  Recent analysis of the viability of solid state-based relic neutrino
detectors has revealed the fundamental necessity for the use of heavy, $A>100$,
$\beta$-decayers as neutrino targets. Of all heavy isotopes, $^{171}$Tm and
$^{151}$Sm stand out for their sufficiently low decay energies, reasonable
half-life times and stable daughter nuclei. However, the crucial bit of
information, that is the soft neutrino capture cross-section is missing for
both isotopes. The main reason for that is a particular type of $\beta$-decay,
which precludes a simple link between the isotope's half-life time and the
neutrino capture rate. Here we propose an experimental method to bypass this
difficulty and obtain the capture cross-section of a soft neutrino by a given
isotope from the isotope's $\beta$-spectrum.
