Technetium(IV) oxide colloids were radiolytically formed by γ irradiation of aqueous solutions of pertechnetate (TcO 4 − ). Pertechnetate solutions (5.5 × 10 −5 -2.9 × 10 −4 M) were irradiated with bremsstrahlung from an electron linear accelerator at 40 and 17 • C. The color of irradiated solutions gradually changed to brownish black, suggesting the formation of Tc(IV) oxide colloids (TcO 2 ·nH 2 O). A transmission electron microscopy (TEM) analysis showed that the size of colloids distributed around 30 to 130 nm in diameter. The characteristic X-rays from technetium and oxygen were simultaneously detected from colloid particles at the TEM measurements. Roundshaped colloids were produced by irradiation at 40 • C, whereas irregular-shaped colloid particles composed of tiny particles (2 nm in diameter) were produced at 17 • C. The concentration of TcO 4 − in the target solution gradually decreased with an increase of the absorbed dose, corresponding to an increase of the colloid yield. The yield of colloids sharply increased in the solution deaerated by Ar bubbling before irradiation, but strongly suppressed in the solution saturated with oxygen (O 2 ) or nitrous oxide (N 2 O) gas. The fact suggests that hydrated electrons play an important role in the course of the reduction of TcO 4 − and that Tc(IV) oxide colloids were formed via successive disproportionation reactions of Tc(VI) and Tc(V). The formation mechanisms of Tc(IV) oxide colloids are discussed.

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