Shape analysis of blocking dips: monte carlo vs. analytical results

Angular blocking dips around the ⟨110⟩ axis in Al single crystal of α-particles of about 2 MeV produced at a depth of 0.2 μm are calculated for several values of the mean transverse displacement υ ⊥ τ of the decaying nucleus within the range 0 ≤ υ ⊥ τ ≤ 260 pm. Calculations have been made both by an extensive multistring Monte Carlo simulation and by a continuum model with diffusion. As far as the Monte Carlo method is concerned, the influence of the (small) solid angle of particles emission and of the “single interaction” approximation has been investigated. The analytical calculations performed on the basis of a Molière (thermally averaged) multistring potential show, for large υ ⊥ τ a clear dependence of the blocking dips on the recoil direction and a sharp peak at very small angles. The shapes of the dips obtained by the two methods are in overall good agreement while a very satisfactory comparison has been found for the dip widths and the relative parameters used in many lifetime measurements.