The Effect of Ionizing Radiations on the Broad Bean Root. Part IV

Abstract

The conditions under which biological material such as bean roots, tissue cultures, tumour fragments, etc., may be exposed to a known dose of α radiation by immersing the tissue in water containing dissolved radon were investigated in some detail. As a result of measurements of the concentration of radon and of the short life active deposit in the root at different times after immersion in radon solution, the following conclusions were reached:— (1) The entry of radon into a root tip is approximately exponential. The average concentration reaches half the equilibrium value 2 minutes after immersion. (2) At equilibrium the concentration of radon in the root tip is the same as in the surrounding solution. (3) The atoms of active deposit are unable either to penetrate into the root or to leave it. There is, however, a very marked tendency for these atoms to accumulate in the surface of the root, which may be almost eliminated if a small quantity of protein (e.g., 2 per cent. gelatine) is added to the radon solution. (4) At points in the root out of reach of the α particles from the surface accumulation of active deposit (i.e., deeper than 70 μ), the dose is related to the radon concentration of the solution by the formula. Dose in energy units = 0·73 × microcuries per c.c. × time of immersion in minutes. By making use of the results outlined above we found the mean lethal dose of α radiation acting on the broad bean root to be 72·3 ± 2·7 energy units. It was shown that this figure was not appreciably influenced by the surface accumulation of active deposit. Thus, when compared on the basis of ionization per unit volume of tissue, α radiation is about equal in efficiency to neutron radiation, and 9 times as efficient as γ radiation in killing the bean root.