Production of Rare-EarthαEmitters with EnergeticHe3Particles; New Isotopes:Er151,Yb156, andYb157

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Abstract
α-active isotopes in the rare-earth region were investigated by bombarding Dy156 and Er162 targets with He3 ions accelerated in the Oak Ridge isochronous cyclotron. In the experiments recoil nuclei ejected from the thin targets were stopped in helium gas, swept through an orifice, and deposited on catcher foils. After each bombardment the activity collected on the catchers was assayed with an α-particle Si(Au) spectrometer. In the case of the Dy156 target relative yields were obtained as a function of bombarding energy for known erbium, holmium, and dysprosium nuclides. Excitation functions determined from statistical-model calculations were found to be in reasonable agreement with the shapes of the experimental (He3,xn) and (He3,pxn) curves but not with those of the (He3,αxn) curves. Experimentally, the yields for (He3,pxn) and (He3,αxn) products were found to be ∼3 to 5 times greater than those for (He3,xn) products. The statistical-model calculations, contrastingly, indicated the (He3,xn) and (He3,pxn) cross sections to be comparable and ∼5 times greater than those for (He3,αxn) products. Similar yields were measured for nuclides observed in bombardments of the Er162 target and were used in the assignment of two new α-emitting isotopes to Yb156 and Yb157. Their half-lives and α-decay energies were measured to be: (1)