= Solution
A fission reactor supplies predominantly <electron antineutrinos>, with typical <energies> of a few MeV. Even if they oscillate to <muon antineutrinos>, producing a real <muon> through a <charged current> requires roughly the <muon> rest <energy>, over $100$ MeV for a free nucleon target. For example,
$$
E_{\rm th}(\bar\nu_\mu p\to\mu^+n)
=\frac{(m_n+m_\mu)^2-m_p^2}{2m_p}\simeq113\ \mathrm{MeV}.
$$
Thus a <muon> spectrometer cannot observe this appearance channel from a reactor. Measure <electron antineutrino> disappearance instead, most commonly through <inverse beta decay>, $\bar\nu_ep\to e^+n$, using the prompt <Positron> and delayed <neutron> signal.
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