Shapes of Allowed and Unique First-Forbidden β-Ray Spectra: 114In, 42K, 86Rb, 90Sr, and 90Y by H. Daniel and others (1964)

Physical Review 136, B1240. Primary experimental study with a double-lens beta spectrometer. The abstract distinguishes the allowed indium spectrum from the unique first-forbidden potassium, rubidium, strontium and yttrium spectra, and reports deviations from their simplest shape models. This is not an experimental measurement of yttrium-91 or the source exam's plotted points.