Foil areal number density ($n_s=nt$)

The number of target chemical atoms per incident area along the beam. For a uniform foil at normal incidence, number density $n$ and thickness $t$ give $n_s=nt$, in ${\mathrm{m}}^{-2}$. At oblique incidence the traversed path must replace $t$. The single-encounter count model requires the chance of two significant deflections to be negligible and energy loss through the foil to be small. A small probability in one detector patch alone does not establish either condition.