By means of a high intensity mass spectrometer we have separated the three isotopes of potassium. Measurements of the radioactivity of the isotopic samples indicate that ${\mathrm{K}}^{40}$ is radioactive. Comparison of the beta-particle emission from the isotopic samples and from KCl shows that ${\mathrm{K}}^{40}$ is responsible for both of the known beta-ray bands.

• Received 25 November 1936

DOI:https://doi.org/10.1103/PhysRev.51.178