A systematic on-line nuclear-orientation study of heavy isotopes by use of anisotropic $\alpha$ emission is reported for the first time. The anisotropies recorded for ${}_{}{}^{199}{}_{}{}^{}\mathrm{At}$, ${}_{}{}^{201}{}_{}{}^{}\mathrm{At}$, and ${}_{}{}^{203}{}_{}{}^{}\mathrm{At}$ are remarkably pronounced and strongly varying. At lower neutron number the $\alpha$ particles are more preferentially emitted perpendicularly to the nuclear-spin direction. This may be interpreted in terms of the high sensitivity of the $\alpha$-emission probability to changes in the nuclear shape.

• Received 5 February 1986

DOI:https://doi.org/10.1103/PhysRevLett.56.1901