  Geo-neutrino studies are based on theoretical estimates of geo-neutrino
spectra. We propose a method for a direct measurement of the energy
distribution of antineutrinos from decays of long-lived radioactive isotopes.
We present preliminary results for the geo-neutrinos from Bi-214 decay, a
process which accounts for about one half of the total geo-neutrino signal. The
feeding probability of the lowest state of Bi-214 - the most important for
geo-neutrino signal - is found to be p_0 = 0.177 \pm 0.004 (stat)
^{+0.003}_{-0.001} (sys), under the hypothesis of Universal Neutrino Spectrum
Shape (UNSS). This value is consistent with the (indirect) estimate of the
Table of Isotopes (ToI). We show that achievable larger statistics and
reduction of systematics should allow to test possible distortions of the
neutrino spectrum from that predicted using the UNSS hypothesis. Implications
on the geo-neutrino signal are discussed.
