Isotopes of americium

Isotopes of americium  (₉₅Am)

Main isotopes [1] Decay abun­dance half-life (t1/2) mode pro­duct 241Am synth 432.6 y α 237Np SF – 242m1Am synth 141 y IT 242Am α 238Np SF – 243Am synth 7350 y α 239Np SF –

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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.

List of isotopes

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: view

1. ^mAm – Excited nuclear isomer.
2. ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
3. # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
4. Modes of decay:

   EC:	Electron capture
   CD:	Cluster decay
   IT:	Isomeric transition
   SF:	Spontaneous fission

5. ( ) spin value – Indicates spin with weak assignment arguments.
6. # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
7. The discovery of this isotope is uncertain due to disagreements between theoretical predictions and reported experimental data. ^[3]

Actinides vs fission products

Actinides and fission products by half-life v t e
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 in the range of 100 a–210 ka ...
	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
₡,  has thermal neutron capture cross section in the range of 8–50 barns ƒ,  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 used in ionization smoke detectors.

Main article: Americium-241

Americium-241 is the most common isotope of americium in nuclear waste. ^[9] 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	241.0568273  Da
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 ²⁴¹Pu, electron capture from ²⁴¹Cm, alpha from ²⁴⁵Bk.

²⁴¹Am alpha decays, with a by-product of gamma rays. Its presence in plutonium is determined by the original concentration of ²⁴¹Pu and the sample age. Due to the low penetration of alpha radiation, ²⁴¹Am 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 pits) may be required.

Americium-242m

Transmutation flow between Pu and Cm in LWR.
Fission percentage is 100 minus shown percentages.
Total rate of transmutation varies greatly by nuclide.
Cm– Cm are long-lived with negligible decay.

^242mAm decay modes (half-life: 141 years) ^[1]

Probability	Decay mode	Decay energy	Decay product
99.55%	isomeric transition	0.049 MeV	242Am
0.45%	alpha decay	5.64 MeV	238Np
(1.5±0.6)×10−10  [11]	spontaneous fission	~200 MeV	fission products

Americium-242m has a mass of 242.0595492 g/mol. It 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 the ground state, americium-242. ^[12]

^242mAm is fissile with a low critical mass, comparable to that of ²³⁹Pu. ^[13] 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. ^{[14] [15]}

²⁴²Am decay modes (half-life: 16 hours) ^[1]

Probability	Decay mode	Decay energy	Decay product
82.7%	beta decay	0.664 MeV	242 Cm
17.3%	electron capture	0.751 MeV	242Pu

Americium-243

A sample of Am-243

Americium-243 has a mass of 243.06138 g/mol and a half-life of 7350 years ^[1] , 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. ^[16] 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 (with a decay energy of 5.27 MeV) ^[16] to become ²³⁹Np, which then quickly decays to ²³⁹Pu, or rarely, by spontaneous fission. ^[17]

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]

References

1. Kondev, F. G.; Wang, M.; Huang, W. J.; Naimi, S.; Audi, G. (2021). "The NUBASE2020 evaluation of nuclear properties" (PDF). Chinese Physics C. 45 (3): 030001. doi:10.1088/1674-1137/abddae.
2. Wang, Meng; Huang, W.J.; Kondev, F.G.; Audi, G.; Naimi, S. (2021). "The AME 2020 atomic mass evaluation (II). Tables, graphs and references*". Chinese Physics C. 45 (3): 030003. doi:10.1088/1674-1137/abddaf.
3. Sun, M. D.; et al. (2017). "New short-lived isotope ²²³Np and the absence of the Z = 92 subshell closure near N = 126". Physics Letters B. 771: 303–308. Bibcode:2017PhLB..771..303S. doi: 10.1016/j.physletb.2017.03.074 .
4. Plus radium (element 88). While actually a sub-actinide, it immediately precedes actinium (89) and follows a three-element gap of instability after polonium (84) where no nuclides have half-lives of at least four years (the longest-lived nuclide in the gap is radon-222 with a half life of less than four days). Radium's longest lived isotope, at 1,600 years, thus merits the element's inclusion here.
5. Specifically from thermal neutron fission of uranium-235, e.g. in a typical nuclear reactor.
6. Milsted, J.; Friedman, A. M.; Stevens, C. M. (1965). "The alpha half-life of berkelium-247; a new long-lived isomer of berkelium-248". Nuclear Physics. 71 (2): 299. Bibcode:1965NucPh..71..299M. doi:10.1016/0029-5582(65)90719-4.
   "The isotopic analyses disclosed a species of mass 248 in constant abundance in three samples analysed over a period of about 10 months. This was ascribed to an isomer of Bk²⁴⁸ with a half-life greater than 9 [years]. No growth of Cf²⁴⁸ was detected, and a lower limit for the β⁻ half-life can be set at about 10⁴ [years]. No alpha activity attributable to the new isomer has been detected; the alpha half-life is probably greater than 300 [years]."
7. This is the heaviest nuclide with a half-life of at least four years before the "sea of instability".
8. Excluding those "classically stable" nuclides with half-lives significantly in excess of ²³²Th; e.g., while ^113mCd has a half-life of only fourteen years, that of ¹¹³Cd is eight quadrillion years.
9. "Americium" Archived 2012-07-30 at the Wayback Machine . Argonne National Laboratory, EVS. Retrieved 25 December 2009.
10. Sasahara, Akihiro; Matsumura, Tetsuo; Nicolaou, Giorgos; Papaioannou, Dimitri (April 2004). "Neutron and Gamma Ray Source Evaluation of LWR High Burn-up UO2 and MOX Spent Fuels". Journal of Nuclear Science and Technology. 41 (4): 448–456. doi: 10.3327/jnst.41.448 .
11. J. T. Caldwell; S. C. Fultz; C. D. Bowman; R. W. Hoff (March 1967). "Spontaneous Fission Half-Life of Am^242m". Physical Review. 155 (4): 1309–1313. Bibcode:1967PhRv..155.1309C. doi:10.1103/PhysRev.155.1309. (halflife (9.5±3.5)×10¹¹ years)
12. 95-Am-242 Archived 2011-07-19 at the Wayback Machine
13. "Critical Mass Calculations for ²⁴¹Am, ^242mAm and ²⁴³Am" (PDF). Archived from the original (PDF) on July 22, 2011. Retrieved February 3, 2011.
14. "Extremely Efficient Nuclear Fuel Could Take Man To Mars In Just Two Weeks" (Press release). Ben-Gurion University Of The Negev. December 28, 2000.
15. Ronen, Yigal; Shwageraus, E. (2000). "Ultra-thin 241mAm fuel elements in nuclear reactors". Nuclear Instruments and Methods in Physics Research A. 455 (2): 442–451. Bibcode:2000NIMPA.455..442R. doi:10.1016/s0168-9002(00)00506-4.
16. "Americium-243" Archived 2011-02-25 at the Wayback Machine . Oak Ridge National Laboratory. Retrieved 25 December 2009.
17. "Isotopes of the Element Americium". Jefferson Lab Science Education. Retrieved 25 December 2009.

Sources

• Isotope masses from:
  • National Nuclear Data Center. "NuDat 3.0 database". Brookhaven National Laboratory.

• Half-life, spin, and isomer data selected from the following sources.
  • National Nuclear Data Center. "NuDat 3.0 database". Brookhaven National Laboratory.

• • IAEA - Nuclear Data Section. Live Chart of Nuclides. Vienna International Centre.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN   978-0-8493-0485-9.