Random Wilson cloud-chamber photographs of cadmium and molybdenum foils show that the double negatron half-life is greater than or equal to 1×${10}^{17}$ years in ${\mathrm{Cd}}^{116}$ and 3×${10}^{17}$ years in ${\mathrm{Mo}}^{100}$, and that the double positron half-life is greater than or equal to 6×${10}^{16}$ years in ${\mathrm{Cd}}^{106}$ and 4×${10}^{18}$ years in ${\mathrm{Mo}}^{92}$. The background appears to be primarily caused by Compton electrons and photoelectrons ejecting other electrons on their way out of the foil and by photons that suffer two Compton scatterings while traversing the foil.

• Received 7 March 1955

DOI:https://doi.org/10.1103/PhysRev.99.88