  We discuss the cross section formula both for massless and massive neutrinos
on stable and radioactive nuclei. The latter could be of interest for the
detection of cosmological neutrinos whose observation is one of the main
challenges of modern cosmology. We analyze the signal to background ratio as a
function of the ratio of the neutrino mass over the detector resolution and
show that an energy resolution less than 0.5 eV would be required for sub-eV
neutrino masses, independently of the gravitational neutrino clustering.
Finally we mention the non-resonant character of neutrino capture on
radioactive nuclei.
