The Third Forbidden Beta Spectrum of Rubidium-87

The use of thin-lens beta spectrometers has been extended to the measurement of an isotope of extremely low specific activity, rubidium 87. The ${\mathrm{Rb}}^{87}$ beta spectrum is third forbidden and not "unique." The spectrum gave a linear Kurie plot when the third forbidden tensor (or vector) correction factor was applied. The extrapolated value $E_{max}=275\mathrm{}$ kev agreed well with the results of recent investigations.