Scattering of High Velocity Electrons by Thin Foils

Abstract

With a narrow, homogeneous beam of electrons, scattering by thin foils of aluminum, silver and gold has been investigated. Voltages up to 145 kv ($\ensuremath{\beta}=0.63$) were used. Under conditions where single scattering was predominate and secondary electrons were absent, the amount of scattering was studied as a function of the primary energy, atomic number and angle. Absolute values were also obtained. The above investigations have lead to the following results: (1) Secondary electrons are defined and a means of eliminating them is proposed and used. (2) Wentzel's criterion for single scattering is tested over a wide range of energies. The value of $\frac{\ensuremath{\theta}}{4{\ensuremath{\omega}}_{min}}$ for aluminum is found to increase from 3.3 at 45 kv to 6.1 at 145 kv. (3) A more critical criterion for single scattering by thin foils is obtained which depends on the shape of the curve connecting $\ensuremath{\rho}$, the amount of scattering, with angle. (4) Dependence of scattering on energy of primary beam is found to agree well with either Mott's equation or with the relation $\frac{k}{{V}^{2}}$, but is at variance with the classical relativistic theory. (5) Comparison of values of scattering for aluminum, silver and gold shows that $\ensuremath{\rho}$ increases faster than ${\mathrm{Z}}^{2}$. (6) Scattering is obtained as a function of angle from 95\ifmmode^\circ\else\textdegree\fi{} to 173\ifmmode^\circ\else\textdegree\fi{}. For aluminum the dependence found experimentally agrees well with either Mott's or Rutherford's equation. The latter also gives the correct dependence on angle for silver and gold. Mott's equation is not applicable for these heavy elements. (7) Absolute values of scattering for aluminum compared with theory give $\ensuremath{\rho}=1.32$ of the value given by Mott's equation. This relation is valid within the ranges $\ensuremath{\theta}={95}^{\ensuremath{\circ}}\ensuremath{-}{173}^{\ensuremath{\circ}}$, $V=56\ensuremath{-}145$ kv. (8) Secondary electrons coming from the foil are distributed according to the simple cosine law. (9) No evidence of loss of energy due to radiation is found up to one-half the energy of the primary beam.