Americium Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide element europium, and thus by analogy was na ...

(₉₅Am) is an

artificial element A synthetic element is one of 24 known chemical elements that do not occur naturally on Earth: they have been created by human manipulation of fundamental particles in a nuclear reactor, a particle accelerator, or the explosion of an atomic bomb; ...

, and thus a

standard atomic weight The standard atomic weight of a chemical element (symbol ''A''r°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, is ...

cannot be given. Like all artificial elements, it has no known

stable isotopes The term stable isotope has a meaning similar to stable nuclide, but is preferably used when speaking of nuclides of a specific element. Hence, the plural form stable isotopes usually refers to isotopes of the same element. The relative abundanc ...

. The first

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

to be synthesized was ²⁴¹Am in 1944. The artificial element decays by ejecting

alpha particles Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be prod ...

. Americium has an atomic number of 95 (the number of protons in the nucleus of the americium atom). Despite being an order of magnitude longer lived than , the former is harder to obtain than the latter as more of it is present in

spent nuclear fuel Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor an ...

. Nineteen

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...

s of americium—²²³Am, ²²⁹Am, ²³⁰Am, and those ranging from ²³²Am to ²⁴⁷Am—have been characterized, with the most stable being ²⁴³Am with a

half-life Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ato ...

of 7,370 years, and ²⁴¹Am with a half-life of 432.2 years. All of the remaining

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

isotopes have half-lives that are less than 51 hours, and the majority of these have half-lives that are less than 100 minutes. This element also has 8

meta state A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited states have ha ...

s, with the most stable being ^242m1Am (t_1/2 = 141 years). This isomer is unusual in that its half life is far longer than that of the ground state of the same isotope.

List of isotopes

, - , ²²³Am"Observation of new neutron-deficient isotopes with Z ≥ 92 in multinucleon transfer reactions" http://inspirehep.net/record/1383747/files/scoap3-fulltext.pdf , style="text-align:right" , 95 , style="text-align:right" , 128 , , 5.2(+12.0-4.4) ms , α , ²¹⁹Np , , - , ²²⁹Am , style="text-align:right" , 95 , style="text-align:right" , 134 , 229.04525(9) , 1.8(1.5) s , α , ²²⁵Np , , - , rowspan=2, ²³⁰Am , rowspan=2 style="text-align:right" , 95 , rowspan=2 style="text-align:right" , 135 , rowspan=2, 230.04609(14)# , rowspan=2, 32(+22-9) s , β⁺ (64.7%) , ²³⁰Pu , rowspan=2, , - , β⁺, SF (35.3%) , (various) , - , rowspan=3, ²³²Am , rowspan=3 style="text-align:right" , 95 , rowspan=3 style="text-align:right" , 137 , rowspan=3, 232.04659(32)# , rowspan=3, 79(2) s , β⁺ (98%) , ²³²Pu , rowspan=3, , - , α (2%) , ²²⁸Np , - , β⁺, SF (.069%) , (various) , - , rowspan=2, ²³³Am , rowspan=2 style="text-align:right" , 95 , rowspan=2 style="text-align:right" , 138 , rowspan=2, 233.04635(11)# , rowspan=2, 3.2(8) min , β⁺ , ²³³Pu , rowspan=2, , - , α , ²²⁹Np , - , rowspan=3, ²³⁴Am , rowspan=3 style="text-align:right" , 95 , rowspan=3 style="text-align:right" , 139 , rowspan=3, 234.04781(22)# , rowspan=3, 2.32(8) min , β⁺ (99.95%) , ²³⁴Pu , rowspan=3, , - , α (.04%) , ²³⁰Np , - , β⁺, SF (.0066%) , (various) , - , rowspan=2, ²³⁵Am , rowspan=2 style="text-align:right" , 95 , rowspan=2 style="text-align:right" , 140 , rowspan=2, 235.04795(13)# , rowspan=2, 9.9(5) min , β⁺ , ²³⁵Pu , rowspan=2, 5/2−# , - , α (rare) , ²³¹Np , - , rowspan=2, ²³⁶Am , rowspan=2 style="text-align:right" , 95 , rowspan=2 style="text-align:right" , 141 , rowspan=2, 236.04958(11)# , rowspan=2, 3.6(1) min , β⁺ , ²³⁶Pu , rowspan=2, , - , α , ²³²Np , - , rowspan=2, ²³⁷Am , rowspan=2 style="text-align:right" , 95 , rowspan=2 style="text-align:right" , 142 , rowspan=2, 237.05000(6)# , rowspan=2, 73.0(10) min , β⁺ (99.97%) , ²³⁷Pu , rowspan=2, 5/2(−) , - , α (.025%) , ²³³Np , - , rowspan=2, ²³⁸Am , rowspan=2 style="text-align:right" , 95 , rowspan=2 style="text-align:right" , 143 , rowspan=2, 238.05198(5) , rowspan=2, 98(2) min , β⁺ , ²³⁸Pu , rowspan=2, 1+ , - , α (10⁻⁴%) , ²³⁴Np , - , style="text-indent:1em" , ^238mAm , colspan="3" style="text-indent:2em" , 2500(200)# keV , 35(10) μs , , , , - , rowspan=2, ²³⁹Am , rowspan=2 style="text-align:right" , 95 , rowspan=2 style="text-align:right" , 144 , rowspan=2, 239.0530245(26) , rowspan=2, 11.9(1) h , EC (99.99%) , ²³⁹Pu , rowspan=2, (5/2)− , - , α (.01%) , ²³⁵Np , - , style="text-indent:1em" , ^239mAm , colspan="3" style="text-indent:2em" , 2500(200) keV , 163(12) ns , , , (7/2+) , - , rowspan=2, ²⁴⁰Am , rowspan=2 style="text-align:right" , 95 , rowspan=2 style="text-align:right" , 145 , rowspan=2, 240.055300(15) , rowspan=2, 50.8(3) h , β⁺ , ²⁴⁰Pu , rowspan=2, (3−) , - , α (1.9×10⁻⁴%) , ²³⁶Np , - , rowspan=3, ²⁴¹AmMost common isotope , rowspan=3 style="text-align:right" , 95 , rowspan=3 style="text-align:right" , 146 , rowspan=3, 241.0568291(20) , rowspan=3, 432.2(7) y , α , ²³⁷Np , rowspan=3, 5/2− , - , CD (7.4×10⁻¹⁰%) , ²⁰⁷Tl, ³⁴Si , - , SF (4.3×10⁻¹⁰%) , (various) , - , style="text-indent:1em" , ^241mAm , colspan="3" style="text-indent:2em" , 2200(100) keV , 1.2(3) μs , SF , , , - , rowspan=2, ²⁴²Am , rowspan=2 style="text-align:right" , 95 , rowspan=2 style="text-align:right" , 147 , rowspan=2, 242.0595492(20) , rowspan=2, 16.02(2) h , β⁻ (82.7%) , ²⁴²Cm , rowspan=2, 1− , - , EC (17.3%) , ²⁴²Pu , - , rowspan=3 style="text-indent:1em" , ^242m1Am , rowspan=3 colspan="3" style="text-indent:2em" , 48.60(5) keV , rowspan=3, 141(2) y , IT (99.54%) , ²⁴²Am , rowspan=3, 5− , - , α (.46%) , ²³⁸Np , - , SF (1.5×10⁻⁸%) , (various) , - , style="text-indent:1em" , ^242m2Am , colspan="3" style="text-indent:2em" , 2200(80) keV , 14.0(10) ms , , , (2+, 3−) , - , rowspan=2, ²⁴³Am , rowspan=2 style="text-align:right" , 95 , rowspan=2 style="text-align:right" , 148 , rowspan=2, 243.0613811(25) , rowspan=2, 7,370(40) y , α , ²³⁹Np , rowspan=2, 5/2− , - , SF (3.7×10⁻⁹%) , (various) , - , ²⁴⁴Am , style="text-align:right" , 95 , style="text-align:right" , 149 , 244.0642848(22) , 10.1(1) h , β⁻ , ²⁴⁴Cm , (6−)# , - , rowspan=2 style="text-indent:1em" , ^244mAm , rowspan=2 colspan="3" style="text-indent:2em" , 86.1(10) keV , rowspan=2, 26(1) min , β⁻ (99.96%) , ²⁴⁴Cm , rowspan=2, 1+ , - , EC (.0361%) , ²⁴⁴Pu , - , ²⁴⁵Am , style="text-align:right" , 95 , style="text-align:right" , 150 , 245.066452(4) , 2.05(1) h , β⁻ , ²⁴⁵Cm , (5/2)+ , - , ²⁴⁶Am , style="text-align:right" , 95 , style="text-align:right" , 151 , 246.069775(20) , 39(3) min , β⁻ , ²⁴⁶Cm , (7−) , - , rowspan=2 style="text-indent:1em" , ^246m1Am , rowspan=2 colspan="3" style="text-indent:2em" , 30(10) keV , rowspan=2, 25.0(2) min , β⁻ (99.99%) , ²⁴⁶Cm , rowspan=2, 2(−) , - , IT (.01%) , ²⁴⁶Am , - , style="text-indent:1em" , ^246m2Am , colspan="3" style="text-indent:2em" , ~2000 keV , 73(10) μs , , , , - , ²⁴⁷Am , style="text-align:right" , 95 , style="text-align:right" , 152 , 247.07209(11)# , 23.0(13) min , β⁻ , ²⁴⁷Cm , (5/2)#

Actinides vs fission products

Notable isotopes

Americium-241

Americium-241 is the most prevalent isotope of americium in nuclear waste."Americium"
. Argonne National Laboratory, EVS. Retrieved 25 December 2009. It is the isotope used in an

americium smoke detector A smoke detector is a device that senses smoke, typically as an indicator of fire. Smoke detectors are usually housed in plastic enclosures, typically shaped like a disk about in diameter and thick, but shape and size vary. Smoke can be detecte ...

based on an

ionization chamber The ionization chamber is the simplest type of gas-filled radiation detector, and is widely used for the detection and measurement of certain types of ionizing radiation, including X-rays, gamma rays, and beta particles. Conventionally, the term ...

. It is a potential fuel for long-lifetime

radioisotope thermoelectric generator A radioisotope thermoelectric generator (RTG, RITEG), sometimes referred to as a radioisotope power system (RPS), is a type of nuclear battery that uses an array of thermocouples to convert the heat released by the decay of a suitable radioacti ...

s. Possible parent nuclides: beta from ²⁴¹Pu, electron capture from ²⁴¹Cm, alpha from ²⁴⁵Bk. Americium-241 decays by

alpha emission Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an atom ...

, with a by-product of

gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

s. Its presence in

plutonium Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibi ...

is determined by the original concentration of plutonium-241 and the sample age. Because of the low penetration of alpha radiation, Americium-241 only poses a health risk when ingested or inhaled. Older samples of plutonium containing plutonium-241 contain a buildup of ²⁴¹Am. A chemical removal of americium from reworked plutonium (e.g. during reworking of

plutonium pit Plutonium is a radioactive chemical element with the symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits ...

s) may be required.

Americium-242m

Americium-242m has a mass of 242.0595492 g/mol. It is one of the rare cases, like ^180mTa, ^210mBi and multiple holmium isomers, where a higher-energy

nuclear isomer A nuclear isomer is a metastable state of an atomic nucleus, in which one or more nucleons (protons or neutrons) occupy excited state, higher energy levels than in the ground state of the same nucleus. "Metastable" describes nuclei whose excited ...

is more stable than the lower-energy one, Americium-242.95-Am-242
^242mAm is

fissile In nuclear engineering, fissile material is material capable of sustaining a nuclear fission chain reaction. By definition, fissile material can sustain a chain reaction with neutrons of thermal energy. The predominant neutron energy may be typ ...

and has a low

critical mass In nuclear engineering, a critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, its nuclear fissi ...

, comparable to that of ²³⁹Pu. It has a very high

cross section Cross section may refer to: * Cross section (geometry) ** Cross-sectional views in architecture & engineering 3D *Cross section (geology) * Cross section (electronics) * Radar cross section, measure of detectability * Cross section (physics) **Abs ...

for fission, and if in a nuclear reactor is destroyed relatively quickly. Work has been done investigating if this isotope could be used for a novel type of

nuclear rocket Nuclear propulsion includes a wide variety of propulsion methods that use some form of nuclear reaction as their primary power source. The idea of using nuclear material for propulsion dates back to the beginning of the 20th century. In 1903 it was ...

Americium-243

Americium-243 has a mass of 243.06138 g/mol and a

of 7,370 years, the longest lasting of all americium isotopes. It is formed in the

nuclear fuel cycle The nuclear fuel cycle, also called nuclear fuel chain, is the progression of nuclear fuel through a series of differing stages. It consists of steps in the ''front end'', which are the preparation of the fuel, steps in the ''service period'' in w ...

neutron capture Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, ...

plutonium-242 Plutonium-242 (242Pu or Pu-242) is one of the isotopes of plutonium, the second longest-lived, with a half-life of 375,000 years. The half-life of 242Pu is about 15 times that of 239Pu; so it is one-fifteenth as radioactive, and not one of the la ...

followed by

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which a beta particle (fast energetic electron or positron) is emitted from an atomic nucleus, transforming the original nuclide to an isobar of that nuclide. For ...

."Americium-243"
. Oak Ridge National Laboratory. Retrieved 25 December 2009. Production increases exponentially with increasing

burnup In nuclear power technology, burnup (also known as fuel utilization) is a measure of how much energy is extracted from a primary nuclear fuel source. It is measured as the fraction of fuel atoms that underwent fission in %FIMA (fissions per init ...

as a total of 5 neutron captures on ²³⁸U are required. If

MOX-fuel Mixed oxide fuel, commonly referred to as MOX fuel, is nuclear fuel that contains more than one oxide of fissile material, usually consisting of plutonium blended with natural uranium, reprocessed uranium, or depleted uranium. MOX fuel is an alt ...

is used, particularly MOX-fuel high in and , more Americium overall and more will be produced. It decays by either emitting an

alpha particle Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produce ...

(with a decay energy of 5.27 MeV) to become ²³⁹Np, which then quickly decays to ²³⁹Pu, or infrequently, by

spontaneous fission Spontaneous fission (SF) is a form of radioactive decay that is found only in very heavy chemical elements. The nuclear binding energy of the elements reaches its maximum at an atomic mass number of about 56 (e.g., iron-56); spontaneous breakdo ...

."Isotopes of the Element Americium"
Jefferson Lab Science Education. Retrieved 25 December 2009. As for the other americium isotopes, and more generally for all alpha emitters, ²⁴³Am is

carcinogen A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive substan ...

ic in case of internal contamination after being inhaled or ingested. ²⁴³Am also presents a risk of external irradiation associated with the gamma ray emitted by its short-lived decay product ²³⁹Np. The external irradiation risk for the other two americium isotopes (²⁴¹Am and ^242mAm) is less than 10% of that for americium-243.

References

Sources

* Isotope masses from: ** * Isotopic compositions and standard atomic masses from: ** ** * Half-life, spin, and isomer data selected from the following sources. ** **IAEA - Nuclear Data Section
Live Chart of Nuclides.
Vienna International Centre. ** {{Navbox element isotopes Americium