Search for solar axions emitted in the M1-transition of 7Li* with Borexino CTF

Results of background measurements with a prototype of the Borexino detector were used to search for 478 keV solar axions emitted in the M1-transitions of ⁷Li^*. The Compton conversion of axion to a photon A+e→e+γ, axioelectric effect A+e+Z→e+Z, decay of axion in two photons A→2γ and Primakoff conversion on nuclei A+Z→γ+Z are considered. The upper limit on constants of interaction of axion with electrons, photons and nucleons – g_Aeg_AN≤(1.0–2.4)×10^-10 at m_A≤450 keV and g_Aγg_AN≤5×10^-9 GeV^-1 at m_A≤10 keV are obtained (90%c.l.). For heavy axions with mass at 100<m_A<400 keV the limits g_Ae<(0.7–2.0)×10^-8 and g_Aγ<10^-9–10^-8 are obtained in assumption that g_AN depends on m_A as for KSVZ axion model. These limits are stronger than obtained in previous laboratory-based experiments using nuclear reactor and artificial radioactive sources.