Agent Name

Actinium

CAS Number

7440-34-8

Formula

Ac

Major Category

Physical/Radiation

Category

Radionuclides

Description

Cubic crystals with a melting point of 1050 degrees C; [Merck Index # 137]

Sources/Uses

All nuclides (Ac-209 through Ac-233) are radioactive. Ac-227 occurs naturally in the decay series of U-235. It is also produced by neutron bombardment of Ra-226. Ac-228 occurs naturally in the decay series of Th-232. [Merck Index # 137] Ac-225 (alpha decay/10 day half-life) and Ac 227 (alpha decay/21.8 years) are attached to monoclonal antibodies to treat cancer; [Waltar, p. 277] With a half-life of 10 days and 4 alpha particles in the decay chain, Ac-225 is ideal for radioimmunotherapy. However, this therapy is limited by the radiotoxicity of the daughter radionuclides. There is limited commercial use of actinium. Exposure may occur in research scientists. [HSDB]

Comments

Of all Actinium radionuclides, Actinium-227 (14952-40-0) has the longest half life at 21.77 years. Mode of decay is alpha, beta, and gamma emission. Ac-228 has a half life of 6 hours and decays by beta and gamma emission. [Merck Index # 137] Internal toxicity is Very High for Ac-227 (ALI = 0.004 mCi) and Ac-225 (ALI = 0.005 mCi) and High for Ac-228 (ALI = 0.02 mCi); Lung clearance half-time is >100 days for oxides and hydroxides, 10-100 days for halides and nitrates, and <10 days for other compounds. [NRC ALIs] The specific gamma ray constant for Ac-227 = ~2.2 [Gusev, p. 587] See "Radiation, ionizing."

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