Fig. 5.

Monte Carlo spectra of ${}^{238}$U decay chain, zoomed on ${}^{222}$Rn and ${}^{218}$Po peaks, obtained by simulating the contaminants in the crystal bulk (left), and on crystal surfaces with depth parameters of 10 $\mu$m (center) and 10 nm (right). The fraction of ${}^{222}$Rn–${}^{218}$Po delayed-coincidences (highlighted in red) over the total events recorded at the Q value peaks decreases as the contaminants are simulated in a thinner surface layer due to $\alpha$-particle escapes. The small peaks appearing in the rightmost plot are due to recoil escapes occurring when contaminants are simulated in a very shallow surface layer ($\lambda =10$ nm)