Backscattering study and theoretical investigation of planar channeling processes. I. Experimental results

Backscattering experiments in planar channeling have been performed on iron single crystals with 1.9-MeV ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ beams; these conditions having been chosen for optimal study of the structure of the spectra. Both for the (110) and (100) planes five equally spaced yield maxima are clearly resolved, the maxima damping out at lower energies. Spectra were also registered at various angles of incidence ${\varphi }_0$ with respect to the planes. Yield maxima are observed up to values of ${\varphi }_0$ twice the half-width at half-minimum ${\psi }_{\frac{1}{2}}$ of an angular scan across the plane [${\psi }_{\frac{1}{2}}={18}^{\prime }$ for the (110) plane]. Except for the first two peaks, these maxima have the same spacing as in the aligned spectrum. They appear to be due to particles belonging to a well-defined transverse energy interval. The mean stopping power for these particles is close to the random stopping power and the mean half-wavelength of their oscillating trajectories in the planar channels calculated from the results is $\lambda =380\mathrm{}$ Å for the (110) plane and $\lambda =320\mathrm{}$ Å for the (100) plane. For ${\varphi }_0>1.2\mathrm{}{\psi }_{\frac{1}{2}}$, the yield on the first maximum is greater than the random yield, reaching ∼1.6 times the latter for $1.4{\psi }_{\frac{1}{2}}<{\varphi }_0<1.8\mathrm{}{\psi }_{\frac{1}{2}}$. The shoulder effect in the angular scans, as observed for various depths, is clearly related to the yield maxima and hence depends strongly on the position and width of the depth interval chosen. The meaning and validity of the assumption of statistical equilibrium for planar channeled particles are discussed in light of the results.