Evidence for an Isomeric State ofY90

Abstract

The recently reported activity induced by neutron bombardment of niobium has been produced by 14-Mev neutrons and by neutrons of energy less than 6 Mev, on both niobium and zirconium. In each case, chemical separation showed the activity to be due to an isotope of yttrium. Two coincident gamma rays having energies of 0.200 and 0.485 Mev and a half-life of 3.1±0.1 hr were observed; these observations were in agreement with earlier results. The activity appears very similar to that which has been previously attributed to the decay of ${\mathrm{Y}}^{92}$. However, threshold considerations and the failure to observe by means of a thin-window Geiger counter any beta emission associated with this gamma activity, point to an isomeric state of ${\mathrm{Y}}^{90}$. Experiments with separated isotopes of ${\mathrm{Zr}}^{90}$ and ${\mathrm{Zr}}^{92}$ support this assignment.