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Americium (95Am) 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 241Am 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 229Am to 247Am with the exception of 231Am, have been characterized; another isotope, 223Am, has also been reported but is unconfirmed. The most stable isotopes are 243Am with a half-life of 7,350 years and 241Am with a half-life of 432.6 years. All of the remaining radioactive isotopes have half-lives that are less than seven days, the majority of which are shorter than two hours. This element also has fourteen meta states, with the most stable being 242m1Am (half-life 141 years). This isomer is unusual in that its half-life is far longer than that of the ground state of the same isotope.
Nuclide [n 1] | Z | N | Isotopic mass (Da) [2] [n 2] [n 3] | Half-life [1] | Decay mode [1] [n 4] | Daughter isotope | Spin and parity [1] [n 5] [n 6] | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Excitation energy [n 6] | |||||||||||||||||||
223Am [n 7] | 95 | 128 | 223.04584(32)# | 10(9) ms | α | 219Np | 9/2–# | ||||||||||||
229Am | 95 | 134 | 229.04528(11) | 1.8(15) s | α | 225Np | 5/2–# | ||||||||||||
230Am | 95 | 135 | 230.04603(15)# | 40(9) s | β+ (<70%) | 230Pu | 1–# | ||||||||||||
β+, SF (>30%) | (various) | ||||||||||||||||||
232Am | 95 | 137 | 232.04661(32)# | 1.31(4) min | β+ (97%) | 232Pu | 1–# | ||||||||||||
α? (3%) | 228Np | ||||||||||||||||||
β+, SF (0.069%) | (various) | ||||||||||||||||||
233Am | 95 | 138 | 233.04647(12)# | 3.2(8) min | β+? (95.5%) | 233Pu | 5/2–# | ||||||||||||
α (4.5%) | 229Np | ||||||||||||||||||
234Am | 95 | 139 | 234.04773(17)# | 2.32(8) min | β+ (99.95%) | 234Pu | 0–# | ||||||||||||
α (0.039%) | 230Np | ||||||||||||||||||
β+, SF (0.0066%) | (various) | ||||||||||||||||||
235Am | 95 | 140 | 235.04791(6) | 10.3(6) min | β+ (99.60%) | 235Pu | 5/2−# | ||||||||||||
α (0.40%) | 231Np | ||||||||||||||||||
236Am | 95 | 141 | 236.04943(13)# | 3.6(1) min | β+ | 236Pu | 5− | ||||||||||||
α (4×10−3%) | 232Np | ||||||||||||||||||
236mAm | 50(50)# keV | 2.9(2) min | β+ | 236Pu | (1−) | ||||||||||||||
237Am | 95 | 142 | 237.05000(6)# | 73.6(8) min | β+ (99.975%) | 237Pu | 5/2− | ||||||||||||
α (0.025%) | 233Np | ||||||||||||||||||
238Am | 95 | 143 | 238.05198(6) | 98(3) min | β+ | 238Pu | 1+ | ||||||||||||
α (1.0×10−4%) | 234Np | ||||||||||||||||||
238mAm | 2500(200)# keV | 35(18) μs | SF | (various) | |||||||||||||||
239Am | 95 | 144 | 239.0530227(21) | 11.9(1) h | EC (99.990%) | 239Pu | 5/2− | ||||||||||||
α (0.010%) | 235Np | ||||||||||||||||||
239mAm | 2500(200) keV | 163(12) ns | SF | (various) | (7/2+) | ||||||||||||||
240Am | 95 | 145 | 240.055298(15) | 50.8(3) h | β+ | 240Pu | (3−) | ||||||||||||
α (1.9×10−4%) | 236Np | ||||||||||||||||||
240mAm | 3000(200) keV | 940(40) μs | SF | (various) | |||||||||||||||
241Am | 95 | 146 | 241.0568273(12) | 432.6(6) y | α | 237Np | 5/2− | ||||||||||||
SF (3.6×10−10%) | (various) | ||||||||||||||||||
241mAm | 2200(200) keV | 1.2(3) μs | SF | (various) | |||||||||||||||
242Am | 95 | 147 | 242.0595474(12) | 16.02(2) h | β− (82.7%) | 242Cm | 1− | ||||||||||||
EC (17.3%) | 242Pu | ||||||||||||||||||
242m1Am | 48.60(5) keV | 141(2) y | IT (99.55%) | 242Am | 5− | ||||||||||||||
α (0.45%) | 238Np | ||||||||||||||||||
SF (<4.7×10−9%) | (various) | ||||||||||||||||||
242m2Am | 2200(80) keV | 14.0(10) ms | SF | (various) | (2+, 3−) | ||||||||||||||
IT | 242Am | ||||||||||||||||||
243Am | 95 | 148 | 243.0613799(15) | 7350(9) y | α | 239Np | 5/2− | ||||||||||||
SF (3.7×10−9%) | (various) | ||||||||||||||||||
243mAm | 2300(200) keV | 5.5(5) μs | SF | (various) | |||||||||||||||
244Am | 95 | 149 | 244.0642829(16) | 10.01(3) h | β− | 244Cm | (6−) | ||||||||||||
244m1Am | 89.3(16) keV | 26.13(43) min | β− (99.96%) | 244Cm | 1+ | ||||||||||||||
EC (0.0364%) | 244Pu | ||||||||||||||||||
244m2Am | 2000(200)# keV | 0.90(15) ms | SF | (various) | |||||||||||||||
244m3Am | 2200(200)# keV | ~6.5 μs | SF | (various) | |||||||||||||||
245Am | 95 | 150 | 245.0664528(20) | 2.05(1) h | β− | 245Cm | 5/2+ | ||||||||||||
245mAm | 2400(400)# keV | 0.64(6) μs | SF | (various) | |||||||||||||||
246Am | 95 | 151 | 246.069774(19)# | 39(3) min | β− | 246Cm | 7− | ||||||||||||
246m1Am | 30(10)# keV | 25.0(2) min | β− | 246Cm | 2(−) | ||||||||||||||
246m2Am | 2000(800)# keV | 73(10) μs | SF | (various) | |||||||||||||||
247Am | 95 | 152 | 247.07209(11)# | 23.0(13) min | β− | 247Cm | 5/2# | ||||||||||||
This table header & footer: |
EC: | Electron capture |
CD: | Cluster decay |
IT: | Isomeric transition |
SF: | Spontaneous fission |
Actinides [4] by decay chain | Half-life range (a) | Fission products of 235U by yield [5] | ||||||
---|---|---|---|---|---|---|---|---|
4n | 4n + 1 | 4n + 2 | 4n + 3 | 4.5–7% | 0.04–1.25% | <0.001% | ||
228 Ra№ | 4–6 a | 155 Euþ | ||||||
248 Bk [6] | > 9 a | |||||||
244 Cmƒ | 241 Puƒ | 250 Cf | 227 Ac№ | 10–29 a | 90 Sr | 85 Kr | 113m Cdþ | |
232 Uƒ | 238 Puƒ | 243 Cmƒ | 29–97 a | 137 Cs | 151 Smþ | 121m Sn | ||
249 Cfƒ | 242m Amƒ | 141–351 a | No fission products have a half-life | |||||
241 Amƒ | 251 Cfƒ [7] | 430–900 a | ||||||
226 Ra№ | 247 Bk | 1.3–1.6 ka | ||||||
240 Pu | 229 Th | 246 Cmƒ | 243 Amƒ | 4.7–7.4 ka | ||||
245 Cmƒ | 250 Cm | 8.3–8.5 ka | ||||||
239 Puƒ | 24.1 ka | |||||||
230 Th№ | 231 Pa№ | 32–76 ka | ||||||
236 Npƒ | 233 Uƒ | 234 U№ | 150–250 ka | 99 Tc₡ | 126 Sn | |||
248 Cm | 242 Pu | 327–375 ka | 79 Se₡ | |||||
1.33 Ma | 135 Cs₡ | |||||||
237 Npƒ | 1.61–6.5 Ma | 93 Zr | 107 Pd | |||||
236 U | 247 Cmƒ | 15–24 Ma | 129 I₡ | |||||
244 Pu | 80 Ma | ... nor beyond 15.7 Ma [8] | ||||||
232 Th№ | 238 U№ | 235 Uƒ№ | 0.7–14.1 Ga | |||||
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Americium-241 (alpha emitter, half-life 432.6 years) is the most common isotope of americium in nuclear waste. [9] It is the isotope used in normal ionization smoke detectors, which work as an ionization chamber. It is a potential fuel for long-lifetime radioisotope thermoelectric generators, with a half-life longer then that of the standard plutonium-238 (87.7 years) or the alternative strontium-90 (28.91 years). Its decay heat is 0.114 W/g; its rate of spontaneous fission 1.2/g/s.
The alpha decay of 241Am is accompanied by a significant emission of gamma rays. Its presence in plutonium is determined by the original concentration of 241Pu (which decays to it) and the sample age. Older samples of plutonium containing plutonium-241 build up 241Am, and chemical separation of americium from such plutonium (e.g. during reworking of plutonium pits) may be required.
Americium-242m (half-life 141 years) is one of the rare cases, like 108mAg, 166mHo, 180mTa, 186mRe, 192mIr, 210mBi, 212mPo and others, where a higher-energy nuclear isomer is more stable than its ground state. While that ground state, 242Am, decays with half-life 16.02 hours by beta emission or electron capture, in a typical example of spin-forbiddenness the isomer does not decay by those modes, but falls to the ground state very slowly (99.55% of decays) or emits an alpha particle (0.45%, partial half-life 31 ky).
242mAm is fissile with a low critical mass, comparable to that of 239Pu. [11] 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. [12] [13]
Americium-243, an alpha emitter, has a half-life of 7350 years [1] , the longest of all americium isotopes. It is formed in the nuclear fuel cycle mainly by neutron capture on plutonium-242 followed by beta decay. [14] Production increases exponentially with increasing burnup as a total of 5 neutron captures on 238U 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 (decay energy 5.439 MeV) [15] to become 239Np, which then quickly goes to 239Pu, or, very rarely, spontaneous fission. The fission rate is about 60% that of americium-241 or about 0.7/g/s. [16]
As for the other americium isotopes, and more generally for all alpha emitters, 243Am is carcinogenic 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. [9]