Elastic Scattering of 188-Mev Electrons from the Proton and the Alpha Particle

The elastic scattering of 188-Mev electrons from gaseous targets of hydrogen and helium has been studied. Elastic profiles have been obtained at laboratory angles between 35\ifmmode^\circ\else\textdegree\fi{} and 138\ifmmode^\circ\else\textdegree\fi{}. The areas under such curves, within energy limits of \ifmmode\pm\else\textpm\fi{}1.5 Mev of the peak, have been measured and the results plotted against angle. In the case of hydrogen, a comparison has been made with the theoretical predictions of the Mott formula for elastic scattering and also with a modified Mott formula (due to Rosenbluth) taking into account both the anomalous magnetic moment of the proton and a finite size effect. The comparison shows that a finite size of the proton will account for the results and the present experiment fixes this size. The root-mean-square radii of charge and magnetic moment are each (0.74\ifmmode\pm\else\textpm\fi{}0.24)\ifmmode\times\else\texttimes\fi{}${10}^{13}$ cm. In obtaining these results it is assumed that the usual laws of electromagnetic interaction and the Coulomb law are valid at distances less than ${10}^{\ensuremath{-}13}$ cm and that the charge and moment radii are equal. In helium, large effects of the finite size of the alpha-particle are observed and the rms radius of the alpha particle is found to be (1.6\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm.