On the Visibility of the Single Collision Peak in BCA-based Simulations

Binary Collision Approximation (BCA) simulations, particularly those using the TRIM/SRIM
framework, are widely used to interpret ion-solid interactions. However, in grazing-incidence scat-
tering of heavy ions at low energies, significant discrepancies often arise between simulated and
experimental energy spectra. A notable issue is the persistent prediction of a single-collision (SC)
peak by standard BCA codes, which is frequently absent [1] or suppressed [2] in experimental
observations, such as in 14 keV Sn scattering from Mo.
This work presents a systematic sensitivity analysis to investigate how internal, typically hard-
coded, algorithmic parameters in the legacy TRIM code influence the visibility of the SC peak. We
employed a modified version of the 1985 Fortran-77 TRIM core to expose and manipulate three
key categories of parameters: (i) electronic energy-loss straggling models, (ii) free-path sampling
statistics, and (iii) the maximum impact parameter (pmax).
Our results indicate that electronic straggling and free-path stochasticity act primarily as sec-
ondary smearing mechanisms. In contrast, the maximum impact parameter emerged as the decisive
control variable. By scaling the mean free path and pmax to explicitly sample long-range, small-
angle encounters, we observed a progressive erosion and eventual suppression of the SC peak
via a cumulative out-scattering mechanism. Furthermore, complementary simulations using the
MARLOWE code are being conducted to assess the influence of target crystallinity and distinct
collision-selection rules on the SC peak visibility, and these results will be presented.
Our findings suggest that the prominence of the SC peak in standard BCA results is largely an
artifact of the default small-angle truncation embedded in the pmax prescription [3]. Understanding
these hidden collision-sampling biases is essential for a reliable comparison between BCA simula-
tions and grazing-incidence experiments.

References
[1] R. A. Wilhelm, et al., Nucl. Instrum. Meth. B 544 (2023) 165123.
[2] S. Rai et al., Nucl. Instrum. Meth. B 482 (2020) 58.
[3] G. M. Azevedo and P. L. Grande, Nucl. Instrum. Meth. B 575 (2026) 166111.