Actinium-208m

Properties and data of the isotope ²⁰⁸Ac.

Content

General data

Name of the isotope:Actinium-208m; Ac-208mSymbol:²⁰⁸Ac or 20889AcMass number A:208 (= number of nucleons)Atomic number Z:89 (= number of protons)Neutrons N:119Isotopic mass:208.01154(6) u (atomic weight of Actinium-208m)
Excitation energy: 506(26) keVNuclide mass:207.9627226 u (calculated nuclear mass without electrons)Mass excess:10.74944 MeVMass defect:1.716012852 u (per nucleus)Nuclear binding energy:1598.45567556 MeV (per nucleus)
7.68488306 MeV (average binding energy per nucleon)Half-life:25(+9-5)msDecay constant λ:27.725887222398 s^-1Specific activity α:8.027365303846 × 10⁺²² Bq g^-1
2169558190228.7 Ci g^-1Spin and parity:
(nuclear angular momentum)(10-)

Radioactive Decay

Half-life T_½ = 25(+9-5)ms respectively 2.5 × 10^-2 seconds s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
α	²⁰⁴Fr	90 %
β⁺	²⁰⁸Ra	10 %

Nuclear Isomers

Nuclear isomers or excited states with the activation energy in keV related to the ground state.

Nuclear Isomer	Excitation Energy	Half-life	Spin
²⁰⁸Ac		95(+24-16) ms	(3+)

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 119) and isobaric (same nucleon number A = 208) with Actinium-208m. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

OZ	Isotone N = 119	Isobar A = 208
73	¹⁹²Ta
74	¹⁹³W
75	¹⁹⁴Re
76	¹⁹⁵Os
77	¹⁹⁶Ir
78	¹⁹⁷Pt	²⁰⁸Pt
79	¹⁹⁸Au	²⁰⁸Au
80	¹⁹⁹Hg	²⁰⁸Hg
81	²⁰⁰Tl	²⁰⁸Tl
82	²⁰¹Pb	²⁰⁸Pb
83	²⁰²Bi	²⁰⁸Bi
84	²⁰³Po	²⁰⁸Po
85	²⁰⁴At	²⁰⁸At
86	²⁰⁵Rn	²⁰⁸Rn
87	²⁰⁶Fr	²⁰⁸Fr
88	²⁰⁷Ra	²⁰⁸Ra
89	²⁰⁸Ac	²⁰⁸Ac
90	²⁰⁹Th	²⁰⁸Th