# Isotopes of americium

Americium ($_{95}$Am) is an artificial element, and thus a standard atomic weight cannot be given. Like all artificial elements, it has no known stable isotopes. The first isotope to be synthesized was $^{241}$Am in 1944. The artificial element decays by ejecting alpha particles. Americium has an atomic number of 95 (the number of protons in the nucleus of the americium atom). Despite $^{243}$ Am being an order of magnitude longer lived than $^{241}$ Am, the former is harder to obtain than the latter as more of it is present in spent nuclear fuel.
Eighteen radioisotopes of americium, ranging from $^{229}$Am to $^{247}$Am with the exception of $^{231}$Am, have been characterized; another isotope, $^{223}$Am, has also been reported but is unconfirmed. The most stable isotopes are $^{243}$Am with a half-life of 7,370 years and $^{241}$Am with a half-life of 432.2 years. All of the remaining radioactive 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 states, with the most stable being $^{242m1}$Am (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

| Nuclide | Z | N | Isotopic mass (Da) | Half-life | Decay mode | Daughter isotope | Spin and parity |
| Spin and parity |||
| $^{223}$Am | 95 | 128 | 223.04584(32)# | 10(9) ms | α | $^{219}$Np | 9/2–# |
| $^{229}$Am | 95 | 134 | 229.04528(11) | 1.8(15) s | α | $^{225}$Np | 5/2–# |
| $^{230}$Am | 95 | 135 | 230.04603(15)# | 40(9) s | β$^+$ (<70%) | $^{230}$Pu | 1–# |
| $^{230}$Am | 95 | 135 | 230.04603(15)# | 40(9) s | β$^+$SF (>30%) | (various) | 1–# |
| $^{232}$Am | 95 | 137 | 232.04661(32)# | 1.31(4) min | β$^+$ (97%) | $^{232}$Pu | 1–# |
| $^{232}$Am | 95 | 137 | 232.04661(32)# | 1.31(4) min | α (3%) | $^{228}$Np | 1–# |
| $^{232}$Am | 95 | 137 | 232.04661(32)# | 1.31(4) min | β$^+$SF (0.069%) | (various) | 1–# |
| $^{233}$Am | 95 | 138 | 233.04647(12)# | 3.2(8) min | β$^+$ (95.5%) | $^{233}$Pu | 5/2–# |
| $^{233}$Am | 95 | 138 | 233.04647(12)# | 3.2(8) min | α (4.5%) | $^{229}$Np | 5/2–# |
| $^{234}$Am | 95 | 139 | 234.04773(17)# | 2.32(8) min | β$^+$ (99.95%) | $^{234}$Pu | 0–# |
| $^{234}$Am | 95 | 139 | 234.04773(17)# | 2.32(8) min | α (0.039%) | $^{230}$Np | 0–# |
| $^{234}$Am | 95 | 139 | 234.04773(17)# | 2.32(8) min | β$^+$, SF (0.0066%) | (various) | 0–# |
| $^{235}$Am | 95 | 140 | 235.047906(57) | 10.3(6) min | β$^+$ (99.60%) | $^{235}$Pu | 5/2−# |
| $^{235}$Am | 95 | 140 | 235.047906(57) | 10.3(6) min | α (0.40%) | $^{231}$Np | 5/2−# |
| $^{236}$Am | 95 | 141 | 236.04943(13)# | 3.6(1) min | β$^+$ | $^{236}$Pu | 5− |
| $^{236}$Am | 95 | 141 | 236.04943(13)# | 3.6(1) min | α (4×10$^{−3}$%) | $^{232}$Np | 5− |
| $^{236m}$Am | 50(50)# keV | 50(50)# keV | 50(50)# keV | 2.9(2) min | β$^+$ | $^{236}$Pu | (1−) |
| $^{236m}$Am | 50(50)# keV | 50(50)# keV | 50(50)# keV | 2.9(2) min | α ? | $^{232}$Np | (1−) |
| $^{237}$Am | 95 | 142 | 237.049995(64)# | 73.6(8) min | β$^+$ (99.975%) | $^{237}$Pu | 5/2− |
| $^{237}$Am | 95 | 142 | 237.049995(64)# | 73.6(8) min | α (.025%) | $^{233}$Np | 5/2− |
| $^{238}$Am | 95 | 143 | 238.051983(63) | 98(3) min | β$^+$ | $^{238}$Pu | 1+ |
| $^{238}$Am | 95 | 143 | 238.051983(63) | 98(3) min | α (1.0×10$^{−4}$%) | $^{234}$Np | 1+ |
| $^{238m}$Am | 2500(200)# keV | 2500(200)# keV | 2500(200)# keV | 35(18) μs | SF | (various) | |
| $^{238m}$Am | 2500(200)# keV | 2500(200)# keV | 2500(200)# keV | 35(18) μs | IT ? | $^{238}$Am | |
| $^{239}$Am | 95 | 144 | 239.0530227(21) | 11.9(1) h | EC (99.99%) | $^{239}$Pu | 5/2− |
| $^{239}$Am | 95 | 144 | 239.0530227(21) | 11.9(1) h | α (0.01%) | $^{235}$Np | 5/2− |
| $^{239m}$Am | 2500(200) keV | 2500(200) keV | 2500(200) keV | 163(12) ns | SF | (various) | (7/2+) |
| $^{239m}$Am | 2500(200) keV | 2500(200) keV | 2500(200) keV | 163(12) ns | IT ? | $^{239}$Am | (7/2+) |
| $^{240}$Am | 95 | 145 | 240.055298(15) | 50.8(3) h | β$^+$ | $^{240}$Pu | (3−) |
| $^{240}$Am | 95 | 145 | 240.055298(15) | 50.8(3) h | α (1.9×10$^{−4}$%) | $^{236}$Np | (3−) |
| $^{240m}$Am | 3000(200) keV | 3000(200) keV | 3000(200) keV | 940(40) μs | SF | (various) | |
| $^{240m}$Am | 3000(200) keV | 3000(200) keV | 3000(200) keV | 940(40) μs | IT ? | $^{240}$Am | |
| $^{241}$Am | 95 | 146 | 241.0568273(12) | 432.6(6) y | α | $^{237}$Np | 5/2− |
| $^{241}$Am | 95 | 146 | 241.0568273(12) | 432.6(6) y | SF (3.6×10$^{−10}$%) | (various) | 5/2− |
| $^{241m}$Am | 2200(200) keV | 2200(200) keV | 2200(200) keV | 1.2(3) μs | SF | (various) | |
| $^{242}$Am | 95 | 147 | 242.0595474(12) | 16.02(2) h | β$^−$ (82.7%) | $^{242}$Cm | 1− |
| $^{242}$Am | 95 | 147 | 242.0595474(12) | 16.02(2) h | EC (17.3%) | $^{242}$Pu | 1− |
| $^{242m1}$Am | 48.60(5) keV | 48.60(5) keV | 48.60(5) keV | 141(2) y | IT (99.54%) | $^{242}$Am | 5− |
| $^{242m1}$Am | 48.60(5) keV | 48.60(5) keV | 48.60(5) keV | 141(2) y | α (.46%) | $^{238}$Np | 5− |
| $^{242m1}$Am | 48.60(5) keV | 48.60(5) keV | 48.60(5) keV | 141(2) y | SF ? | (various) | 5− |
| $^{242m2}$Am | 2200(80) keV | 2200(80) keV | 2200(80) keV | 14.0(10) ms | SF | (various) | (2+, 3−) |
| $^{242m2}$Am | 2200(80) keV | 2200(80) keV | 2200(80) keV | 14.0(10) ms | IT ? | $^{242}$Am | (2+, 3−) |
| $^{243}$Am | 95 | 148 | 243.0613799(15) | 7,350(9) y | α | $^{239}$Np | 5/2− |
| $^{243}$Am | 95 | 148 | 243.0613799(15) | 7,350(9) y | SF (3.7×10$^{−9}$%) | (various) | 5/2− |
| $^{243m}$Am | 2300(200) keV | 2300(200) keV | 2300(200) keV | 5.5(5) μs | SF | (various) | |
| $^{243m}$Am | 2300(200) keV | 2300(200) keV | 2300(200) keV | 5.5(5) μs | IT ? | $^{243}$Am | |
| $^{244}$Am | 95 | 149 | 244.0642829(16) | 10.01(3) h | β$^−$ | $^{244}$Cm | (6−) |
| $^{244m1}$Am | 89.3(16) keV | 89.3(16) keV | 89.3(16) keV | 26.13(43) min | β$^−$ (99.96%) | $^{244}$Cm | 1+ |
| $^{244m1}$Am | 89.3(16) keV | 89.3(16) keV | 89.3(16) keV | 26.13(43) min | EC (0.0364%) | $^{244}$Pu | 1+ |
| $^{244m2}$Am | 2000(200)# | 2000(200)# | 2000(200)# | 900(150) μs | SF | (various) | |
| $^{244m2}$Am | 2000(200)# | 2000(200)# | 2000(200)# | 900(150) μs | IT ? | $^{244}$Am | |
| $^{244m3}$Am | 2200(200)# | 2200(200)# | 2200(200)# | ~6.5 μs | SF | (various) | |
| $^{244m3}$Am | 2200(200)# | 2200(200)# | 2200(200)# | ~6.5 μs | IT ? | $^{244}$Am | |
| $^{245}$Am | 95 | 150 | 245.0664528(20) | 2.05(1) h | β$^−$ | $^{245}$Cm | 5/2+ |
| $^{245m}$Am | 2400(400)# | 2400(400)# | 2400(400)# | 640(60) ns | SF | (various) | |
| $^{245m}$Am | 2400(400)# | 2400(400)# | 2400(400)# | 640(60) ns | IT ? | $^{245}$Am | |
| $^{246}$Am | 95 | 151 | 246.069774(19)# | 39(3) min | β$^−$ | $^{246}$Cm | (7−) |
| $^{246m1}$Am | 30(10)# keV | 30(10)# keV | 30(10)# keV | 25.0(2) min | β$^−$ | $^{246}$Cm | 2(−) |
| $^{246m1}$Am | 30(10)# keV | 30(10)# keV | 30(10)# keV | 25.0(2) min | IT ? | $^{246}$Am | 2(−) |
| $^{246m2}$Am | 2000(800)# keV | 2000(800)# keV | 2000(800)# keV | 73(10) μs | SF | (various) | |
| $^{246m2}$Am | 2000(800)# keV | 2000(800)# keV | 2000(800)# keV | 73(10) μs | IT ? | $^{246}$Am | |
| $^{247}$Am | 95 | 152 | 247.07209(11)# | 23.0(13) min | β$^−$ | $^{247}$Cm | 5/2# |
| This table header & footer: view ||||||||
* ↑ $^m$Am – Excited nuclear isomer.
* ↑ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
* ↑ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
* ↑ Modes of decay: CD:Cluster decay EC:Electron capture IT:Isomeric transition SF:Spontaneous fission
* ↑ ( ) spin value – Indicates spin with weak assignment arguments.
* 1 2 # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
* ↑ The discovery of this isotope is uncertain due to disagreements between theoretical predictions and reported experimental data.

## Actinides vs fission products

Actinides and fission products by half-life vte :
| Actinides by decay chain | Actinides by decay chain | Actinides by decay chain | Actinides by decay chain | Half-life range (a) | Fission products of $^{235}$U by yield | Fission products of $^{235}$U by yield | Fission products of $^{235}$U by yield |
| 4n | 4n + 1 | 4n + 2 | 4n + 3 | Half-life range (a) | 4.5–7% | 0.04–1.25% | <0.001% |
| Ra$^№$ | | | | 4–6 a | | Eu$^þ$ | |
| Bk | | | | > 9 a | | | |
| Cm$^ƒ$ | Pu$^ƒ$ | Cf | Ac$^№$ | 10–29 a | Sr | Kr | Cd$^þ$ |
| U$^ƒ$ | | Pu$^ƒ$ | Cm$^ƒ$ | 29–97 a | Cs | Sm$^þ$ | Sn |
| | Cf$^ƒ$ | Am$^ƒ$ | | 141–351 a | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| | Am$^ƒ$ | | Cf$^ƒ$ | 430–900 a | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| | | Ra$^№$ | Bk | 1.3–1.6 ka | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| Pu | Th | Cm$^ƒ$ | Am$^ƒ$ | 4.7–7.4 ka | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| | Cm$^ƒ$ | Cm | | 8.3–8.5 ka | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| | | | Pu$^ƒ$ | 24.1 ka | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| | | Th$^№$ | Pa$^№$ | 32–76 ka | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... | No fission products have a half-life in the range of 100 a–210 ka ... |
| Np$^ƒ$ | U$^ƒ$ | U$^№$ | | 150–250 ka | Tc$^₡$ | Sn |
| Cm | | Pu | | 327–375 ka | | Se$^₡$ |
| | | | | 1.33 Ma | Cs$^₡$ |
| | Np$^ƒ$ | | | 1.61–6.5 Ma | Zr | Pd |
| U | | | Cm$^ƒ$ | 15–24 Ma | | I$^₡$ |
| Pu | | | | 80 Ma | ... nor beyond 15.7 Ma | ... nor beyond 15.7 Ma | ... nor beyond 15.7 Ma |
| Th$^№$ | | U$^№$ | U$^{ƒ№}$ | 0.7–14.1 Ga | ... nor beyond 15.7 Ma | ... nor beyond 15.7 Ma | ... nor beyond 15.7 Ma |
| ₡, has thermal neutron capture cross section in the range of 8–50 barnsƒ, fissile№, primarily a naturally occurring radioactive material (NORM)þ, neutron poison (thermal neutron capture cross section greater than 3k barns) |||||||||

## Notable isotopes
### Americium-241

Americium-241 is the most common isotope of americium in nuclear waste. It is the isotope used in an americium smoke detector based on an ionization chamber. It is a potential fuel for long-lifetime radioisotope thermoelectric generators.
| Parameter | Value |
| Atomic mass | 241.056829 u |
| Mass excess | 52930 keV |
| Beta decay energy | −767 keV |
| Spin | 5/2− |
| Half-life | 432.6 years |
| Spontaneous fissions | 1200 per kg s |
| Decay heat | 114 watts/kg |
Possible parent nuclides: beta from $^{241}$Pu, electron capture from $^{241}$Cm, alpha from $^{245}$Bk.
$^{241}$Am alpha decays, with a by-product of gamma rays. Its presence in plutonium is determined by the original concentration of $^{241}$Pu and the sample age. Due to the low penetration of alpha radiation, $^{241}$Am only poses a health risk when ingested or inhaled. Older samples of plutonium containing plutonium-241 contain a buildup of $^{241}$Am. A chemical removal of americium from reworked plutonium (e.g. during reworking of plutonium pits) may be required.

### Americium-242m

$^{242m}$Am decay modes (half-life: 141 years) :
| Probability | Decay mode | Decay energy | Decay product |
| 99.54% | isomeric transition | 0.05 MeV | $^{242}$Am |
| 0.46% | alpha decay | 5.64 MeV | $^{238}$Np |
| (1.5±0.6) × 10$^{−10}$ | spontaneous fission | ~200 MeV | fission products |
Americium-242m has a mass of 242.0595492 g/mol. It is one of the rare cases, like $^{108m}$Ag, $^{166m}$Ho, $^{180m}$Ta, $^{186m}$Re, $^{192m}$Ir, $^{210m}$Bi, $^{212m}$Po and others, where a higher-energy nuclear isomer is more stable than the ground state, americium-242.
$^{242m}$Am is fissile with a low critical mass, comparable to that of $^{239}$Pu. It has a very high fission cross section, and is quickly destroyed if it is produced in a nuclear reactor. It has been investigated whether this isotope could be used for a novel type of nuclear rocket.
$^{242}$Am decay modes (half-life: 16 hours) :
| Probability | Decay mode | Decay energy | Decay product |
| 82.70% | beta decay | 0.665 MeV | $^{242}$Cm |
| 17.30% | electron capture | 0.751 MeV | $^{242}$Pu |

### Americium-243

Americium-243 has a mass of 243.06138 g/mol and a half-life of 7,370 years, the longest lasting of all americium isotopes. It is formed in the nuclear fuel cycle by neutron capture on plutonium-242 followed by beta decay. Production increases exponentially with increasing burnup as a total of 5 neutron captures on $^{238}$U are required. If MOX-fuel is used, particularly MOX-fuel high in $^{241}$ Pu and $^{242}$ Pu, more americium overall and more $^{243}$ Am will be produced.
It decays by either emitting an alpha particle (with a decay energy of 5.27 MeV) to become $^{239}$Np, which then quickly decays to $^{239}$Pu, or rarely, by spontaneous fission.
As for the other americium isotopes, and more generally for all alpha emitters, $^{243}$Am is carcinogenic in case of internal contamination after being inhaled or ingested. $^{243}$Am also presents a risk of external irradiation associated with the gamma ray emitted by its short-lived decay product $^{239}$Np. The external irradiation risk for the other two americium isotopes ($^{241}$Am and $^{242m}$Am) is less than 10% of that for americium-243.