Spectrum of Helium-3

Abstract

THE possibility of obtaining samples of helium in which the isotope of mass 3 has been enriched allows the isotope shifts in the arc spectrum of this element to be investigated. The first samples, which had kindly been made available to us by Dr. Rollin and Mr. Hatton¹, had been prepared by means of thermal diffusion, and contained helium-3 in a concentration of 1 : 700. With an ordinary Fabry Pérot étalon, the detection of satellites several hundred times weaker than the main line is impossible, owing to the finite background intensity of this instrument. This difficulty was overcome by the use of two étalons, of equal spacings, in series. By suitable choice of apertures, distances and orientations of the wedge-shaped étalon plates, it was found comparatively easy to avoid secondary reflexions. The method appears to be very useful for the investigation of faint satellites very close to strong lines.