  We calculate the response of a lead-based detector, such as the Helium and
Lead Observatory (HALO) or its planned upgrade HALO-1kt to a galactic
core-collapse supernova. We pay particular attention to the time dependence of
the reaction rates. All reaction rates decrease as the neutrino luminosity
exponentially drops during the cooling period but the ratio of one-neutron (1n)
to two-neutron (2n) event rates in HALO is independent of this overall
decrease. Nevertheless, we find that this ratio still changes with time due to
the changing character of neutrino flavor transformations with the evolving
conditions in the supernova. In the case of inverted hierarchy, this is caused
by the fact that the spectral splits become less and less sharp with the
decreasing luminosity. In the case of normal hierarchy, it is caused by the
passage of the shock wave through the Mikheyev-Smirnov-Wolfenstein resonance
region. However, in both cases, we find that the change in the ratio of 1n to
2n event rates is limited to a few percent.
