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.
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