= Attenuation count model with an additive background
{title2=$\mu_n=t[N_0e^{-bn}+B]$}
For a constant-rate radiation measurement through $n$ identical absorbing sheets, a simple <Poisson distribution> model has expected counts $\mu_n=t[N_0e^{-bn}+B]$. Here $N_0$ is the unattenuated signal rate, $b$ is attenuation per sheet, $B$ is an additive background rate and $t$ is exposure. At low attenuation the signal is approximately exponential; at strong attenuation the observed rate approaches $B$. Consequently a logarithmic plot of raw counts bends towards a plateau rather than remaining a straight line. Measure background independently with the source absent, or fit it with its uncertainty; subtracting it and then taking a logarithm is unreliable when the net count is small or negative.
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