= Foil areal number density
{title2=$n_s=nt$}
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.
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