma rays, which require a more elaborate protection. If
consumed, curium is excreted within a few days and only 0.05% is
absorbed in the blood. From there, ~45% goes to the liver, 45% to the
bones, and the remaining 10% is excreted. In bone, curium accumulates
on the inside of the interfaces to the bone marrow and does not
significantly redistribute with time; its radiation destroys bone
marrow and thus stops red blood cell creation. The biological
half-life of curium is about 20 years in the liver and 50 years in the
bones. Curium is absorbed in the body much more strongly via
inhalation, and the allowed total dose of 244Cm in soluble form is 0.3
μCi. Intravenous injection of 242Cm- and 244Cm-containing solutions to
rats increased the incidence of bone tumor, and inhalation promoted
lung and liver cancer.

Curium isotopes are inevitably present in spent nuclear fuel (about 20
g/tonne). The isotopes 245Cm-248Cm have decay times of thousands of
years and must be removed to neutralize the fuel for disposal. Such a
procedure involves several steps, where curium is first separated and
then converted by neutron bombardment in special reactors to
short-lived nuclides. This procedure, nuclear transmutation, while
well documented for other elements, is still being developed for
curium.


                            Bibliography
======================================================================
*
* Holleman, Arnold F. and Wiberg, Nils ' Lehrbuch der Anorganischen
Chemie', 102 Edition, de Gruyter, Berlin 2007, .
* Penneman, R. A. and Keenan T. K.
[http://www.osti.gov/bridge/purl.cover.jsp?purl=/4187189-IKQUwY/ The
radiochemistry of americium and curium], University of California, Los
Alamos, California, 1960


                           External links
======================================================================
* [http://www.periodicvideos.com/videos/096.htm Curium] at 'The
Periodic Table of Videos' (University of Nottingham)
*
[http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@na+@rel+curium,+radioactive
NLM Hazardous Substances Databank - Curium, Radioactive]


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