The beta spectrum of carbon 14 was measured with radioactive methane as the source in a proportional counter. To reduce end and side wall effects a pressure of 6 atmos (argon) was employed and a magnetic field of 3500 gauss was applied parallel to the counter axis. The spectrum of sulfur 35 was observed with the same equipment. The theoretical allowed spectrum for this source was modified according to calculations made to take into account the instrumental limitations. The modified theoretical form agreed excellently with the observations down to energies below 5 kev. With the sulphur spectrum as an experimental standard for an allowed shape very accurate empirical correction factors could be obtained, and these were applied to the data for ${\mathrm{C}}^{14}$. The adjusted data for ${\mathrm{C}}^{14}$ were found to follow the theoretical shape for an allowed transition to within 1 percent accuracy at energies above about 3 kev.

• Received 28 June 1954

DOI:https://doi.org/10.1103/PhysRev.96.395