Number 18 (Story #1), January 23, 1991 by Phillip F. Schewe and Ben Stein THE MOST PRECISE ISOTOPE SHIFT measurement ever made for a multi-electron atom has been performed by a group of scientists at Harvard (Francis M. Pipkin, 617-495-2910). Observing the same atomic transition in both helium-3 and helium-4, the scientists can study the isotope shift, whose chief components are the "mass shift," the shift due to the different motion of the two nuclei (the He-3 nucleus has two protons and one neutron, whereas He-4 has an extra neutron), and the "volume shift," the shift due to the different shape of the electric charge distribution for the two nuclei. The isotope shift was measured using laser-microwave spectroscopy techniques to a precision of one part per million, 100 times better than previous measurements, allowing checks on various theoretical calculations. (Phys. Rev. Lett., Feb. 4)