Electron Slowing-Down Spectrum in Cu of Beta Rays fromCu64

Abstract

The electron slowing-down spectrum of $^{64}\mathrm{Cu}$ beta rays in an effectively infinite isotropic copper medium has been measured from 56 keV to 11 eV above the bottom of the conduction band with a spherical electrostatic electron spectrometer utilizing a Faraday cup and an electrometer as detector. The black-body cavity source was in the shape of two parallel coaxial disks of radioactive copper. Electrons emerged into the spectrometer through the opening between the disks. The flux varied from about 2.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ electrons ${\mathrm{cm}}^{\ensuremath{-}2}$ e${\mathrm{V}}^{\ensuremath{-}1}$ per primary beta-ray ${\mathrm{cm}}^{\ensuremath{-}3}$ at 56 keV to 3.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ at the peak of the secondary-electron maximum at 15 eV above the bottom of the conduction band. A step at about 7 keV is identified as the sum of the $\mathrm{Ni} \mathrm{KLL}$ Auger electrons caused by electron capture in $^{64}\mathrm{Cu}$, $\mathrm{Cu} \mathrm{KLL}$ Auger electrons from the filling of $K$ vacancies in copper atoms ionized by beta rays, and photoelectrons produced by ${K}_{\ensuremath{\alpha}}$ x-rays from both Ni and Cu. Less prominent steps at 800 and 250 eV are probably $\mathrm{LMM}$ Auger electrons and ${L}_{\ensuremath{\alpha}}$ photoelectrons, and the Compton edge from ${K}_{\ensuremath{\alpha}}$ x rays, respectively. The spectrum was compared above 35 eV with our extension of the Spencer-Attix continuous slowing-down theory and above 1.8 keV with the tabulation of the Spencer-Fano theory by McGinnies. Agreement with both theories was found everywhere within experimental uncertainties.