Search for the (n^th, α) reaction on¹⁸⁰Hf,¹⁸¹Ta and²⁰⁹Bi

The (n_th, α) reaction on¹⁸⁰Hf,¹⁸¹Ta and²⁰⁹Bi was studied at the thermal neutron guide of the Grenoble high flux reactor. Upper limits of σ_α≦13μb, ≦μb and ≦0.3μb, respectively, were obtained for the a branches to the ground states in contrast to the values of ≈100μb to ≈500 mb reported in the literature. Possible reasons for these discrepancies are discussed. Furthermore, the present upper limits of the cross sections are compared with the values calculated from the statistical model.