The direct measurement of gamma‐rays only yields about half the picture in the study of nuclear energy levels. The other, and indeed to most physicists more familiar, half concerns the β‐rays, including in this term both the continuous β‐ray spectrum and the line spectrum by “conversion”, either internal or external. Ever since 1948, therefore, we have been much occupied with the design and construction along rather novel lines of a high precision helical focusing magnetic β‐ray spectrometer planned as a companion instrument to the crystal diffraction spectrometer as regards absolute precision and accuracy. We have only very recently completed this instrument and made the first tests on it which indicate that it will meet all our expectations both as to high absolute accuracy and high luminosity and sensitivity to weak sources.
The beta‐ray spectrometer discussed in the following pages has recently been completed at the California Institute of Technology. It is shown in cross section on the cover.