The momentum spectra of the negatrons and positrons emitted by ${\mathrm{Cu}}^{64}$ have been measured with high resolution under such conditions that the usual distortions resulting from source thickness and backing, scattering, and finite counter window are completely negligible for almost the entire range of energies. Whereas the Fermi theory predicts the correct distribution for the high energy region, it is found that there are more negatrons and positrons than are predicted by the theory at low energies. The shapes of the spectra and the fact that no nuclear gamma-rays are observed preclude the possibility that they are complex spectra. Deviation from the Fermi theory occurs for energies below 0.270 Mev for the positrons and for energies below 0.190 Mev for the negatrons.

Incidental to these measurements, the end-point energies have been determined as 0.657±0.004 Mev for the positrons and 0.571±0.002 Mev for the negatrons. The ratio of the total number of electrons to positrons is 2.0.

• Received 10 December 1947

DOI:https://doi.org/10.1103/PhysRev.73.601