Elastic and Inelastic Scattering of 18-Mev Alpha Particles from Neon, Argon, and Xenon

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Abstract

The scattering of 18-Mev alpha particles from neon, argon, and xenon was studied with a multiplate reaction chamber. The scattered particles were defined within an rms angular width of 0.45\ifmmode^\circ\else\textdegree\fi{} by a system of slit pairs, spaced every 2\textonehalf{}\ifmmode^\circ\else\textdegree\fi{} from 10\ifmmode^\circ\else\textdegree\fi{} to 170\ifmmode^\circ\else\textdegree\fi{}. The elastic scattering from neon and argon show the pronounced maxima and minima characteristic of diffraction scattering but are equidistant in $\ensuremath{\varphi}$, not $sin(\frac{\ensuremath{\varphi}}{2})$. The ratio to Rutherford scattering varies as much as 25-fold between successive maxima and minima in the case of neon, the well-defined structure indicating a small mean free path for absorption of alpha particles in the nucleus. Nuclear interaction radii calculated by the formula $2kR\ensuremath{\Delta}[sin(\frac{\ensuremath{\varphi}}{2})]=\ensuremath{\pi}$ were found to be 6.36\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm for neon and 6.95\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm for argon. Xenon, investigated chiefly for control purposes, showed no definite deviation from Rutherford scattering up to 50\ifmmode^\circ\else\textdegree\fi{}. Groups corresponding to the excitation of the 1.63-, 4.25-, 4.97-, 5.81(5.63)-, and 7.2-Mev levels of ${\mathrm{Ne}}^{20}$ and the 1.46-Mev level of ${\mathrm{A}}^{40}$ were observed. No excited states were observed in xenon. Notably absent was excitation of the 6.74-Mev ${(0}^{+}$) level in ${\mathrm{Ne}}^{20}$. As predicted by direct-interaction theories, the cross sections for inelastic scattering leading to the first excited ${(2}^{+}$) states of neon and argon could be approximated by the squares of spherical Bessel functions of the second order with interaction radii of 6.71\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm for neon and 6.60\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}13}$ cm for argon. These cross sections do not tend toward small values in the forward direction, which is interpreted as evidence for distortion of the incident and scattered waves. No fit was possible for any of the other excited states.

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