Structure of theHe4Nucleus from Elastic Electron Scattering

We have measured the cross section for elastic electron scattering from ${\mathrm{He}}^4$ at values of $q^2$, the four-momentum transfer squared, ranging from 0.5 to 20.0 ${\mathrm{F}}^{-2\mathrm{}}$. Previous Stanford measurements extended up to $q^2=6.2\mathrm{}$ ${\mathrm{F}}^{-2\mathrm{}}$ and were consistent with a Gaussian shape for the charge distribution of the ${\mathrm{He}}^4$ nucleus. Our results confirm the earlier data but show a deviation from the Gaussian model for $q^2>6\mathrm{}$ ${\mathrm{F}}^{-2\mathrm{}}$. At $q^2=10\mathrm{}$ we observe a diffraction minimum of the scattering cross section. Among the models which have been found to fit the new data is the Fermi three-parameter charge distribution. The rms radius ${〈r^2〉}^{\frac{1}{2}}$, the half-density radius $r_{\frac{1}{2}}$, and the 90% to 10% skin thickness $t$ of the charge distribution have been calculated for the models which adequately fit the data. The results indicate that the surface of the ${\mathrm{He}}^4$ charge distribution is less diffuse than implied by the nuclear harmonic-oscillator model.