RADIOLYTIC PRODUCTS FORMED IN CRYSTALLINE CsBrOa BY
eocO^/-RAYS
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Production, Identity, and Annealing of the Radiolytic Products Formed in Crystalline Cesium Bromate by Cobalt-60 Y-Raysl
by G. E. Boyd and Q. V. Larson Oak Ridge Natwnal Laboratmy, Oak Ridge, Tennessee 97891
(Received November 24, 1964)
Yields for bromate ion decomposition and for the production of oxygen gas, bromide ion, and oxidizing fragments in the radiolysis of crystalline CsBrOs byoC@ ' y-rays were determined as a function of dose, temperature, and dose rate. At 57" all of the yields except those for Oz(g) increased nonlinearly with dose. For large doses the concentration of oxidizing fragments became constant, and further radiolysis produced bromide and oxygen. Radiolysis at 300" gave only bromide and oxygen ; at lower temperatures increasing amounts of oxidizing fragments including hypobromite, bromite, and bromine species with higher oxidation numbers were formed, and the amounts of Br- and Oz(g) decreased. The radiolytic decomposition of BrOs- increased from -196 to 40" and then decreased with increasing temperature until a maximum stability was reached at ca. 100"; a t 200" the decomposition was nearly the same as at 40", and a t 300" it was significantly greater. Post-irradiation heating of the crystals removed their color, alkaline reaction, and oxidizing properties, increased their bromide content, and decreased the initial bromate decomposition. The yields of all the radiolysis products were independent of the dose rate. The behavior of the thermally labile oxidizing fragments was explained in terms of their decomposition to bromide and oxygen and their back-reaction to re-form bromate ion. The reaction of BrOz- ion with lattice oxygen produced thermally or by y-rays is proposed to account for the annealing of the chemical radiation damage produced in CsBr03.
The decomposition modes exhibited by molecular ions in crystals when they are exposed to energetic ionizing radiations have been the object of an increasing number of investigations. Researches on the radiolysis of the alkali metal halates, nitrates, sulfates, and phosphates, for example, have been directed toward identifying the nature of the products formed and toward establishing a mechanism for the decomposition. Quite generally, ionizing radiations act to degrade oxygenated anions by breaking their oxygen-central atom bonds to form oxygen gas and a reduced valence state of the central atom. Diverse, partially fragmented species including free radicals, positively charged molecular ions, etc., may be created and stabilized in the crystal lattice. In the study to be reported in this paper, bromate ions in highly purified CsBrOa crystals were radiolyzed by 8oCo -prays. An attempt was made to identify the species formed and to measure the variation of their
yields with dose, temperature, and dose rate. Irradiations were carried out a t low temperatures to enhance the stabilization of the highly labile radiolytic intermediates and a t elevated temperatures to study the thermal annealing of the radiation damage. Aqueous solutions of the yellow-colored irradiated solid were slightly alkaline and possessed oxidizing properties. Readily detectable quantities of bromide ion were produced, and oxygen gas was released on dissolving the crystals. Part of the oxidizing power of the solution was attributed to hypobromite ion which was identified spectrophotometrically. A preliminary account of some of these observations has been given2sa; in this (1) Presented before the Division of Physical Chemistry, 148th National Meeting of the American Chemical Society, Chicago, Ill., Sept. 1964. Research sponsored by the U . S. Atomic Energy Commission under contract with Union Carbide Corp. (2) G.E . Boyd, E. W. Graham, and Q. V..Larson, J . Phy8. Chem., 66, 300 (1962). (3) G. E. Boyd and Q. V. Larson, ibid., 68, 2627 (1964).
Volume 69,Number 4
April 196&
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research detailed nieasurements were made with Csbecause this salt was the most easily radiolyzed of all the alkali metal bromat,es.
Experimental Two ca. 175-g. preparations of pure, crystalline CsBr03 were formed by treating Cs2C03with HBr03 prepared from reagent grade I
