Cosmic-Ray Electron Flux above 4.7 BV

The cosmic-ray electron flux was measured at 2.16 g/${\mathrm{cm}}^2$ and 4.35 g/${\mathrm{cm}}^2$ residual atmosphere, using a lead-plate spark chamber to detect the soft showers. Both flights were made from Palestine, Texas, where the vertical geomagnetic cutoff is 4.7 BV. A twelve-gap spark chamber containing seven lead plates of 6.28 g/${\mathrm{cm}}^2$ each and five magnesium of 1.1 g/${\mathrm{cm}}^2$ was used to detect the electrons. Six hundred and eighty five "electron-type" events were detected at 4.35 g/${\mathrm{cm}}^2$, and 197 at 216 g/${\mathrm{cm}}^2$. The number of electron showers was deduced from the data by utilizing the difference between the cascade length and the nuclear interaction length. In the first three lead plates, 97% of the electrons initiate substantial showers. The nuclear particles interact almost equally in the seven lead plates, and the number of nuclear interactions in the first three plates is deduced from the observed rate in 4 through 7. The excess interactions observed in the first three lead plates are caused by electrons. The electron flux was determined at 4.35 g/${\mathrm{cm}}^2$ residual atmosphere to be 16.3±5.3 electrons/sr ${\mathrm{m}}^2$ sec and at 2.16 g/${\mathrm{cm}}^2$ residual atmosphere to be 4.8±5.4 electrons/sr ${\mathrm{m}}^2$ sec. The upper limit to the primary flux is 5.5 electrons/sr ${\mathrm{m}}^2$ sec when both statistical and instrumental uncertainties are included. The penetrating particle flux was monitored throughout the flight at 4.35 g/${\mathrm{cm}}^2$ as 684±40 penetrating particles/sr ${\mathrm{m}}^2$ sec and at 2.16 g/${\mathrm{cm}}^2$ as 750±41 penetrating particles/sr ${\mathrm{m}}^2$ sec.