Americium-231

Properties and data of the isotope ²³¹Am.

Contents

General Data

Radioactive Decay

Parent Nuclides

Isotones und Isobares

Literature

Americium-231 is a radioisotope of the chemical element americium, which has 136 neutrons in its atomic nucleus in addition to the element-specific 95 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 231. The very short-lived, only artificially produced, unstable and thus radioactive nuclide has no practical significance; the study of ²³¹Am is exclusively for academic purposes.

Americium-231 was first detected - according to a 2026 report [1] - as an alpha-decay product during the production of berkelium-235 via the nuclear reaction ¹⁹⁷Au(⁴⁰Ar,2n)²³⁵Bk.

See also: List of individual Americium isotopes (and general data sources).

General data

Name of the isotope:Americium-231; Am-231Symbol:²³¹Am or 23195AmMass number A:231 (= number of nucleons)Atomic number Z:95 (= number of protons)Neutrons N:136Nucleon pairing (Z - N):odd - evenNuclear ratio (N/Z ratio):1.4315789473684 (= neutron-proton ratio)Neutron excess (N-Z):41Isotopic mass:231.04553(32) u (atomic weight of Americium-231)Nuclide mass:230.9934221 u (calculated nuclear mass without electrons)Mass excess:42.41092 MeVMass defect:1.876276616 u (per nucleus)Nuclear binding energy:1747.74041014 MeV (per nucleus)
7.5659758 MeV (average binding energy per nucleon)Separation energy:S_N = 8.533(332) MeV (first neutron)
S_P = 1.811(300) MeV (first proton)Half-life:75 sDecay constant λ:0.0092419624074659 s^-1Specific activity α:2.408893109353 × 10⁺¹⁹ Bq g^-1
651052191.71711 Ci g^-1Spin and parity:
(nuclear angular momentum)(5/2-)Year of discovery:2026

Radioactive Decay

Half-life T_½ = 75 s respectively 7.5 × 10¹ seconds s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
β⁺	²³¹Pu	ca. 83 %	3.929(82) MeV
α	²²⁷Np	ca. 17 %	7.234(18) MeV

Radioactive decay scheme of the isotope americium-231

Parent Nuclides

Direct parent isotope is: ²³⁵Bk.

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 136) and isobaric (same nucleon number A = 231) with Americium-231. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

Z	Isotone N = 136	Isobar A = 231
80	²¹⁶Hg
81	²¹⁷Tl
82	²¹⁸Pb
83	²¹⁹Bi
84	²²⁰Po
85	²²¹At
86	²²²Rn
87	²²³Fr	²³¹Fr
88	²²⁴Ra	²³¹Ra
89	²²⁵Ac	²³¹Ac
90	²²⁶Th	²³¹Th
91	²²⁷Pa	²³¹Pa
92	²²⁸U	²³¹U
93	²²⁹Np	²³¹Np
94	²³⁰Pu	²³¹Pu
95	²³¹Am	²³¹Am
96	²³²Cm
97	²³³Bk

External data and identifiers

Adopted Levels, Gammas:NuDat 231Am

Literature and References

[1] - J. G. Wang, Z. C. Li, Z. Y. Zhang et al.:
Decay properties of new isotopes ²³⁵Bk and ²³¹Am.
In: Physics Letters B, 875, 140365, (2026), DOI 10.1016/j.physletb.2026.140365.