Precision Determination of the Energy of the Gamma Ray of Potassium-40

The gamma ray of potassium-40 was compared with the 1368-keV line of sodium-24 in a scintillation spectrometer. The lines of yttrium-88 were simultaneously recorded, and linear interpolation between these lines was corrected to bring the sodium-24 value into accord with the accepted standard value. A proportional correction was applied to the interpolated value for the energy of the potassium-40 line. Sources consisted of (i) yttrium-88 mixed with potassium bicarbonate, and (ii) yttrium-88 mixed with sodium bicarbonate, with the sodium activated by a small neutron source. About seven kilograms of bicarbonate were used in each case; to obtain sufficient potassium-40 activity in (i), and to simulate approximately the small-angle Compton scattering in a substance of equal density in (ii). Full-energy peaks in the pulse-height spectra were fitted to a normal distribution by an iterative method of least squares. With about 100 000 counts in each peak, energy determinations were reproducible to about 0.5 keV. The energy of the potassium gamma ray is 1459.7±1.0 keV. The dependence of the result upon changes in the standards is also given.