Radiation Effects on Aqueous Thiourea Solutions

Abstract

Thiourea and elemental sulfur (1), as well as other compounds containing sulfur in labile organic and inorganic linkage, play an important part in radiation chemical reactions in vitro and in vivo (2-5), owing to their protective action against radiation effects. The protective power of a substance in aqueous solution means its ability to decrease or abolish the effect of radiation on another solute, and this depends on the relative probabilities with which the two substances eliminate radicals formed from the solvent (water) by radiation. Very low concentrations of solute decrease the probability of collision between solute and radicals and therefore favor the recombination of radicals. At higher concentrations of solute this competing recombination gradually becomes negligible; i.e., the radiation yield increases. For some substances a constant yield is reached; for others the yield continuously increases with concentration. This variation in concentration dependence is not yet fully understood, as pointed out in previous communications (6-9). A possible explanation is that with good protectors, i.e., good acceptors of radicals, the recombination of radicals will become more negligible at lower concentrations than with inefficient ones, leading to a constant number of molecules changed per 100 ev (G). Other factors, e.g., molecular weight and shape of molecules, in particular in proteins, may be additional causes. It was observed in preliminary experiments that the amounts of sulfur liberated by ionizing radiations from thiourea increased with concentration without reaching a constant value, although this substance is a good protector. Because of this apparent contradiction in the yield/concentration relationship, it seemed of interest to investigate the radiation chemistry of thiourea in greater detail. The results obtained exceeded the initial purpose of the investigation and revealed various interesting and unusual features in the radiation reaction. Since the preliminary experiments were carried out with the crude technique of weighing very small amounts of sulfur, a more quantitative procedure was adopted 35