Actinium-208

Properties and data of the isotope ²⁰⁸Ac.

Contents

Actinium-208 is a radioisotope of the chemical element actinium, which, in addition to element-specific 89 protons, has 119 neutrons in the atomic nucleus, resulting in the mass number 208. The very short-lived, only artificially producible, unstable and therefore radioactive nuclide has no practical significance; dealing with ²⁰⁸Ac is for academic purposes only.

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

General data

Name of the isotope:Actinium-208; Ac-208Symbol:²⁰⁸Ac or 20889AcMass number A:208 (= number of nucleons)Atomic number Z:89 (= number of protons)Neutrons N:119Nucleon pairing (Z - N):odd - oddNuclear ratio (N/Z ratio):1.3370786516854 (= neutron-proton ratio)Neutron excess (N-Z):30Isotopic mass:208.01154(6) u (atomic weight of Actinium-208)Nuclide 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)Separation energy:S_N = 8.46(8) MeV (first neutron)
S_P = 8(8) MeV (first proton)Half-life:95(+24-16) msDecay constant λ:7.2962861111573 s^-1Specific activity α:2.112464553643 × 10⁺²² Bq g^-1
570936365849.65 Ci g^-1Spin and parity:
(nuclear angular momentum)(3+)Year of discovery:1994

Radioactive Decay

Half-life T_½ = 95(+24-16) ms respectively 9.5 × 10^-2 seconds s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
α	²⁰⁴Fr	99 %	7.72(5) MeV
EC	²⁰⁸Ra	1 %	9.03(6) MeV

Parent Nuclides

Direct parent isotope is: ²¹²Pa.

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
^208mAc	506(26) keV	25(+9-5)ms	(10-)

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-208. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

Z	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
91	²¹⁰Pa