Isotopes of americium

Actinides and fission products by half-life
Actinides by decay chain				Half-life range (y)	Fission products of 235U by yield
4n	4n+1	4n+2	4n+3
						4.5–7%	0.04–1.25%	<0.001%
228Ra№				4–6	†		155Euþ
244Cm	241Puƒ	250Cf	227Ac№	10–29		90Sr	85Kr	113mCdþ
232Uƒ		238Pu№	243Cmƒ	29–97		137Cs	151Smþ	121mSn
248Bk	249Cfƒ	242mAmƒ		141–351	No fission products have a half-life in the range of 100–210 k years ...
	241Amƒ		251Cfƒ	430–900
		226Ra№	247Bk	1.3 k – 1.6 k
240Pu	229Th№	246Cm	243Amƒ	4.7 k – 7.4 k
	245Cmƒ	250Cm		8.3 k – 8.5 k
			239Puƒ№	24.1 k
		230Th№	231Pa№	32 k – 76 k
236Npƒ	233Uƒ№	234U№		150 k – 250 k	‡	99Tc₡	126Sn
248Cm		242Pu		327 k – 375 k			79Se₡
				1.53 M		93Zr
	237Np№			2.1 M – 6.5 M		135Cs₡	107Pd
236U№			247Cmƒ	15 M – 24 M			129I₡
244Pu№				80 M	... nor beyond 15.7 M years
232Th№		238U№	235Uƒ№	0.7 G – 14.1 G
Legend for superscript symbols ₡  has thermal neutron capture cross section in the range of 8–50 barns ƒ  fissile m  metastable isomer №  naturally occurring radioactive material (NORM) þ  neutron poison (thermal neutron capture cross section greater than 3k barns) †  range 4–97 y: Medium-lived fission product ‡  over 200,000 y: Long-lived fission product

Americium (Am) is an artificial element, and thus a standard atomic mass 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 into alpha particles. Americium has an atomic number of 95 (the number of protons in the nucleus of the americium atom).

Nineteen radioisotopes of americium have been characterized, with the most stable being ²⁴³Am with a half-life of 7,370 years, and ²⁴¹Am 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 ^242mAm (t_1/2 141 years). The isotopes of americium range in atomic weight from 231.046 u (²³¹Am) to 249.078 u (²⁴⁹Am).

Some notable isotopes

Americium-241

Americium-241 used in a smoke detector

Main article: Americium-241

Americium-241 is the most prevalent isotope of americium in nuclear waste. It is the americium 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 ²⁴¹Pu, electron capture from ²⁴¹Cm, alpha from ²⁴⁵Bk.

Americium-241 decays by alpha emission, with a by-product of gamma rays. Its presence in plutonium 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 pits) may be required.

Americium-242m

Transmutation flow between ²³⁸Pu and ²⁴⁴Cm in LWR.
Fission percentage is 100 minus shown percentages.
Total rate of transmutation varies greatly by nuclide.
²⁴⁵Cm–²⁴⁸Cm are long-lived with negligible decay.

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

Probability	Decay mode	Decay energy	Decay product
99.54%	isomeric transition	0.05 MeV	242Am
0.46%	alpha decay	5.64 MeV	238Np
(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 ^180mTa, where a higher-energy nuclear isomer is more stable than the lower-energy one, Americium-242.

^242mAm is fissile (because it has an odd number of neutrons) and has a low critical mass, comparable to that of ²³⁹Pu. It has a very high cross section for fission, and if in a nuclear reactor is destroyed relatively quickly. Another report claims that ^242mAm has a much lower critical mass, can sustain a chain reaction even as a thin film, and could be used for a novel type of nuclear rocket.

²⁴²Am decay modes (half-life: 16 hours)

Probability	Decay mode	Decay energy	Decay product
82.70%	beta decay	0.665 MeV	242Cm
17.30%	electron capture	0.751 MeV	242Pu

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 ²³⁸U are required.

It decays by either emitting an alpha particle (with a decay energy of 5.27 MeV) to become ²³⁹Np, which then quickly decays to ²³⁹Pu, or infrequently, by spontaneous fission.

²⁴³Am is a hazardous substance, because it can cause cancer. ²³⁹Np, which is formed from ²⁴³Am, emits dangerous gamma rays, making ²⁴³Am the most dangerous isotope of Americium.

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)	daughter isotope(s)	nuclear spin
	excitation energy
229Am	95	134		3.7 s	α	225Np
231Am	95	136	231.04556(32)#	30# s	β+	231Pu
					α (rare)	227Np
232Am	95	137	232.04659(32)#	79(2) s	β+ (98%)	232Pu
					α (2%)	228Np
					β+, SF (.069%)	(various)
233Am	95	138	233.04635(11)#	3.2(8) min	β+	233Pu
					α	229Np
234Am	95	139	234.04781(22)#	2.32(8) min	β+ (99.95%)	234Pu
					α (.04%)	230Np
					β+, SF (.0066%)	(various)
235Am	95	140	235.04795(13)#	9.9(5) min	β+	235Pu	5/2−#
					α (rare)	231Np
236Am	95	141	236.04958(11)#	3.6(1) min	β+	236Pu
					α	232Np
237Am	95	142	237.05000(6)#	73.0(10) min	β+ (99.97%)	237Pu	5/2(−)
					α (.025%)	233Np
238Am	95	143	238.05198(5)	98(2) min	β+	238Pu	1+
					α (10−4%)	234Np
238mAm	2500(200)# keV			35(10) µs
239Am	95	144	239.0530245(26)	11.9(1) h	EC (99.99%)	239Pu	(5/2)−
					α (.01%)	235Np
239mAm	2500(200) keV			163(12) ns			(7/2+)
240Am	95	145	240.055300(15)	50.8(3) h	β+	240Pu	(3−)
					α (1.9×10−4%)	236Np
241Am	95	146	241.0568291(20)	432.2(7) y	α	237Np	5/2−
					CD (7.4×10−10%)	207Tl, 34Si
					SF (4.3×10−10%)	(various)
241mAm	2200(100) keV			1.2(3) µs
242Am	95	147	242.0595492(20)	16.02(2) h	β− (82.7%)	242Cm	1−
					EC (17.3%)	242Pu
242m1Am	48.60(5) keV			141(2) y	IT (99.54%)	242Am	5−
					α (.46%)	238Np
					SF (1.5×10−8%)	(various)
242m2Am	2200(80) keV			14.0(10) ms			(2+,3−)
243Am	95	148	243.0613811(25)	7,370(40) y	α	239Np	5/2−
					SF (3.7×10−9%)	(various)
244Am	95	149	244.0642848(22)	10.1(1) h	β−	244Cm	(6−)#
244mAm	86.1(10) keV			26(1) min	β− (99.96%)	244Cm	1+
					EC (.0361%)	244Pu
245Am	95	150	245.066452(4)	2.05(1) h	β−	245Cm	(5/2)+
246Am	95	151	246.069775(20)	39(3) min	β−	246Cm	(7−)
246m1Am	30(10) keV			25.0(2) min	β− (99.99%)	246Cm	2(−)
					IT (.01%)	246Am
246m2Am	~2000 keV			73(10) µs
247Am	95	152	247.07209(11)#	23.0(13) min	β−	247Cm	(5/2)#
248Am	95	153	248.07575(22)#	3# min	β−	248Cm
249Am	95	154	249.07848(32)#	1# min	β−	249Cm

Notes

• Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses – ( ).
• Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC, which use expanded uncertainties.

Sources

• Isotope masses from:
• Isotopic compositions and standard atomic masses from:
• Half-life, spin, and isomer data selected from the following sources.

Isotopes of plutonium	Isotopes of americium	Isotopes of curium
Table of nuclides