The precise determination of the number of muons in extensive air showers is a key issue for being able to separate showers that have been initiated by different primary particles. In the context of planned muon detector extensions of the Pierre Auger and Telescope Array Observatories, we analyze CORSIKA shower simulations at energies above 10 18 eV to quantify expectations on the separability of primary particles. We observe that an improved separation power, described in terms of the figure

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... merit, is achieved if muons of low momenta are rejected, i.e. the detector energy threshold is set to ∼ 1 GeV. Investigating the impact of photonuclear reactions in the shower development, we find that there are less muons produced by photonuclear precursor reactions in iron initiated showers than in proton initiated showers. At low momenta, these muons considerably lower the figure of merit and hence worsen the discrimination of primary masses.