Production of Rare-Earth α Emitters with Energetic 3He Particles; New Isotopes: 151Er, 156Yb, and 157Yb

α-active isotopes in the rare-earth region were investigated by bombarding ¹⁵⁶Dy and ¹⁶²Er targets with ³He 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 ¹⁵⁶Dy 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 (³He,xn) and (³He,pxn) curves but not with those of the (³He,αxn) curves. Experimentally, the yields for (³He,pxn) and (³He,αxn) products were found to be ~3 to 5 times greater than those for (³He,xn) products. The statistical-model calculations, contrastingly, indicated the (³He,xn) and (³He,pxn) cross sections to be comparable and ~5 times greater than those for (³He,αxn) products. Similar yields were measured for nuclides observed in bombardments of the ¹⁶²Er target and were used in the assignment of two new α-emitting isotopes to ¹⁵⁶Yb and ¹⁵⁷Yb. Their half-lives and α-decay energies were measured to be: (1) ¹⁵⁶Yb, 24 +/- 1 sec, 4.80 +/- 0.01 MeV, and (2) ¹⁵⁷Yb, 34 +/- 3 sec, 4.50 +/- 0.01 MeV. The assignment of the 4.80-MeV α group to ¹⁵⁶Yb was clearly established by noting a generic relationship between it and its known α-decay daughter, ¹⁵²Er. A similar clear-cut confirmation, based on a generic relationship, was not possible for ¹⁵⁷Yb. The previously unknown nuclide, ¹⁵¹Er, was found in the ¹⁵⁶Dy bombardments by noting an initial growth period in the 4.60-MeV α-particle group known to follow the decay of its daughter, ^151mHo. From this growth the half-life of ¹⁵¹Er was found to be 23 +/- 2 sec. The existence of 5.3-min ¹⁵⁵Er, previously observed by us in ⁴He bombardments of ¹⁵⁶Dy, was confirmed. Its α-decay energy was measured to be 4.01 +/- 0.01 MeV, in agreement with the value reported earlier.