Molecular Product and Free Radical Yields of Ionizing Radiations in Aqueous Solutions

Ionizing radiations of different energies and masses cause a perplexing variety of chemical and biological effects. Generally speaking, the chemical yield per unit expenditure of energy in aqueous solutions is higher for light-particle than for heavy-particle radiations. This is due in part to a pronounced effect for systems irradiated with X-rays, y-rays, and electrons. Normally, heavy-particle radiation is as effective in promoting chemical changes in the presence of oxygen as in its absence. However, there is a marked selectivity of action in the systems studied, and generalizations, unless carefully stated, may be misleading. The decomposition of water affords an excellent example of a striking difference between the actions of the a-rays and X-rays. Pure water, if irradiated in vessels containing no gas phase, was found by Fricke and Brownscombe (1) to be stable to X-rays, whereas water containing certain types of impurities of the order of 10-5 M is decomposed. Consequently, it is not surprising that Lea (2), as late as 1947, found the evidence for the decomposition of water by X-rays most contradictory. Alpha rays, on the other hand, decompose pure water in high yield in either the presence or absence of oxygen. These characteristics of ionizing radiations may be explained by the differences existing in the ion density along the track of the particle and by the interaction of free radicals with molecular products. The energy necessary for ion-pair formation in air is only slightly lower for X-rays than for a-rays (32.5 and 35 ev/ip, respectively). Within the limits of experimental error, the energy required per measurable water molecule decomposed in dilute aqueous solutions is 29 ? 3 ev, regardless of the type of radiation. The appreciable differences found in the chemical effects produced by different types of radiation depend solely on the relative numbers of the water molecules yielding molecular products (hydrogen and hydrogen peroxide) and hydrogen and hydroxylfree radicals.