Differences in the Distribution of Zirconium-95 and Niobium-95 in the Rat

Abstract

ZIRCONIUM-95, one of the γ-ray emitting isotopes in fall-out, has a physical half-life of 65 days. It decays to niobium-95 which emits γ-rays and has a physical half-life of 35 days. Therefore, both these isotopes contribute to the total radiation hazard of fall-out. During the first year, these two isotopes share no less than 20 per cent of the total activity of fission products aged from a few days to one year¹. A knowledge of the metabolism of the parent isotope (zirconium-95) in relation to that of the daughter (niobium-95) is essential in order to estimate the total internal radiation hazard due to ⁹⁵Zr–⁹⁵Nb because the parent and the daughter decay at different rates. Further, the photopeak energies of zirconium-95 (0.72 MeV) and niobium-95 (0.764 MeV) are so close to one another that it is not possible to resolve and identify the individual photopeaks of the parent and daughter isotopes by γ-scintillation spectrometry¹. Therefore, we have determined the physical decay patterns of ⁹⁵Zr–⁹⁵Nb accumulated by rat tissues to determine whether some tissues show preference for zirconium-95, while others concentrate niobium-95 in four days after intraperitoneal administration of ⁹⁵Zr–⁹⁵Nb in transient equilibrium (Fig. 1).